An Introduction to Metallurgy [4 ed.]

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AN INTRODUCTION TO METALLURGY Complete and Unexpurgated Revised and Enlarged (Banned In Boston)

4th Edition

.....Being a Compendium of Knowledge, Folklore, Whimsy, and Metaphysical Ruminations; Garnered from Hitherto Lost, Mystic Treatises of the Ancients and Substantially Plagiarized from Modern Day Texts; Designed to Guide the Innocent Traveler Unscathed Through the Labyrinth of Pitfalls, Misconceptions, Archaic Ideas, Abstruse Theories, and Pseudo-Experts that Inhabit the Great Void and Contrive to Waylay the Unsuspecting Pilgrim at Every Chance Encounter Whilst on His Journey Towards Metallurgical Enlightenment.

Concocted by J.D. Dufour Manager, Metallurgy Cameron Division Cooper Cameron Corporation Superbly compiled by Cecilia A. Hobbs Cameron Division Cooper Cameron Corporation

"In that direction," the Cat said, waving its right paw round, "lives a Hatter: and in that direction," waving the other paw, "lives a March Hare. Visit either you like; they're both mad." "But I don't want to go among mad people," said Alice. "Oh, you can't help that," said the Cat: "we're all mad here. I'm mad. You're mad." "How do you know I'm mad?," said Alice. "You must be," said the Cat, "or you wouldn't have come here." Lewis Carroll Alice's Adventures in Wonderland (1865)

RAVE REVIEWS FOR The First Edition Of

AN INTRODUCTION TO METALLURGY!

.....To gain a clear understanding of the complexities of metallurgy, read Elements of Material Science by L.H. Van Vlack. Don't read this book! CHICAGO TIMES .....A crime against humanity! THE VATICAN PRESS

.....It is difficult to select the most appropriate adjective for this book - vulgar, contemptible, vicious, pusillanimous, obscene, crass, blasphemous, noxious. All immediately come to mind and all apply equally well. BOSTON GLOBE .....Detente has been set back 5 years. TASS

.....Seldom has there been so prurient and pernicious a volume. CHRISTIAN HERALD

.....Humbug! LONDON EXAMINER

.....In making this contribution to world peace, the author has joined the ranks of Ivan the Terrible, Stalin, the Marquis de Sade, Genghis Khan, Hitler, Idi Amin, and Attila the Hun. W ASHINGTON REPORTER .....America has finally avenged Pearl Harbor. TOKYO STAR

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CRITICAL ACCLAIM FOR The Second Edition Of

AN INTRODUCTION TO METALLURGY! .....We previously stated that the first edition was the nadir of scientific thought in this century. After reading the second edition, we apologize for having slandered the first. SCIENTIFIC DEVELOPMENT .....Great literature withstands the test of time. I don't look for this book being around tomorrow. PARIS COURIER .....Fertilizer! AMERICAN HORTICULTURALIST

.....Highly recommended for insomnia due to its soporific properties, however, too great a dose results in patient despondency and general blathering. Do not administer to anyone having previous scientific or technical training! Studies show that these patients develop homicidal tendencies towards the author that, if left untreated, develop into psychoses typical of the criminally insane. JOURNAL OF AMERICAN PHYSICIANS .....Morally depraved! Mentally disturbed! MANCHESTER DISPATCH

.....Bombed again! NAGASAKI NEWS

.....One questions the author's motive - Maliciousness? Trying to kill time? Out to make a fast dime? Some budding psychiatric student will find this fertile ground for a future thesis on deviate behavior. TAMPA TIMES .....Takes metallurgy out of the Dark Ages and into the Stone Age. JOURNAL OF METALLURGICAL ENGINEERING

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AND NOW BOUQUETS OF ACCOLADES FOR The Third Edition Of

AN INTRODUCTION TO METALLURGY!

.....One can only wonder at a society that can put a man on the moon and yet produce the author of this book. THE PHYSICS SOCIETY OF AMERICA .....Three strikes...you're out! U.S. SPORTS MAGAZINE

.....Whoever said that the third time's the charm has not yet read this book. LITERARY MAGAZINE .....The author has done to metallurgy what Alferd Packer did to American cuisine. CULINARY ARTS .....Male bovine fecal matter. AMERICAN STOCKMAN

.....The author has declared total war! No punches are pulled. No mercy is shown. No prisoners are taken. There will be few survivors of this devastating course. W EEKEND W ARRIOR MAGAZINE .....The great abyss between the covers of this book is matched only by the one between the author's ears. JOURNAL OF UNEXPLAINED PHENOMENA .....Along with the Titanic, the Hindenberg, federal income tax, and API Spec 6A, one of the great disasters of modern times. GLOBAL CURRENT EVENTS

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PAEANS OF PRAISE FOR The Fourth Edition of

AN INTRODUCTION TO METALLURGY!

...One of the foundations of Darwin’s theory of evolution is that man has evolved from lower life forms. The author of this book is living proof that evolution has now gone full circle. NEW BIOLOGY MAGAZINE ...Dante said it best, “All hope abandon, ye who enter here!” LITERARY SOCIETY

...While we have always deplored the occasional book burning that has stained our past, we would gladly strike the match at an auto-de-fe’ given in honor of this book and its author. NATIONAL LIBRARIAN ...Plumbs new depths of depravity. AMERICAN SURVEYOR

...This book rates alongside the Piltdown man, water witching, ether, and perpetual motion as one of the great hoaxes of modern science. ...The whirling sounds you hear when you open the covers of this book are the great scientists of ages past turning over in their graves. SCIENCE QUARTERLY ...What to read when you have five minutes of free time, but don’t want to be overly distracted from more pressing matters? This book is the ticket. A highly entertaining volume that deserves a place alongside everyone’s commode. Look for it in the pulp fiction section of your local bookstore. SLEEZE MAGAZINE ...The author has boldly gone where no man has gone before - at least no one in their right mind. W ORLD NEWS ...The malignant machinations of a myopic, moronic, monstrous, machiavellian, metallurgist. METALS MONTHLY

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TABLE OF CONTENTS

FORWARD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix CHAPTER I - THE BASICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 CRYSTALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 GRAIN SIZE, SHAPE, AND ORIENTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 COLD AND HOT WORK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 PHASES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 DISLOCATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 STRENGTHENING MECHANISMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 CHAPTER II - HEAT TREATMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ANNEALING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NORMALIZING, AUSTENITIZING, QUENCHING & TEMPERING OF STEELS . . . . . . . . . . . AGE HARDENING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T-T-T DIAGRAMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CONTINUOUS COOLING TRANSFORMATION DIAGRAMS . . . . . . . . . . . . . . . . . . . . . . . . . ODDS & ENDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15 15 16 18 20 29 33

CHAPTER III - ALLOYING ELEMENTS OF STEEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 CHAPTER IV - MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED . . . . . . . . . TENSILE TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IMPACT TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HARDNESS TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FRACTURE MECHANICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FRACTURE TOUGHNESS TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FACTORS AFFECTING FRACTURE TOUGHNESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FATIGUE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . THE FRACTURE MECHANICS APPROACH TO FATIGUE . . . . . . . . . . . . . . . . . . . . . . . . . .

47 48 52 55 59 63 69 73 75

CHAPTER V - MAKING METALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIRGINS AND ORES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . STEEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NICKEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

81 81 83 94

CHAPTER VI - SURVEY OF METALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 THE UNIFIED NUMBERING SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 CARBON AND LOW ALLOY STEELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 STAINLESS STEELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 NICKEL BASE ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 ALLOYING ELEMENTS OF NICKEL BASE ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 HEAT TREATING NICKEL BASE ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 SOME COMMON NICKEL BASE ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 BITS AND PIECES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 CHAPTER VII - FORGING, CASTING, & POWDER METALLURGY . . . . . . . . . . . . . . . . . . . . . FORGING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FORGING DEFECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ADVANTAGES AND LIMITATIONS OF FORGING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CASTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CASTING DEFECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ADVANTAGES AND LIMITATIONS OF CASTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . POWDER METALLURGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

129 129 137 138 139 145 147 148

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TABLE OF CONTENTS ADVANTAGES AND DISADVANTAGES OF CONVENTIONAL POWDER METALLURGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HOT ISOSTATIC PRESSING (HIP’ing) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HIP’ing AS A FORMING PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HIP’ing AS A CLADDING PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ADVANTAGES OF FORMING BY HIP’ing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ADVANTAGES OF CLADDING BY HIP’ing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LIMITATIONS OF HIP’ing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

150 151 151 153 155 156 157

CHAPTER VIII - NONDESTRUCTIVE EXAMINATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RADIOGRAPHY (RT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RT - General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RT - Radiation Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X-ray Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RT - Imaging and Image Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RT - Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ULTRASONIC EXAMINATION (UT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UT - General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UT - Physics of Wave Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UT - Transducers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UT - Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UT - Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UT - Calibration and Standard Reference Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . UT - Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAGNETIC PARTICLE EXAMINATION (MT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MT - General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MT - Materials That Can Be Examined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MT - Types of Magnetic Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MT - Magnetizing Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MT - Magnetizing Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MT - Odds and Ends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MT - Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LIQUID PENETRANT EXAMINATION (PT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PT - General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PT - Types of Penetrants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PT - Developers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PT - Liquid Penetrant Examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PROCESSING FLOW DIAGRAMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PT - Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

161 162 162 164 168 171 177 178 178 179 186 194 198 201 202 203 203 206 206 208 211 215 216 217 217 218 220 221 221 221

CHAPTER IX - SPECIAL PROCESSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 ELECTROPLATING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 ELECTROLESS NICKEL PLATING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 THERMAL SPRAY COATINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 PHOSPHATE COATING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 FLAME HARDENING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 INDUCTION HARDENING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 NITRIDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 CARBURIZING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 BORIDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 CHAPTER X - WELDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 WELDING PROCESSES - GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 page - vi

TABLE OF CONTENTS ARC WELDING PROCESSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OXYFUEL GAS WELDING (OFW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ELECTRON BEAM WELDING (EBW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . WELDMENT TERMINOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . WELD DEFECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . WELDING METALLURGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CORROSION RESISTANT WELD CLADDING PROCESSES . . . . . . . . . . . . . . . . . . . . . . . HIP CLADDING VS WELD CLADDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . BRAZING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . BRAZING METALLURGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

250 257 257 259 262 264 267 269 270 271

CHAPTER XI - CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . THE BASICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ELECTROCHEMISTRY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TYPES OF GALVANIC CELLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . POLARIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PASSIVITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TYPES OF CORROSION DAMAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SULFIDE STRESS CRACKING (SSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . BACTERIA AND CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CO2 CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CORROSION PREVENTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CHEMICAL ADDITIVES USED IN PETROLEUM PRODUCTION . . . . . . . . . . . . . . . . . . . . . MARINE CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . THE MARINE ENVIRONMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SEAWATER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FORMS OF MARINE CORROSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PROTECTIVE MEASURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CATHODIC PROTECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ISOLATING THE EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . INHIBITION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SELECTING INHERENTLY CORROSION RESISTANT MATERIALS . . . . . . . . . . . . . . . . . ADDING A CORROSION ALLOWANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ALTERING THE ENVIRONMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

275 276 280 287 292 292 294 298 298 299 300 301 302 303 305 307 311 312 321 337 339 339 340

APPENDIX A - GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345 APPENDIX B - ABBREVIATIONS & SYMBOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389 APPENDIX C - REGISTERED PRODUCTS REFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 APPENDIX D - BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 APPENDIX E - A CHECKLIST OF FACTORS TO BE CONSIDERED WHEN SELECTING A MATERIAL FOR MARINE ENVIROMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 APPENDIX F - FACTORS TO CONSIDER WHEN SELECTING A COATING SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399 APPENDIX G - TRIM SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401 INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411

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AN INTRODUCTION TO METALLURGY FORWARD

Aaaaaaaaaaaaarrrrrrrrgggggghhh!!!! How often have we heard that plaintive cry just as some production control planner got the word from one of the heat treat foreman that his 410 stainless steel body cracked in half during heat treat and, although it can't be used to build a valve anymore, the halves would make great matching bookends? How many times has the month's business been bushwhacked by parts made out of 4130 material that punched OK at heat treat only to crater at final hardness testing after machining? What has caused the cancerous growth of our material specifications that once averaged only one page long, but are now pushing 15? Fracture toughness, VAR, K Ic, tensile strength, hardness, stringers, J-integral, cracks, laps, elongation, banding, calcium treating, reduction of area, ESR, macro etch, micro etch, heat testing, heat per heat treat lot testing, equivalent heat per heat treat lot testing, lateral expansion, QTC, PSL 6 there seems to be no end to this parade of potential alligators that are stomping out of the swamp in our direction with a mean and hungry look in their eye! All of us in Cameron are well aware of the tremendous impact that materials have on our business. We've all seen customer requirements become increasingly complex. We've all seen what we thought were well planned projects get stopped dead in their tracks after having been unexpectedly snake bit by a material problem at the most inopportune moment. Are you confused by this metallurgical madness? Don't feel bad, I'm confused too and I'm a metallurgical engineer by trade. How is the average person in Cameron who is deficient in metallurgical acumen (but is otherwise a well educated, respectable, and personable individual) to cope with these metal related problems and avoid becoming part of the gators' cuisine?

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A.

Make like a tree and leaf!

B.

Ignore them and maybe they'll go away.

C.

Accept the inevitable and hope that they at least get indigestion.

D.

Hope they just ate and are too full for more than a nibble or two.

E.

Read An Introduction to Metallurgy by your humble servant.

The correct answer, of course, is E (for those of you who picked A, B, C, or D, I can only wish those rascally reptiles bon appetit!). Perusal of this slender volume will not make you a metallurgist. After reading it, you probably won't be able to select an appropriate material for all the different services that our equipment may see. You probably won't be able to specify a specific heat treatment or perform a failure analysis. But you will have a good, basic understanding of the fundamentals of metallurgy. You may not have all the answers, but you will be able to ask the right questions and be able to raise a flag when you come across a potential problem. You have within the covers of this book a potent weapon, a hefty club, that when expertly wielded and judiciously applied, is a powerful persuader for getting those gators to do a 180 and slink back to the swamp where they belong.

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A NOTE ON THE TEXT Every effort has been made to ensure that the text of AN INTRODUCTION TO METALLURGY is complete and as accurate as possible. If you would like to see some other area of metallurgy covered in a future edition that is not addressed in the present one or if you have any comments regarding the text please let me know so that the next edition can better serve your needs. I can personally vouch for the absolute perfection of the original draft of this book. Cecilia's word processing was flawless. The text, like Caesar's wife, is above reproach. Any typographical errors or technical inaccuracies that may have crept into the finished book could only have done so through the slovenliness of the printer. If you find any discrepancies, please let me know so that they can be corrected. The printer will be drawn and quartered and our future business taken elsewhere. Jim Dufour Cameron P. O. Box 1212 Houston, Texas 77251-1212 Phone: (713) 939-2141

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CHAPTER I

THE BASICS Simply Amazing!

A metallurgist is a practitioner of the ancient and honorable profession of metallurgy. He is concerned with the various methods of manipulating a metal's microstructure in order to obtain the most desirable properties. Metallurgy is as much an art as it is a science: at times it may seem to border on the supernatural. Metallurgy is an extremely powerful tool that should not be given into the hands of the irresponsible. It should be regarded with a mixture of reverence and awe, admiration as well as trepidation. Let us embark upon our journey into the realm of metallurgy by examining the metallurgical magnificence of steel. The lessons we learn will be applicable to all metals. Steel is essentially an alloy of iron and carbon. An alloy is a metallic substance that consists of at least two elements, at least one of which is metal. Carbon may be present in a steel in amounts of up to 2%, but most commercial steels will not have more than 0.5%. If the carbon content exceeds 2%, then the alloy is classified as a cast iron. Other elements may be present in varying amounts either as intentional additions used to enhance properties or as impurities. If you look at a piece of polished and etched steel under a microscope, you'll notice several things. First is that it has a crystalline structure. The size, shape, and orientation of these crystals, or grains as they are more commonly called, play an important role in determining a steel's properties. The second thing that you'll notice is that not all the grains look alike. This is because each grain is composed of one or more phases. Different phases have different properties, consequently, the macroscopic (overall) properties of steel are strongly dependent upon the type and the relative amounts of phases present. There is a third factor that influences a steel's properties, but this one you may not be able to detect under a microscope: the amount of cold work. We're going to examine each of these factors in some detail; how they affect a metal's properties and how they can be controlled so that we can obtain the properties we want in a metal. We must first, however, become familiar with crystals.

CRYSTALS A crystal is a group of atoms that has a particular arrangement that is repeated over and over again in three dimensions. The smallest repetitive volume of a crystal is known as the unit cell. The particular arrangement of atoms in a unit cell is called the crystal lattice. Metals are usually polycrystalline, that is, they consist of a multitude of small crystals rather than a single large one. The junction between two crystals is called a grain boundary. Grain boundaries are what delineate the size and shape of an individual crystal. Pure iron may form one of two crystal structures depending on temperature: body-centered cubic (BCC) or face-centered cubic (FCC). The lattice of each crystal structure is illustrated in Figure 1. page - 1

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A Body Centered Cubic (BCC)

B Face Centered Cubic (FCC)

Figure 1: Crystal Structures of Iron Iron at room temperature has a BCC structure. If we heat the BCC iron, or alpha iron as it is called, to 1674(F the iron atoms will rearrange themselves into a FCC structure called gamma iron. Gamma iron is the stable form of iron up to 2541(F. At this temperature the iron atoms revert back to the BCC structure. The BCC iron at this temperature is referred to as delta iron in order to distinguish it from BCC iron at room temperature. Delta iron melts at 2800(F. Metals, such as iron, that have different crystal structures over different temperature ranges are known as allotropic. As might be surmised, the properties of an allotropic metal are directly related to the particular crystal structure that happens to be stable at the temperature of interest. Vastly different properties can be obtained in these types of metals merely by changing the temperature a few degrees so that the metal goes into a temperature range where a new crystal structure appears.

GRAIN SIZE, SHAPE, AND ORIENTATION Grain size is often denoted by an ASTM grain size number. The ASTM (American Society for Testing and Materials) grain size number, n, is obtained from the formula: n 1 N 2 Where, N = the number of grains observed per square inch when the metal is viewed under a microscope with a linear magnification of 100X n = ASTM grain size number Note that the larger the grain size number, the more grains there are per square inch, consequently the smaller the grains. A "fine" grain steel has an average grain size number of five or higher. A grain size number of five corresponds to 160,000 grains per square inch. As a general rule of thumb, virtually all the mechanical properties of a metal at room temperature increase as grain size decreases. We can control grain size in a page - 2

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metal by regulating the heat treatment and the amount of working it receives, and by special alloy additions. One way we can minimize grain size is to cool the molten metal as rapidly as possible. This causes a greater number of nucleation sites (places where crystals start to grow) to appear in the molten metal thus increasing the number of crystals and decreasing their size. Grain size will increase when a metal is held above a certain temperature, and we can prevent grain growth simply by avoiding temperatures greater than this. Another way of inhibiting grain growth is by adding alloying elements such as aluminum or vanadium. These elements form hard compounds that will "pin" the grain boundaries in place or act as innoculants that form additional nucleation sites. Once you have a certain average grain size in a metal, there are only two ways to refine the structure (decrease the grain size): through a phase transformation or by recrystallization. We'll discuss grain refinement by phase transformations a little later, but right now let's talk about recrystallization. If we take a sledge hammer and pound on a chunk of steel for an hour or so, two things will happen: 1) the steel will be internally strained, and 2) we won't be able to move our arms again for a week. We, in effect, mashed the crystals so that they've become distorted and broken. Note that we have not made the grains any smaller. We greatly increased the internal energy of the structure. If we now heat the material up to an elevated temperature and hold it for a period of time, new, strain-free grains will start to form at many different sites along the old grain boundaries. These new grains will be smaller than the old ones as long as we don't hold the metal too long at temperature so that excessive grain growth occurs. These new, strain-free grains formed because the structure was in an unstable state at that temperature because of its high internal energy. It lowered its internal energy when the atoms rearranged themselves into new grains, thus returning to a more stable condition. This phenomenon is called recrystallization. The temperature that recrystallization occurs at, after a specified holding time, is the recrystallization temperature. This temperature is generally around one-third to one-half that absolute melting temperature in degrees Rankine or Kelvin ((Rankine = (F + 460( and (Kelvin = (C + 273() of a metal, but varies with the amount of internal energy of the material. Grain size, shape, and orientation are all closely related because obviously all the grains in a metal must interlock so that all space is completely filled. Of the three factors, grain size has the greatest influence on a metal's mechanical properties. The shape of a grain can affect some of a metal's properties such as its formability. Grains may be spherical, columnar, plate like, dendritic (shaped like a tree), or any number of other forms. The orientation of grains in most metals is random (orientation here refers to how an individual grain is situated in space, not the grainflow that occurs in forming operations). It is sometimes desirable to orient the crystal lattices of the grains in a metal's microstructure in a common direction, or a preferred orientation. This orientation can be brought about by mechanically working the metal in a certain manner and by special heat treatments. Special kinds of steels for electrical applications are often made with preferred grain orientations because the electromagnetic properties of grains are very directional. page - 3

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COLD AND HOT WORK Cold working is deforming a metal below its recrystallization temperature while hot working is deforming it at a higher temperature. By cold working a material, you introduce a considerable amount of strain into its structure. The more strain you put into the crystal lattices, the harder it will be for the atoms to relocate in the deformation process. As a consequence the material will become harder and stronger, but also less ductile. Let's do an experiment. Crush an empty beer can in the middle (see Figure 2). We can bend it in half lengthwise and then straighten it out again without too much difficulty. If we keep on bending it and then straightening it, in time we'll notice that it gets harder and harder to bend the can: the portion of the can that has been bent has gotten stronger. If we keep on bending and straightening the can, it will eventually crack and then completely break, indicating a loss of ductility. NOTE: Do not try this experiment with bottled beer!

Figure 2: Cold Work

Hot working takes place above the recrystallization temperature, consequently, any strain induced in the crystal structure will be removed as the distorted grains instantly recrystallize. Strain does not accumulate in the metal as with cold working and the metal will not be strengthened. This is why most forming operations such as rolling, extruding, and forging are done above the recrystallization temperature. Here the metal can be shaped with a minimum amount of energy and without worry of cracking it. Metals are often hot worked first and then cold worked. The hot work is done to move the bulk of the metal into roughly the desired form. It is then cold worked into the final form, thus strengthening the metal.

PHASES A phase is a portion of a pure metal or an alloy that is chemically and physically homogeneous and has a distinct boundary. An understanding of phases is essential to an understanding of the behavior of metals. By regulating the type and relative amounts of phases present on a microscopic level, we can tailor the macroscopic properties of a metal to a specific usage. Two different phases may be chemically identical and differ page - 4

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THE BASICS

only in crystal structure (e.g. alpha and gamma iron) or, conversely, may have identical crystal structure and differ only in chemistry. This is getting complicated. Better go grab another beer for a backup before reading on. And cheer up, you're already familiar with phases! An example should make this clear. Consider an ice cube floating in a glass of water. Here we have a two phase system: a solid and a liquid, both having the same chemistry, but differing radically in their mechanical properties. Each of the phases is homogeneous, that is, any given portion of the liquid is identical to the remainder of the liquid and any part of the solid is identical to the rest of the ice cube. There is clearly a distinct boundary between the ice and the water. We can control the relative amounts of both phases at a particular instant in time by either heating or cooling the system. Iron is somewhat more complicated than our glass of water. Each of the different crystal structures represents a different phase with each phase having different properties. The rearrangement of atoms at 1674(F (BCCFCC), at 2541(F (FCCBCC), and at 2800(F (BCCliquid) represents a phase transformation. Let's go back to our glass of water, assume the ice cube has melted, and add a teaspoon full of salt. Here we're adding another dimension to our system, that of chemistry. If we stir the mixture long enough, all the salt will dissolve (go into solution). How many phases do we have now? Because any given portion of the salt solution is physically and chemically identical to the remainder, we still have just one phase. If we keep adding salt to the solution, eventually we will reach a point where no more salt will dissolve. We now have two phases: a liquid phase consisting of a salt solution, and a solid phase consisting of granules of salt. If we heat the water, we can get more salt to dissolve. We have thus learned about two ways of controlling the types and relative quantities of phases. We can change the chemistry or the temperature of the system. Steel, as we have already mentioned, is an alloy of iron and carbon. At very minute concentrations, carbon atoms can occupy the spaces in between the iron atoms in alpha iron without distorting the crystal lattice. Alpha iron that contains carbon atoms is called ferrite. Ferrite is a very soft and ductile material. The solubility of carbon atoms in gamma iron is much greater than in alpha iron because the interatomic spacing of the iron atoms is larger. Gamma iron that contains carbon atoms is referred to as austenite. Most forging and rolling operations are done while steel is in the austenitic range because it's easily worked there. But just as salt has a limited solubility in water, carbon has a limited solubility in iron, regardless of the crystal structure or temperature. If the carbon exceeds this limit, a second phase must be formed. The excess carbon atoms will combine with some of the iron atoms to form cementite. Cementite (or iron carbide) has a stoichiometric formula of Fe3C and is a very hard, brittle substance. A combination of ferrite and cementite has significantly better mechanical properties than either phase by itself. So far we have learned (hopefully!) that the phases present in a carbon steel are dependent on temperature and carbon content. The iron-carbon equilibrium diagram, or phase diagram, indicates which phases are stable for any given combination of temperature and carbon content (see Figure 3). Equilibrium means that we allow enough time during heating or cooling for any possible reaction to completely finish. page - 5

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THE BASICS

Let's discuss what happens to a molten steel containing 0.30% carbon as we slowly cool it to room temperature. Referring to Figure 3, we see that the steel remains a liquid until the temperature reaches approximately 2750(F at point A. Here the liquid starts to solidify into delta iron. From point A to point B (2723(F) we have a two phase system: particles of solid delta iron immersed in a liquid. At point B, all the delta iron (BCC) transforms into austenite (FCC). More and more liquid solidifies as we approach point C (about 2650(F), but now it solidifies directly into austenite rather than delta iron. At point C, the liquid has completely disappeared and we have a one phase system containing austenite only. Decreasing the temperature further, we remain in one phase region until we reach point D, about 1510(F). Here the austenite begins to transform into ferrite (BCC). Once again we have a two phase mixture. At point E (1341(F), the remainder of the austenite will transform, but because the solubility of carbon is so much less in ferrite than in austenite, the ferrite that forms will not be able to accommodate all the carbon that the austenite held. As a consequence, the remaining austenite decomposes into a mixture of ferrite and cementite (iron carbide). The horizontal line at 1341(F is called the eutectoid isotherm and point F is the eutectoid point. A eutectoid point is where a single solid phase (in this case austenite) transforms isothermally (at a specific, constant temperature) into two different solid phases (in this case ferrite and cementite). Point G is the eutectic point. A eutectic point is where a liquid transforms isothermally into two different solid phases (austenite and cementite in this instance). The horizontal line at 2098(F is the eutectic isotherm. When we cooled our 0.30% carbon steel below point E, we obtained a mixture of ferrite and cementite, but if we look at the microstructure under a microscope we will be able to see a difference in the ferrite that formed above 1341(F and that which formed at a lower temperature. The ferrite that formed from austenite above 1341(F is called proeutectoid ferrite and we can see individual grains of this substance in the microstructure. The austenite remaining just above 1341(F is very rich in carbon and corresponds to the carbon content at the eutectoid point. When it transforms, it will produce a mixture of about 88% ferrite and 12% cementite by weight. This mixture is lamellar, that is, it's composed of alternating layers of ferrite and cementite. The resulting micro constituent is called pearlite.

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CHAPTER I

THE BASICS

Figure 3: Iron-Carbon Equilibrium Diagram Pearlite is not a phase, but a specific mixture of two phases formed by transforming austenite of eutectoid composition into ferrite and cementite. The lamellar shaped structure of pearlite gives it many unique properties: the hard, brittle cementite reinforces the soft, ductile ferrite thus forming a natural composite. The properties of pearlite can be changed by changing the spacings between the layers of ferrite and cementite through an appropriate heat treatment. At room temperature our 0.30% carbon steel has a microstructure that looks something like Figure 4.

Figure 4: Microstructure of a 0.30% C Steel page - 7

CHAPTER I

THE BASICS

In the austenite to ferrite transformation, iron atoms must rearrange themselves. By cooling a metal slowly from an elevated temperature, as we did in the previous example, we allow the iron atoms sufficient time to move and arrange themselves into a different crystal structure. Suppose now, that instead of slowly cooling a metal down from the austenite region, we cool it very rapidly or quench it. If we exceed a certain critical cooling rate, depending on the thickness of the metal as well as its chemistry, the iron atoms will not transform from a FCC structure into a BCC structure as they did during a slow cool. Instead they will form a body-centered tetragonal structure. This structure is similar to BCC, except that it is not cubic: one dimension of the unit cell is longer than the other two. The iron atoms will instantly try to form a BCC structure. The carbon atoms in the austenite will not, however, have time to diffuse out before the metal temperature drops, consequently, they are locked in place. This is because diffusion (or the movements of an atom) decreases rapidly as temperature decreases. You'll remember that carbon has a low solubility in BCC iron because of the small distances between atoms in the BCC structure. The resulting structure of the metal we quenched will then be essentially a BCC structure that is distorted in order to accommodate all the carbon atoms. This structure is called martensite. Martensite is a metastable phase. A metastable phase is one that exists in a nonequilibrium condition, but still will not change spontaneously as would most other nonequilibrium conditions. Martensite is a very hard, strong, brittle substance that is one of the chief hardening agents in steel. We have one more topic to discuss that involves phases: grain refinement through phase transformations. If we heat BCC iron to a temperature higher than 1674(F, grains of FCC iron will begin to appear all along the grain boundaries of the BCC iron. The temperature of the metal remains constant while this is going on. Once the transformation is complete, we will have an entirely FCC structure, but with a finer grain size. The temperature of the metal can now begin to rise. If we now cool the metal back down below transformation temperature, the process of the grain refinement will repeat itself, only this time it will be smaller BCC grains that form. This is the basis of grain refinement of a steel by normalizing. Normalizing is one type of heat treatment and we'll discuss it in the next section.

DISLOCATIONS Football players will more than likely experience at least one knee dislocation during their professional careers. Trying to pat yourself on the back too often may result in a severe dislocation of the shoulder. Watching a pretty blonde pass by in a speeding car may cause a neck dislocation (with complications if you are accompanied by your wife). Metals are also subject to dislocations, but of a different sort. A dislocation in a metal is a linear defect of the metal's crystal structure. Dislocations are the primary reason why the actual tensile strengths (typically 10,000 300,000 psi) of common engineering metals are so much lower than the theoretical strengths (1,000,000 - 3,000,000 psi) that are calculated from the stress necessary to break all the bonds between two adjacent planes of atoms. Ignoring those related to page - 8

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THE BASICS

human anatomy, there are two basic types of dislocations: edge and screw (see Figure 5).

Figure 5: Dislocation Defects An edge dislocation can be thought of as an extra plane of atoms pushed in between two existing planes of atoms in a crystal lattice. A screw dislocation is a little harder to visualize at the atomic level. Figure 5(b) shows that a portion of the upper part of the lattice is displaced relative to the lower. This displacement of one atomic distance causes the lattice planes of atoms near the dislocation to spiral around the dislocation line much like the threads of a screw hence the name screw dislocation. Dislocations can move! Apply a large enough shear stress (see Figure 6) and the dislocation (be it an edge or a screw) will move completely through the lattice. Note in Figure 6 that the top part of each lattice has been displaced by one atomic distance relative to the bottom after the dislocation has passed through. The lattice is said to have undergone slip. Dislocations tend to move on certain crystallographic planes in which the atoms are most closely packed together. These planes are known as slip planes.

A Screw Dislocation Movement

B Edge Dislocation Movement

Figure 6: Dislocation Movement Do you understand all this? You do? Who are you trying to kid? After reading it through the first time? Hey, Einstein died back in 1955 so Einstein you're not. You obviously didn't read it carefully enough or you're so lost that you don't know you're lost. Go back and read it again. Dislocation movement is the primary factor why the actual tensile strength of metals is just a fraction of the theoretical strength. The shear stress required to move a page - 9

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dislocation through a lattice so that the upper part of the lattice is displaced by one atomic spacing relative to the lower is much, much smaller than breaking all the atomic bonds between two parallel planes of atoms in a perfect crystal and then moving the top half one atomic space from the bottom. A common analogy for why this is so is the effort it takes to move a large, heavy rug a short distance across a floor. It will take a great deal of work to grab an edge and just drag the rug the desired distance. It's much easier if you put a ripple in the rug near the edge and then merely push the ripple to the other side. The rug will have been displaced by a small distance. It takes much less effort to push the ripple across the rug and cause the displacement than it does to drag the whole thing because only a small portion of the rug moves at any one time as the ripple is pushed. A dislocation is just like that ripple in that only a small part of the crystal lattice has to move at any one time in order to get a net displacement of the entire lattice. The amount of deformation that a lattice undergoes when a dislocation passes through is only one atomic spacing. This by itself is insignificant. It has been estimated, however, that the dislocation density of a strain-free metal is 10 6 to 10 8 dislocations per cm2. Thus the cumulative effect of dislocation movements is very significant because of the vast numbers involved. There is obviously some distortion in the lattice adjacent to a dislocation. In other words, a dislocation has a strain field associated with it. This strain field is very important because it can interact with the strain fields from other dislocations as well as with grain boundaries, precipitates, foreign atoms, etc. These interactions may make it difficult or impossible for a dislocation to move. In essence, a dislocation can become locked in place. The primary reason why the actual strength of metals is so much lower than the theoretical is that metals contain dislocations which are free to move. Impede the movement of dislocations and you will increase the strength of a metal.

STRENGTHENING MECHANISMS How can we keep those pesky dislocations from moving around and lowering the strength of our metal? Through a combination of one or more of the following mechanisms: 1. 2. 3. 4.

reduction of grain size cold work solid solution strengthening dispersion strengthening

We've already discussed the first two mechanisms. The strain field associated with a dislocation can interact with a grain boundary and prevent further motion of the dislocation. By reducing the grain size, we increase the total amount of grain boundary area thus increasing the interactions with the dislocation strain fields and strengthening the metal. Cold work induces a great deal of strain into the crystal lattice which can interact with the dislocation strain fields and impede the movement of the dislocations. In addition, cold working will increase the number of dislocations. These dislocations will not all form on the same slip plane. Under an applied stress, some of the page - 10

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dislocations will move and eventually the strain field of one dislocation will bump into the strain field of another and both dislocations will be locked in place. The dislocations have become entangled.

A Interstitial Solid Solution

B Substitutional Solid Solution

Figure 7: Solid Solutions The last two mechanisms are new so we'll look at them in detail. A solid solution is a homogeneous mixture of two or more kinds of atoms in the solid state. The most abundant atom is called the solvent and the least abundant the solute. Solute atoms can occupy one of two possible positions within the solvent atoms' crystal structure (see Figure 7). The solute atoms may take the place of some of the solvent atoms within the crystal structure. This is known as a substitutional solid solution. The other possibility is that the solute atoms may occupy a site in between the solvent atoms in the crystal. This is known as an interstitial solid solution. If the solute atoms are sufficiently large in either a substitutional or interstitial solid solution, they will cause the solvent crystal structure to become distorted. The strain induced by the mismatch in atomic sizes can interact with dislocation strain fields thus preventing dislocation movement and strengthening the metal. Note that the solute atom cannot be too large or else the solvent lattice will not be able to accommodate very many and strengthening may not be achieved. The large solute atoms are, in effect, outside the solvent lattice and therefore exert little influence on it. Dispersion strengthening takes place as a hard, second phase is finely precipitated throughout a softer matrix of another phase. The hard precipitates act as barriers to dislocation movement. They also introduce strain fields into the lattice that can interact with dislocation strain fields. Just as with solute atoms, the precipitates must not be too large or else strengthening will not be achieved. Dispersion hardening is the chief strengthening mechanism for precipitation hardenable alloys. By now you're probably thinking all kinds of nasty thoughts about metallurgy and metallurgists. Despite what your intuition tells you, metallurgy is not a devil's invention meant to confound the innocent. It is, rather, a way of life, an elegant science that is worthy of many hours of quiet meditation and reflection. A firm grasp of its great principles will break the chains that tether your mind to the commonplace and will allow it to soar. The great epochs of man are inextricably linked to the state of their metals' technology hence the Bronze Age, Iron Age, and the Age of Steel. Indeed the progress of our civilization may be said to have always followed the progress of metallurgy. This gives you, gentle reader, an awesome responsibility. Significant progress in metallurgy page - 11

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can only be achieved when the common man, such as yourself, has been exposed to its basic truths, understands their importance, and has a burning desire to preach them amongst the heathen (mechanical engineers, Texas A&M graduates, plant managers, etc.). Have you scaled Olympus? Have you entered the Elysian Fields? No? Then go back and diligently reread this chapter lest you be accused of shackling the progress of civilization. This chapter is the foundation upon which the rest of the course will be built. If, after several rereadings, you still have not attained Nirvana, then grab a beer before going on, it's the next best thing.

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NOTES:

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CHAPTER II

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If you managed to muddle through the last section, congratulations! You are well on your way to becoming an honorary metallurgist. Soon you'll be able to amaze your friends, your spouse, and yes, even your kids with the breadth and depth of your new knowledge! Soon you'll be able to shovel your way through any snow-job that a Cameron metallurgist throws your way! But not yet. First, we have to talk about heat treating. Hopefully you remember that the properties of metals are determined by: 1. the size, shape, and orientation of grains, 2. the types and relative amounts of phases present, 3. the amount of cold working. By controlling the above three items, we can produce the particular properties we want in a metal. The best way to exert this control is through alloy additions and through heat treatment. In this section, we'll examine the heat treatments that are commonly used in manufacturing our oil tool equipment. We use a wide variety of metals in our products and, of course, many different heat treatments. The specific heat treat procedure used in manufacturing a given part will be dependent on the alloy involved, the size of the part at the time of heat treatment, the required properties, the available heat treating equipment, and any customer requirements. The vast majority of parts in our products are carbon, low alloy, or stainless steels or nickel base alloys. Carbon and low alloy steels are typically austenitized, quenched, and tempered or normalized, austenitized, quenched, and tempered. Stainless steels, depending on the variety, may be annealed, age hardened, or austenitized, quenched, and double tempered. Nickel base alloys are typically used in the annealed or age hardened condition depending on the specific alloy and the required strength level. We'll examine each of these heat treatments.

ANNEALING Annealing consists of heating a material to a suitable temperature, holding for a specified time, and then cooling (usually at a slow rate). If this seems like somewhat of a vague definition, that's because there are many different types of anneals. The purpose of annealing may be to: 1. 2. 3. 4. 5. 6.

soften stress relieve refine the grain size remove gases alter mechanical properties obtain a desired microstructure

The term full anneal, when applied to steel, involves heating the metal into the austenitic region and then slowly cooling it (often the metal is left in the furnace and the page - 15

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furnace shut off). This produces a very soft, homogeneous microstructure free from residual stresses. A solution anneal involves heating a metal up to a temperature sufficiently high for a time sufficiently long to dissolve one or more microconstituents into a solid solution with the matrix material. The metal is then cooled rapidly enough to keep the microconstituents in solution. A process, or subcritical anneal is an in-process heat treatment used to soften steel that has been cold worked during a forming process. The steel is heated up to a temperature just below the start of the ferrite to austenite transformation and then held until the desired degree of softening has been attained through recrystallization and grain growth. Many alloys that cannot be substantially hardened through heat treatment are often used in the annealed condition. Examples include Monel® 400, Inconel® 600, and 316 stainless steel. These alloys, as well as many other nickel base alloys, austenitic and duplex stainless steels, and certain other nonferrous metals, are typically annealed to provide a homogeneous microstructure, to stress relieve, to remove the effects of cold work, and to provide the optimum microstructure for corrosion resistance. Following an anneal, parts may be air cooled down to room temperature or quenched in water or oil to prevent the precipitation of undesirable phases.

NORMALIZING, AUSTENITIZING, QUENCHING AND TEMPERING OF STEELS Normalizing is a special type of anneal that involves heating a steel to a temperature about 100(F into the fully austenitic region, then removing the material from the furnace and allowing it to cool in still air. Normalizing accomplishes several things. First, it produces a fine grained pearlitic structure thus enhancing the uniformity of mechanical properties. Second, it can greatly improve notch toughness. Third, it removes some of the stresses that occur in the material during forging or rolling operations. And finally it homogenizes the material making it easier to austenitize in a subsequent heat treat operation. The austenitizing operation consists of heating the material to a temperature about 50(F into the austenitic range and holding the material at temperature until the transformation to austenite is complete. Once the material is fully austenitic, it is then "quenched." Quenching consists of removing the material from the furnace and immersing it in an agitated fluid. This causes the material to cool rapidly. Water, oil, or a polymer solution are the fluids used for quenching low alloy steels. 410 stainless steels are quenched in oil, polymer solution, or air depending on section thickness. The quenching operation is used to harden the material. It does this through the formation of martensite from the austenite. This transformation will take place only if the cooling rate exceeds a certain limit. Because some of the material may not exceed the page - 16

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critical cooling rate (material near the surface of a forging will cool faster than the material at mid-wall or the center), some of the austenite may not form martensite. The austenite at locations receiving a subcritical cooling rate will transform into ferrite, pearlite, and/or bainite (we'll talk about bainite later on in this section) in varying amounts, depending on the distance from the quenched surface. This is the reason why parts made out of the same material, in the same heat treated condition, but with different wall thicknesses may have vastly different properties. It also explains why the surface hardness of a part may be much higher than the hardness just below the surface. The quenching operation will increase the hardness and strength of a steel, but will drastically decrease ductility and notch toughness. As a consequence, the quenching operation for steels is always followed by another heat treat operation: tempering. Tempering consists of reheating the quenched steel to a temperature below the austenitic range, holding the material at temperature for a specified period of time, and then cooling at a specified rate. The purpose of tempering is to improve the toughness and ductility of the quenched material. It does this by causing the brittle, hard martensite to dissociate into ferrite and iron carbide. The resulting structure, called tempered martensite, is not lamellar like pearlite, but contains many fine particles of carbide dispersed throughout the ferrite. This gives the material the optimum combination of strength, toughness, and ductility. Tempering will reduce the strength of a material from that found in the asquenched condition, however, it will considerably improve toughness and ductility while still maintaining a higher strength level than could be attained in the normalized condition. The final properties attained will be dependent on the tempering temperature and the holding time at temperature. In some highly hardenable alloy steels, such as 410 stainless steel, there is a possibility that not all of the austenite will transform during the quenching operation. Due to local variations in chemistry, there may be pockets of austenite that are quite happy with the status quo and have no intention of altering their lifestyle merely because some metallurgist pours water or oil on them. This austenite that failed to transform during the quench is referred to as retained austenite and can be quite troublesome. So, after the quenching operation, we may have a material that consists of newly formed martensite interspersed with pockets of retained austenite. In order to improved ductility and toughness, we must temper the material so that the brittle martensite will dissociate into ferrite and carbide. During this tempering cycle, some of the retained austenite may decide that being martensite isn't so bad after all and go ahead and transform. This fickleness on the part of the retained austenite causes complications because now after tempering, our material has a structure consisting of a strong, but ductile, tempered martensite and pockets of fresh martensite (transformed from the retained austenite) that are hard and brittle. We are going to have to retemper the material in order to eliminate the pockets of fresh martensite. This page - 17

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is why a "double temper" is called out in many of our material specifications. The second temper will always be at a lower temperature than the first so that we don't overtemper the bulk of the metal.

AGE HARDENING Age hardening, or precipitation hardening as it is sometimes called, is a three step heat treatment that is used to increase the strength and hardness of an alloy. Not all alloys can be effectively age hardened. We'll explain the reason for this as well as examine each of the three steps a little later, but first, we have to look at a typical phase diagram of an age hardenable alloy (and you thought you were finished with phase diagrams after the first lesson!). Figure 1 shows a phase diagram for elements A and B (at least one of which is a metal). Pure A has a stable phase of  up to a temperature of T1, while pure B is a  phase up to a temperature of T2. Remember the salt and glass of water in the first lesson? There we stirred a teaspoonful of salt into a glass of water. The water dissolved all the salt (i.e. the salt went into solution) and we still had a one phase system. When we continued to add salt we eventually reached a point where no more salt would dissolve: the solution had become saturated. We then had a two phase system consisting of a liquid (the salt solution) and granules of salt. By cooling the system, the solution would not be able to hold as much salt and some salt would have to come out of solution. In other words, there is a limit on the solubility of salt in water at a particular temperature and this solubility decreases with decreasing temperature. The  phase in Figure 1 can only accommodate a certain amount of element B at a given temperature (just as water can only dissolve so much salt) before a second phase, , is formed. The amount of B that the  phase can contain (the solubility limit!) increases up to a temperature of T3. The solvus line in Figure 1 represents the solubility limit of B in the  phase. At temperature T4, the  phase can hold 1% B atoms. At a higher temperature T5, the  phase can hold 2% B atoms in solution. Summing up, there is a limit on the solubility of B in  at any given temperature and this solubility limit decreases with decreasing temperature. All age hardenable alloys exhibit this behavior, however, not all alloys exhibiting this behavior are age hardenable. Enough of the preliminaries, let's get down to business. The three steps in age hardening are: 1) solution anneal, 2) quenching, and 3) controlled reheating (aging). We'll examine the heat treatment of an age hardenable alloy consisting of 98% A and 2% B. At room temperature we have a two phase system:  and .

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Figure 1: Age Hardening

1. Solution Anneal - The first step in age hardening is to heat the alloy up into a one phase region so that all the  will dissolve and the B atoms go into solution. This requires that we heat our alloy to a minimum temperature of T5. We'll heat it up to T6 just to be on the safe side and be sure all the B is in solution. 2. Quench - Once we've gotten all the B into solution at T6, we're going to quench the alloy to some temperature well below T5, say T4. The B atoms will still remain in solution in the  phase even though the amount of B atoms (2%) exceeds the solubility limit at T4 (1%). By rapidly cooling the alloy to a low temperature, we have effectively "frozen" the B atoms in place before they had a chance to move. We now have a supersaturated solution. 3. Controlled Reheating - The final step in age hardening is the aging process itself. This consists of reheating the supersaturated solution to T7, a temperature below T5, holding for a period of time, and then cooling. This causes some of the B atoms to come out of solution (precipitate out). These precipitates are finely dispersed throughout the  matrix and introduce a great deal of strain into the  crystal lattices. page - 19

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The extra strain in the crystal structure makes it difficult for dislocations to move in response to an applied load, consequently the alloy is hardened and strengthened. Note that although the B atoms do form a precipitate, this precipitate may not necessarily have the same structure as the  phase. The aging time and temperature determine how much aging, or hardening, actually takes place. We can overage the alloy by holding it for too long a time at the aging temperature or by using too high an aging temperature. This causes the precipitate particles to agglomerate and grow, resulting in a decrease in hardness and strength.

T-T-T DIAGRAMS In Chapter I we discussed what happened to a carbon steel that was slowly and continuously cooled through the austenitic region down to room temperature. You'll recall that we ended up with a mixture of ferrite and pearlite. Will austenite always transform to ferrite and pearlite? Obviously a loaded question. The answer is no. We've already talked about one case - the formation of martensite - but there are others as well. One of the chief means of studying the decomposition of austenite is through the metallographic examination of isothermal transformation products. As an example, if we quickly quench a small test specimen that has been austenitized down to a temperature where austenite is unstable and then hold the sample at that temperature, eventually the austenite will transform into something else (isothermal here refers to the fact that we hold the austenite at a constant temperature until it transforms). We can then quench our sample down to room temperature, examine it under a microscope, and see what the austenite decomposed into. Suppose we take a large number of small test specimens of a carbon steel containing 0.64% carbon and 1.14% Manganese (AISI 1566) and heat them up into the austenitic region, quench them to various temperatures, hold them at those temperatures for various lengths of time, and then quench them to room temperature. We can then look at each specimen under a microscope and determine what microconstituents are present and in what quantities. This data will allow us to make a plot of the type and quantity of the austenite transformation products as a function of transformation temperature and holding time. This type of plot is called a T-T-T diagram (the T's stand for time-temperature-transformation). It is sometimes referred to as an isothermal transformation curve. The T-T-T diagram for our AISI 1566 carbon steel is shown in Figure 2. The first line (from the left) in Figure 2 shows where the austenite first starts to decompose after it has been quenched to a specific temperature. The dotted line shows where 50% of the austenite has decomposed. The solid line on the right shows where the austenite transformation is complete. The horizontal line marked Ac1 is the temperature above which austenite first starts to form upon heating. The line marked Ac3 is the temperature at which the transformation to austenite is complete. Figure 2 page - 20

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shows that for our carbon steel, austenite may transform into pearlite, martensite, or something we haven't talked about yet: bainite. We'll examine each of these transformations in some detail. 1. Pearlite - Assume we can instantaneously quench a small piece of austenitized 1566 carbon steel down to 1200(F (point 1 in Figure 2). After holding it at that temperature for about ten seconds, ferrite will start to form along the austenite grain boundaries (point 2). This is the proeutectoid ferrite that we talked about in Chapter I. Proeutectoid ferrite precedes the formation of pearlite in hypoeuctectoid steels (steel having less that the eutectoid carbon content of 0.77%). In hypereutectoid steels (steels having more than the eutectoid carbon content of 0.77%), however, the iron-carbon phase diagram in Chapter I tells us that some of the austenite will first transform to cementite before the remaining austenite transforms to pearlite.

Figure 2: AISI 1566 T-T-T Diagram

As we continue to hold our specimen at 1200(F, more and more of the austenite transforms into ferrite until we reach point 3 after approximately forty seconds. At this point, all the austenite that is going to transform into proeutectoid ferrite has done so. The remaining austenite has become increasingly rich in carbon because of the

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limited solubility of carbon in ferrite. Point 3 also marks the start of pearlite formation. Pearlite forms when iron carbide begins to nucleate out in the remaining austenite along grain boundaries. The subsequent growth of the carbides depletes the carbon content of the adjacent areas so that in these areas the austenite transforms into ferrite. Further carbide nucleation will result in a pearlite "colony" having alternate layers of cementite and ferrite. This process is illustrated in Figure 3. All the austenite that did not transform into proeutectoid ferrite will have transformed to pearlite by the time we have reached point 4. No further changes in the quantities or the types of microconstituents in our specimen will occur after point 4. The final structure in our specimen consists of proeutectoid ferrite and pearlite. Note in Figure 2 that the lower the temperature at which pearlite forms, the "finer", or more closely spaced, the layers are. Fine pearlite is stronger and tougher than coarse pearlite.

Figure 3: Pearlite Formation 2. Bainite - Assume we can instantaneously quench a small piece of austenitized 1566 carbon steel down to 800(F (point 5 in Figure 2). After holding our specimen for about 3 seconds at this temperature (point 6), ferrite will start to nucleate along austenite boundaries. The lattices of the ferrite grains are coherent with the austenite matrix (this means that the lattices match, see Figure 4). As the ferrite forms, the austenite adjacent to it becomes richer in carbon until it becomes saturated. At this point cementite will begin to precipitate out. The carbides form parallel to the longitudinal axis of the bainite "needle." The austenite will have completely transformed to bainite at point 7. No further transformations will occur past this point. The resulting structure has a feathery appearance under the microscope and is known as upper bainite. Figure 5 shows the formation of upper bainite. Austenite that isothermally transforms between the knee of the curve and approximately 660(F will form upper bainite.

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Coherent Lattices

Incoherent Lattices

Figure 4: Coherency Austenite that isothermally transforms between approximately 660(F and the line in Figure 2 identified "MS" forms lower bainite. Lower bainite forms when ferrite, supersaturated with carbon, forms along austenite grain boundaries. Again the ferrite is coherent with the austenite matrix. Carbides will precipitate out within the bainite needles(see Figure 6). Lower bainite has an acicular (needle shaped) structure that is similar to tempered martensite. The fact that the ferrite in both upper and lower bainite forms coherently within the austenite matrix distinguishes it from the ferrite in pearlite which forms incoherently. Bainite is considerably stronger than pearlite because with coherency, perfect matching of the two lattices is seldom achieved. This results in considerable amounts of induced strain that will hinder the movement of dislocations under a load thus strengthening the material. Bainite is hard and brittle in the asquenched condition and consequently must be tempered.

Figure 5: Upper Bainite Formation 3. Martensite - Martensite will form if we quench our 1566 carbon steel fast enough so that we miss the knee of curve and go down below the "MS" (martensite start) temperature line in Figure 2. As we discussed in Chapter 1, martensite has a distorted body centered tetragonal structure. The austenite is so unstable at these low temperatures that it will instantaneously transform. Atomic diffusion is extremely slow at low temperatures so martensite does not form through a nucleation and growth process like bainite and pearlite do. Instead the austenite page - 23

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Figure 6: Lower Bainite Formation will transform in a shear reaction involving minimal atomic movement. The ferrite matrix that forms is distorted because it must accommodate the extra carbon atoms that were in solution in the austenite (remember that carbon has a much lower solubility in a BCC structure than a FCC). This distortion is accompanied by a considerable amount of induced strain which hardens and strengthens the material. Martensite has an acicular structure. Note in Figure 2 that the amount of austenite that transforms into martensite is dependent solely on the temperature to which we quench our austenite to. The percent of austenite that transforms is independent (unlike bainite or pearlite formation) of the time that the quenched austenite is held at temperature. Because of this, martensite formation is referred to as an athermal reaction, as opposed to the isothermal reactions that we have been talking about. The amount of austenite that transforms to martensite increases as we quench to lower temperatures until we reach the "MF" (martensite finish) temperature line in Figure 2. At this point all the austenite has transformed to martensite. We can use T-T-T diagrams to show the effect of alloying elements on the heat treat response of a steel. Our T-T-T diagram for AISI 1566 steel can be thought of as three superimposed curves (see Figure 7A). By alloying with different elements, we can separate these curves in order to make it easier to obtain the desired transformation product (see Figure 7B and Figure 7C). Increasing the carbon, manganese, nickel, and silicon content of a steel will move the pearlite and bainite curves further to right, but do not separate them to any extent. Molybdenum, chromium, and vanadium move the pearlite curve up and further to the right while moving the bainite curve to lower temperatures. Figure 8 shows these effects for an AISI 4340 steel (nominal composition, 0.40% carbon, 0.80% manganese, 1.80% nickel, 0.25% molybdenum, and 0.80% chromium).

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Figure 7: T-T-T Diagrams The significance of all this is that by moving the ferrite and pearlite curves to the right we have more time in which to quench our steel from the austenite region to the bainitic or martensitic region without having any of the austenite transform into the softer proeutectoid ferrite or pearlite. This is desirable because the rate at which we can quench our steel is limited by the quenching medium and the size of the piece we are quenching. The farther the ferrite and pearlite curves are moved to the right, the more time we have to quench our steel in order to obtain the desired martensite or bainite, the slower our cooling rate can be, and consequently the larger the piece of steel we are quenching can be and still obtain the desired microstructure. We're going to examine this a little further later on. T-T-T diagrams are also useful for illustrating the types of heat treatments performed on steel. Figure 9 shows an annealing or normalizing heat treat of our 4340 steel. The steel is heated up into the austenitic region and then slowly cooled. This slow cooling will result in a microstructure of ferrite and pearlite. Figure 10 shows an austenitize, quench, and temper heat treatment. Note that two cooling curves are shown: one for the surface and one for the center of the part being quenched. The differences in cooling rates can be appreciable depending on the size of the part being quenched and the quenching medium. Ideally the part is quenched so that the bainite and pearlite curves are missed: only martensite is formed. After the part is cooled below MF, it is heated up again for the tempering operation.

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Figure 8: AISI 4340 T-T-T Diagram Figure 11 illustrates a martempering heat treatment. Here the part is rapidly quenched down to a temperature just above MS and held until the temperatures of the center and surface of the part become equalized. The part is then quenched into the martensitic range. Typically this "interrupted quench" is done by quenching and holding the part in a salt bath or martempering oil until the temperature equalizes throughout the part and then is air cooled into the martensitic range. The purpose of this heat treatment is to harden the material by forming martensite, but at the same time, minimize the distortion and residual stresses associated with the martensite transformation. We use this heat treatment for heat treating Colmonoy® coated parts. It is always followed by tempering. Figure 12 shows an austempering heat treatment. Here we again quench our steel down to a temperature just above MS and hold it there until the austenite has completely transformed into bainite. It is then air cooled. Like martempering, it is usually performed in a salt bath. The purpose of austempering is to harden the steel by forming bainite. Although this will not give the same strength as martensite, bainite formation produces much less distortion and residual stresses than martensite formation. As a consequence, warping and quench cracking are minimized. Austempering is usually followed by a tempering operation in order to restore ductility.

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Figure 9: Anneal/Normalize

Figure 10: Austenitize, Quench, & Temper EGADS! We're finished with T-T-T diagrams. Unfortunately T-T-T diagrams are of limited use in the real world of heat treating because they are based on the assumption that we can instantaneously quench a piece of steel down to a desired temperature and also because most heat treatments do not involve isothermal page - 27

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reactions. But let's not be too disparaging about T-T-T diagrams. They have served their purpose well in helping us examine the decomposition of austenite.

Figure 11: Martemper

Figure 12: Austemper To accurately predict the heat treat response of different steels, having different sizes, and quenched in different mediums, we are going to have to dive into continuous page - 28

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cooling transformation diagrams. This, like our T-T-T diagrams, is another transcontinental topic that metallurgists like to wax rhapsodic about. So take a deep breath before we submerge.

CONTINUOUS COOLING TRANSFORMATION DIAGRAMS Continuous cooling transformation (CCT) diagrams show the changes that steels undergo when quenched from the austenitic condition. There are two common ways of presenting this information. The first is a plot of transformation products as a function of transformation temperature and cooling time (time after quenching). This is illustrated for an AISI 4140 steel in Figure 13. Transformation temperature is always plotted on the vertical axis and the cooling time along the horizontal axis. Time is plotted on a logarithmic scale in order to make the diagram compact.

Figure 13: AISI 4140 CCT Diagram

Suppose we want to determine what type of structure we would have in the center of 2¼" diameter bar of 4140 after austenitizing and then oil quenching. If we superimpose a plot of temperature versus time corresponding to the cooling rate at the center of a 2¼" diameter bar during an oil quench onto Figure 13 (curve A), we can predict the final structure. In this case we see that the austenite begins to transform to proeutectoid ferrite at point 1 (after 20 seconds of cooling). By the time we have reached point 2 (after approximately 45 seconds), 5% of the austenite has transformed into ferrite. At point 2, the remaining austenite begins to transform into bainite. The page - 29

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bainite transformation is complete by the time we reach point 3. We now have roughly 5% ferrite, 55% bainite, with the remainder being untransformed austenite. Point 3 marks the start of martensite formation. Virtually all the remaining austenite will transform to martensite by the time we reach room temperature so our final structure consists of roughly 5% proeutectoid ferrite, 55% bainite, and 40% martensite. A second way of presenting CCT diagrams is illustrated in Figure 14 for an AISI 4130 steel. Here the horizontal axis represents the size of bar being quenched. There are three scales: one for each type of quenching medium. Again the horizontal scales are logarithmic to keep the diagram compact. The vertical axis is the transformation temperature. This type of CCT diagram is a convenient means of determining the microstructure in the center of a round bar that has been austenitized and then air, oil, or water quenched. Let's look at an example. Assume we take a 6" diameter bar of 4130, austenitize it, and then water quench it. We can find out what the resulting microstructure will be by drawing a vertical line through the 6" diameter marker on the horizontal axis for water quenching (line A in Figure 14). Roughly 30% of the austenite has transformed into proeutectoid ferrite (as indicated by point 1), roughly 45% has transformed into pearlite (as indicated by point 2 where 75% of the total austenite has transformed), and the balance of the austenite has transformed into bainite. CCT diagrams are extremely useful tools. We can use the type illustrated in Figure 13 to accurately predict what the microstructure (and consequently the mechanical properties) of a steel part will be at any location within the part as long as we know what the cooling rate is at that location. The type of CCT diagram illustrated in Figure 14 is useful for determining the maximum size bar in which we can develop the desired mechanical properties throughout its entire cross section. So far we have learned that in order for a steel to be strengthened (or hardened) though heat treatment, it must undergo several phase transformations. The desired transformation products for most of the types of steels that we deal with are martensite, bainite, or a combination of the two. Martensite is harder and stronger that bainite: both are significantly harder and stronger that pearlite or ferrite. When we talked about the quenching operation, we stated that austenite will transform into martensite and/or bainite (thus hardening the steel) only if the cooling rate exceeds a certain critical value. This cooling rate is one indication of a steel's hardenability. Hardenability can be thought of as a measure of how easy it is for a particular steel to develop a given hardness at a given location. A steel with low hardenability requires a faster cooling rate (a more severe quench) to attain the same hardness at a given location than a steel with high hardenability. Note that hardenability does not refer to the maximum hardness that can be developed in a steel.

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Figure 14: AISI 4130 CCT Diagram A steel's hardenability is primarily a function of its chemistry. Increasing the carbon content in a steel greatly increases its hardenability. Manganese also increases hardenability, but to a smaller degree. Other common alloying elements that are used for increasing hardenability are chromium, molybdenum, vanadium, columbium, and nickel. These elements increase hardenability by decreasing the critical cooling rate necessary to form martensite or bainite. The effects of alloying elements on hardenability can be shown by comparing the CCT diagrams for two different low alloy steels: 4130 and a 2¼Cr-1Mo steel. Figure 15 shows the CCT diagram for the 2¼Cr-1Mo steel. Both these steels are typically water quenched. Figure 14 shows that a 4130 steel (0.30% carbon, 1.00% chromium, 0.20% molybdenum) will have a microstructure consisting of 100% martensite and bainite up to approximately 2" in diameter. Figure 15 shows that a 2¼Cr-1Mo steel (0.14% carbon) will have a microstructure consisting of 100% martensite and bainite up to approximately 10" in diameter. The 2¼Cr-1Mo alloy clearly has the greater hardenability. Figure 14 and Figure 15 illustrate why we are using more and more 2¼Cr-1Mo alloy in our shops. Our steels must meet certain minimum strength requirements (and page - 31

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consequently certain minimum hardnesses) established by API and our engineers. At the same time, there is a limit of 22 Rockwell C imposed by NACE on low alloy steels for use in sour service (we'll talk about this later in the chapter on Corrosion). There is thus a fairly narrow hardness window that our parts must fall into after heat treating and machining. A water quench is one of the most severe quenches that can be obtained. This establishes the maximum cooling rate with which we can quench a steel bar. The cooling rate at the surface (which is in contact with the quenching fluid) will always be greater than the cooling rate experienced by the center of the bar. As a consequence, quenched and tempered steel bars will always show a hardness drop from the surface to the center of the bars. Because the surface hardness of our low alloy steels is fixed at a maximum of 22 Rockwell C, the rate at which the hardness drops will determine the maximum size bar that we can obtain the minimum strength requirements in the center. The greater the hardenability, the lower the rate that the hardness decreases will be.

Figure 15: 2¼Cr-1Mo CCT Diagram All gate valve bodies and bonnets made to API 6A must meet API 36K, API 45K, API 60K, or API 75K material strength requirements depending on the pressure rating. API 75K material requires a minimum tensile strength of 95 KSI which corresponds to a hardness of approximately 197 HB. The hardenability of 2¼Cr-1Mo steels is such that we can consistently through-harden a bar up to roughly 12" in diameter and fall within the 22 Rockwell C maximum at the surface and 197 HB minimum (for 75K properties) in the center after tempering. 4130, on the other hand, is limited to roughly 4" in page - 32

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diameter for developing 75K properties in the center of bars. Does this mean that 4130 can never be used for heavy wall parts requiring 75K yield strength? No, because seldom do our parts require 75K properties in the center of their heaviest cross section. It does mean that 4130 parts may have to be preheat treat machined to reduce the wall sections in these areas that will be highly stressed in service and thus require optimum properties. Whether or not 4130 should be used for a heavy wall part is frequently an economic decision based in part on the cost of preheat treat machining versus the cost of going to a higher alloyed steel such as 2¼Cr-1Mo. Figure 15 Shows that 2¼Cr-1Mo steels will have a predominately bainitic structure when quenched in the thicknesses typical of our equipment. Why settle for a bainitic structure? Why not specify an alloy steel that will develop a martensitic structure and has even better hardenability? Because in order to obtain a fully martensitic structure we would have to use a steel having a significantly higher alloy content than our 2¼Cr-1Mo steel. This would cost a lot more bucks and really wouldn’t buy us anything. Our 2¼Cr-1Mo alloy can do the job. A higher alloy steel may actually hurt us as far as machinability, welding, etc., is concerned.

ODDS & ENDS Damn! This turned out to be a long winded section! No doubt both your brain and your bottom are numb from sitting and trying to digest all this stuff. If you think it is excruciating to read you ought to try writing about it. You have no cause for complaint! At least you’re getting educated. All I’m getting is a sore writing hand and fingers that are permanently stained with ink. Cecilia is the only person who gets my sympathy. With great patience and fortitude, she has managed to get this volume typed from a mass of notes that were written in a semi-legible scrawl; arranged in no particular order; stained with coffee, lunch, Snickers bars, etc.; and heavily crisscrossed with arrows indicating where to insert words or sentences that I wanted to add and had squeezed into the margins so that they could only be deciphered with a magnifying glass. She didn’t bat an eye when I asked her to change the “finished” manuscript an average of fourteen times per page. She could have filed a complaint with the NLRB for unfair labor practices. She could have given me a lecture on doing it right the first time. Should could have..., but she didn’t. I nominate Cecilia Hobbs for sainthood! But I fear I’m rambling too much — back to the matter at hand: odds and ends. If you thought we had covered just about everything that has anything to do with heat treating, you thought wrong. Here we’re going to discuss those items that either I couldn’t fit nicely into one of the other categories or that I forgot to include in the first fourteen drafts. I’m afraid if I go back to Cecilia for a fifteenth time to include what I overlooked, she’ll become unglued (Cecilia can swear like a sailor when she gets mad) and blow her chance for sainthood. The austenite to martensite phase transformation is accomplished by a slight increase in the volume of the material because the unit cell of martensite is slightly page - 33

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larger than that of austenite. As a result, a highly hardenable steel that is quenched at too fast a rate can crack because the speed and extent of martensite formation induces a lot of stress into the metal. Whether or not these quench cracks occur is also very dependent on part configuration and size during the quenching operation. Quench cracks will often initiate at sharp corners, surface nicks, or other stress risers. Intersecting bores in alloy steel parts containing more than about 0.20% carbon can also result in quench cracking. An example of this would be a 4130 cross forging with both bores machined in it prior to heat treat. As the cross is quenched, the metal near the intersection bores is hit with quenchant from several directions simultaneously and consequently forms martensite before the bulk of the material transforms. The metal that transforms into martensite has an increase in volume. The bulk of material that has not yet transformed will constrain the metal that is transforming thus generating stresses high enough to crack the forging in half. Quench cracks will always initiate at a surface and then grow inward. Quench cracks can be prevented by minimizing stress risers, by martempering, eliminating intersecting bores, or by selecting a quench medium that will give a cooling rate just fast enough to obtain the desired transformation products, but no faster. Let’s talk about this last means of prevention. If we take a 4140 bar and water quench it after austenitizing, chances are it will crack. The cooling rate is so fast and 4140 so hardenable that the stresses induced by the martensite formation will crack the bar. We can’t change the chemistry of 4140, but we can change the cooling rate during the quenching by switching from water to oil. Although providing a slower cooling rate than water, oil will still quench 4140 fast enough to produce the desired microstructure and, at the same time, minimize stresses. Water and brine provide the fastest quenches and consequently are used for steels with relatively low hardenability. Oil, air, molten salt, and polymer solutions are the quenching mediums used for highly hardenable steels that can tolerate a slower cooling rate. We’ll examine each of these mediums. Water is a medium most frequently used for quenching low alloy and carbon steels having a carbon content of no more than 0.30%. Next to brine, it provides the fastest cooling rate of any of the common quenching mediums. The advantages of water are obvious: it provides a fast quench, it’s cheap and readily attainable, and it’s easily disposed of. The main disadvantage is the fact that the cooling rate is so high that it may not be suitable for thin or asymmetric parts where distortion could be a problem and it may not be suitable for steels containing over approximately 0.30% carbon for fear of quench cracking. Brine is a solution of sodium or calcium chloride in water. Typically the salt concentration is around 5-10%. Brine provides the fastest quench of the commonly used mediums. It is typically used when through-hardening must be attained, hence the cooling rate must be maximized. Distortion is less of a problem in a brine quench then with water. Because the salt increases the boiling point of water, steam pockets are less likely to form than in a water quench. Steam pockets are detrimental because they act as a an insulating barrier that will slow down the cooling rate and result in a soft spot in the metal. The disadvantages of using brine as a quenchant are the same as for water, plus brine is highly corrosive to low alloy and carbon steels. A hood may be page - 34

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necessary to keep fumes from corroding nearby equipment. The quench tank and associate equipment must be corrosion resistant which can significantly increase costs. Quenching oils can be categorized as conventional, fast, and martempering (or hot quenching) depending on their quenching effect and the temperature at which they are used. All quenching oils are mineral oils. Oils provide a much slower quench than water or brine and consequently are more suitable for medium and high carbon containing steels or for parts where distortion could be a problem. Most quenching oils will contain antioxidants to prevent the oil from breaking down over a period of time. The cooling rate provided by oil varies depending on how the oil is formulated and by the amount of agitation in the quench tank. Conventional quenching oils are the most frequently used. Fast quenching oils have a lower viscosity than conventional oils and provide a faster initial cooling rate. Martempering (or hot quenching) oils are formulated for good thermal and oxidation stability at 200-450(F. These oils are heated to this temperature range and used in the quenching of steels down to just above the MS temperature in martempering operations. The disadvantages of quenching oils are their cost, they cannot quench fast enough for low hardenability steels, and there is always a danger from fires or explosions because of water contamination. Polymer quenchants have the greatest degree of flexibility as far as cooling rates are concerned. There are many different polymeric quenchants on the market. The ones most commonly used work on the principle of inverse solubility. The quenchant consists of a solution of liquid organic polymers and corrosion inhibitors in water. At ambient temperatures, the polymer is completely soluble in water. At temperatures above roughly 200(F, the polymer separates from the water as an insoluble phase. Inverse solubility refers to the fact that rather than increasing with temperature as with most materials, the solubility of these polymers in water decreases. When hot metal is quenched in a polymer quenchant of this type, a film of liquid polymer will deposit on its surface. The rate at which the part then cools is dependent on the thickness of the polymer deposit. By controlling the thickness of the deposit through controlling the concentration of polymer, the bath temperature, and the amount of agitation, we can get a wide range of cooling rates. Polymer quenchants are frequently used instead of oils for safety reasons. Their main disadvantages are cost and the fact that the bath must be carefully monitored to ensure that the correct concentration is always maintained. Molten salt is used almost exclusively for martempering or austempering. It provides a much slower quench rate than oil. Typically salt baths are held around 400(F, which is just above the MS temperature for the type of steel that get martempered or austempered. Because of its slow cooling rate, molten salt quenching minimizes the stresses associated with bainite or martensite formation, but also limits the size bar that can be fully hardened. Air quenching is employed for highly hardenable steels that would crack in any other medium because the cooling rate would be too fast. Air quenching generally consists of nothing more than using large fans to air cool austenitized parts. Needless to say, it is a slow process. page - 35

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4130 is usually water quenched. 4140 is usually oil quenched. For some steels, we will specify a different quenching medium, depending on section size. 410 stainless steel is a good example. Some of our specifications for 410 specify that small diameter bars be air quenched while large diameter bars be oil quenched. By tailoring the quench medium to the size of the bar, we can get a quench rate that is just fast enough to obtain a martensitic structure and thus minimize stresses and the possibility of quench cracking. All the quenching business that we have talked about so far has referred to the rapid cooling operation following the austenitization of steels or the solutionizing treatment given age hardenable alloys. We also quench after other heat treating operations. Many carbon and low alloy steels are susceptible to a form of embrittlement when slowly cooled through the 700-1000(F temperature range. This is know as temper embrittlement and is believed to result from the precipitation of certain impurities along grain boundaries. Tempered embrittled steels will have very poor notch toughness, but fortunately the effects can be reversed by merely reheating the steel above the embrittling range. By water quenching after tempering, the steel will quickly pass through the 700-100(F temperature range and not have time to become embrittled. Quenching may also be done after aging precipitation hardening materials. This is so the aging process is immediately ended at the desired point. Some additional aging may occur if a part is slowly cooled from the aging temperature. Many nonhardenable alloys (316 stainless steel for example) are quenched from the annealing temperature to prevent the precipitation of undesirable phases. API 6A imposes controls on quench media temperature for materials that are quenched and tempered. Water used for quenching must not exceed 100(F at the start of the quench nor exceed 120(F at the end. The purpose of the limits is to insure that the cooling rate provided by the water is maximized. The rate at which heat is extracted from an austenitized steel part during quenching is dependent on many things including the temperature difference between the part and the quench medium. Maximize this difference and the cooling rate is maximized. By limiting the final water temperature to 120(F, API 6A effectively establishes the minimum size of quench tank, the minimum amount of quenchant agitation, and whether or not heat exchangers must be used on the quench tank when quenching a given size load. For quenchants other than water, it is up to the manufacturer to set temperature limits. There are other embrittling mechanisms that can occur during heat treatment besides temper embrittlement. Table 1 shows some of these as well as the alloys that are susceptible. These embrittling mechanisms are not only a concern during heat treatment, but effectively limit the maximum service temperature of a susceptible alloy. Fortunately, as with temper embrittlement, the embrittling effects can be reversed by reheating the part above the embrittling region and then rapidly cooling back down through the region. Most of these embrittlements occur because of the precipitation of a new, brittle phase (such as sigma phase in duplex stainless steel) or because impurities segregate out along the grain boundaries.

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TYPE OF EMBRITTLEMENT

SUSCEPTIBLE STEELS

TEMPERATURE RANGE 450-700(F

Blue Brittleness

carbon and some types of alloy steel

Temper Embrittlement

carbon and alloy steels having appreciable amounts of Mn, Ni, or Cr, and impurities of Sb, Sn, P, or As

Tempered Martensite Embrittlement

high strength, low alloy steels with a martensite or bainitic structure

400-700(F

885(F Embrittlement

ferritic and martensitic stainless steels containing 15% or more Cr

700-950 (F

Sigma Phase Embrittlement

ferritic and duplex stainless steel

700-1050(F

1050-1800(F

While most metals can be safely heat treated in a furnace atmosphere consisting of air, there are some reactive alloys that should be heat treated in a vacuum or under a protective coating or atmosphere (argon, etc.) so that they do not react with the oxygen, nitrogen, water vapor, etc., in the air. There are many special processes such as carburizing and nitriding that require the use of a special furnace atmosphere. Almost all metals heat treated in air will form oxides as the oxygen present in the air combines with the surface of the metal at elevated temperatures. Oxides may take the form of a tightly adherent layer for some metals or brittle, loose flakes for others. Visible oxides formed during the heat treatment are referred to as heat treat scale. The reaction with oxygen may cause the loss of certain elements at the surface of a metal. For example, the carbon in steel is very reactive with oxygen at high temperatures and will combine with it to form CO2 or CO gas which is then lost to the atmosphere. The resulting decrease in carbon content on the surface of the part is referred to as decarburization. This usually presents no problems because it is a skin effect and is often removed during final machining. The decarburized surface layer will have lower hardness and strength than the bulk of the metal. As a consequence, when hardness testing low alloy or carbon steel in the as-heat treated condition, it is important to grind or machine off roughly 1/16" of the surface in order to get down below the decarburized layer and obtain a valid hardness for the bulk of the material. When decarburization cannot be tolerated (such as when retempering a steel part that has already been machined), heat treating can be done either in a vacuum furnace or under a protective atmosphere, or the part may be coated with a ceramic paint, etc., that isolates it from the air in the furnace. Most of our heat treating is done in batch furnaces — furnaces that process one heat treat load at a time. Continuous furnaces, on the other hand, are capable of continuously (hence the name!) heat treating as many parts as desired. Continuous furnaces are typically in-line, open ended furnaces. The product being heat treated usually passes through the furnaces at a constant speed. Quenching is usually done utilizing a spray ring. There may be several different temperature zones in a continuous page - 37

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furnace each designed for a different purpose (e.g. the first zone may be at the highest temperature to provide the greatest heat-up rate). Furnace temperatures are typically higher in continuous furnaces than in batch furnaces and the corresponding times at temperature shorter. Continuous furnace heat treating is generally limited to solid parts less than 12" in diameter and to plate and pipe of any size.

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NOTES:

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CHAPTER III

ALLOYING ELEMENTS OF STEEL What's It Doing In There?

A deeply religious person is full of grace. A college cheerleader is full of school spirit. An avid sports fan rooting for the home team is full of enthusiasm. A feisty codger is full of vinegar. And metallurgists have been accused of being full of …… many interesting and little known facts about metals. (It is sad, but true, gentle reader, that some of the baser, unastute members of our society or perhaps those with severe personality disorders subconsciously completed the previous sentence with a two syllable word for a highly disagreeable, odoriferous substance that is the by-product of feeding fodder to a male member of the genus Bos. Rest assured, gentle reader, that my confidence in your intelligence, sensitive nature, and intolerance of all that is vulgar persuades me that you were not among them.) If metallurgists are full of facts, what are steels full of? Alloying elements, of course! In this section we will examine the different alloying elements used in steels and the purpose they serve. Pure metals have a rather restricted range of properties and this limits their usage as engineering materials. By making intentional additions of selected elements to our base metal (the starting metal that will form the bulk of our material), we can obtain a vast array of properties. Alloying elements may be added to increase strength, improve toughness, enhance corrosion resistance, deoxidize, control grain size, and for many other reasons. All commercial metals contain more than one alloying element. Different alloying elements may be added to enhance different properties or to work in combination with each other to enhance one specific property. Alloying elements do not always work in isolation within the base metal: sometimes the effect of alloying with a combination of elements exceeds what would be expected from the sum of the effects of the individual alloying elements. This interaction of alloying elements is known as the multiplicative effect. For example, both chromium and molybdenum may be added individually to a steel in order to strengthen it. A small amount of molybdenum used in conjunction with chromium will result in a much greater strengthening effect than the sum of their individual contributions. Steels can be somewhat arbitrarily divided into two groups: carbon steels and alloy steels. Steels that have, in addition to carbon, a maximum of 1.65% Mn, 0.60% Si, and 0.60% Cu and no other intentionally added elements (except when a deoxidizer or boron is specified or when small amounts of lead, sulfur, or phosphorus are specified for improved machinability) are classified as carbon steels. Other elements may be present in the form of residuals (impurities). Alloy steels exceed any or all of the above limits for Mn, Si, and Cu and may contain other alloying elements as well. Some of the more common alloying elements and their effect on the properties of steel are presented below. Carbon - This is the principal hardening element in steel. As carbon content increases, so will the steel hardness and tensile strength. Carbon, page - 41

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of course, is essential to the formation of cementite, pearlite, bainite, and martensite. Columbium - Columbium (also known as niobium) in very small quantities can significantly increase a steel's yield strength, toughness, and, to a lesser extent, the tensile strength by imparting a fine grain size. It can improve the creep and tensile properties of a metal at elevated temperatures by inhibiting grain growth through the formation of very stable carbides. It is used to tie-up carbon in some stainless steels in order to prevent sensitization (see Stainless Steels in Survey of Metals). Manganese - This element is present in all commercial steels. Next to carbon, it is the alloying element that provides the greatest increase in the hardenability of a steel (it solid solution strengthens). Manganese is an active deoxidizer. It combines readily with sulfur preventing "hot shortness" (see sulfur). Phosphorus - This is generally considered an impurity, although it may be added in amounts up to 0.12% in some free machining steels for better machinability. While it increases hardness and strength, it significantly decreases ductility and toughness, consequently in most steel specifications it is limited to a fairly low amount. Sulfur - Like phosphorous, sulfur may be intentionally added to free machining steels in order to improve machinability, but is otherwise considered to be detrimental. It can impair a steel's ductility, toughness, and weldability. Sulfur can combine with iron to form FeS. FeS can, in turn, combine with iron to form an eutectic that has a melting point of about 1812(F, well below the melting point of steels, and thus tends to segregate out along grain boundaries. Most steels are hot worked (forged, extruded, etc.) above 1812(F, consequently, if significant amounts of sulfur are present in a steel, intergranular (between the grains) cracking may result. The molten film of Fe-FeS eutectic has virtually no strength so stresses associated with forming will cause cracks to propagate throughout the material. This phenomenon is called hot shortness. Manganese additions can prevent hot shortness because manganese forms a stable, high melting temperature compound with sulfur. If the sulfur is tied up with the manganese, it is unavailable to form the low melting temperature eutectic. Silicon - This is generally used as a deoxidizing agent in amounts up to 0.35%. Dissolved oxygen in molten steel is detrimental because as the steel solidifies, oxygen can form blow holes or porosity or combine with other elements to form hard, brittle oxides that would impair the steel's workability and ductility.

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Nickel - Nickel increases the hardenability of steels. When it is present in appreciable amounts, it increases toughness (particularly at low temperatures) and corrosion resistance. Nickel provides for a simplified and more economical heat treatment to obtain higher strengths in steel and lessens the chance of distortion during quenching. Nickel is a powerful austenite stabilizer and is used in the 300 series stainless steels to make an austenitic structure that is stable from cryogenic temperatures up to the melting point. Chromium - Chromium is a very effective hardening agent in steels and is used for this purpose in amounts up to 4.0% Chromium will combine with carbon to form an extremely hard and stable carbide, thus imparting a high degree of abrasion and wear resistance to a steel. Steels having chromium in excess of 4% are often used in high temperature applications because of good creep and oxidation resistance. High levels of chromium can impart excellent corrosion resistance to steels. Stainless steels, for example, derive much of their corrosion resistance from the 10.5% minimum chromium content. The chromium forms a protective oxide film on the surface of the stainless steel that isolates from the environment. Molybdenum - Molybdenum increases the hardenability and the high temperature tensile and creep strengths of steel. It is added to corrosion resistant alloys to reduce pitting susceptibility. Relatively small amounts can significantly reduce a steel's susceptibility to temper embrittlement. It has a strong multiplicative effect on hardenability when used in conjunction with chromium and nickel. Vanadium - This increases the strength and toughness of heat treated steels through its ability to inhibit grain growth over a broad temperature range. It is added in amounts of up to roughly 0.05% to increase hardenability. At higher amounts it may cause problems with welding. Copper - Copper may be added to some steels in order to improve atmospheric corrosion resistance. It is usually limited to 0.2 - 0.5%. Boron - Boron can significantly increase the hardenability of steel in amounts as low as 0.0005%. It enhances the effects of other alloying elements used for strengthening. Boron is generally restricted to very small amounts because it can cause cracking problems during welding. Lead - Lead does not form an alloy with steel. When added to some free machining steels in the amount of 0.15-0.35%, it forms a fine dispersion of elemental lead within the steel which enhances machinability. Nitrogen - Nitrogen, in amounts over 0.004%, will react with other elements (particularly aluminum) to form nitrides. These nitrides increase page - 43

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the steel hardness and tensile strength, but lower the toughness and ductility. Nitrogen is added to many duplex and other types of stainless steels that aren't hardenable by heat treatment in order to solid solution strengthen the matrix. Nitrogen also tends to enhance corrosion resistance in stainless steels (particularly pitting resistance) through a mechanism that is not clearly understood. Aluminum - This is used to deoxidize steel and to inhibit grain growth. It is sometimes added to steels that will be nitrided because it forms a hard, stable nitride. It is usually added in the amount of 0.9-1.30% when used for this purpose. Titanium - Titanium may be added to steels for several different reasons. It is a powerful carbide former. It is used to form stable carbides in some stainless steels to prevent sensitization (see Stainless Steels in Survey of Metals). The stable carbides also inhibit grain growth thus improving high temperature properties. Like columbium, titanium is used in small quantities in some micro-alloyed steels to improve yield strength and, to some extent, tensile strength through grain refinement.

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CHAPTER IV

MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

Was It Really Worth It? We've come a long way, baby! We've doped up our steel with a pinch of this and a smattering of that alloying element. We've beat the ever livin' shit..... daylights out of it in a forge press to get it in the shape we want. We threw it in a fire to heat it up. We took it out of the fire and threw it in a bucket of water to cool it down. And then, incredibly, we threw it into the fire one more time! ENOUGH OF THIS ABUSE! The tortures of the Inquisition pale beside what we have just done to our steel! And just what have we accomplished with all this brutality? That's what mechanical testing will tell us. What are mechanical properties? They are the characteristics of a metal that describe how the metal will react to an applied stress. Some of the mechanical properties that we'll discuss and that you should become familiar with are hardness, toughness, and the tensile properties of tensile and yield strengths, percent reduction of area, and percent elongation. Mechanical testing is the means we use to determine these properties. When you apply a stress to a metal it will deform. The type and amount of deformation is dependent on the amount of stress and how it is applied. Metal deformation can be characterized as either elastic or plastic (inelastic). All mechanical tests involve measuring the elastic and plastic deformation of a metal in response to a known stress. An example will make the difference between the two clear. Suppose we suspend a wire from the ceiling and then attach a small weight to its free end. The wire will "stretch" a certain distance over its original length. If the weight is sufficiently small, the wire will return to its original length when we take the weight off. In other words, the wire acted like a spring when subjected to a small stress: there was no permanent deformation. This behavior is called elastic deformation. Now suppose we increase the weight we hang on the wire. Again the wire will stretch past its original length. But if the additional weight is large enough, the wire will not return to its original length when the weight is taken off. Instead, immediately upon removal of the weight, the wire contracts slightly. This again indicates elastic behavior. But because the wire did not contract all the way back to its original length, the wire must have been permanently stretched or deformed. This permanent elongation of the wire indicates plastic deformation.

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MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

TENSILE TESTING The tensile properties of a metal, not too surprisingly, are determined by a tension test done in a tensile tester. A tensile tester is a machine that applies a load to a standard size metal specimen at a given rate. The specimen is clamped between a fixed and a movable cross head. As the movable cross head moves away from the fixed cross head, the specimen is put in tension (or stretched). The movable cross head is driven by either a hydraulic mechanism or by a motor through a screw drive. The test specimen will elongate to a certain point and then break, at which point the test is finished. Two basic measurements are made during the test: the load put on the test specimen at a particular instant of time and the corresponding elongation of the specimen. On most test machines this data is continuously plotted out on an X-Y plotter. Load is measured by sensors in the tensile tester while the elongation is measured by an extensometer, a device that clamps directly onto the specimen. The extensometer detects any small movement of the sample and sends out an electrical signal proportional to the amount of movement. The test specimen itself may be one of several different standard types (see Figure 1). Samples taken from plate are generally cut into flat specimens while bar or forging samples are generally turned into round specimens. You'll notice that each type of specimen has a reduced section in the center and that there are two points marked on each specimen. These points mark the gage length of the specimen. The gage length is an arbitrarily specified distance (generally 2" for full-size turned specimens and 8" for flat) that designates the part of the specimen that we will consider in our calculations. We'll use the values of load and elongation from the test to develop an engineering stress-strain curve. Engineering stress is the load applied to the test sample divided by the original cross-sectional area of the test specimen’s gage length. It is expressed in terms of pounds per square inch, or psi. Strain can be defined as the percent increase in the gage length during the test. It can also be expressed as the elongation (in inches) per inch of gage length. Let's look at a stress-strain curve for a typical, ductile metal in Figure 2.

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MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV

Standard Round Specimens Dimensions Per ASTM E8 Standard Specimens

Nominal Diameter G - Gage Length D - Diameter R - Radius of fillet, min A - Length of reduced section, min

Sub-size Specimens

in.

in.

in.

0.500

0.350

0.250

2.000 ± 0.005 0.500 ± 0.010

1.400 ± 0.005 0.350 ± 0.007 ¼ 1¾

1.000 ± 0.005 0.250 ± 0.005 3/16 1¼

G

2¼

Standard Plate Specimen, 1½" Wide Dimensions Per ASTM E8 in. G - Gage length W - Width T - Thickness R - Radius of fillet, min L - Over-all-length, min A - Length of reduced section, min B - Length of grip section, min C - Width of grip section, approx

8.00 ± 0.01 1½ + , -¼

F

1 18 9 3 2

Figure 1: Tensile Test Specimens

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MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV

Figure 2: Engineering Stress-Strain Curve The significant parts are as follows: Section AB - This portion of the curve shows that in the initial part of the test, the metal deformed elastically and that the stress is directly proportional to strain. In other words, if we double the load on the test specimen, the amount that the test specimen will elongate, or stretch, will double. If we remove the load, the specimen will return to its original length. Note that the volume of material in our test specimen does not change during the test. As a consequence, if the specimen elongates as a load is applied in one direction, then the specimen must contract in another direction so that its volume is always the same. This contraction does in fact take place and occurs at 90 ( to the direction of elongation. This is known as the Poisson effect. If we were to find the amount of strain in contraction and divide it by the amount of strain that the specimen undergoes during elongation at any point in Section AB, we'll get a constant. This constant is the Poisson's ratio for the particular material. Point B - This marks the point on the curve where stress is no longer proportional to strain and is consequently called the proportional limit. Up to the proportional limit, if we pick any point on the curve and take the corresponding stress and divide it by the corresponding strain, we'll get a

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constant. This constant is the modulus of elasticity or Young's modulus. It reflects the rigidity of the metal. Point C - Point C is the elastic limit. Once we increase the load on the test specimen (consequently increasing the stress) beyond this point, the specimen will become permanently (or plastically) deformed. In many materials the elastic limit and the proportional limit virtually coincide. Point D - Point D is the yield strength of the metal. Yield strength is the stress at which a specified amount (usually 0.2%) of permanent strain occurs. It is a measure of a metal's ability to resist being plastically deformed. It is found by drawing a line from 0.2% strain (or whatever is specified) on the strain axis parallel to section AB. Where this parallel line intersects the curve is the yield strength. This method of determining yield strength is called the 0.2% offset method. Segment CE - Plastic deformation is uniform along this portion of the curve. This means that the cross-sectional areas of all the points along the gage length decrease the same amount as the specimen is stretched during the test. Point E - Point E is the ultimate tensile strength, or just tensile strength. It is the highest point on the curve and thus represents the maximum load or the maximum stress that the specimen can withstand before breaking. Tensile strength is a measure of the metal's maximum resistance to deformation. Segment EF - Once we get beyond the maximum load on the curve (the tensile strength), continued pulling of the test specimen will result in nonuniform deformation. There will be some point on the specimen where localized deformation will exceed that of all other points. This is referred to as necking (see Figure 3). Point F - Necking will continue in a localized area on the specimen until the specimen finally breaks. The fracture point is F on the curve. We've extracted a lot of useful information from the stress-strain curve. Now it's time we focus our attention on the broken test specimen. If we fit the broken pieces back together and measure the distance between the gage marks, we can get an idea of how much plastic deformation the specimen underwent prior to the fracture. Percent elongation is the percentage increase of the gage length at fracture in comparison to the original length. We can also measure the cross-sectional area at the fracture end of one of the broken specimen halves and compare it to the original area. The percent reduction of area is the percentage decrease in the cross-sectional area of the test specimen during the test. Like percent elongation, it is a measure of plastic deformation. Percent elongation and reduction of area are two different ways of

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measuring a metal's ductility, its ability to resist fracture while being plastically deformed.

Figure 3: Necking

IMPACT TESTING Impact testing is one means of determining the notch toughness of a metal. Notch toughness is the amount of energy required to fracture a specimen that contains a stress-riser in the form of a notch by applying a sudden load to it. The type of impact testing that is normally done to our materials is the Charpy V-notch. Figure 4 illustrates the standard Charpy V-notch test specimen. The "V" describes the shape of the notch. Great care must be taken in preparing the specimen because any slight machining flaw can significantly affect test results. The specimen is supported at both ends in a cradle in the horizontal position at the bottom of the impact tester. The axis of the V-notch is oriented in the vertical position. A free swinging pendulum strikes the specimen in the middle of the face opposite the notch. The pendulum is released from a fixed height so it always strikes a specimen with the same energy. The pendulum will continue its swing after breaking the specimen and rise to a certain height opposite the side it was released from. The difference between the height that the pendulum was released from and the height that it swung to after breaking the sample is multiplied by the weight of the pendulum to obtain the energy it took to fracture the sample. This energy is usually reported in footpounds. Materials are often characterized as having "good impact strength." This is really a misnomer because the impact test measures absorbed energy, not strength. However, "impact strength" has become the common terminology. If one material required more absorbed energy to fracture it than another material at a particular page - 52

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temperature, then the first material is said to be "tougher" than the second. Conversely, the second material is more "brittle" than the first.

Figure 4: Charpy V-notch Test The fracture surface of a broken impact sample may look shiny or dull or have areas of both. A shiny surface is indicative of a brittle (or cleavage) fracture. This type of fracture absorbs comparatively little energy. The dull surface is indicative of a ductile (or shear) fracture. This type of fracture absorbs the most energy. Of course, fracture surfaces showing both shiny and dull areas indicate an intermediate absorbed energy level. We can measure the percent shear by comparing the fracture face against photographic standards in ASTM. The impact toughness of some materials is very dependent on the test temperature. This is particularly true of body-centered-cubic materials such as carbon and low alloy steels. If we make a number of specimens out of one of these materials and test them at various temperatures, we can develop a curve that shows the relationship between temperature and impact energy. A typical curve is presented in Figure 5.

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Figure 5: Transition Curve There are several important things you should note about this curve. First is that at temperatures T1 and below, the impact energy is constant and at its lowest value. It doesn't matter how much we lower the test temperature, impact energy will not go below that value shown on the curve. This is because the impact specimens fracture in a completely brittle fashion in this region of the curve: the worse possible case for absorbing energy. The second thing to notice about the curve is that at temperatures T2 and higher, the impact energy is again constant, but this time it is at it's highest value. This portion of the curve represents the impact energies of specimens that broke in a completely ductile manner. (Of course, there may be some scatter in the energies reported in the upper and lower regions of the curve due to small variations within the test specimens or in the testing itself.) The curve in the region between temperatures T1 and T 2 shows that there is a transition region where the impact specimen fracture changes mode. As the test temperature is increased above T1, the fracture becomes more and more ductile until T 2 is reached. Because of this transition region, this type of curve is known as a "transition curve." The region at temperatures T1 and below is called the "lower shelf" and the region at temperatures T2 above the "upper shelf." Besides absorbed energy and the percent shear of the fracture surface, we can make one more type of measurement from a broken impact specimen. This third measurement is lateral expansion. Lateral expansion is the increase in the specimen's width on the compression side, opposite the notch. It is generally given in mils and is one more indication of how brittle (or ductile) the fracture was. See Figure 6.

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MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV

Lateral Expansion (Larger of X1 or X2 )  (Larger of Y1 or Y2 )

Figure 6: Lateral Expansion (L.E.)

HARDNESS TESTING A material's hardness is a measure of its ability to resist penetration of its surface. The most common of the various hardness tests is the indentation type. This involves applying a specified load onto an indenter of fixed geometry which then makes an impression in the surface of the metal being tested. The degree of hardness is measured by the size of the indentation. Hardness tests that use an indenter include Brinell, Rockwell, and Vickers. The Vickers Test is also known as the Diamond Pyramid Hardness test. The indenter is made out of a diamond cut in the form of a square-based pyramid that has an angle of 136( between opposite faces. The loads applied can vary from 1 to 120 kilograms with 1, 5, 10, 30, and 50 kg being the most common. The specimen to be tested is placed on an anvil and brought to within a millimeter of the indenter. When the loaded indenter is released, it goes down and penetrates the specimen's surface and then returns to its original position by means of a cam and weight arrangement. Both the rate at which the indentation is made and the amount of time that the indenter is on the surface are controlled. The lengths of both of the diagonals in the resulting square-shaped indentation are measured under a microscope. The Vickers hardness number is equal to the applied load divided by the surface area of the indentation or

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MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV HV

2 P sin (/2)

d2

Where, HV = Vickers Hardness P = applied load  = angle between faces of the indenter (136() d = average length of diagonals The applied load is typically reported along with the Vickers number, e.g. 250 HV10 where the “10" subscript refers to a 10 kg applied load. The Brinell Test consists of applying a constant load (usually 500 or 3000 kg) onto a hardened steel ball-type indenter, 10mm in diameter. The 3000 kg load is used for testing steel and cast irons while the 500 kg load is used for testing softer materials. The ball indenter penetrates the surface of the test specimen and is held for a specified period of time before the load is released. This generally runs 10-15 seconds for steels and 30 seconds for softer materials. This time insures that all plastic deformation taking place in the specimen has stopped. The average diameter of the indentation is calculated from two measurements taken at right angles to each other. The Brinell hardness number is defined as the applied load divided by the surface area of the indentation, or

HB

P (% D/2) [D

D 2 d 2]

Where, HB P D d

= = = =

Brinell Hardness applied load diameter of ball, in mm diameter of indentation, in mm

The Rockwell Test is different from the other two types of hardness tests in that the hardness is determined from the depth of the indentation. One of the most common type of indenters used in Rockwell testing is a diamond ground to a 120( cone with a spherical apex having a 0.2mm radius. This is known as the Brale indenter. The specimen to be tested is brought up to the indenter and a minor load is applied. The minor load is 10 kg and is applied in order to eliminate backlash in the load train. It also causes the indenter to break through slight surface roughness or foreign matter. The minor load thus greatly increases the accuracy of the test. The depth of the indenter after the minor load is applied is used as a reference point on the dial gage of the tester. A major load is then applied consisting of a 60, 100, page - 56

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or 150 kg load depending on which scale you're using. This, of course, causes the indenter to penetrate even further into the material. The difference in depth between the major and minor loads is automatically registered on the dial gage in the form of a Rockwell Hardness Number. Each division on the gage represents a difference in indentation depth of 0.002 mm. Various combinations of indenters and loads can be used depending on the hardness and the thickness of the material being tested. Each standard combination of indenter and load constitutes a Rockwell scale. When testing metals, the "C" and "B" scales are the most frequently used. The "C" scale uses a Brale indenter with a 150 kg major load, and the "B" scale a 1/16" diameter ball indenter with a 100 kg load. The "B" scale is used for testing materials too soft to be tested using the "C" scale. The applicable scale must be reported when reporting a Rockwell hardness number. For example, 22 HRC tells us that the metal tested has a Rockwell hardness number of 22 when tested using the "C" scale. Superficial Rockwell testing is similar to the standard Rockwell test except the minor load is 3 kg and the major load is 15, 30, or 45 kg. Both can use the same indenters. Superficial Rockwell testing is used to determine the hardness of thin sheets, plating, case hardening, and other surface effects. The Versitron® hardness tester that we sometimes use in our shops is a type of Rockwell tester. Unlike conventional Rockwell machines that use weights to provide the necessary load on the indentor, the Versitron's® load is provided by a spring. This allows the test head to be moved down to the part thus allowing large parts to be tested that would not have fit into a conventional Rockwell machine. Which is the preferred method of hardness testing? Depends on what you want tested! The Vickers test can check the greatest range of hardness without changing scales, however, it is essentially a laboratory test. Vicker test specimens have to be flat, have exactly parallel faces, and be carefully ground or polished. It is often used for making hardness traverses across weldment cross sections or for other hardness measurements that must be made in a precise location: it is seldom used for production parts. The Brinell test is often used in our shops. The large indentation gives an accurate hardness of the bulk of the material because it is less sensitive to minor surface irregularities. This also makes surface preparation less critical. The indentation can be measured at any time thus it becomes a permanent record of hardness. Large parts are easily tested. Finally the Brinell hardness test can measure a wide range of hardness values without changing scales. Its main disadvantages are that some parts cannot tolerate the large indentation because of seal areas, thin wall sections, or surface finish requirements, and the fact that a hardness tester must physically measure this indentation. The Rockwell test can quickly and easily test parts that are small enough to fit into the machine. Its small indentation will not deform or disfigure most parts. The main disadvantages are large parts cannot be tested on most machines and many different scales may have to be utilized depending on the hardness of the material to be tested.

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MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV

Microhardness testing is hardness testing performed on a very localized area of a specimen using indentor loads of 1 kg or less. The purpose of performing a microhardness test may be to: 1. Measure the hardness of parts that are too small, thin, etc., for regular hardness testing. 2. Measure the hardness of individual microconstituents in a metal's structure. 3. Measure the hardness of plating, coatings, etc., on the surface of a metal. 4. Measure the effects of carburizing and nitriding operations. Most microhardness testers are very similar in principle to the Vickers test. The test specimen is ground flat the and test surface ground parallel to the back side. The surface to be tested is then polished. Small specimens may be mounted in a Bakelite® or plastic mold for convenience. The test piece is placed on the X-Y stage of the tester and the stage raised up to the level of the indentor. Looking through a microscope integral with the hardness testing machine, the hardness tester locates the specific area to be hardness tested. He then shifts the stage with the test piece to a point directly under the indenter and trips a lever which causes the indenter to come down, make an indentation, and then retract (all through a cam and weight system). The stage is then shifted back under the microscope and the indentation measured. Either a Vickers or a Knoop indenter can be used in microhardness testing. The Vickers indenter is identical to the one used in the standard Vickers test. The Knoop indenter is a diamond ground to a pyramidal form that makes a diamond-shaped indentation having an approximate ratio of seven to one between the long and short diagonals (see Figure 7). The pyramid has an included longitudinal angle of 172 ( 30' and included transverse angle of 130(. The depth of indentation is about 1/30 of its length. The Knoop indenter, because of its shape, can produce accurate results with very light loads. The Knoop hardness number is the ratio of the load applied to the indenter to the projected area of the indentation or

HK P /A P/CL 2 Where, HK P A C L

= = = = =

Knoop hardness applied load projected area of indentation 0.07028 measured length of long diagonal in millimeters

The applied load must always be referenced. Microhardness values may change with the applied load at these low load levels because of the differences in the rate of strain hardening as the indenter penetrates the surface.

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MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV

Figure 7: Knoop Hardness Indentation

We frequently use microhardness testing (done with a Knoop indenter) to perform hardness surveys in order to check for banding. Banding is a type of chemical segregation. A banded material, instead of having uniform chemistry, will have alternating layers of material having different chemistries. Banding may arise from many processing factors including melting practice, hot work, heat treat, etc. It may be detrimental because the differences in chemistry between the alternating bands may result in non-uniform mechanical properties and less than optimum corrosion resistance. By performing a hardness survey by making several closely spaced Knoop hardness indentations in a row, we can check the degree of banding in a material. If the hardness results are fairly uniform, then banding is not a problem. If the hardness values show a wide range, then a banding problem exists.

FRACTURE MECHANICS Material toughness can be thought of as the resistance of the material to crack propagation in the presence of a notch. Fracture mechanics is a method of characterizing fracture behavior in terms of material toughness, flaw size, and stress level. This is heavy duty stuff. The faint of heart, the unastute, the unstable, and all Texas A & M graduates should turn back now before we dive into the morass of fracture mechanics and they suffer mental overload. We'll meet you again at the start of the next section. All metals contain flaws. These flaws may be the result of processing, alloying, etc. They may be macroscopic and thus easily detectable by nondestructive examination, or they may be microscopic and be undetectable. Fracture mechanics permits us to examine the stability of flaws in materials, analyze their growth, and to predict the size at which catastrophic failure will occur. The fracture mechanics approach that we’ll look at first is based upon the linear-elastic theory of metals. The linear-elastic fracture mechanics approach (or LEFM) is suitable for materials that are normally ductile, but become brittle in the presence of a flaw. These materials are often referred to as ductile, crack sensitive. Ductile, crack sensitive materials include high strength ferritic steels, titanium alloys, and high strength aluminum alloys.

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MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV

Now some preliminaries. Plane strain describes the state of stress that characterizes thick or brittle parts in which the stress adjacent to a flaw is tri-axial tension (material next to a crack tip, for example, is under a tensile stress in the X, Y, and Z directions). A part in a plane strain condition that contains a flaw (such as a crack) is subject to rapid and complete fracture if it is loaded such that the stress intensity adjacent to the flaw exceeds a certain critical value. If such a part is slowly loaded, the flaw will grow because the restraining effects of the bulk of the material and the Poisson effect of metal minimizes the amount of local yielding that can occur. The strain energy will be absorbed by the material up to a certain point after which any additional stress will result in rapid fracture. The basis of fracture mechanics is that the stress field ahead of a sharp crack can be described by a single parameter K, the stress intensity factor. K has units of ksi in and is a function of the stress level and the flaw size. Unstable crack growth will occur whenever K reaches a certain critical value (designated Kc). There are three possible displacement modes for crack propagation (see Figure 8). Mode I, in which the applied stress is normal to the crack surfaces, has received the greatest attention because it is associated with many catastrophic structural failures. Mode I

Mode II

Mode III

Figure 8: Displacement Modes The elastic-stress field distribution at the tip of a crack in Mode I is shown in Figure 9. This figure shows that the distribution of the stress field adjacent to the crack tip is invariant for structural components that are loaded in Mode I. The stress intensity factor that describes the magnitude of the elastic stress field in Mode I is designated KI. The magnitude of KI is affected by the applied stress, the crack size and configuration, and the structural configuration of the part, however, none of these factors will change the stress field distribution ahead of the crack tip. As a consequence, this analysis can be applied to different structural configurations (see Figure 10). Note that in all the examples in Figure 10, KI is dependent on the nominal stress and the square root of the flaw size.

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MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV

Fracture toughness is a measure of a material's resistance to crack propagation in the presence of a notch. It is a material property just like tensile strength or hardness. Fracture toughness values are dependent on the type of material, environment, loading rate, and the type of constraint. The fracture toughness value that we test our materials for is KIc. KIc is the critical stress intensity factor for static, Mode I loading and plane strain conditions.

)y

K1 2 %r

COS

 2

1sin

3  sin 2 2

Figure 9: Elastic Stress Field At A Crack Tip

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MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV

Through Thickness, Center Crack

K1

) a 1.77 1.77

2a W

BW

2

Through Thickness Crack In Pressurized Cylinder

K1

PR t

2 %a 1  1.61 a

½

Rt

Double Edge Crack

K1

) a 1.99  0.36 2a BW

w

2.12 2a w

2

 3.42 2a

3

w

Figure 10: KI For Different Crack Geometries

"Sir, while I'm not an old mossback, I'm no spring chicken either. How dare you try to foist this rigmarole off on me as having some import? I am sure that there are legions of academicians working for our customers who make a handsome living by perpetuating this nonsense amongst themselves. Undoubtedly their survival is attributable only to the fact that they have managed to convince their superiors that this claptrap is vital to their common well-being. By obfuscating the already difficult to understand so that only they can interpret its meanings, this cabal has permanently page - 62

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entrenched themselves within our customers organizations. I am a simple, but honest man. What does this technical trivia have to do with me? Either enlighten me as to its meaning or let us go forth together and root out these charlatans before the disease spreads! What does it all mean?" Good question. Here's the answer. Just as our engineers must always select a material that has a yield strength well above the nominal stress that a given part will see in service, they must select a material that has a KIc value well above the maximum KI that will be induced in the part as it is loaded in service. In order to calculate the maximum KI value, the engineer assumes that the part may contain a flaw up to a certain size, but no larger. This can be verified in the actual part through nondestructive examination. As long as the value of KI (which is dependent on the magnitude of the applied stress and the size of flaw for a given part) is below the KIc value of the material used to make the part, the flaw will not grow catastrophically. If, however, the combination of flaw size and applied stress is such that the K I value exceeds K Ic in a given part, the flaw will propagate through the part in a brittle fracture mode (see Figure 11).

Figure 11: Material Selection Based On KIc

FRACTURE TOUGHNESS TESTING There are many different types of fracture toughness tests. We typically use three for our types of materials. ASTM E399 is used to determine the plain strain fracture toughness (KIc). For ductile materials that are not particularly crack sensitive (this includes most of our materials), ASTM E399 is not a valid test method and we will have to estimate fracture toughness using other test methods such as ASTM E813 (JIc determination) and BS 5762 (CTOD). We’ll examine ASTM E399 in detail and then briefly look at the other two test methods.

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CHAPTER IV 1.

MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

ASTM E399 — Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials This test utilizes precracked specimens that are loaded either in tension or three point bending. The load versus displacement across the notch at the specimen edge is recorded on an X-Y recorder. The load corresponding to a 2% apparent increment of crack extension is determined from the load versus displacement curve. This value is then used to calculate KIc using equations derived from the elastic stress analysis of the particular type of specimen being tested. There are a number of possible test specimen configurations. The two most commonly used are standard bend and compact tension specimens (see Figure 12). We typically use compact tension specimens. We’ll examine how a compact tension specimen is tested to determine KIc. We defined fracture toughness as the resistance of a material to crack propagation. If we are going to measure the fracture toughness of our specimen, we must first intentionally introduce a very sharp crack of known size and orientation in our specimen. This "precracking" is done by fixturing the test specimen into a load frame (similar to a tensile tester) and cyclically loading it in order to produce a fatigue crack at the tip of the slot. Because the validity of the test is dependent on the establishment of a sharp crack condition at the tip of the fracture crack, the stress intensity level at which the fatigue precracking is done is limited. This means that the test specimen must see a large number of loading cycles in order to achieve a fatigue crack of the desired size (this can take hours to accomplish). Let’s go through a fracture toughness test using a compact specimen. The precracked test specimen is loaded in the load frame and an environmental chamber to control temperature is placed around it. An extensometer is placed in the slot of the specimen in order to monitor displacement as the specimen is loaded in tension. The temperature in the chamber is adjusted to the test temperature (0(F is commonly used). Testing begins when the specimen reaches the desired temperature (as shown by an attached thermocouple). The specimen is loaded in tension at a rate such that the rate of increase in stress intensity at the crack tip is within 30-150 ksi in per minute . An X-Y plot of load versus displacement is made during the test. The test is over once the specimen cannot withstand any additional increase in load.

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CHAPTER IV

(A) Bend Test Specimen

(B) Compact Specimen

Notes for A & B: W B

1. 2 2. A = 0.45W to 0.55W 3. Fatigue crack extension must be at least 0.25W or 0.050 inch, whichever is larger Figure 12: Common ASTM E399 Test Specimens page - 65

MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV

Typical forms that the load versus displacement curve can take are shown in Figure 13. The first step in calculating K Ic value is to obtain the value of the load PQ graphically from the load versus displacement curve. If the curve has the form shown in A of Figure 13, a line is drawn from the origin with a slope equal to 0.95 x slope of the linear portion of the curve. PQ is where the line intersects the curve. If the load versus displacement curves have the forms shown in B or C of Figure 13, again a line will be drawn through the origin that has a slope of 0.95 x slope of the linear portion of the curve. The largest load preceding the intersection of the line with the curve will be PQ.

Figure 13: Load Versus Displacement Curves Next we must use the value of PQ to calculate KQ, the conditional value of KIc (it will be KIc once we’ve proven our test results to be valid). For a compact specimen:

Where,

f (a/w)

page - 66

KQ

=

(PQ/BW 1/2) x f(a/w)

a B w

= = =

crack length thickness of specimen width of specimen

(2  a/w) (0.886  4.64a/w 13.32a 2/w 2  14.762a 3/w 3 5.6a 4/w 4) (1 a/w)3/2

MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV

This rather awesome formula for calculating KQ is greatly simplified by using the tabulated values of f(a/w) for various ratios of crack lengths to specimen widths found in ASTM E399. KQ will equal KIc provided the following two conditions are met: 1. 2.

Pmax/PQ < 1.10 KQ 2 2 .5 < the specimen width and < the crack length

)ys

Many of the metals that we use will not meet the second condition. For example a low alloy steel having a yield strength of 75 ksi will have a KQ of about 200 ksi in . Our compact tension specimen would require a minimum width of about 18" in order to have a valid KIc test. We’d get a hernia trying to lift a specimen this size and probably couldn’t find a load frame big enough to break the damn thing. Fortunately we have an out: we can estimate the KIc value of low alloy steels and other metals by first determining the JIc value and then converting it to KIc. What is a JIc value? Read on! 2.

ASTM E813 — Standard Test Method for JIc, A Measure of Fracture Toughness A J-integral is a mathematical expression used to characterize the stressstrain field around a crack tip in a metal that is either too ductile or lacks sufficient thickness to be validly tested for KIc. JIc is the critical value of J near the onset of stable crack growth in such a material when tested to the requirements of ASTM E813. In practical terms for elastic materials such as the low alloy steels that we commonly use, the J-integral is equal to the crack extension force — the elastic energy per unit of new separation area that is made available at the tip of an ideal crack during incremental crack growth. It is thus a measure of fracture toughness. Jintegral testing utilizes the same type of test specimens and the same equipment as we described for KIc testing. A load versus displacement curve is generated. The J-integral is determined (it’s directly proportional to the area under the curve) for a selected displacement level. A plot of J versus crack growth is then plotted using at least four data points within specified limits of crack growth. The value of JIc is found graphically from this curve. The evaluation of the test data is very complex and involves a great deal of mysticism, ritual dancing, chanting, etc., and consequently will not be dealt with here. JIc has units of inch-pounds per square inch (kilojoules/square meter).

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British Standard BS 5762 — Methods for Crack Opening Displacement (COD) Testing COD (or more commonly referred to in this country at CTOD — crack tip opening displacement) testing is another way to measure the fracture toughness of ductile materials that cannot be validly KIc tested. This test method utilizes a three point bend specimen that has been notched and fatigue precracked. The preferred test specimen is a rectangular block having a thickness B (where B is the cross section thickness of the material under examination), a width equal to 2B, and a length equal to 4.6 W. This can result in huge test specimens for product forms other than plate so BS 5762 allows other, smaller size specimens (referred to as subsidiary specimens) when agreed upon by the manufacturer and purchaser. The test consists of slowly bending the test specimen in three point bending. The load is applied opposite the notch in the center of the bar. An extensometer records displacement in the width of the notch as the load is applied. COD values are derived mathematically from certain critical values on the load versus displacement curve. A COD value is the displacement of the crack surfaces normal to the original (unloaded) crack plane at the tip of the fatigue precrack. It typically has units of millimeters. Depending on the shape of the load versus displacement curve, one or more different COD values ( ) may be derived. They are as follows:

c — COD value at either unstable fracture or the onset of arrested brittle crack growth (“pop-in”) when no slow crack growth has occurred.

i — COD value at which slow crack growth commences.

m — COD value at the first attainment of maximum load plateau.

u — COD value at either unstable fracture or the onset of arrested brittle crack growth (“pop-in”) when slow crack growth has occurred. COD testing is much more commonly used in Europe than in the U.S.

FACTORS AFFECTING FRACTURE TOUGHNESS What affects the fracture toughness of a metal? Will the fracture toughness value derived from a laboratory test always be valid for field conditions? What can we page - 68

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do to improve the fracture toughness of our metal? What is the relationship between fracture toughness and the other mechanical properties of a metal? Lots of good questions. Here are the answers. Fracture toughness is a material property just like tensile or yield strength, but this does not mean that it’s always constant for a particular metal. Its value may vary depending on temperature, strain rate, and environment. The fracture toughness of most metals decreases with decreasing temperature over their common service temperature range. This temperature effect is most pronounced for ferritic and other BCC materials. These materials will undergo a distinct shift from ductile to brittle as the mechanisms for crystallographic separation changes. At high temperatures, separation occurs by fibrous tearing (also called microvoid coalescence or dimpled rupture). Small voids will start to nucleate (particularly around particles of second phases or impurities) as the metal is stressed and tears (see Figure 14).

Figure 14: Fibrous Tearing These voids will enlarge with increasing stress and eventually combine (coalesce) until fracture is complete. At low temperatures, the crystallographic separation occurs by cleavage. Metals having a non-BCC structure generally will a much smaller decrease in fracture toughness as temperature decreases than BCC metals. Strain rate affects fracture toughness. For example in steels and other BCC metals higher strain rates result in lower fracture toughness (see Figure 15). This shift is more pronounced at higher temperatures. Increasing strain rate has the effect of shifting the KIc versus temperature curve to the right (with increasing temperature on the x-axis).

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Figure 15: Effect of Strain Rate on KIc

The environment can have a profound effect on the fracture toughness of a metal. A steel, for instance, may exhibit ductile behavior and have high fracture toughness when tested in air, but become embrittled in service when exposed to H2S resulting in much lower fracture toughness. We’ll look at some of the types of environments that are detrimental to metals in the chapter Corrosion. For now it is important to realize that the fracture toughness of certain metals may be much less in certain environments than in air because of embrittling mechanisms and stress corrosion cracking. It is essential that an engineer be fully aware of the environment that a part will be exposed to. As a general rule of thumb, as yield strength of a given metal in a particular heat treated condition increases, fracture toughness decreases. Alloying elements must be looked at individually for their effects on specific metals. Increasing carbon steel, for example, will increase strength, but decrease fracture toughness. Increasing nickel in steel will increase fracture toughness. Grain size influences fracture toughness — the smaller the grain size, the greater the fracture toughness. A metal’s cleanliness can greatly influence its fracture toughness. Foreign particles may act as stress risers. Impurities may form brittle compounds or films at grain boundaries (e.g. phosphorous or sulfur in steel) that greatly reduce fracture toughness. By using a clean melting practice or going to a remelt grade (VAR or ESR) of metal, we can significantly increase fracture toughness.

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Specific types of heat treatment can greatly alter the fracture toughness of a metal. Annealing, spheroidizing, and tempering are all designed to improve the toughness of steel. Quenching, on the other hand, increases strength, but decreases toughness. The temperatures selected for a specific heat treatment play a critical role in determining a metal’s fracture toughness. For example, generally as the tempering temperature for a steel is increased, strength decreases and fracture toughness increases. The exception to this is when increasing the tempering temperature puts it into an embrittling range (see Heat Treating). Cooling rates during heat treating can also affect fracture toughness. For example, duplex stainless steels must be rapidly quenched after annealing to prevent the formation of the embrittling sigma phase. The fracture toughness of a metal can vary with the direction of working in the metal (grain flow). Many materials show a significant increase in toughness when a fracture toughness specimen is taken such that the crack propagates in a direction transverse to the grain flow. It is important therefore to be able to identify the orientation of a fracture toughness specimen in relation to the direction of greatest working. ASTM E616 has established some nomenclature for specimen and crack orientation. It consists of a two letter code. The first letter represents the direction normal to the crack plane. The second designates the expected direction of crack propagation. The letters and their meaning vary by product form. Figure 16 show E616 nomenclature for rectangular cross sections and for round bar. Well, there it is. We’re done with fracture mechanics (almost). Are there any survivors? Are you ready to raise the white flag, throw in the towel, abandon ship, skedaddle out of the county? Do you find yourself wondering what ever possessed you to become an engineer in the Oil Patch and subject yourself to all this abuse? Do you long for a simpler, less complicated life as a shoe salesman or a bowling alley attendant? Do you keep imagining that things can’t possibly be as bad as they seem and that good times are just around the corner? Get real! Things are that bad and wishful thinking won’t change a thing. It doesn’t get any better than this so you might as well get used to it. Hey, shit happens. Fracture mechanics is here to stay. Ignoring it won’t make it go away. What ya don’t know can hurt you! So grow a backbone. It’s time to belly up to the bar, plunk your money down, and have one neat. Fracture mechanics is a lot like having a shot of Jack Daniels: when first encountered it may burn a little, but after you’ve been acquainted awhile you’ll be left with a warm glow!

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(A) Orientations for Rectangular Sections

(B) Orientations for Round Bar & Cylinders Figure 16: ASTM E616 Test Specimen Orientation page - 73

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FATIGUE Fatigue accounts for more failures of metal parts than any other source. It is the failure of a metal part under a cyclic load. The overall stresses induced by the load may be well below the static yield strength of the metal. The cyclic load may be due to vibrations, pressure pulses, thermal cycling, mechanical loading and unloading, etc. Fatigue is a three stage process. The first stage is crack initiation. Localized strain in a metal is not completely reversible when a load is reversed or removed. This is because the stress around a microscopic imperfection may be considerably higher than the overall stress that the part sees and results in permanent strain adjacent to the imperfection. As the part is repeatedly loaded, this localized strain will accumulate until a crack forms. The second stage is crack propagation. Newly formed cracks grow larger in a direction normal to the maximum tensile stress as the cyclic loading causes the material adjacent to the crack tip to work harden. Fracture, the last stage, occurs when cracks have grown to the extent that the effective cross section of the part can no longer support the load. The number of cycles that a given part can withstand before fracture is dependant on the amplitude of the applied stress (the difference between the maximum and minimum stresses for each cycle) and the average stress. The higher the amplitude, the fewer the cycles to failure. As the average or mean stress becomes increasingly tensile, the smaller the amplitude has to be to maintain the same number of cycles to failure. The behavior of a metal under a cyclic load is often presented in the form of a S/N curves (see Figure 17). This is a plot of applied stress (S) versus the number of cycles to failure (N). Figure 17 illustrates that as the stress decreases, the number of cycles to failure increases. The curve for steel in Figure 17 has a value of stress below which failure will not occur regardless of the number of cycles. This stress is known as the endurance or fatigue limit. Most nonferrous alloys have no fatigue limit and thus can only be subjected to a finite number of cycles however small the applied stress. The number of cycles (the fatigue life) that a part can withstand in a given application is greatly reduced if there is a stress riser in the part. Nicks, scratches, grooves, tool marks, sharp radii on internal corners, etc., can all be detrimental to fatigue life. Porosity, shrinkage, micro cracks, and inclusions in the microstructure of a metal can also be stress risers and reduce fatigue life. Fatigue cracks typically (but not always) initiate at the surface of a part. Fatigue life can often be greatly increased by improving the surface finish by polishing or grinding so that stress risers such as nicks page - 74

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and tool marks are eliminated. Fatigue life can also be improved by imparting a compressive residual stress to the surface of a part through cold work or by shot peening. Carburizing, nitriding, boriding, and shot peening can increase fatigue life both by strengthening the metal and by imparting a compressive, residual stress in the surface of the part. Metals that have a fine, rounded microstructure such as tempered martensite or tempered bainite will generally have a better fatigue life than those having an angular microstructure such as pearlite. The grain flow orientation also impacts fatigue life. Typically the longitudinal direction gives better fatigue properties than the transverse.

Figure 17: Fatigue Curves It is important not to try to use a S/N curve for a given metal that was developed under room conditions in designing for fatigue in another environment. The combination of cyclic loading in a corrosive environment can result in failure much earlier than would be expected by either fatigue or corrosion acting alone. This phenomenon is called corrosion fatigue. In corrosion fatigue, the endurance limit disappears; all metals including steels have a finite life. S/N curves are developed by counting the number of cycles that a metal specimen can withstand until failure under a known load. Typically 6 to 12 specimens may be run under different loads to completely describe the curve. Fatigue tests are often characterized by the mode of loading: direct axial stress, plane bending, rotating beam, alternating torsion, or a combination of modes. Test specimens vary in size, shape, surface finish, and whether or not they are notched. There are a number of ASTM specifications that cover fatigue testing.

THE FRACTURE MECHANICS APPROACH TO FATIGUE

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Alright, I lied. Mea culpa. We’re not quite finished with fracture mechanics. Fracture mechanics can be used as a powerful analytical tool to evaluate fatigue life. Suppose we find a crack in a metal part that’s been in service under a constant load and we want to know if the part is still usable. We can analyze the part to see if the crack induces a KI < KIc for the metal. If it does, it’s safe to return the part to service. Now suppose that instead of just being under a constant load, the part is also subject to cyclic load in service. The question we must now answer is; if we return the part to service, how many cycles of loading can the part be subjected to before the crack grows (through fatigue) to a size where it induces a KI > KIc and fractures occur? To answer this question we will need to know something about fracture mechanics, fatigue, and a little bit of calculus. Figure 18 shows a logarithmic plot of crack growth per load cycle, da/dN, versus the stress intensity factor range, k, for a precracked metal specimen subject to a cyclic load. The vertical dashed lines separate the three stages of crack growth. Note that the stable crack growth stage is by far the longest and that the curve is linear in this section.

Figure 18: Fatigue Crack Growth The linear portion of the curve in Figure 18 Can be represented by the following equation for most common, structural materials:

da

C ( k)n dN

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(1)

MECHANICAL PROPERTIES AND HOW THEY ARE DETERMINED

CHAPTER IV

where C and n are material constants. We can integrate equation (1) as follows: Ni

af

f

ai

1 dN

C N where,

Ni Nf ai af

= = = =

k nda

(2)

initial number of cycles final number of cycles initial flaw size final flaw size

Assuming that the cyclic stress have a constant amplitude:

k G )

(3)

a

)

where,

= the change in stress G = a correction factor based upon part geometry

Substituting this expression for k into equation (2), we get the following: Ni

dn

Nf

( ) c

) n

ai

G n # a n/2 da

(4)

af

If G does not vary much over the range of crack lengths we can consider it a constant and equation (4) becomes:

N Nf N i

)

G n(

) n

(

a

n 1)

(5)



n 1

c

ai

2

2

     

af

Equation (5) is a very powerful analysis tool. It can tell us: —

The number of cycles a part containing a known-size flaw (ai) will last before the flaw grows to a critical size (af) and fracture occurs or

a f acritical

KIc

2

G)c

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—

The inspection limit (ai) that must be placed on a part so that the maximum allowable flaw size that can be tolerated is not exceeded after a given number of cycles.

—

The maximum level of stress that a component containing a flaw (ai) can tolerate without the flaw growing to critical size for a given number of cycles.

This is heavy duty shit guaranteed to grow hair on your chest and curl your toes. “Oh, be still my palpitating heart!” Hey not to worry, you don’t need to understand all this stuff. The important thing to remember is that one of the most successful applications of fracture mechanics is fatigue analysis. You can leave all the heavy duty number crunchin’ to our Engineering Analysis Group.

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NOTES:

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NOTES:

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NOTES:

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MAKING METALS Basic Recipes "Cry Havoc!" and let slip the dogs of war.

This oft-quoted line from Shakespeare's Julius Caesar has nothing whatsoever to do with this chapter, but I thought it might add a touch of class to the text and besides I like the sound of it. Shakespeare had precious little to say about metallurgy so I must turn to a lesser known bard for a more appropriate quotation. The bard I'm referring to was an affable heat treat foreman in the Critical Services Plant in Houston and although his works are perhaps not quite as extensive as Shakespeare's, he managed to turn out some memorable lines. One in particular has remained with me throughout the years because it succinctly describes what a metallurgist is all about: "A metallurgist is a person who can tell the difference between a virgin metal and a common ore." While not Shakespeare, the above quotation will serve admirably to introduce the subject of how metals are made. We probably should have talked about this earlier, but I put the cart before the horse and decided to talk about basic physical metallurgy first. No matter. We can squeeze old Dobbin in here and be none the worse for it.

VIRGINS AND ORES How does one go about separating virgin metals from common ores? This is the job of an extractive metallurgist. An ore is a rock that contains metal-bearing minerals in sufficient quantity to make it economical to mine and process. Taconite, for example, is an iron ore that comes from the Lake Superior District of the United States. It may contain the mineral hematite (Fe2O3) or magnetite (Fe3O4). Bauxite, an ore of aluminum, is rock containing hydrated aluminum oxide. The first step in processing ore is to mechanically enrich the content of the mineral of interest. This is known as beneficiation or mineral dressing. The ore is typically crushed and pulverized in a series of mills until the desired particle size is obtained. We are now ready to begin enriching the ore by separating and removing the gangue (the junk rock, clay, sand, etc.) from the mineral of interest. There are many ways to do this depending on the nature of the ore. Most make use of the difference in densities between the gangue and the good stuff. Simple washing may be used to remove clay, dirt, etc. from the ore. Heavy media separation consists of dumping the pulverized ore into a container filled with a fluid that has a specific gravity between that of the trash rock and that of the mineral of interest. One will float and the other will sink thereby becoming separated. Floatation separation uses a liquid with a chemical additive known as a collector to separate out the junk. The collector has a strong affinity for either the mineral of interest or the gangue, and for air. The collector is added to a liquid suspension of ore particles and promptly attaches itself to either the gangue or the good stuff. Air is bubbled through page - 82

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the mixture and individual bubbles will become attached to the particles coated with the collector causing them to rise to the surface. The froth (with one type of particles) on top is periodically decanted off thus separating it from the other class of particles still in suspension. Magnetic separation is used to separate out magnetite from gangue in some iron ores. The pulverized ore is passed over a rotating drum that is magnetized. The ferromagnetic magnetite particles will cling to the drum while the non-magnetite gangue falls off. The magnetite particles are scraped off the rotating drum after being carried away from the gangue particles. There are many other methods of mineral dressing. Most ores require the use of a combination of methods before the ore is enriched enough for further processing. The final step in mineral dressing is often an agglomeration process. In order to process the ore through the various separating methods, it may have been necessary to pulverize the starting material into a powder. This powder made it easier to separate the gangue from the mineral of interest, but it may not be suitable for the next step in our quest for virgin metal. In the case of iron, the next step is the blast furnace. A blast furnace (which we'll discuss in detail shortly) utilizes a flow of hot gases in contact with the furnace charge to cause certain chemical reactions to occur. The powdered product from some mineral dressing operations is too fine to use as blast furnace feed and so must be agglomerated by pressing into brignettes, mixing the powder with a fuel such as coke and then sintering the mixture together by exposing it to a flame, or by pelletizing. Pelletizing is done by loading a balling mill with the powdered ore and a binder such as bentonite (a type of clay). A solid fuel is sometimes added to improve sintering. As the drum rotates, the particles will grow larger due to a "snowball" effect. Once the particles have reached the desired size (typically 1/2" - 1" in diameter), they will be taken from the mill and sintered in a continuous furnace to improve hardness and strength. These pellets are sufficiently large to let the gases in a blast furnace flow readily in between them, but not so large as to reduce the effective surface area exposed to the gases. Thus far we have produced an ore rich in the metal-bearing mineral of interest, but no virgin metal. In the rest of this chapter we'll examine the processes used to extract the virgin metal from the ore, how the metal is refined, and how the metal is alloyed into a useful engineering material. We'll limit our discussion to the two classes of metal most commonly used in the Oil Patch: steels and nickel base alloys. We'll look at how steel is made first.

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STEEL 1. The Blast Furnace - The enriched iron ore (often in the shape of pellets) that we obtained from mineral dressing is transformed into metallic iron in a blast furnace. A blast furnace is essentially a big refractory-lined pot up to 10 stories tall. The process begins by charging the top of the furnace with a mixture of iron ore, coke, and limestone. A continuous "blast" of hot, preheated air enters near the bottom of the furnace and reacts with the coke to form carbon monoxide gas. The carbon monoxide then combines with oxygen from the iron oxides in the ore to form carbon dioxide gas thus reducing the ore to metallic iron. The limestone is added as a flux. It facilitates the removal of impurities in the iron ore by making them more fusible and combining with them to form a slag on top of the molten iron. The molten iron will collect at the bottom of the furnace where its will eventually be tapped into a ladle. The metallic iron produced in a blast furnace contains substantial amounts of impurities such as carbon, sulfur, phosphorous, and many others. These impurities make the iron very brittle and useless without further processing. 2. The Porcine Beginnings - Steels are direct descendants of pigs. Pigs, besides being the domesticated quadrupeds responsible for bacon and the covers on footballs, are the ingots that result from pouring the molten iron produced in the furnace into small, block-shaped molds. The iron produced in the blast furnace, as a consequence, is referred to as pig iron. The steel making process consists of refining pig iron or ferrous scrap in order to remove undesirable elements and then adding the required alloying elements in the correct proportion. There are three basic types of steel making processes: open-hearth, basic oxygen, and electric-arc furnace. Most high quality steels are made by the basic oxygen or electric furnace process. A. Open-Hearth Process - A schematic of an open-hearth furnace is shown in Figure 1. It consists of a large, shallow basin lined with refractory material, an arched roof lined with fire-brick, gas or oil burners, and regenerators called checkers for preheating the air used for combustion.

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Figure 1: Open Hearth Process The furnace is charged with scrap, limestone, solid pig iron, and sometimes iron ore (used in conjunction with the limestone flux). The charge, laying on the "open" hearth, is swept by flames from the burners. When this solid charge begins to melt, molten pig iron/scrap may be added. Oxygen is frequently introduced by lances through the roof. The charge is refined through several mechanisms. Impurities such as manganese, silicon, and phosphorus are oxidized and float to the top of the molten metal and become part of the slag. Carbon will be oxidized to CO. Sulfur and other impurities will combine with limestone in forming the slag. The refining time ranges from 4 to 10 hours. When the molten metal is sufficiently pure, the furnace is tapped from the bottom into a ladle. The molten metal in the ladle may be treated with deoxidizers and alloying elements. The open-hearth process can make a wide variety of steels in 100-500 ton melts. The open hearth process is seldom used now because of the long refining time and the generally dirty steel it produces. B. Basic Oxygen Process (BOP) - Figure 2 is an illustration of a basic oxygen furnace. It is essentially a big, refractory lined pot that is charged with molten pig iron and scrap. The furnace is on trunnions so it can be tilted in order to facilitate the charging process. A water-cooled oxygen lance is brought down through the top opening in the furnace and high-velocity stream of oxygen is directed onto the charge. This causes the rapid oxidation of carbon, manganese, and silicon in the charge: reactions which give off heat and help to melt the scrap and aid in the refining process. At the same time that the oxygen is being blown in, burnt lime and fluorspar are being added. These act as fluxes page - 85

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and form a slag which floats to the top of the molten metal. The impurities in the metal are removed by oxidation and/or interaction with the slag. With the BOP process, heats as large as 300 tons can be made in less than an hour and having quality comparable to open-hearth steel. The molten metal is tapped into a ladle where deoxidizers and alloying elements are added.

Figure 2: Basic Oxygen Process C. Electric-Arc Furnace Process - An electric-arc furnace (see Figure 3) is a refractory lined pot with three carbon or graphite electrodes entering through the roof. The furnace is charged with solid scrap and pig iron. The electrodes are brought down to a point near the charge and an arc is established which causes the charge to melt. When the charge is roughly 70% melted, iron ore, burnt lime, and other ingredients are added to form a slag. Impurities are removed through interaction with the slag which floats on top of the metal. In comparison with the BOP and open-hearth process the electric-arc furnace process has the advantage of being able to start with a solid charge. The process permits extremely close control over temperature, composition, and refining conditions for each heat making it particularly useful for specialty steels. A heat may vary form a few hundred pounds to over 200 tons. It can take 3 to 8 hours to produce one heat depending on the type of steel. Alloying elements and deoxidizers are added once the molten metal is tapped from the bottom of the furnace into a ladle.

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Figure 3: Electric Arc Furnace Process 3. Deoxidation - In the three types of steel making processes that we have described, one of the primary reactions taking place is the combination of oxygen and carbon to form a gas. If the oxygen available for this reaction is not removed prior to the molten steel solidifying, then gaseous products will continue to evolve as the metal freezes in the ingot mold and may form blowholes, porosity, etc. in the solidified steel. There are many applications where these cavities cannot be tolerated because of their effect on the steel's mechanical properties. Steels destined for use in these critical applications will be deoxidized. The amount of oxygen removed will determine the type of steel. Deoxidizers such as aluminum or ferro-silicon are added to the ladle or the ingot mold to control the amount of available oxygen by combining with it to form non-metallic oxides or silicates. If no gas is evolved during solidification, the steel is termed killed. Steels having increasing amounts of gas evolution are referred to as semi-killed, capped, or rimmed. A. Killed Steels - Killed steels are sufficiently deoxidized so that very little gas evolves during solidification. The top surface of the ingot freezes first. As the rest of the ingot solidifies, the metal shrinks and forms a large cavity or pipe in the center near the top. The portion of the ingot containing the pipe will later be cropped off. A killed steel has the most uniform chemistry and properties. B. Semi-killed Steels - These have an intermediate level of deoxidation. Enough oxygen is retained so that there will be a page - 87

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sufficient amount of gas evolution during solidification to replace the pipe by an approximately equal volume of deep-seated blowholes. C. Rimmed Steels - Rimmed steels are only slightly deoxidized. In rimmed steels, the evolution of gas is sufficient to keep the top of the ingot a liquid until a side and bottom rim of substantial thickness has solidified. The gas evolution is a result of the interaction between carbon and oxygen in the molten steel at the boundary between the solidifying rim and the remaining molten metal. This causes the decarburization of the rim which, in turn, makes it very ductile. The center portion of the ingot, which solidifies last, will generally have a great deal of segregation. The low carbon surface layer of a rimmed steel makes it ideal for cold forming applications and where surface is of major importance. Rimmed steels contain less than 0.25% carbon and 0.60% manganese. The rimming action is very slow or nonexistent in steels above these levels. D. Capped Steels - These steels are the same as rimmed steels, except that the rimming action was stopped before it was complete. The amount of gas entrapped in the solidifying metal causes it to raise to the top of the mold. When the rising metal comes in contact with the heavy metal cap on top of the mold, the rimming action is cut short. Rimming may also be stopped by the addition of more deoxidizers. Capped steels have a thin, low carbon rim with the remainder of the ingot cross-section having a uniformity typical of semi-killed steels. This gives capped steels good cold forming characteristics and uniform properties. 4. Vacuum Treating - Molten steel contains other gases besides oxygen such as nitrogen and hydrogen. These entrapped gases can be sources of non-metallic inclusions, porosity, metal embrittlement, and a host of other evils. Vacuum degassing will frequently be specified for critical steel applications (those that require the greatest internal soundness, structural uniformity, or some other property that could be adversely affected by uncontrolled amounts of dissolved gases). Vacuum degassing is used in conjunction with deoxidizers. The deoxidizers are generally added late in the vacuum cycle. Degassing may take place in a vacuum furnace as the steel components are being melted (i.e. vacuum induction melting), in a ladle or as a ladle is being poured (i.e. stream, continuous circulation, or ladle degassing), page - 88

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or in a vacuum furnace in which the steel is being remelted (i.e. vacuum are remelting). A. Vacuum Induction Melting (VIM) - An induction furnace consists of a crucible surrounded by an electric coil. The crucible is charged with scrap, pure iron, and alloying elements in proportions correct for the chemistry of the desired steel. VIM has the crucible in a vacuum chamber. An alternating current passing through the coil induces currents in the crucible's charge. These induced currents generate heat and cause the charge to melt. The molten metal is thoroughly mixed by electromagnetic forces resulting from the induced currents. This insures good chemical homogeneity as well as exposing more and more molten metal to the surface where the vacuum draws off the dissolved gases. B. Stream Degassing - In this process a ladle full of molten steel is placed on top of a vacuum chamber that contains an empty ladle directly underneath the full one. The molten metal is tapped off the bottom of the full ladle through a spout that is connected to an opening in the chamber. As the metal stream enters the vacuum, the low pressure in the chamber causes the stream to break up into droplets. This increases the surface area of the molten metal exposed to the vacuum facilitating the removal of gases. This teeming (pouring) practice is often referred to as "ladle-to-ladle". C. Continuous Circulation Degassing - This process is illustrated in Figure 4. A ladle full of molten metal is placed beneath a vacuum chamber. The chamber is lowered so that two refractory tubes are immersed in the liquid metal. Inert gas is bubbled into one of the tubes and this creates a density differential between the two tubes. This allows atmospheric pressure to force the molten metal up one tube into the vacuum chamber, where the vacuum draws off the gases, and down the other tube back into the ladle. This is also known as the Ruhrstahl-Heraeus (R-H) process.

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Figure 4: Continuous Circulation Degassing D. Ladle Degassing - In this process, the ladle is placed inside a vacuum chamber. The molten metal is usually stirred in order to expose the maximum amount of steel to the vacuum where the gases can be drawn off. Stirring is accomplished by electrical induction or by bubbling argon up through the bottom of the ladle. E. Vacuum Arc Remelting (VAR) - The VAR process utilizes a consumable electrode that is cast out of the steel to be refined. This electrode is remelted inside a vacuum furnace (see Figure 5) by creating an electrical arc between the top of the electrode and a water-cooled copper mold at the bottom. As the arc causes the tip of the electrode to melt, metal droplets are transferred down through the vacuum to the bottom of the mold. Here they form a molten pool. The bottom portion of the molten pool that is in contact with the water-cooled mold will start to solidify and form an ingot. This ingot will continue to grow as more and more metal is transferred across the arc into the molten pool on top of the ingot and then solidifies.

The VAR process results in a greatly reduced gas content in the steel. Some of the more volatile impurities may also be removed. This virtually eliminates porosity and can significantly improve the cleanliness of the steel. The toughness and ductility are thus optimized. Another benefit of the VAR process is the elimination of the gross chemical segregation found in conventionally cast ingots. Segregation refers to the nonuniform distribution of alloying elements, impurities, or phases during solidification. The center section of a conventionally cast ingot is much richer in low melting temperature impurities and certain alloying elements than the areas of the ingot next to the page - 90

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mold walls because the center is last to freeze. In the VAR process, the metal droplets become well mixed in the molten pool on top of the forming ingot and quickly solidify. This increases homogeneity and limits the time available for segregation to occur. It also eliminates shrinkage cavities found in conventional ingots. Shrinkage cavities are the voids that occur in the center of a conventionally cast ingot that result from the fact that the solidified metal occupies less volume that does molten metal. The properties of a VAR steel are thus more uniform that a conventionally processed steel.

Figure 5: Vacuum Arc Remelting F. Vacuum Arc Degassing (VAD) - This degassing process utilizes a vacuum chamber that has electrodes extending into the chamber through the top somewhat like an electric furnace. Molten metal from the steel making furnace is tapped into a chrome magnesite slag-lined ladle. Burnt lime, fluorspar bauxite, and grain aluminum are added to form a slag. The ladle and its contents are then placed in the VAD unit and the unit closed up. The slag is fluidized by the simultaneous actions of an argon purge, an electrical arc passing between electrodes through the charge, and the drawing of a vacuum. After the slag is fluidized, arcing and purging are stopped and the charge is allowed to degas under the vacuum for about 8 minutes. The temperature of the charge is then adjusted by a second argon purge which permits a sulfur-slag reaction to take place. A final 8 minute degas cycle is made resulting in extremely low page - 91

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residual gas content in the charge. Sulfur is considerably reduced through interaction with the slag. 5. Other Refining Techniques - There are many other refining techniques that are used to reduce the content of undesirable elements in a steel that do not utilize vacuum treating. We'll examine two of the more important ones: electroslag remelting and argon-oxygen decarburization. A. Electroslag Remelting - Electroslag remelting is a steel refining process similar to VAR except that it is not done under a vacuum (see Figure 6). A specially designed slag is placed in the ESR furnace. The molten slag will float on top of the liquid metal. The tip of the electrode is immersed into the slag. As the arc melts the tip of the electrode, metal droplets will fall through the slag and collect at the bottom of the furnace in a watercooled mold. The ingot forms just as it did in the VAR process. The metal droplets are refined through interaction with the slag. The slag also keeps the molten metal from being oxidized by the surrounding air.

Figure 6: Electroslag Remelting The ESR process does not remove as much of the gaseous impurities as the VAR process, but has a significant advantage in removing non-gaseous ones. Like VAR material, ESR material is chemically homogeneous and has uniform properties page - 92

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throughout the ingot's cross-section. It also results in an ingot free from shrinkage cavities. B. Argon-Oxygen Decarburization (AOD) - As the name implies, the AOD process is used to reduce the carbon content of a metal. It revolutionized the manufacture of many specialty stainless steels when it first appeared in the early 1970's because it was the first refining process that removed carbon to ultra-low levels without a substantial loss in chromium. AOD refining is also used on nickel base alloys to reduce carbon content. AOD refining generally follows electric furnace melting, although BOP may also be used. The molten metal from the electric furnace is poured into the AOD vessel, essentially a refractory lined pot with one or more tuyeres located near the bottom. Tuyeres (pronounced twe-yars) are nozzles for gas injection. A mixture of oxygen and argon are blown into the melt through the tuyeres. This causes the molten metal to circulate in the vessel and increases exposure to the gases. The oxygen combines with the carbon to form carbon monoxide which is lost to the atmosphere thus removing the carbon from the system. The argon helps to stir the mixture and to lower the partial pressure of the carbon monoxide which facilitates carbon removal. The gas mixture starts out at approximately 3 to 1 oxygen to argon. This ratio changes as carbon is removed until almost pure argon is injected. The oxygen not only oxidizes carbon, it also oxidizes some of the chromium. To recover this chromium, a reducing slag is introduced to the vessel during the final argon blow. The oxygen from the chromium oxide combines with the slag thus freeing the chromium. The metal can now be tapped. Sometimes an additional slagging step is utilized to remove sulfur. The reducing slag is removed and a basic slag of lime, fluorspar, and deoxidizers is added to the vessel. As the argon mixes the slag with the metal, sulfur will react with the slag materials. When the argon is stopped, the slag will float to the top of the melt where it (and the sulfur) can be removed. A variation of the AOD process is vacuum decarburization. Here the vessel is placed in a vacuum chamber. Argon is again injected through tuyeres in the bottom of the vessel. A reducing slag is utilized. Oxygen is added not through the tuyeres, but through a lance that comes through the top of the chamber. The lance directs the flow of oxygen into (or onto in some variations of the process) the molten metal/slag mixture. Carbon is page - 93

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removed from the system as it reacts with the oxygen to form carbon monoxide which is drawn off by the vacuum. 6. Calcium Treating - We have one last topic to discuss in this section and that is calcium treating. As previously mentioned in the section on alloying elements, one of the reasons manganese is added to steel is combine with sulfur, thereby preventing "hot shortness." While effectively tying up the sulfur, the manganese sulfides may create another problem: stringers. Stringers are non-metallic inclusions that have a high length to width ratio. There are many different types of stringers, but we'll limit or discussion to those involving manganese sulfides. Manganese sulfides are soft and ductile, consequently, rather than being broken up during whatever hot work the steel sees, they will deform. If the metal is extruded into bar, these soft and ductile inclusions will be stretched out in the direction of greatest working. Stringers are often found during magnetic particle testing. Here they appear as fine, straight lines. For most applications, stringers are not detrimental to the steel's performance. For some critical applications, stringers cannot be tolerated because they may result in the functional failure (e.g. a stringer on the surface of a ring groove is a potential leak path) or the structural failure (e.g. a stringer may be considered a "crack" in the material because of its very low strength) of the equipment. What does all this have to do with calcium treating? Calcium, when added to molten steel, will combine with the manganese sulfides to form hard, spherical-shaped inclusions. These inclusions will not deform when the metal is subsequently worked. The spherical shaped is ideal because it minimizes the stress associated with stringers (i.e. no "notch" effect) and it restricts the inclusion to a localized area in the metal. This will also greatly increase the transverse mechanical properties of the finished product. Note that calcium treating does not reduce the sulfide content of the steel, but it does change the sulfide morphology (shape) in the finished product. Calcium is typically injected (often in wire form) into molten steel in the ladle.

NICKEL Most commercially important nickel-bearing ores contain the minerals milleriteNiS, pentlandite-(Ni, Fe)11S10, or garnierite-H2(Mg,Ni)SiO4. Copper is often found in association with nickel in ores. The largest nickel producing area in the world is Canada's Sudbury District. Nickel ores are mechanically enriched using techniques similar to what we discussed for iron ores. The manufacture of nickel alloys can be broken down into three stages: smelting, refining, and melting practice.

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1. Smelting - I went to school at Michigan Tech in the Upper Peninsula of Michigan. One of the joys of spring was for a group of people to take a keg of beer and go smelting. A smelt, for those of you who have never experienced this particular pleasure, is a small fish about eight inches long that runs in schools up streams early in the year to breed. Its main purpose in life is to be caught, deep fried in a batter, and eaten with beer. They are easily caught with nets, buckets, etc. Often when the level of beer in the keg went down and our spirits went up, we'd resort to grabbing them with our hands. This method of capture, although cold, was not as inefficient as it sounds. The water literally seethed with fish and it was relatively easy to snag one or two with each swipe. The beer, of course, served admirably as antifreeze. There is another form of smelting, unrelated to fish and certainly less appetizing, that is more pertinent to our discussion of metals. Smelting is a thermal process used to produce molten metal from enriched ore. Nickel ores that contain sulfides are matte smelted in reverberatory furnaces. A reverberatory furnace has a shallow refractory lined hearth that contains the ore and a flux. As the furnace is fired, the flames are directed across the hearth and heat is reflected off the refractory lined roof onto the charge. The heat causes the charge to fuse. The molten metal sulfides will collect into a "matte" on the bottom of the hearth while the slag-forming oxides will float to the surface. The slag is drawn off leaving a matte rich in Ni3S2 as well as other metal sulfides (such as Cu2S and FeS depending on the ore). The molten matte is poured into a converter. A converter is essentially a refractory lined pot with one or more tuyeres near the bottom that permit air to be blown through the charge. This oxidizes iron sulfides in the reaction: 2FeS

 302  2Fe0  2S02

The SO2 is a gas that escapes to the atmosphere. Flux is added to slag the FeO. Air is blown only until all the FeS is oxidized and the FeO slagged: the Ni3S2 is not oxidized. The slag is then poured off leaving a mixture of copper and nickel sulfides called Bessemer matte. The most common way to separate copper and nickel sulfides in Bessemer matte is to cast the molten matte into large ingot molds and then slowly cool until they solidify. The copper sulfides will separate from the nickel sulfides because of their difference in density. After the matte has solidified, it is then crushed and the two sulfides separated by flotation. 2. Refining - Nickel sulfide is usually refined using one of two methods. The electrolytic method uses a cast anode made from the impure nickel in an electrolytic cell with a thin sheet of pure nickel as the page - 95

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cathode. The bath is a slightly acid, nickel sulfate solution that also contains some boric acid. When a direct current is applied to the cell, the anode dissolves and goes into solution. The solution is pumped from the cell, treated chemically to remove various impurities, and returned at the cathode. The nickel ions in the purified solution will plate out on the cathode. The carbonyl process is the other commonly used refining method. The Bessemer matte is first roasted and then the copper is leached out with dilute sulfuric acid. This leaves NiO. The NiO is reduced by heating it in a water gas atmosphere according to the reaction:

NiO  H2  Ni  H2O The resulting nickel is still relatively impure. Next it is exposed to carbon monoxide gas at 120-200(F which causes the formation of volatile nickel carbonyl:

Ni  4CO  Ni(CO)4 The nickel carbonyl is carried out in the gas stream into a vessel containing a column of pure nickel pellets. The 350(F temperature in the vessel causes the nickel carbonyl to decompose into pure nickel and carbon monoxide. The nickel will deposit out onto the nickel seed pellets in the column. 3. Melting Practice - Nickel base alloys are typically made in an electric furnace and then undergo one or more VAR or ESR cycles depending on the degree of cleanliness required. Vacuum induction melting (VIM) is also sometimes used followed by one or more VAR or ESR cycles. AOD may be employed to reduce the carbon content. The general principles of these processes are the same as we discussed for steels.

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NOTES:

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NOTES:

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Metal Mania A name serves the useful purpose of conveniently identifying a person, place, or thing without the need for a lengthy description. For instance, I may want to refer to a person who is a red neck, a braggart, provincial, lives 50 years behind the times, has teeth permanently stained by chewing tobacco, is totally devoid of any social graces, massacres the English language every time he opens his mouth, is inordinately proud of where he comes from, is short on wits (but long on drawl), is suspicious of all outsiders and Yankees in particular, who refuses to acknowledge the defeat of the South in the Civil War, and, like Voltaire's Candide, believes that his is the best of all possible worlds. This time consuming description does serve to uniquely identify this individual, but is extremely cumbersome in a conversation especially if repeated references must be made. It's much easier to jettison all this excess baggage and simply refer to this person as a Texan. Similarly, rather than having to say 0.28-0.33% carbon, 0.40-0.60% manganese, 0.035% maximum phosphorous, 0.040% maximum sulfur, 0.15-0.35% silicon, 0.80-1.10% chromium, and 0.15-0.25% molybdenum every time that we want to refer to a particular low alloy steel, it's much easier to say AISI 4130. In this chapter we are going to examine the nomenclature of some of the metals used in our products and briefly look at the properties of some of the more important alloys. There are many different ways to classify metals including composition (carbon or alloy steel), product form (forging or plate), heat treatment (annealed or quenched and tempered), finishing operation (hot or cold finished bar), strength level (API type 60K or 75K material), microstructure (martensitic or austenitic stainless steels), melting practice (open hearth or electric furnace), or deoxidation practice (killed or rimmed) just to name a few. Some of the organizations that have tried to put some method into the madness of steel classification are: the American Society for Testing and Materials (ASTM), the American Iron and Steel Institute (AISI), the American National Standards Institute (ANSI), the Alloy Castings Institute (ACI), the American Society of Mechanical Engineers (ASME), the Society of Automotive Engineers (SAE), a host of government agencies that have produced Military and Federal Specifications, and a myriad of foreign institutes. The specifications written by the organizations just mentioned are intended for general usage and may not be adequate for a particular user's needs. Consequently many major companies, such as Cameron have developed their own material specifications.

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THE UNIFIED NUMBERING SYSTEM The number of aliases that a particular metal may have are legion. It may go under a myriad of trade names. It may be assigned various specification numbers by a trade association, government agency, or commercial user. These numbers often change by product form (plate, for instance, would have a different specification number than pipe). But no matter what you call it, it's the same metal. It may have any number of disguises that make it unrecognizable to you, yet it's the same old stuff. If you want to refer to a particular metal regardless of product form, heat treatment, manufacturer, mechanical properties, quality, etc., what should you call it? Tough question. So tough, as a matter of fact, that the Society of Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM) got together in the late 1960's, knocked heads, and came up with the idea of a single numbering system that could be used to identify metals. The U.S. Army eventually got into the act along with several trade organizations and many individuals from industry. After many close calls and harrowing experiences, SAE and ASTM issued SAE/ASTM Recommended Practice for Numbering Metals and Alloys in 1974. This document (SAE J1086 and ASTM E527) outlines the Unified Numbering System (UNS). The Unified Numbering System assigns a six character, alpha-numeric code to alloys of "commercial standing." Commercial standing means that the alloys are commonly used in industry. A UNS number cannot be assigned to a newly developed alloy with no commercial usage. There are eighteen series of UNS numbers (see Table 1). A UNS number is not a specification: you can't order material to it. It serves only to identify an alloy for which controlling limits (such as composition or mechanical properties) have been established in government or industry specifications. Although SAE and ASTM administer the UNS program, the actual numbers may be assigned by a trade organization. For example, AISI has number assignment responsibilities for the G, H, S, and T series while The Aluminum Association is responsible for the A series. In 1975, SAE and ASTM issued the first edition of Metals & Alloys in the Unified Numbering System (SAE HS J1086 and ASTM DS-56) which was a compilation of all the assigned UNS numbers with a brief description (usually typical composition ranges) of the metals they represent. Revisions to this handbook are issued periodically. Many standards (such as NACE MR0175) and material specifications now reference UNS.

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UNS SERIES

METAL

Nonferrous A00001 - A99999 C00001 - C99999 E00001 - E99999 L00001 - L99999 M00001 - M99999 N00001 - N99999 P00001 - P99999 R00001 - R99999 Z00001 - Z99999

Aluminum and Its Alloys Copper and Its Alloys Rare Earth Metals and Alloys Low Melting Metals and Alloys Miscellaneous Nonferrous Metals and Alloys Nickel and Its Alloys Precious Metals and Alloys Reactive and Refractory Metals and Alloys Zinc and Its Alloys

Ferrous D00001 - D99999 F00001 - F99999 G00001 - G99999 H00001 - H99999 J00001 - J99999 K00001 - K99999 S00001 - S99999 T00001 - T99999

Steels with Specified Mechanical Properties Cast Irons AISI and SAE Carbon and Alloy Steels AISI H-Steels Cast Steels Miscellaneous Steels and Ferrous Alloys Heat and Corrosion Resistant Steels Tool Steels

Welding Filler Metals W00001 - W99999

Welding Filler Metals, Covered and Tubular Electrodes, Classified By Weld Deposit Composition

CARBON AND LOW ALLOY STEELS Carbon steels, if you'll remember back to the lesson on Alloying Elements, are steels that have a maximum of 1.65% Mn, 0.60% Si, and 0.60% Cu. No other intentionally added elements are permitted except when deoxidizers, boron (for increased hardenability) or sulfur, phosphorus, or lead (for better machinability) are specified. As a side note, when we say that a steel has a certain percentage of alloying elements in it, this percentage does not include carbon. A "steel" is by definition an alloy of carbon and iron. Alloying elements are those that are added to enhance the properties of the iron-carbon alloy. The American Iron and Steel Institute is the Linnaeus of the ferrous metallurgy world. AISI in conjunction with SAE, has developed the most widely used system of classifying steels; one based on composition. AISI divides carbon steels into four categories or grades. Each steel within a particular grade is assigned a four digit code with the first two digits designating the grade. The carbon steel grades are:

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Plain (Nonresulfurized), Mn 1.00% Max Resulfurized Resulfurized and Rephosphorized Plain (Nonresulfurized), Mn 1.00-1.65%

Resulfurizing and rephosphoring mean restoring the sulfur and phosphorus contents of the steel to a level high enough to improve machinability. The last two digits of the four digit code designate the nominal carbon content of the steel in hundredths of a percent thus a 1018 steel is a plain carbon steel with approximately 0.18% carbon. A letter "B" between the second and third digits refers to a boron steel (a steel in which boron is added for greater hardenability) while the letter "L" refers to a leaded steel (a steel in which lead is added for better machinability). If the four digit code is prefixed by the letter "E", it means that the steel is electric furnace melted. A letter "M" prefix refers to merchant quality carbon steels, steels that are produced to broad carbon and manganese ranges for use in non-critical applications. A suffix "H" is added to the four digit code if the steel is ordered to specific hardenability requirements. Carbon steels represent the greatest tonnage of all the types of steels produced. They are inexpensive, readily available, and are easy to form and fabricate. Carbon steels having a carbon content of less than 0.35% are easily welded. The major limitation of carbon steels is their limited hardenability or strength. Where high strength levels or other special properties such as corrosion resistance in a particular environment are required, it may be necessary to specify an alloy steel. Alloy steels are somewhat arbitrarily divided into low, medium, or high alloy steels depending on total alloy content. Low alloy steels have 1½ - 2¾% alloying elements by weight, medium alloy steels have 2¾ - 10%, and high alloy steels have over 10% alloying elements by weight. These limits are not absolute, but vary according to which textbook you read, consequently they're used only for guidelines. AISI has also established a four digit (usually!) code for the various types of alloy steels. Like the code for carbon steels, the last two digits refer to the nominal carbon content in hundredths of a percent. In the case of the five digit 50XXX, 51XXX, and 52XXX series, the last three digits refer to carbon content. The letters "E", "H", "B", and "L" are utilized in the same position and have the same meaning as they did in the four digit carbon steel codes. The letter "V" between the second and third digits designates a vanadium addition for increased hardenability. The AISI classifications for alloy steels are listed in Table 2.

Table 2 AISI Designation

Type of Steel

Nominal Alloy Content

13XX

Manganese

1.75% Mn

23XX

Nickel

3.50% Ni

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AISI Designation

Type of Steel

Nominal Alloy Content

25XX

Nickel

5.00% Ni

31XX

Nickel-Chromium

1.25% Ni; 0.65% & 0.80% Cr

32XX

Nickel-Chromium

1.75% Ni; 1.07 Cr

33XX

Nickel-Chromium

3.5% Ni; 1.50 & 1.57% Cr

34XX

Nickel-Chromium

3.00% Ni; 0.77% Cr

40XX

Molybdenum

0.20% & 0.25% Mo

41XX

Chromium-Molybdenum

0.50%, 0.80%, & 0.95% Cr; 0.12%, 0.20%, 0.25%, & 0.30% Mo

43XX

Nickel-Chromium-Molybdenum

1.82% Ni; 0.50% & 0.80% Cr; 0.25% Mo

43BVXX

Nickel-Chromium-Molybdenum

1.82% Ni; 0.50% Cr; 0.12% & 0.25% Mo; 0.03% (Min.) V

44XX

Molybdenum

0.40% & 0.52% Mo

46XX

Nickel-Molybdenum

0.85% & 1.82% Ni; 0.20% & 0.25% Mo

47XX

Nickel-Chromium-Molybdenum

1.05% Ni; 0.45% Cr; 0.20% & 0.35% Mo

48XX

Nickel-Molybdenum

3.50% Ni; 0.25% Mo

50XX

Chromium

0.27%, 0.40%, 0.50%, & 0.65% Cr

50XXX

Chromium

0.50% Cr; 1.00% (Min.) C

51XX

Chromium

0.80%, 0.87%, 0.92%, 0.95%, 1.00% & 1.05% Cr

51XXX

Chromium

1.02% Cr; 1.00% (Min.) C

52XXX

Chromium

1.45% Cr; 1.00% (Min.) C

61XX

Chromium-Vanadium

0.60%, 0.80%, & 0.95% Cr; 0.10% & 0.15% (Min.) V

72XX

Tungsten-Chromium

1.75% W; 0.75% Cr

81XX

Nickel-Chromium-Molybdenum

0.30% Ni; 0.40% Cr; 0.12% Mo

86XX

Nickel-Chromium-Molybdenum

0.55% Ni; 0.50% Cr; 0.20% Mo

87XX

Nickel-Chromium-Molybdenum

0.55% Ni; 0.50% Cr; 0.35% Mo

88XX

Nickel-Chromium-Molybdenum

0.55% Ni; 0.50% Cr; 0.35% Mo

92XX

Silicon-Manganese

2.00% Si; 0.75%, 0.82%, & 0.87% Mn; 0.00% 0.18% & 0.32% Cr

93XX

Nickel-Chromium-Molybdenum

3.25% Ni; 1.20% Cr; 0.12% Mo

94XX

Nickel-Chromium-Molybdenum

0.45% Ni; 0.40% Cr; 0.12% Mo

97XX

Nickel-Chromium-Molybdenum

0.55% Ni; 0.20% Cr; 0.20% Mo

98XX

Nickel-Chromium-Molybdenum

1.00% Ni; 0.80% Cr; 0.25% Mo

The alloying elements in these steels give them an incredibly wide range of properties. Specific elements may be added in order to increase a steel's strength, hardness, corrosion resistance, high temperature properties, or some other property that is desired in a particular environment. These alloying elements impart the desired properties in an alloy steel at a price: alloy steels can cost considerably more than carbon steels both in terms of the initial price and the increased costs in forming, machining, and welding. page - 104

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Let's look a little closer at four of the most commonly used low alloy steels in the Oil Patch: 4130, 4140, 4340, and 2¼Cr-1Mo alloys. These steels have achieved stardom because they are easily heat treated to API strength levels, they're cheap and readily available, they're easy to cast or forge, surface properties are easily altered by special processes (e.g. weld overlaying, thermal spraying, etc.), and they are quite easy to machine. All of these metals are generally used in the quenched and tempered or normalized, quenched, and tempered conditions. 4130 - 0.28-0.33% C, 0.40-0.60% Mn, 0.20-0.35% Si, 0.80-1.10% Cr, 0.15-0.25% Mo 4130 is probably the most widely used steel for wellhead equipment and for good reasons. It's inexpensive, widely available, easily processed, easily welded, and easily machined. It is typically water quenched, although some special applications may call for an oil quench. 4130 is not subject to temper embrittlement. It is suitable for sour service as defined by NACE MR0175 provided that the hardness does not exceed 22 HRC. The main drawback of 4130 is it is relatively low hardenability. This makes it difficult to through-harden heavy cross-sections to meet API minimum strength levels while not exceeding 22 HRC on the surface. It also means that preheat treat machining must sometimes be performed in order to ensure that a finished part falls within the required hardness range. There is generally little problem meeting API impact properties with 4130 in the cross-section sizes in which API strength levels can be attained without exceeding 22 HRC. Yield strengths of 75 ksi and above can be attained in the centers of the bars approximately 4" in diameter or less while still meeting a maximum hardness of 22 HRC. 4130 is widely used for bodies, bonnets, flanges, and other components. 4140 - 0.38%-0.43% C, 0.75-1.00% Mn, 0.20-0.35% Si, 0.80-1.10% Cr, 0.15-0.25% Mo A quick glance at its composition shows that 4140 is very similar to 4130 except for its increased carbon and manganese content. The higher levels of these two elements greatly increases the hardenability of 4140 over that of 4130. Like 4130, 4140 may be used in sour service up to a maximum hardness of 22 HRC. Yield strengths of 75 ksi and above can be attained in the centers of bars approximately 8" in diameter or less without exceeding 22 HRC at the surface. 4140 must be oil or polymer quenched because of its hardenability. Water quenching will crack it. While not subject to temper embrittlement, 4140 is generally tempered outside of 450-700(F to avoid blue brittleness. The increased hardenability makes 4140 more difficult to form and to weld in comparison to 4130. Its impact properties are generally low particularly at sub-zero temperatures. 4140 is used for many high strength applications such as hangers, BOP rams and operating pistons, etc.

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4340 - 0.38-0.43% C, 0.60-0.80% Mn, 0.20-0.35% Si, 0.70-0.90% Cr, 1.65-2.00% Ni, 0.20-0.30% Mo 4340 is essentially a 4140 chemistry with a nickel addition. The nickel gives this alloy deep hardenability and excellent toughness. Unfortunately the 1.65-2.00% Ni makes 4340 unsuitable for sour service (NACE MR0175 only permits up to 1% Ni in low alloy steels). 4340 can be welded, but this is considerably harder to do than with 4130. Quenching is typically done in oil or polymer. It is not subject to temper embrittlement. 75 ksi and over yield strengths are easily obtainable in the largest commercially available bars: there is no hardness limitation of 22 HRC because 4340 cannot be used in sour service. Although 4340 cannot be directly exposed to H2S, it still finds many applications in sweet service including bodies, bonnets, flanges, hangers, etc. It is frequently specified for high yield strength applications where it is not directly exposed to sour service well fluid. 2¼Cr-1Mo - 0.15% Max. C, 0.30-0.60% Mn, 0.50% Max. Si, 2.00-2.50% Cr, 0.87-1.13% Mo The chemistry that I have given for this alloy is that of ASTM A182, grade F22 material. There are many variations of this basic composition being used in the Oil Patch including higher carbon versions. The high chromium and molybdenum contents give this alloy excellent hardenability. The relatively low carbon content makes it easy to weld. Yield strengths of 75 ksi or more can be attained in the center of bars 12" or less in diameter without exceeding 22 HRC on the surface. This outstanding hardenability negates the need for preheat treat machining in most cases and can make 2¼Cr-1Mo cheaper to manufacture than 4130 even though the starting material costs may be higher. It is usually water quenched. 2¼Cr-1Mo alloys are widely used for bodies, bonnets, flanges, etc.

STAINLESS STEELS Stainless steels are a special class of high alloy steels. To be "stainless," a steel needs to have a minimum of 10.5% chromium. The chromium at the surface of stainless steels reacts with oxygen to form a tightly adherent oxide film. We need a minimum of 10.5% chromium to insure that this oxide film is continuous over the entire surface of the steel. The oxide film is what makes stainless steels "stainless." It is a protective barrier that keeps the corrosive medium from coming into contact with the base metal. Destroy the oxide film in a reducing environment, for example, and you destroy the corrosion resistance of the stainless steel. In oxidizing environments, if the oxide layer is damaged, it will reform and continue to provide protection. Increasing the chromium page - 106

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content increases the oxide film stability. Both chromium and molybdenum additions increase the resistance to chloride penetration. Nickel additions will improve the corrosion resistance of the film in strong acids. Stainless steels are divided into four major grades: austenitic, ferritic, martensitic, and precipitation hardening. The austenitic, ferritic, and martensitic grades of stainless steels are so classified because of their microstructure while the precipitation hardening stainless steels are grouped together because of their unique strengthening mechanism. There is a newly developed group of stainless steels known as duplex stainless that we'll also briefly cover. 1. Austenitic Stainless Steels - The austenitic stainless steels have a face-centered cubic structure from ambient temperatures up to their melting point, consequently they cannot be strengthened through heat treatment. Strengthening these alloys is accomplished by cold working them. The austenitic structure of this grade of stainless steels results from the high nickel content. The austenitic stainless steels are the most widely used stainless steels. They are characterized by excellent corrosion resistance, good formability, good strength (by cold working), and excellent ductility and notch toughness even at cryogenic temperatures. The austenitic stainless make up the 300 series of AISI stainless steels. Type 304 is the general purpose alloy. Another common austenitic type is 316. This is similar to type 304 but has a 2-3% molybdenum addition for pitting resistance. They are often referred to as "18-8" stainless steels because they have a minimum chromium content of 18% and a nickel content of 8%. When austenitic stainless steels are exposed to a temperature between 800-1650(F, they can become sensitized. Carbon and chromium have a strong affinity for each other: they love to react and form various types of carbides such as M23C6 (the "M" stands for chromium with a smidgen of iron). Carbon will diffuse to the grain boundaries in this temperature range and combine with the chromium to form carbides. Chromium that is tied-up in the form of carbides is no longer available to form the protective oxide layer that gives stainless their corrosion resistance. Each carbon atom will suck up almost four chromium atoms (i.e. M23C6) from the areas on either side of a grain boundary. The situation is potentially grim. The areas adjacent to the grain boundaries have become chromium depleted as a result of the carbide precipitation at the grain boundaries. To make matters worse, we have a large chromium-rich area in contact with a small chromium-depleted area. This creates a galvanic cell in which the depleted area becomes the anode and will rapidly corrode if exposed to many corrosive environments. We'll talk more about galvanic cells in the chapter on Corrosion. Because the page - 107

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resulting corrosion follows the chromium-depleted areas along the grain boundaries, this type of corrosion is called intergranular attack (IGA). Obviously IGA puts a limit on the maximum service temperature an austenitic stainless steel such as 304 or 316 can be exposed to in a corrosive environment. It presents problems during welding because some of the base metal will fall within the sensitizing temperature range. 304 and 316 stainless steels are sure to be sensitized if they are annealed or stress relieved after welding by heating to an elevated temperature and then slowly cooling. Sensitization is particularly rapid when the metal is held at 1200(F. Fortunately, there are several ways of preventing sensitization or restoring corrosion resistance once sensitization has occurred. The first method is to lower the carbon content down to 0.030% maximum. Although this does not completely prevent sensitization, with less carbon available, it takes longer for a sufficient quantity of carbon to diffuse to the grain boundaries to cause sensitization. At 1100(F, it will take 8 or 9 hours before an austenitic stainless with this carbon content becomes sensitized. This is more than enough time to make a weld and not have to worry about the effect on corrosion resistance. Austenitic stainless steels having a restricted carbon content are thus frequently specified for applications that require welding. These low carbon alloys are designated with a letter "L" (e.g. 304L, 316L). A second method of preventing sensitization is by making alloying additions of titanium or columbium and tantalum. If there's anything that carbon loves more than chromium, it's titanium (Ti), columbium (Cb), or tantalum (Ta). These elements combine with carbon to form very stable carbides. With all the carbon tied-up in carbides, the chromium is free to go about its business forming the protective oxide film. Austenitic stainless steels that contain Ti, Cb, or Ta are designated as stabilized grades. Type 347 is one example. It contains a combined tantalum and columbium content equal or greater than 10 times the carbon content. The stabilized grades are used in applications requiring welding or where the service temperature falls within the sensitizing temperature range of 800-1650(F. The last method we're going to talk about restores corrosion resistance to already sensitized parts. We can heat the sensitized part up to a temperature above the sensitizing range and allow the carbides to dissolve back into solution. We will then cool the part at a very rapid rate through the sensitizing range by quenching it. There will not be enough time for the carbon atoms to diffuse back to the grain boundaries and react with the chromium, consequently the original corrosion resistance will be restored. page - 108

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Austenitic stainless steels are subject to another type of corrosion known as stress corrosion cracking. Some metals under a constant tensile load will crack when exposed to a specific environment. Austenitic stainless steels are liable to do so when exposed to an environment containing chlorides at temperatures over roughly 120(F. Great care should be exercised when specifying an austenitic stainless for use in saltwater above ambient temperatures. 316 stainless steel is often used for bodies and bonnets, ring gaskets, etc. in mildly corrosive environments. Austenitic stainless steels are usually provided in the cold worked or annealed conditions. Annealing consists of holding the steel at 1800-2000(F in order to eliminate stresses, then water or oil quenching. Note that quenching does not make the austenitic stainless steels any harder (as it would carbon steels) because they always have an austenitic structure regardless of temperature. They are quenched to prevent sensitization or the precipitation of undesirable phases. Austenitic stainless steels may also be provided in the cold worked condition for maximum strength. It is important to remember that cold worked steels will recrystallize at elevated temperatures and in doing so suffer a degradation in strength. This could limit the usefulness of a cold worked austenitic stainless steel for a particular application. NACE MR0175 allows the use of austenitic stainless steels in the 300 series in sour service up to a maximum hardness of 22 HRC provided they are not cold worked to enhance properties. 2. Ferritic Stainless Steels - As the name implies, ferritic stainless steels have a BCC structure. They contain approximately 12% or more chromium. Chromium, manganese, and silicon are the only major alloying elements in these steels. Mo, Cb, Ta, and Ti are added to some types of ferritic stainless steels to improve corrosion and heat resistance while Pb, S, and Se are added to others for improved machinability. The ferritic stainless steels are used where moderate strength, good corrosion resistance, and good heat resistance are required. They exhibit particularly good resistance to stress corrosion cracking when compared to the other types of stainless steels. Because of their comparatively low alloy content, they are cheaper than many of the martensitic or austenitic grades. The ferritic stainless steels remain essentially ferritic up to their melting point. This means that they cannot be hardened using the quench and tempering process we've talked about under Heat Treatment. Certain types of ferritic stainless steels can be strengthened, but at a tremendous loss of ductility, through an embrittling mechanism when held for long periods of time in certain temperature ranges. Ferritic stainless steels are generally provided in the annealed condition. page - 109

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NACE MR0175 allows the use of ferritic stainless steels in sour service provided they are annealed and do not exceed 22 HRC. 3. Martensitic Stainless Steels - Martensitic stainless steels are similar in composition to the ferritic stainless steels. The carbon and chromium contents of these two grades of stainless steels determine their microstructure at any given temperature. The ferritic stainless steels are essentially body-centered cubic up to their melting points. The martensitic types are body-centered cubic in the fully annealed condition at room temperature, but undergo a phase transformation at about 1475(F to become face-centered cubic (or austenitic). They develop a martensitic structure after being quenched in oil or air from the austenitic region. Water is seldom used as the quenching medium because of the possibility of quench cracking. The martensitic structure is always tempered to obtain the best combination of strength and toughness. The martensitic stainless steels are utilized wherever moderate corrosion resistance and strength are needed. They have excellent machinability. The notch toughness of many of the martensitic stainless steels is very low in comparison to the austenitic grades. The martensitic stainless steels, along with the ferritic stainless steels make up the AISI 400 series of stainless steels. 410 is the most commonly specified martensitic stainless steel. It can have a chromium content as low as 11.5%. This is precariously close to the bare minimum (10.5%) required to make a stainless "stainless." Not surprisingly, 410 has very poor corrosion resistance in comparison to other types of stainless steels (but significantly better than most alloy steels). 410 is particularly susceptible to pitting corrosion. As in the case with other steels that can be hardened (or strengthened) through quenching and tempering, the mechanical properties of the martensitic stainless steels are primarily dependent on the tempering time and temperature. The as-quenched structure will be essentially martensite and has the maximum strength level, but also the lowest ductility and toughness that can be attained. The asquenched steel is tempered at 1150-1400(F in order to transform the martensite into another microstructure (tempered martensite) that has better ductility, but accompanied with a slight decrease in strength. The higher the tempering temperature or the longer the steel is held at the tempering temperature, the better the ductility and the lower the strength of the tempered martensite will be. We usually double temper the martensitic stainless steels because of the possibility of retained austenite transforming into fresh martensite during the first temper. This double temper is required by NACE MR0175 for martensitic stainless steels for sour service. Martensitic page - 110

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stainless steels are subject to temper embrittlement if tempered in the 700-1050(F range consequently all of our specifications require a minimum tempering temperature well above this. NACE requires a minimum tempering temperature of 1150(F for 410 stainless steel. 410 has rather poor impact properties at low temperatures. For North Slope type of environments, a modified 410 called CA6NM or F6NM is frequently specified. This has a 410 chemistry with a four percent nickel addition, which significantly improves notch toughness, and a small addition of molybdenum. Both materials are used for bodies, bonnets, flanges, stems, gates, hangers, and many other parts in the Oil Patch. Both materials are suitable for sour service: 410 at 22 HRC maximum and CA6NM at 23 HRC maximum. 4. Precipitation Hardening Stainless Steels - We talked about the principles of age hardening (or precipitation hardening) in the lesson on Heat Treatment. Stainless steels are one type of metal where these principles are put into use. Essentially age hardening involves making a supersaturated solid solution (solution annealing followed by quenching) and then aging by heating it to a relatively low temperature. This causes a second phase to precipitate out, thus strengthening the material. The final structure may be austenitic, martensitic, or semiaustenitic depending on the particular type of precipitation hardening stainless steel. Copper, aluminum, tantalum, titanium, columbium, and nitrogen are the usual elements added to form the precipitates. These types of stainless steels can develop very high strength levels while retaining a fair amount of ductility. They are used where good corrosion resistance (comparable to the austenitic) and high strength are required. They have excellent high temperature properties. One of the advantages of using a precipitation hardening stainless steel is that forming and machining can be done while the steel is in the solution annealed condition making these operations easy to perform. The steel can then be hardened by aging. There is virtually no distortion accompanying the aging process, unlike strengthening by quenching. One of the more commonly used martensitic precipitation hardening types is 17-4PH®. This alloy contains 15.5-17.5% Cr and 3.0-5.0% Ni. It is acceptable to use in sour service up to 33 HRC maximum in the solution annealed and aged condition although it is seldom used for highly stressed parts in severe sour environments because of potential cracking problems. It is occasionally used for valve stems. A-286 is a commonly used austenitic precipitation hardening stainless steel. It has a chromium content of 13.5-16.0% and 24.0-27.0% nickel. It has high strength and excellent toughness. It is acceptable for sour page - 111

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service in the solution annealed and aged condition up to 35 HRC. It is sometimes used for valve stems and bolting in the Oil Patch. 5. Duplex Stainless Steels - The duplex stainless steels, as the name suggests, have a microstructure consisting of two predominate phases in roughly equal proportions. By far the most commonly used duplex stainless steels are the austenitic-ferritic varieties. These steels were developed in order to combine the general corrosion resistance of the austenitic grades with the chloride stress corrosion cracking resistance of the ferritic grades. They typically have "islands" of austenite in a ferrite matrix. There are many duplex stainless steels (DSS) on the market. One of the most commonly used DSS is 2205. This variety is used for bodies, bonnets, flanges, etc. in the Oil Patch. It contains 21.0-23.0% Cr, 4.56.5% Ni, 2.5-3.5% Mo and 0.08-0.20% N. It is used in the solution annealed and quenched condition and is suitable for sour service up to 28 HRC. The outstanding corrosion resistance of this alloy makes it a prime candidate for many oil field environments particularly when there is a high chloride and CO2 presence. Duplex stainless steels are subject to sigma phase embrittlement if improperly heat treated or hot worked. Sigma is a phase in the ironchromium system that has a tetragonal structure with 30 atoms per unit cell. It is a hard, brittle material. If it forms in significant quantities, the hardness of the duplex stainless will increase while the toughness suffers a drastic decrease. Sigma forms in the 1050-1800(F region with the most rapid growth taking place near the higher temperature range. It is essential that duplex stainless steels not be exposed to temperatures within the sigma forming range for any longer than necessary. This is the reason why a quench is specified after annealing. If sigma formation does take place, properties can be restored by re-annealing above 1800(F and then quenching.

NICKEL BASE ALLOYS Nickel base alloys are extremely important in the Oil Patch because many of them have a combination of high strength and outstanding corrosion resistance. The excellent corrosion resistance is attributable to a tenacious oxide film that forms on the surface. This film acts as a barrier that separates the underlying metal from the environment. Pure nickel, as well as its alloys, has a face centered cubic structure (FCC). Nickel base alloys have excellent toughness even down to cryogenic temperatures. Being FCC, they do not have a transition region as BCC steels do where toughness falls dramatically with temperature.

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Nickel base alloys, depending on their composition, may be hardened by solid solution strengthening, precipitation hardening, and/or cold working. The FCC structure of the nickel matrix is designated gamma (or by the Greek letter ). The gamma phase can be solid solution hardened by alloying with aluminum, chromium, molybdenum, tungsten, as well as other elements. The precipitation hardening process is the most important for developing high strength in many nickel alloys. Nickel, like iron and cobalt, is ferromagnetic. Nickel base alloys can be precipitation hardened because of the formation of the gamma prime (or 1) phase during the aging cycle. It is essentially a compound of nickel, aluminum, and titanium having the composition Ni3(Al, Ti). In complex alloys, other metals may substitute for some of the nickel and/or aluminum and titanium. It is amazing stuff. The strength of 1 actually increases with temperature. It is inherently ductile so even if it were to precipitate out extensively in a localized area, a grain boundary for instance, the metal would probably not become embrittled.

ALLOYING ELEMENTS OF NICKEL BASE ALLOYS We talked about alloying elements in steels in Chapter 3. Now let's look at the major alloying elements in nickel base alloys. Cobalt - Cobalt solid solution hardens (strengthens)  and 1. It reduces the amount of Al and Ti that can dissolve in the matrix thus making them available for the formation of more 1. Cobalt increases the hot workability of nickel base alloys. Chromium - Chromium is used extensively in nickel base alloys. It forms part of the matrix in conjunction with the nickel. The chromium oxide and nickel oxide surface layer gives many nickel base alloys their outstanding corrosion resistance. Chromium is a strong solid solution hardening element for both  and 1. Like cobalt, it increases the amount of 1 that can form by decreasing the solubility of Al and Ti in the matrix. Chromium is a strong M23C6 type of carbide former. Columbium - Columbium may be added to tie up carbon in the form of very stable carbides. It strengthens the matrix through solid solution strengthening. It can strengthen through the precipitation of Ni3Cb (gamma double prime or 2). Iron - Iron is essentially an inexpensive "filler" material for many nickel base alloys. It does slightly increase the strength of the matrix through solid solution hardening. Iron can combine with carbon to form stable MCtype carbides in certain temperature ranges. This can increase ductility by suppressing the formation of other types of carbides that may cause embrittlement. This can also increase the corrosion resistance by tying up the carbon so that the carbon cannot form carbides with the other alloying elements.

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Molybdenum - Molybdenum greatly improves the pitting and crevice corrosion resistance of nickel base alloys. It is a strong solid solution strengthening element for both  and 1. It facilitates the formation of MC (at low temperatures) and M6C (at high temperatures) types of carbides. This increases ductility by suppressing M23 C6 carbide formation along grain boundaries. Titanium - Titanium is an important alloying addition for precipitation hardenable nickel base alloys because it is one of the constituents of 1. It will solid solution strengthen 1. As titanium increases, so will the amount of 1. Titanium is a strong M23C6 type of carbide former. Carbon - Carbon is, of course, required in carbide formation. There are many types of carbides that can form in nickel base alloys, but the most important ones are MC, M6C, and M23C6. Carbides tend to precipitate out along grain boundaries in nickel base alloys although M6C type carbides may also precipitate out within the grains in what is known as Widmanstatten Structure. A Widmanstatten structure consists of plate or needle shaped precipitates that form along certain crystallographic directions in the matrix. M23C6 carbides that are continuous along grain boundaries and Widmanstatten structures are detrimental to ductility and the stress rupture properties of nickel base alloys. Stress rupture is a measure of a metal's ability to carry a load at a high temperature for a certain period of time without failing. Just as carbide precipitation can sensitize some types of stainless steels, it can also cause "denuded" zones along grain boundaries in nickel base alloys. The concentration of M atoms is greatly reduced in these zones as the M atoms precipitate out with the carbon in the grain boundaries. Some carbides are desirable in the grain boundaries because they improve the mechanical properties at elevated temperatures. Vanadium - Vanadium can be substituted for some of the aluminum in 1. It thus increases the amount of 1. It is also a weak solid solution strengthening element of the matrix. Tungsten - Tungsten helps improve corrosion resistance in certain environments. It is a strong solid solution strengthening element for both  and 1. Tungsten improves ductility by forming MC or M6C type carbides thus suppressing the embrittling M23C6 type. Increasing tungsten increases the amount of 1. Zirconium - Zirconium is added in small amounts to suppress the formation of M23C6 type carbides at grain boundaries. It also improves creep resistance. Creep is the time dependent permanent deformation that occurs in a metal under a constant load especially at elevated temperatures.

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Aluminum - Aluminum, like titanium, is necessary in order to form 1: as aluminum increases so does the amount of 1. It is a strong solid solution strengthening element of the matrix. It promotes the formation of M23C6 type carbides at elevated temperatures. Tantalum - Tantalum can be substituted for some of the aluminum. It increases the amount of 1 and also solid solution strengthens it. Tantalum is a strong MC and M23C6 type carbide former at high temperatures and M6C type carbide former at low temperatures. Magnesium - Magnesium is sometimes used in conjunction with zirconium or boron. It improves creep resistance and suppresses the formation of M23C6 type carbides at grain boundaries. Boron - Small amounts of boron are added to improve creep resistance and to stabilize 1 and grain boundaries. It suppresses the formation of M23C6 type carbides at grain boundaries.

HEAT TREATING NICKEL BASE ALLOYS As with steels, all nickel base alloys may be annealed in order to stress relieve, homogenize, or soften through recrystallization. The precipitation hardenable alloys will go through a solution anneal, a quench, and one or more aging cycles. Let's examine the precipitation hardening process in detail for nickel base alloys. The purpose of solution annealing is to dissolve carbides that may be present and to put 1 into solution. The properties of some nickel base alloys (718 for example) are strongly influenced by the solution annealing temperature. A high solution anneal temperature in these types of alloys will dissolve more carbides, but may also result in significant grain growth. A low temperature solution anneal may not dissolve high temperature forming carbides, but will minimize grain growth. To keep the 1 in solution, parts are quenched after solution annealing. Most nickel base alloys can be air quenched although some, such as some of the Hastelloys®, require a rapid quench in water or oil. The aging process in nickel base alloys is very complex. The precipitation of 1 is the chief hardening agent. There are, however, many other phases that can precipitate out as well depending on the composition and heat treatment. Among these are eta (Ni3Ti), 2 or gamma double prime (Ni3Cb in a body centered tetragonal structure), various carbides, nitrides, carbonitrides, borides, as well as other phases. An aging heat treatment must take into account which phases will precipitate out, their morphology (size and shape), their relative amounts, and whether or not they will precipitate out inter- or intragranularly. Some of these precipitates are desirable (1, 2, or some fine M23C6 type carbides evenly dispersed along grain boundaries for instance). Others are not. Among the bad actors are Widmanstatten M6C, interconnecting M23C6 type carbides along grain boundaries, and Laves phase (M2Ti). The aging temperature page - 115

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must often be a compromise between what gives the optimum formation of 1 and what best suppresses the undesirable phases. In some alloys, X-750 for example, a two step aging process is performed in order to control the type of carbide that precipitates, its morphology, and its location as well as to precipitate 1. In other alloys, 625 Plus® for example, a two step age is performed merely to precipitate out as much 1 as possible. Let's look at a two stage aging process for a nickel base alloy Inconel® X-750. Inconel® X-750 is usually solution annealed around 1800(F and then air quenched. In order to precipitate out 1, this alloy is typically aged at 1300(F, however, if directly aged at this temperature, a cellular form of M23 C6 will also precipitate out. This depletes the surrounding areas of chromium and suppresses the formation of 1. We can avoid this problem by first aging at 1550(F. This will cause the precipitation of a blocky form of M23C6 that does not denude grain boundaries. The subsequent 1300(F aging cycle will result in a fine dispersion of 1 right up to the grain boundaries. The 1550(F age may be as long as twenty hours. Parts are then generally furnace cooled down to 1300(F where they are then aged for up to twenty additional hours. Nickel and its alloys can become embrittled when exposed to low melting temperature elements such as sulfur or lead at elevated temperatures. This necessitates that lubricants, marking paints, greases, etc. (all of which may contain sulfur) be thoroughly cleaned off nickel parts prior to heat treatment. Many material specifications impose a limit on sulfur that may be present in fossil fuels used to fire furnaces in which nickel base alloys are to be heat treated. Contact with steel scale should also be avoided at elevated temperatures.

SOME COMMON NICKEL BASE ALLOYS There are many nickel base alloys used in the Oil Patch. We'll examine a few of the more important ones in this section. The compositions given are typical for commercial grades and do not necessarily reflect the limits in our material specifications. I am identifying these alloys by their most widely recognized trade names just as a matter of convenience. Most of them are available from more than one supplier under various alloy names. Monel® 400 (UNS N04400) - 63.0% min. Ni, 28.0 - 34.0% Cu, 2.5% max. Fe, 2.0% max. Mn, 0.3% max. C, 0.5% max. Si The Monels® are essentially nickel-copper alloys containing approximately copper and nickel. They have excellent corrosion resistance in sea water, in many acids, alkalies, and wellhead environments. Monel® 400 was the first to be developed. It has the good corrosion resistance typical of the family and is frequently specified for heat exchangers, pumps, shafts, valves, and piping in the chemical and marine industries where corrosion is a primary concern. NACE MR0175 allows Monel® 400 to be used in sour service up to 35 HRC. Monel® 400 can only be hardened by cold work. It is typically used in the annealed

D

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condition. A typical anneal is done at 1600-1800(F and is followed by an air quench. The yield strength of annealed Monel® 400 is around 25-50 ksi. This relatively low value frequently limits its use in wellhead equipment to low stress applications such as seals. Monel® K-500 (UNS N05500) - 63.0% min. Ni, 27.0-33.0% Cu, 2.303.15% Al, 0.35-0.85% Ti, 2.0% max. Fe, 0.25% max. C, 1.5% max. Mn, 0.5% max. Si This alloy is the age hardenable version of Monel® 400. In the solution annealed and aged condition the yield strength is around 110 ksi: a considerable improvement over Monel® 400. A typical heat treatment for the Oil Patch would be to solution anneal at 1700-1800(F, water quench, and then age around 1100(F. NACE MR0175 allows the use of Monel® K-500 in sour service in the hot worked and age hardened condition, the solution annealed condition, and the solution annealed and age hardened condition up to a maximum of 35 HRC. The high strength and good corrosion resistance of Monel® K-500 make it a popular choice for valve trim, propeller and pump shafts, fasteners, etc. Monel® K-500 should not be used for cathodically protected parts because it is subject to hydrogen embrittlement. Inconel® 600 (UNS N06600) - 72.0% min. Ni, 14.0-17.0% Cr, 6.0-10.0% Fe, 0.15% max. C, 1.0% max. Mn, 0.5% max. Si, 0.5% max. Cu The Inconel® family consists of nickel-chromium-iron alloys. Inconel® 600 has outstanding corrosion and heat resistance which make it a commonly used material in the chemical, power, and aerospace industries. Its high nickel content gives it excellent toughness even down to cryogenic temperatures. NACE MR0175 allows the use of Inconel® 600 up to 35 HRC maximum in sour service. Inconel® 600 cannot be hardened through heat treatment. It can be strengthened, of course, through cold working. Inconel® 600 is typically annealed at 1700-1800(F and then air cooled. Yield strength is around 30-35 ksi in the annealed condition. The low strength level relegates the use of Inconel® 600 in the Oil Patch to low stress parts such as seals, trim, etc. Inconel® X-750 (UNS N07750) - 70.0% min. Ni, 14.0-17.0% Cr, 5.0-9.0% Fe, 2.25-2.75% Ti, 0.40-1.00% Al, 0.70-1.20% Cb, 0.08% max. C, 1.00% max. Mn, 0.50% max. Si, 0.50% max. Cu, 1.00% max. Co Inconel® X-750 is the age hardenable version of Inconel® 600. Depending on the amount of work and specific heat treatment, X-750 in the solution annealed and aged condition can have a yield strength over 100 ksi. NACE MR0175 allows X-750 to be used up to 35 HRC in the solution annealed and aged condition, solution annealed condition, hot worked condition, hot worked and aged condition. We've already discussed a typical heat treatment with a two step aging cycle. Inconel® page - 117

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X-750 has been used for stems, springs, and other highly stressed parts in the Oil Patch. It is subject to hydrogen embrittlement under cathodic protection. Inconel® 625 (UNS N06625) - 58.0% min. Ni, 20.0-30.0% Cr, 8.0-10.0% Mo, 3.15-4.15% Cb, 5.0% max. Fe, 0.10% max. C, 0.50% max. Mn, 0.50% max. Si, 0.40% max. Al, 0.40% max. Ti Inconel® 625 has outstanding corrosion resistance. It is very resistant to pitting and chloride stress corrosion cracking in chloride solutions. It is frequently the alloy of choice for severe wellhead environments such as found in Mobile Bay and the North Sea. 625 is widely used for main pressure containing parts, hangers, stems, gates, etc. in the Oil Patch. We use 625 to clad low alloy steels by HIP’ing or welding in order to make these steels suitable for highly corrosive environments. NACE MR0175 allows 625 to be used up to 35 HRC. It is usually solution annealed at 2000-2200(F. Yield strength is relatively low in the solution annealed condition (around 40-60 ksi). In wrought products, some strengthening can be obtained by aging the solution annealed material at 1100-1300(F where a sluggish precipitation occurs. Yield strengths can be increased in relatively thin parts up to around 75 ksi by aging. Amazingly, HIP’ed 625 (we'll talk about HIP’ing in detail in the Chapter on Forging, Casting, and Powder Metallurgy) can develop yield strengths well over 100 ksi without exceeding the NACE hardness limit in relatively large parts after aging. Why this occurs is not altogether understood, but is related to the fine grain size of the HIP’ed product, the use of a more restrictive chemistry, as well as other reasons. Inconel® 718 (UNS NO7718) - 50.0-55.0% Ni, 17.0-21.0% Cr, 4.75-5.50% Cb, 2.80-3.30% Mo, 0.65-1.15% Ti, 0.20-0.80% Al, 1.0% max. Co, 0.08% max. C, 0.35% max. Si, 0.006% B, 0.30% Cu, Balance Fe 718 is one of the workhorses of the Oil Patch. It has excellent corrosion resistance and can attain high strength levels through aging. This makes it useful for main body components, gates, stems, hangers, etc. Wrought 718 can be used in sour service in the following conditions as permitted by NACE MR0175: 1. 2. 3. 4.

solution annealed to 35 HRC maximum hot worked to 35 HRC maximum hot worked and aged to 35 HRC maximum solution annealed and aged to 40 HRC maximum

Annealing is generally done at 1700-1950(F. Yield strength may vary from about 45 ksi to 80 ksi depending on annealing temperature and the amount of working. Like X-750, 718 has many possible aging heat treatments. The one most commonly used in the Oil Patch is a single aging cycle at 1200-1500(F. The primary hardening agent is gamma page - 118

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double prime. Yield strengths well above 100 ksi can be obtained in the aged condition without exceeding the maximum NACE hardness. Equipment made from 718 is used in many hostile wellhead environments. At temperatures over roughly 300(F, it can become susceptible to sulfide stress corrosion cracking in high chloride solutions if free sulfur is present. 625 Plus® (UNS NO7716) - 59.0-63.0% Ni, 19.0-22.0% Cr, 2.75-4.00% Cb, 0.15% max. Cu, 7.00-9.50% Mo, 1.00-1.60% Ti, 0.20% max. Si, 0.20% max. Mn, 0.03% max. C, 0.35% max. Al, Balance Fe This rising star in the Oil Patch is essentially a 625 alloy that has been modified to have a better aging response. In the solution annealed and aged condition, yield strengths in excess of 120 ksi can be obtained without exceeding the 40 HRC maximum hardness imposed by NACE MR0175 for use in sour service. 625 Plus® is typically solution annealed at 1800-1900(F. A two step aging process is used: age at 1350-1375(F, furnace cool down to 1150-1200(F, and then age again. The high strength and good corrosion resistance of 625 Plus® make it a natural for main body components, hangers, stems, etc. Inconel® 725 (UNS NO7725) - 55.0-59.0% Ni, 19.0-22.50% Cr, 7.009.50% Mo, 2.75-4.00% Cb, 0.35% max. Al, 1.00-1.70% Ti, 0.20% max. Si, 0.03% max. C, 0.35% max. Mn, Balance Fe This alloy is INCO's version of 625 Plus®. The two alloys have similar chemistries, heat treatments, and properties. NACE has approved its use in sour service in the solution annealed condition up to a maximum of 40 HRC. Hastelloy® C-276 (UNS NO10276) - 14.5-16.5% Cr, 0.02% max. C, 4.007.00% Fe, 1.00% max. Mn, 15.0-17.0% Mo, 2.5% Co, 0.08% max. Si, 0.03% max. V, 3.0-4.5% W, Balance Ni The Hastelloys® are a group of Ni-Mo-Cr alloys with outstanding corrosion resistance that make them widely used in the chemical and petroleum industries. C-276 is not considered to be hardenable through heat treatment, although some minor aging may occur at elevated temperatures. It is typically annealed at 2050(F and then rapidly quenched. The yield strength is about 50 ksi in the annealed condition. NACE permits its use in sour service in the following conditions: 1. solution annealed or solution annealed plus cold worked up to 35 HRC maximum page - 119

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2. cold worked and unaged condition at 45 HRC maximum when used at a minimum temperature of 250(F C-276 is typically used for seals or other low stressed parts that must perform satisfactorily in a highly corrosive environment.

BITS AND PIECES The brief survey of steels and nickel base alloys that we just plowed through covers the lion's share of materials that we use in our equipment. There are, however, many other types of metals not included in these two alloy families that are used for special applications. In this section we'll give a quick look at some of these specialized metals that tonnage-wise make only a minuscule part of our products, but without which many of our products could not be made. 1. Elgiloy® (UNS R30003) - 39.0-41.0% Co, 19.0-21.0% Cr, 15.0-16.0% Ni, 1.0% max. Be, 0.15% max. C, 1.5-2.5% Mn, 6.0-8.0% Mo, Balance Fe Elgiloy® is a cobalt base alloy that is strengthened by a combination of cold work and precipitation hardening. It is frequently specified as a spring material in the Oil Patch because it is corrosion resistant and is capable of developing extremely high strength levels. It is typically annealed around 2150(F and then cold worked until roughly a 50% reduction in area is obtained. If it is going to be used for a spring, it is next coiled, stamped, or otherwise fabricated. Next comes the aging process at about 850-950(F to develop the desired strength. NACE MR0175 allows the use of Elgiloy® springs up to 60 HRC in the cold worked and age hardened condition. Small diameter spring wire can easily have yield strengths over 225 ksi. NACE also allows the use of Elgiloy® bar in any condition up to 35 HRC. 2. MP35N® (UNS R30035) - 0.025% max. C, 19.0-21.0% Cr, 1.00% max. Fe, 0.15% max. Mn, 9.00-10.50% Mo, 33.0-37.00% Ni, 0.15% max. Si, 1.00% max. Ti, Balance Co Like Elgiloy®, MP35N® finds its greatest use as spring material. It is also used for high strength bolting, tie-down screws, and the like. MP35N® can attain yield strengths over 200 ksi in small diameter wire that has been cold worked and aged. NACE permits MP35N® springs to have a hardness up to 55 HRC in the cold worked and aged condition. NACE allows the use of MP35N® for other components in sour service at a maximum hardness of 51 HRC and in one of the following cold worked and aged conditions. A. Aged at 1300(F for at least 4 hours page - 120

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Aged at 1350(F for at least 4 hours Aged at 1425(F for at least 6 hours Aged at 1450(F for at least 4 hours Aged at 1475(F for at least 2 hours Aged at 1500(F for at least 1 hour

MP35N's® high nickel, cobalt, chromium, and molybdenum contents gives it outstanding corrosion resistance. MP35N® and Elgiloy® can usually be substituted for another: an important point to remember because the availability of either one of them in a given product form is often limited. MP35N® springs are generally limited to applications that see temperatures below 400(F because of embrittlement problems at higher temperatures. 3. Titanium Alloys - Titanium alloys are becoming more and more common in the Oil Patch because of their high strength, low weight, and excellent corrosion resistance. Titanium has a hexagonal close packed (HCP) structure (see Figure 1) at room temperature, but transforms into body centered cubic at 1625(F. Titanium alloys may be divided into alpha, beta, and alpha-beta groups. Alpha alloys have a HCP structure that has been stabilized by tin, aluminum, zirconium, or a combination of any of the three. Oxygen, nitrogen, and carbon also help to stabilize the alpha structure (note the alpha structure in titanium is HCP not BCC as in steels). Beta alloys have a body centered cubic structure (BCC) that has been stabilized with cobalt, columbium, vanadium, iron, manganese, chromium as well as some of the other transition elements. A combination of alpha and beta stabilizing elements are used in the alpha-beta alloys where both phases may be present. Alpha alloys cannot be hardened by heat treatment. Their stable alpha structure makes them easy to weld in comparison to the other types. They have excellent corrosion resistance and find frequent use in the chemical industry because of it. Alpha alloys are typically used in the annealed condition. Yield strengths are typically 100 ksi and above. Alpha-beta alloys are precipitation hardenable. They are typically solution annealed high in the alpha-beta region. Depending on a variety of factors, the beta phase that was present during solutionizing will either be retained or undergo a martensite type of transformation after quenching. Aging is typically done somewhere between 9001200(F. During aging a fine precipitate of alpha comes out of solution within the retained or transformed beta phase. The final microstructure is typically a mixture of primary alpha (untransformed alpha) and a fine page - 121

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mixture of alpha precipitates within the beta phase. Aging can increase the strength by 50% over the annealed condition. The more beta stabilizers in the alloy, the greater the hardenability. Alpha-beta alloys are the most widely used titanium alloys.

Figure 1: Hexagonal Close Packed (HCP) Beta alloys are heat treatable and have much better hardenability than the alpha-beta alloys. Beta alloys are typically aged from 850-1200(F. Aging causes a partial decomposition of the beta phase into alpha. The alpha precipitates out as finely dispersed particles within the retained beta and thus greatly strengthens the material. Cold work or holding at a slightly elevated temperature may result in the formation of additional alpha. Let's look at a couple of titanium alloys that are used in the Oil Patch. A. Titanium, Grade 12 (UNS R53400) - 0.08% max. C, 0.30% max. Fe, 0.015% max. H, 0.2-0.4% Mo, 0.03% max. N, 0.6-0.9% Ni, 0.25% max. O, Balance Ti This commercially pure titanium is not hardenable through heat treatment. It is often used for ring gaskets in highly corrosive environments because of its outstanding corrosion resistance and its relatively low hardness. NACE allows its use in sour environments when annealed at 1400-1450(F for two hours followed by air cooling and at a maximum hardness of 92 HRB. B. Beta-C® (UNS R586401) - 0.05% max. C, 0.03% max. N, 0.03% Fe, 0.14% max. O, 3.0-4.0% Al, 7.5-8.5% V, 5.5-6.5% Cr, 3.5-4.5% Mo, 3.5-4.5% Zr, Balance Ti This heat treatable beta-alloy is finding increasing use in the Oil Patch for highly stressed parts in corrosive environments. It is often used for high strength springs, load rings, etc. Several oil page - 122

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companies are specifying Beta-C® tubulars. Beta-C® is solution annealed at 1500-1700(F. Aging is done at 850-1000(F. Yield strengths over 170 ksi are obtainable in bar that has been aged. Spring wire may have yield strengths in excess of 200 ksi in the cold drawn and aged condition. NACE allows the use of Beta-C® at hardnesses up to 42 HRC. It is used in the annealed or the solution annealed and aged conditions. C. Ti-6Al-2Sn-4Zr-6Mo (UNS R56260) - 0.04% max. C, 0.04% max. N, 0.15% max. Fe, 0.15% max. O, 5.5-6.5% Al, 1.75-2.25% Sn, 3.5-4.5% Zr, 5.5-6.5% Mo, 0.0125% max. H, Balance Ti This alpha-beta alloy is capable of being strengthened though heat treatment. It is solution annealed at 1550-1650(F and aged around 1100(F. Yield strengths over 150 ksi can be obtained in bar. NACE allows its use in sour service at 45 HRC maximum in one of the following conditions: A. annealed B. solution annealed C. solution annealed and aged It is sometimes used for gaskets in the annealed condition and load rings, etc. in the aged. 4. Cemented Tungsten Carbides - A cemented tungsten carbide is a composite material consisting of hard, brittle tungsten carbide (WC) particles in a soft, ductile metal matrix. Cobalt is the binder most commonly used to "cement" the tungsten carbide particles together and is the binder for all of our cemented tungsten carbides. We use cemented tungsten carbides for choke needle tips and seats because of the outstanding erosion and wear resistance afforded by the tungsten carbide particles. As with all composite materials, the overall properties of cemented tungsten carbides are very dependent on the properties and relative amounts of matrix element (cobalt) and the reinforcing element (tungsten carbide). The higher the tungsten carbide content, the higher the hardness, compressive strength, and abrasion resistance of the overall composite. A price is paid, however, in lower transverse strength and toughness. As the percentage of soft, ductile cobalt binder is increased, the opposite is true. Small amounts of tantalum and titanium carbides may be added to refine the grain size thus increasing the toughness. The cemented tungsten carbides are fabricated into shapes through a powder metallurgy process (see the chapter on Forging, Casting, and Powder Metallurgy). We use several different grades of cemented tungsten carbide in our products including 94% WC-6% Co and 87% WC-13% Co. Hardness of the 94% WC-6% Co grade is typically about 92 HRA while that of page - 123

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the 87% WC-13% Co is about 89 HRA. The cemented tungsten carbide is brazed into choke bean or onto the end of the needle. NACE allows the use of these alloys at any hardness. 5. Stellites® 3 and 4 - Stellites® are cobalt based alloys used for their high wear and galling resistance. We use Stellite® 3 and Stellite® 4 for seat rings in gate valves. In addition to cobalt, both of these alloys contain approximately 30% Cr, 11-14% W, and small amounts of Si, Fe, Ni, and Mo. The big difference is in carbon content. Stellite® 4 contains up to 1% C while the Stellite® 3 may contain up to 2.7% C. The carbon combines with the chromium to form M7C3 type carbides and with the tungsten to form M6C-type carbides. It is these carbides that give the Stellites® their outstanding wear resistance. The higher carbon content of Stellite® 3 gives it a higher hardness and better wear resistance than the Stellite® 4. Stellite® 4, on the other hand, has better corrosion resistance. Our Stellite® seat rings are made from centrifugal castings (see the chapter on Forging, Casting, and Powder Metallurgy). The only heat treatment they undergo is a stress relief at 1650(F followed by furnace cooling. The stress relieved Rockwell C hardness of Stellite® 3 is typically in the mid to high fifties while that of the Stellite® 4 is in the high forties to low fifties. 6. Tribaloy® T-800 - This is another cobalt based wear-resistant alloy. It has a nominal composition of 3.0% max. Ni and Fe, 28.5% Mo, 17.5% Cr, 3.4% Si, 0.08% max. C, with the balance being cobalt. Instead of relying on hard carbides, this alloy gets its wear resistance from the formation of a hard, intermetallic (Laves) phase that is dispersed throughout a softer matrix. We use this alloy as a hardfacing for gates in gate valves. It is applied by either plasma transferred arc spray (see the chapter on Special Processes) or through HIP’ing powder directing to the gate (see the chapter on Forging, Casting, and Powder Metallurgy). The overall hardness of the alloy is 54-62 HRC. 7. Union Carbide LW-45® - This is a hard facing coating that we use on gate valve gates. Its nominal composition is 78% W, 12% Co, 5% Cr, and 4% C. It gets its wear resistance from various carbides (such as tungsten carbide, M6C) that are dispersed throughout a cobaltchromium matrix. LW-45® is applied using the Union Carbide D-Gun Process®. We will discuss this in detail in the chapter on Special Processes. The bonding of the LW-45® coating to the base metal is through the mechanical interlocking of the particles with the substrate. 8. Colmonoy® #5 - This is a hard facing material that we use to provide wear resistance on gates, stab pins, and BOP operating pistons. It is nickel based with approximately 10-14% Cr, 2-3% B, 3-4.5% Si, 3-5% Fe, 0.4-0.8% C, and 0.25% maximum Co. It can be applied by several different methods, but we typically use an oxy-acetylene weld process (see the chapter on Special Processes). Once applied using this page - 124

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method, the hardfaced part must undergo a full heat treatment. This causes the hardfacing particles to fuse together and to the base metal thus forming a true metallurgical bond. Colmonoy® #5 has a hardness of 45-50 HRC. 9. Ultimet® (UNS R31233) -Nominal composition 9% Ni, 3% Fe, 26% Cr, 5% Mo, 2% W, 0.06% C, 0.08% N, 0.3% Si, 0.8% Mn, Balance Co. Ultimet® is a cobalt base alloy that has excellent corrosion resistance and excellent galling resistance. We use it for parts that must come into moving contact with nickel base alloys (that are exceptionally prone to galling). It can also be used as a weld overlay. Ultimet® is nonheat treatable, but can be cold worked to high strength levels. In the annealed condition it has a yield strength of about 75 ksi. NACE allows its use in sour service in the annealed condition at 22 HRC maximum. 10. Unobtainium (UNS XXXXXX) - This remarkable alloy has half the specific gravity of steel, is immune to environmental cracking and general corrosion, is easily welded, and is capable of developing a high yield strength while maintaining excellent notch toughness down to cryogenic temperatures. It is easy to cast, forge, and machine. It can be used in sour service at any hardness. Galling has never been known to occur. It is not subject to any form of embrittlement. Post weld heat treating is not required. Bars up to 36" O.D. through harden. Chemistry varies. It is made by mills world-wide and costs only pennies per pound. This material is frequently specified by our larger customers.

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CHAPTER VII

FORGING, CASTING, & POWDER METALLURGY And the National Bird of Texas

Everything is big in Texas, but perhaps nothing is on so grand a scale as the common cockroach, affectionately known as the national "bird" of Texas. The small ones are quite capable of carting off your kids or pets and doing all sorts of horrible things to them if you are not careful, while the larger ones are frequently seen saddled in rodeos. Up North where I come from, having roaches in your house is a sure sign of slovenly living, but not in Texas. In fact it's just the opposite: if you don't have roaches in your house you're ostracized by the community. People figure that you're not just worth socializing with if even the roaches won't have anything to do with you. There is a "live and let live" attitude towards roaches down here, but being a Yankee, I have not yet reached this degree of tolerance. On the contrary, I have declared total, unrestricted warfare on my domestic roaches and do my very best to bring the little heathens to their just reward. But despite years of chemical warfare and vicious sneak attacks during the night with a rolled up newspaper, Cockroachus domicilus is not yet an endangered species in my house. Many Texans have a certain sentimental attachment for the roaches in their kitchens. It is the Texas equivalent of having a "cricket in the hearth" for good luck. Some Texans even go so far as to spread choice morsels of food around their kitchens to keep their semi-domesticated, six-legged pets in the pink of condition. I consider this to be casting pearls before swine, however. This interesting and illuminating entomological discussion naturally leads us into our next topic: forging, casting, and powder metallurgy. Pearls are not the only thing that can be cast, so can metals. Casting is one of the three manufacturing methods that we will talk about in this section that will help us to get metal into the desired shape. The other methods are forging and powder metallurgy. We will examine each of these as they apply to our products and compare their advantages and disadvantages.

FORGING Forging is a method of permanently deforming a metal into a desired shape by applying a compressive force usually at an elevated temperature. Forging methods are classified by the equipment and tooling that are utilized. Some common methods that we utilize are: open-die hammer forging, closed-die hammer forging, open-die press forging, closed-die press forging, rolling, extrusion, and ring rolling. 1. Open and Closed Die Forging - A die is a block of metal used to strike the starting material and impart some desired shape to it through plastic deformation. A die may or may not have a cavity in it. The forge that utilizes a set of dies is generally classified as either a hammer or a press forge.

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Hammer Forging is shaping a metal by repeated blows between two dies. The upper die is fastened to a weighted ram that moves in the vertical direction while the lower die is fastened to a stationary anvil. The type of forging hammer that is most commonly used for our size of forging is the power drop hammer. In this type of hammer, compressed air or steam is used to raise the ram and upper die up and then supplements the force of gravity in the downward stroke. Press Forging involves shaping a metal by "squeezing" it between two dies. Unlike hammer forging, generally only one work-stroke is utilized for each set of dies. Presses may be mechanical or hydraulic. Hydraulic presses have by far the greatest rated capacity. Hammers and presses may utilize open or closed dies. A closed die is one in which the metal is shaped completely within the cavity of the dies as the dies come together. The cavity may be in either the upper or lower die or may be in both. Open dies may be either flat or have a cavity in them, but differ from closed dies in that there is little or no restraint on the lateral movement of the metal being forged. Closed dies produce forgings that require the least amount of subsequent machining (they get the metal as close as possible to the final, desired shape) in comparison to open dies. Closed dies can also produce more complex shapes, closer tolerances, and better control over grain flow than open dies do. Open dies are used for simple shapes and for forgings too large to be forged in closed dies. The big advantage of open dies over closed dies is cost. Closed dies cost considerably more to make and maintain because of the complex cavities in them, consequently an open die forging (even with more subsequent machining) may be cheaper if the quantity is small. In closed die forging, you always start out with more material than required to fill up the die cavity. Naturally there will be excess material left over after the dies are brought together. The excess material extends out from the body of the forging in a plate-like fashion along the parting line (the junction where the dies come together). This excess material is known as flash and serves several important functions. First, because it is relatively thin, flash cools more rapidly than the metal in the die cavity and thus restricts the flow of metal out of the cavity during forging. This helps to insure that the cavity will be completely filled with metal. Second, flash keeps the faces of the two dies from actually striking and damaging each other. Third, as the dies strike the part, pressure is generated within the metal that will cause it to flow. Because of friction with the surface of the dies and the fact that it cools rapidly, flash will flow only to a limited extent. This allows the pressure to build up within the part and eventually force the bulk of the metal to flow into all the nooks and crannies of the die. The flash is trimmed off after forging has been completed.

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There is a lot of jargon used by forge shops to describe certain operations. We will go over some of the more common terms that you may encounter. Blocking refers to the initial operation in a forging sequence in which the bulk of the starting material is roughly distributed to the required locations. Several sets of different blocking dies (or different blocking cavities within the same set of dies) may be required to produce a given part. Finishing will then move the metal into the desired configuration. Blocking dies save wear and tear on the more expensive finishing dies by minimizing the amount of metal that must be moved as well as the distance that the metal must be moved during finishing operations. Cogging is a term used in open die forging that refers to an operation in which the cross section of the starting ingot or billet material is reduced uniformly along its length. A billet is a solid, semifinished product with a uniform cross section (typically round or square) that is used as the starting material for other forging operations. Upsetting is a term used in open die forging that refers to an operation in which the cross section of the starting material is increased. The billet material is stood up on end on the lower die. As the upper die strikes the top, the middle of the billet will "barrel" out. A blacksmith forging is an open die, hammer forging of simple configuration made just large enough to machine (or otherwise produce) a desired part from. 2. Rolling - This is a forging process in which the cross section of the starting material is reduced in size and/or altered in shape as the material is passed between two or more rollers. The shape of the rollers and the size of the gap in between them will determine the final cross section of the product. The machine that performs this forging operation is called a rolling mill. The four basic configurations of rolling mills are shown in Figure 1. The production of a rolled product typically starts out with a cast ingot being processed on a primary mill. A primary mill is a heavy duty machine used to provide sufficient hot work to convert the cast, ingot structure into a wrought one and to produce a semi-finished product with the desired cross section. The semi-finished product is referred to as either a slab, bloom, or billet, depending on its cross section. A slab

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Figure 1: Rolling is always rectangular in cross section and is typically 2-9" thick by 2-5' wide. A bloom is square (or slightly oblong) in cross section and is typically 6-12" on a side. Billets are usually square in cross section and are typically 2-5" on a side. Most primary mills have a two high configuration, although some three high types are also used. Primary mills are sometimes classified by their semi-finished product (e.g. slabbing mill or blooming mill). A small rolling operation may have just one primary mill. The starting material will have to be passed multiple times through the rollers with each pass making only a slight reduction in order to obtain the necessary hot work. It is not practical to roll an ingot into the semifinished product in just one step because of the enormous power requirement it would take and because of the excessive wear on the mill. To produce slabs, the rollers will be moved closer together after each pass. Billets and blooms must be processed on multiple pass rollers in single mill operations (see Figure 2). After each pass, the material being rolled is moved to a different location along the rollers where the next reduction of area will take place. The disadvantages of having a single mill are readily apparent. In

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order to make multiple passes on a two high mill, the material being rolled must be brought back after each pass and fed from the same side (a cumbersome process) or the mill must be reversed (which wastes a great deal of energy). Three high mills allow the rolled material to be fed through opposite side by utilizing the bottom set of rollers for the first pass and then the top set for the next, but a mechanism for raising or lowering the material to the appropriate set of rollers is required. Large rolling operations will always have many mills with each mill dedicated to making a given reduction in a single pass. The material being rolled will go from mill to mill (sometimes being heated back up to rolling temperature in between) until the desired cross section is obtained. Mills may have contoured rollers so that different size products can be rolled using the same set of rollers. Product mills take the bloom, billet, or slab produced on the primary mills and work the material into the final, desired cross section. They are also used to straighten the finished product, impart any required cold work, and produce the required surface finish. The final rolled product may be plate, sheet, bar, structural shapes (I, T, H beams), etc. The sequence of rolling passes required to take a billet, slab, or bloom and make it into a final product can be divided into four operations: breakdown, roughing, intermediate, and finishing passes. A breakdown pass, as the name implies, makes a heavy reduction of the starting material and involves the bulk redistribution of material. Roughing passes make further bulk distributions of material into the areas where it is needed. Intermediate passes make much smaller reductions than breakdown or roughing passes and are used to shape the work piece into its final configuration. The finishing pass will take the work piece into its final form and impart the desired finish. The mills used in the manufacture of the final rolled product tend to take much smaller reductions of areas than do primary mills so they are not as heavy duty. Two high and three high mills are often used in the production of plate, bar, sheet, and structural parts. Four high and cluster mills are also used for plate and sheet. The smaller rollers in these mills are directly in contact with the work piece and provide a much better surface finish than large diameter rollers. The large rollers are used as backup for the small rollers to prevent them from deflecting. A universal mill is one that has two or more vertical rollers (in addition to horizontal ones) that are used to roll the edges of plate as it passes through the mill. This controls the width, gives a square edge, and provides a better surface finish. 3. Extrusion - Extrusion is a special forging process in which a metal billet, usually at an elevated temperature, is squeezed through a die in order to produce a uniform cross section of some desired shape. An page - 133

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extrusion may be hollow (by forcing the metal over a mandrel while it goes through the die) or it maybe solid. The two main types of extrusions are direct and reverse. In direct extrusion, the ram travels in the same direction as the extruded product while in reverse extrusion, the ram travels in a direction opposite to that of the product (see Figure 3). By using an appropriately shaped die, many different configurations may be extruded including round or square bar and structural bar (T, H, I-bar, etc.). Extrusion presses may be vertical (the extruded product moves in a vertical direction) or, more typically, horizontal (the extruded product moves in a horizontal direction). Extrusions are generally made in one operation. To prevent excessive wear, powdered glass or glass fabric is often used to line the die. The glass becomes molten in contact with the metal and acts as a lubricant. A glass "sock" is sometimes placed over the mandrel used in the production of hollow extrusions for the same reason.

Figure 3: Extrusion The amount of work that a metal undergoes during extrusion varies considerably from the surface to the center of the extrusion. The center, of course, may see relatively little hot work. To insure a wrought structure in the center, most extruding operations will use forged billet material as the starting stock: they do not rely solely on the extrusion process to impart the necessary hot work. Different materials may be co-extruded together. A stainless steel liner, for example, may be co-extruded inside a low alloy steel pipe for corrosion protection. We use extruded pipe and bar in many of our products. The bodies of our annular blow out preventers are made by reverse extrusion. 4. Ring Rolling - The next type of forging process we will talk about is ring rolling. Ring rolling is a process for making seamless rings out of page - 134

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doughnut shaped preforms. The preform may be made by forging a round bar and then punching a hole in it. The preform is placed between two rollers: one idler roller and one driven. The power driven roller causes the preform to rotate. At the same time, the idler roller is brought increasingly closer to the power driven roller, squeezing the preform in between them. This causes a reduction in the thickness of the ring and also an increase in the ring diameter. The rollers are sometimes shaped to produce a contoured ring of a desired cross sectional shape. Ring rolling is illustrated in Figure 4. Virtually any size ring can be made. Additional rollers may be utilized to help control tolerances.

Figure 4: Ring Rolling In single pass rolling, the height of the ring is controlled by the contour of the rolls. In two pass rolling, the axial rollers control the height. We use rolled rings for a variety of parts such as large gaskets, flanges, seat rings, etc. 5. Drawing - Drawing is a method of producing small diameter bar, wire, and tube by pulling the starting stock through a die which imparts the desired cross section. Drawing is generally done at room temperature so starting material is usually in the annealed condition. Depending on the total reduction required and the starting material, drawing operations may use just one die or a series of dies. Parts may be annealed in between passes as they become work hardened. Round, square, rectangular, as well as other cross sections may be drawn. The first stage in drawing is to get the bulk of the starting stock into roughly the desired shape. The starting material for wire is generally a rod or bar that has been rolled or forged into shape. Drawn tube may use rod, bar, or larger diameter tube as the starting material. The starting material must be descaled and thoroughly cleaned before drawing. The leading end or edge is often pointed (chamfered) to page - 135

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facilitate entry into the die. Pointing is usually done by swaging, rolling, turning or grinding. Figure 5A shows how flat bar can be drawn through undriven rollers. Figure 5B illustrates the drawing of rod, round bar, or wire. Rod and bar are usually drawn on single die machines that produce a straight product of limited length. Wire is usually drawn continuously on a multiple die machine and then coiled.

A

B Figure 5: Drawing Solids

There are several different drawing techniques that can be utilized to manufacture tube. If the starting stock is tube, smaller diameter tube can be drawn as shown in Figure 6A. This process is sometimes called sinking (note that no mandrel is used). Large diameter tube is generally made utilizing a fixed plug (see Figure 6B). The plug is initially pushed into the deformation zone where it will be pulled forward by the frictional forces of the deforming tube. A mandrel or plug bar keeps the plug from being pulled through the die. For long sections of small diameter tube, a plug bar may not have sufficient strength to hold the plug in place. A floating plug (see Figure 6C) can be used when this is the case.

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A

B

C

Figure 6: Drawing Hollows

FORGING DEFECTS Forging can be a very effective way of redistributing the metal in a billet into some desired shape, but things don't always go according to plan. In this section we'll look at a veritable rogue's gallery of defects that may arise as result of the forging process. 1. Laps - A lap is a surface defect where metal has folded over upon itself during forging, but was insufficiently worked so that is did not become welded to the bulk of the forging. Laps often occur at corners or other sharp transitions. They may first appear to be cracks because only the edge of the fold may be visible during inspection. Laps can generally be prevented by proper die design. 2. Surface Cracks - Surface cracks may come from many different sources during forging. Among these are poor die design, poor lubrication, insufficient starting stock, strain hardening, too heavy a reduction in cross section, and improper forging temperature. 3. Bursts - Bursts, or center cracks, occur in forgings and extrusions as a result of poor lubrication, too heavy a reduction in cross section, and poor die design. When metal is squeezed in a die, its cross section will be reduced and it will flow in a direction 90( to the applied force. If the frictional force between the work piece and the surface of the die is too high, the flow of metal at the interface will be constrained. The squeezing action of the dies will generate a hydrostatic pressure within the work piece that may lead to tensile stresses sufficiently large to cause the center of the forging to rupture. 4. Raised Parting Line - This ridge of metal around a closed die forging is the result of a poor trimming job of the flash.

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5. Underfill - This describes the condition where metal has failed to completely fill a die during forging. This may be due to insufficient starting stock, improper forging temperature, or poor die design. 6. Mismatch - Mismatch is a gross dimensional error resulting from the misalignment of the upper and lower dies during closed die forging.

ADVANTAGES AND LIMITATIONS OF FORGING A properly forged part will have a wrought structure. A wrought structure means that the part has been sufficiently hot worked so that the starting ingot's cast structure has been completely broken up. The high temperature and pressure during forging will cause any cracks, porosity, shrinkage, gas voids, etc. that may be present in the starting material to be compressed and welded shut. Alloys solidify over a range of temperatures. There is a difference in the composition of the metal that first freezes compared to the metal that freezes last. This variation in chemical composition is known as segregation. The forging process can help to homogenize the material by breaking up areas of segregation. The high temperature at which parts are forged facilitates the diffusion of atoms of a given element from areas of high concentration to those of low so that a more uniform distribution is obtained. The large, dendritic grains that so often characterize a cast structure are eliminated as the material is hot worked and recrystallized. A wrought structure is thus substantially free of the discontinuities, chemical segregation, and large grain size that are frequently encountered in castings. The more homogeneous nature of a forging optimizes mechanical properties (toughness and ductility in particular) and may also improve corrosion resistance. Forging may be used to cold work a material that cannot otherwise be strengthened. There are some limitations to this because it is not always possible to uniformly cold work thick parts. Naturally the center of a thick part will see less work than the outer portions. This may result in a significant variation of strength throughout the cross section of a large part. Forgings are often cleaner than many types of castings. In many casting processes there is always a possibility that the molten metal may break off particles of mold material as it is poured into the mold and thus create non-metallic inclusions. These inclusions can decrease ductility and toughness. Metal flows during a forging process. The metal grains will tend to align themselves parallel to the direction of greatest metal-flow, the longitudinal direction. A transverse direction is 90( to the direction of the greatest metal-flow. Mechanical properties, toughness in particular, often vary by direction in a wrought material. A Charpy impact specimen removed from a forging such that its longitudinal axis is parallel to the direction of greatest metal-flow (the longitudinal orientation) will have a significantly higher toughness value than a specimen taken perpendicular to the direction of greatest metal flow (the transverse orientation). Forgings can often be made such that the grain flow provides the optimum properties in the most highly stressed directions in service. The longitudinal direction for a plate is the primary rolling direction. page - 138

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For an extrusion, the longitudinal direction is parallel to the centerline of the part. There is often no single, longitudinal direction in a closed die forging: grain flow will generally follow a direction parallel to the surface of the die. How much hot work is necessary to convert a cast structure into a wrought one? We obviously don't want to hot work a metal any more than we have to because of increased costs due to die and equipment wear, furnace time, etc. There is no one answer to this question. A lot will depend on the size and condition of the starting material as well as the particular forging process itself. Many forge shops use a rule of thumb that calls for a reduction of area of 4 to 1 or sometimes 3 to 1. In other words, if the starting ingot's cross sectional area has been reduced by a factor of 4 (or whatever factor is selected) during forging, a wrought structure is assured. A 24" diameter ingot, for example, must be reduced to 12" diameter product. The reduction of area that we require in our specifications often varies by starting material. A VAR ingot, for example, requires much less hot work than a air melt ingot because it is already essentially free from porosity, shrinkage, blow holes, etc. A particular forging process, such as closed die forging, may not always provide sufficient reduction to meet a 4 to 1 criteria. In these cases, the starting billet material must already be in the wrought condition. The conversion of cast to wrought structure takes place during primary forging: the closed die forging process is utilized primarily to obtain the desired shape. There are, of course, some limitations to forging. For small numbers of parts, it may be uneconomical because of the high cost of dies. Maintenance of the dies can be expensive. Available press or hammer forge sizes or roll sizes can limit the size of product that can be made on a given piece of equipment. Extremely large parts may not be capable of being forged because they exceed the power capacity of any forging equipment. It is extremely difficult and many times impossible to forge internal cavities, yet cavities are easily made in castings. Finally there are many alloys that cannot be forged because of their brittle nature or their propensity to work harden rapidly. Virtually all metals can be cast.

CASTING Casting is a process for making a part by pouring molten metal into a mold having a cavity of the desired shape and then allowing it to solidify. The cavity will be slightly larger than the finished part because the metal contracts as it solidifies and cools. Casting processes are generally classified by the types of molds that are utilized. We'll examine four different casting processes used in making our products. 1. Sand Casting - In sand casting, sand mixed with water and suitable binders is the molding material. The molding material is packed around a pattern, a form (often made of wood) having the desired shape and slightly larger than the finished part. When the pattern is removed from the molding material, a cavity having the same shape as the pattern remains. Green sand molds are the most common type. "Green" means that the sand mixture is used damp: it is not dried in an oven page - 139

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before molten metal is poured in. Let's look at how a typical green sand mold is made. We'll make a cast connecting rod as an example (see Figure 7). Of course, we are not in the connecting rod business, but it is a whole lot easier for me to draw a connecting rod rather than a gate valve body.

Figure 7: Connecting Rod The first step is to make a pattern corresponding to the shape of the casting we want. Due allowances must be made for the fact that molten metal shrinks as it solidifies, consequently our pattern will be slightly larger than the finished part. Because we plan on making a lot of these connecting rods, we are going to make a cope and drag pattern. This is a split pattern with each part mounted separately on plates. We are going to make our sand mold in two parts: a top half and a bottom half. These will be connected together when we pour the metal. The top half of the mold is called the cope and the bottom half is the drag. We need a hole in our mold in which to pour the molten metal and we are going to need some channels to direct the molten metal into the mold cavity. We also want to have a reservoir of molten metal next to the cavity so that when the molten metal in the cavity solidifies and consequently shrinks, any voids will be immediately filled with more molten metal. The hole that we pour the molten metal in is called the pouring basin. The molten metal is directed down to the channels or runners by the sprue. The runners will channel the liquid metal into the reservoirs (or risers). The place where the molten metal enters the mold cavity is called the gate. We will incorporate the runners, risers, and gating system into our pattern. We will worry about the sprue and pouring basin later. We will start making our green sand mold by taking a metal flask (basically a retaining wall) and putting it over the drag portion of our pattern. Molding sand is then rammed around the pattern in a series of page - 140

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steps designed to insure uniform density. When the flask is filled we will invert it and remove the pattern, leaving a replica of its shape in the sand. We will repeat the process for the cope half of the mold. The flasks are positioned over the cope and drag pattern plates by locating pins. This is necessary in order to insure that the cavities in the cope and the drag match up when the two halves are brought together. After removing the cope pattern, we are going to poke a hole all the way through the cope half of the mold where we want the sprue to be. Then we will invert it and cut out an area for the pouring basin. Our connecting rod has a small hole in each end. It would be very difficult to use a pattern having corresponding holes in it and trying to compact the molding sand in the holes so that is had sufficient strength. When we pull the pattern out of the flask we would probably damage the compacted sand in the holes anyway. Instead, we won't put the holes in our pattern. To get the holes in our casting, we will use cylindrical cores. A core is a sand (or some other material) preform that is inserted into the mold wherever you want an internal surface. Some cores may required the use of chaplets, metal supports, to hold them in place. Chaplets, if used, will fuse in contact with the molten metal and become part of the finished casting. After inserting the two cylindrical cores into the drag where we want the holes to be, we are ready to put the cope and the drag together with the aid of alignment pins. Our mold is now complete and should look something like Figure 8. Metal of the desired composition is heated well above its melting point in order to insure good fluidity. If this is not done, the molten metal may turn "slushy" when it is poured into the cold mold and consequently be unable to completely fill it before it solidifies. Many casting grades of steels will have a higher specified amount of silicon than the corresponding wrought grades because silicon also helps to increase the fluidity of the molten metal. The molten metal is poured into the pouring basin, runs down the sprue, and is channeled into the riser. Runners distribute the molten metal to the gating system where it is then fed into the mold cavity. After the metal has solidified, the finished casting can be shaken or knocked out of the mold (which can be used only once). There will be excess metal attached to the cast part where the molten metal froze in the riser, runners, and sprue, etc., but this is easily knocked off while the casting is still hot or may be cut off. Cores will be broken out. Sand castings represent the greatest tonnage of castings. Virtually any size casting can be produced. Extremely large sand castings have been made by fabricating a sand mold in an excavation in the ground. Large sand castings are most often made using a dry sand mold. Dry sand molds are made similar to green sand molds except a refractory page - 141

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paste is used to coat the surface of the mold cavity and then the mold is allowed to dry. The refractory paste strengthens the surface of the mold and prevents mold material from breaking off. The advantages of sand castings in comparison to other casting processes include low cost, high flexibility, and the fact that it is a simple, expedient way to make a wide size range of castings. It is generally not economical for very small castings and may not be suitable for castings having long, thin sections. Other disadvantages are that the tolerances are not as tight as some of the other processes and the possibility of grains of sand being torn off the mold and becoming inclusions in the cast metal. We use sand molding for some gate valve bodies, handwheels, firesafe clamps, etc.

Figure 8: Green Sand Mold 2. Investment Casting - Investment casting is also known as the "lost wax" process or as "precision casting." The term investment means an outer layer of covering, in this case a refractory mold, surrounding a refractory-covered wax pattern. The detailed steps of the process are as follows: A. A metal die is made for casting the wax patterns in. Dies are blocks of metal that have cavities machined into them that correspond to the shape of the desired part. B. Melted wax is injected into the die under pressure and allowed to solidify. C. The wax pattern is removed from the die and then dipped into a slurry of refractory coating material. After dipping, the refractory coated wax pattern is sprinkled with fine silica sand and the assembly allowed to dry. D. The assembly is then invested in the mold. This consists of inverting the assembly onto a flat surface, placing a paper lined metal tube over it, and filling up the tube with the molding mixture. page - 142

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E. The mold is allowed to air set until it is sufficiently hardened. The wax is removed by heating the mold to 200-300(F while it is in an inverted position. The wax melts and runs out of the mold leaving a cavity having the same shape as the desired part. The mold is then further heated until it reaches a temperature compatible with the pouring temperature of the particular metal being cast. This serves to insure that any wax trapped in the mold is burnt out (if it remained, the wax would cause gas pockets) and slows down the cooling rate of the molten metal so that it stays molten longer and consequently can fill all the nooks and crannies of the mold before it solidifies. F. The molten metal is poured in and allowed to solidify. G. The cast part is removed from the mold and any flashing cleaned off. Investment casting has many advantages. Extremely fine detail can be reproduced and close dimensional tolerances can be held. This means that many investment castings often require little or no machining. Surface finishes are superior to many other casting or forging processes. There is no parting line as with sand castings. Investment casting is readily adapted for use in a vacuum or in an inert gas atmosphere (some molten metals cannot be exposed to oxygen, hydrogen, etc. found in air because of potential embrittlement problems). The main limitation of the process is that the size and weight of castings is restricted by both physical and economic constraints. Most investment castings are under 10 pounds although castings as large as several hundred pounds have been made. We have used investment castings for gate valve seat rings as well as for wellhead seals and other small parts. 3. Centrifugal Casting - A centrifugal casting is made by pouring molten metal into a horizontal, cylindrically shaped carbon, metal, or refractory mold that is rotating about its own axis. The spinning action of the mold creates a centrifugal force on the molten metal that throws it up against the mold surface. Porosity and slag inclusions migrate towards the center of the mold as they are displaced by the heavier metal. The metal against the side of the wall will solidify first and freezing proceeds towards the center. The hollow shaped casting must be bored out to remove the porosity, inclusions, etc. that have accumulated near the casting's ID. Centrifugal castings offer several significant advantages over static castings. They are generally cleaner because most inclusions are removed in the boring operation.

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Shrinkage voids are virtually eliminated because of the pressure that the molten metal exerts against the solidifying metal as solidification progresses inwardly from the ID surface of the mold. Centrifugal castings have a finer, more randomly orientated, and more uniform grain structure than static castings. This results in better and more homogenous properties. We currently use centrifugal castings in the manufacture of some gate valve and ball valve seats. 4. Shell Casting - Shell casting makes use of a mold that is made by compacting a thin layer of a thermosetting resin/sand mixture onto a heated metal pattern. Depending on the type of resin utilized, the pattern may be heated to 300-600(F. The resin will set when the mold mixture is brought into contact with the hot pattern thus binding the sand particles together. The relatively thin mold (the "shell") is removed from the pattern and placed upside down in a flask. Sand (or some other material) is compacted onto the back of the shell up to the top of the flask. The sand acts as a backing material that supports the thin, somewhat fragile shell. A cope and drag type mold is typically made. Shell casting has several advantages over sand casting. The thermosetting resin binds the sand particles together with a much stronger bond than the binders used in sand casting. Thinner and more intricate shapes then can be cast without worry that the flowing molten metal will dislodge mold material. Closer tolerances can be maintained in shell casting than in sand casting. Shell molds can be rapidly produced on automated molding machines.

CASTING DEFECTS Castings are liable to contain more than their fair share of defects. Having cut my technical teeth with Cameron Iron Works, a company that took great pride in having allwrought products, I am naturally somewhat prejudiced against castings. Every now and then we would stray from the path and specify a casting as a "cost saving" measure. Much of my antipathy towards castings stems from being the presiding medical examiner during the autopsy of numerous cast bodies. Most had suffered multiple and varied wounds, any one of which could have been fatal. The majority of these wounds were self-inflicted: we were experts at ordering forgings, but babes in the woods when it came to castings. Ignorance is not always bliss. We paid dearly for ours when we replaced many defective castings in the field. If you correctly specify a casting, odds are that the following "wounds" will not contribute to the demise of your part.

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1. Porosity and Blowholes - These voids are the result of entrapped gas in the solidified metal. Porosity consists of small bubbles of gas that have been frozen in place. Blowholes are larger pockets of gas that get trapped in the solidifying metal. There are many potential sources of gas including volatile matter in the mold material, absorbed gases in the molten metal as it is poured, etc. Molten metal that is contacted with the cool mold surface will form a "skin" as it solidifies. Any absorbed or entrained gas in the underlying molten metal will thus become entrapped. 2. Shrinkage - Metal contracts as it solidifies and cools. The volume occupied by a given weight of solid metal at room temperature is thus less than the same weight in the molten state. When we pour molten metal into a mold and completely fill the cavity, the metal that comes into contact with the surface of the mold will be the first to freeze. This establishes an envelope that defines the overall volume of our casting. Solidification then proceeds towards the center of the casting. Because the volume of the solidifying metal is less than the molten metal, a void may develop in the center of the cast section unless we are able to continuously add more molten to make up the difference in volume. This void is known as shrinkage. Shrinkage tends to be more of a problem in heavy cross sections. It can be minimized or prevented by proper mold design, by using a process such as centrifugal casting that closes up voids by applying pressure to the solidifying metal, and by following good pouring practice. 3. Hot Tears - As the solidifying metal shrinks as it cools, tensile stresses may appear as the result of the metal being contained at sharp radii or transition areas. The hot metal has relatively little strength so the tensile stresses may be sufficiently high to cause the constrained metal to tear. 4. Cold Shuts - Large castings may require two or more sprues with separate gating and risering systems in order to adequately fill the mold cavity. Obviously the different streams of molten metal from the sprues must converge upon each other somewhere within the cavity. If the leading edges of the streams have cooled to the point where they are covered by a "skin" of solidified metal, then there is a possibility that when these leading edges meet they will be too cool to metallurgically bond to each other. If this happens, the unbonded juncture is referred to as a cold shut. Cold shuts can be prevented by heating the mold to slow down the cooling rate, using a higher pouring temperature, or by improving the gating and risering systems in the mold.

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5. Inclusions - Inclusions are non-metallic, foreign particles that were entrapped in the solidifying metal. They may originate from several different sources. Mold material that breaks off while the molten metal is being poured in a common source. Core material that breaks off is another. Of course some inclusions stem not from the casting process, but from the starting material. 6. Underfill - This describes the condition where molten metal did not completely fill the mold. If the leading edge of the molten stream of metal solidifies before reaching the extremities of the mold cavity, the solidified metal will act as a plug that keeps any additional molten metal from getting by and completely filling the cavity. Underfill may result from several different causes including too low a pouring temperature, poor gating and risering design, and trying to cast too intricate a part for the particular casting process. Parts with long, thin sections would naturally be prone to underfill. In some casting processes air may become trapped in the cavity and prevent molten metal from filling it. Underfill can be prevented by raising the pouring temperature, heating the mold to reduce the heat transfer rate through the walls, and by good mold design. 7. Mismatch - Mismatch is the dimensional error that occurs when the cope and drag halves of a mold are not properly aligned. The top half of the resulting casting will thus be shifted some distance from the bottom. 8. Large grain size - Castings often are characterized by a large grain size just below the surface. This is particularly true for heavy wall castings. The molten metal in contact with the surface of the mold cools more rapidly than the metal in the center of the cavity. Many nucleation sites for grains will appear at the surface because of the faster cooling rate and will result in a relatively fine grain size. Just below the surface, however, the cooling rate is much lower resulting in very little grain nucleation. Instead, the grains that formed just below the surface will start to grow as the metal solidifies. This growth is primarily towards the center of the cavity because lateral growth is restricted by the presence of other grains and because of the heat flow. The final cast structure will have large, columnar grains extending to the center of the section. Large grain sizes can often be prevented through the use of grain refiners, controlled cooling rates, or by special casting techniques.

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The obvious advantage of casting is that it is a relatively inexpensive way to make a part that is close to finished dimensions. A pattern and mold can be designed and manufactured faster and less expensively than a forging die. Thin, intricate parts are generally difficult to forge, but can often be readily cast. Internal cavities and holes are easily cast into a part through the use of cores, but are very difficult or impossible to make in forging. The net result of all this is that a finished part made from a casting is often cheaper than one made from a forging because start-up costs are less, less machining is required , and less material is lost as "chips" during machining. When identically shaped parts can be produced by either forging or casting, it is almost always cheaper to go with a casting for small production runs. There are many materials that are either extremely difficult or impossible to forge because they work harden too rapidly or because of their brittle nature. Virtually all metals can be easily cast. The size of a casting is limited only by the size of the mold that can be constructed. Forgings, on the other hand, are much more restricted in size due to the enormous power requirements of the equipment. There is no expensive die maintenance associated with casting. Patterns are easily modified to accommodate changes in design, while altering forging dies is a major and expensive undertaking. The primary disadvantage of casting is that it is difficult or impossible to totally prevent the formation of the defects that we have previously discussed. Extensive nondestructive testing may be required to insure the integrity of a casting.

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POWDER METALLURGY Powder metallurgy is a metal fabricating process in which metal powder is compacted into some desired shape and sintered together. Sintering (in powder metallurgy) refers to the metallurgical bonding that takes place between individual powder particles as a result of pressure and elevated temperature (the sintering temperature is well below the melting temperature of the alloy). Conventional powder metallurgy processes usually involve four steps: powder production, powder blending, compaction, and sintering. 1. Powder Production - Metal powders may be made by many different methods. Atomization of molten metal is probably the most common. Metal of the desired composition is melted in a crucible (often in a vacuum induction furnace) that sits on top of the cooling tower. The molten metal is fed into the cooling tower through a nozzle at the top. Molten metal is atomized into spherical globules as it passes through the nozzle. These globules will solidify as they fall to the bottom. Many alloys require that the melting and atomization be done in a vacuum or an inert gas atmosphere to prevent an oxide layer from forming on the powder particles. A counterflow of inert gas is sometimes employed within the cooling tower to help break up the stream of molten metal coming out of the nozzle and to help control the cooling rate. The solidified powder is collected on the bottom of the tower. Other ways of manufacturing powder include the mechanical processing (such as crushing, grinding, etc.), chemical precipitation or decomposition, and electrolytic deposition. Depending on the material and the powder production technique, powder particles may be spherical, acicular (shaped like a needle), in the shape of flakes, angular, or irregular in shape. The morphology (size and shape) of the powder plays an important role in how well the particles compact and what the ultimate density of the part will be. After the powder has been produced, it will be screened into the desired size ranges. 2. Blending - The next step in the manufacture of a powder metallurgy (P/M) part is to blend the powder with any necessary die lubricants, alloying additions, etc. Heats of powder are generally rather small and, as a consequence, several heats may be necessary to fill a large order. To insure uniformity of the product, it is often desirable to manufacture all the product from the same powder material. This can be accomplished by taking as many different heats of powder as required by the size of the order and mixing them into a homogeneous blend. Each heat will be divided up and distributed equally into different lots and then mixed. The lot size is dependent on the amount page - 148

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of powder that can be efficiently blended together at one time. Each lot, or sub-blend as it is often called, thus has the same proportion of each heat of powder. Together the sub-blends constitute the master blend. Traceability and qualification testing (mechanical properties) are on a master blend basis. 3. Compaction - The powder can be compacted in a die or by extrusion, rolling, or numerous specialized methods. Compaction serves three basic purposes: A. It gets the powder into the desired shape and roughly the desired size. B. It increases density. C. It imparts enough strength to the work piece for subsequent handling. As an example, we'll examine compaction in a closed die. There is a substantial amount of empty space in the volume occupied by the powder because the apparent density of the powder is only 20-40% of the theoretical density of the metal. This, of course, means that we will have to add significantly more powder to the die than what is required to fill the actual die cavity. A punch (either mechanically or hydraulically driven) is used to compress the powder into the die. The end of the punch may be shaped. The powder particles are squashed together and become mechanically interlocked. The density of the powder increases as a result of the compaction to typically 75-90% of the theoretical density. The work piece, or green compact as it is now referred to, is ejected from the die. It will undergo a slight increase in volume because of the elastic recovery of the powder particles. It is held together strictly by the interlocking of the powder particles. This gives the green compact sufficient strength to be handled, but drop it and in most cases it will crack or shatter. There are many variations in how powder is compacted. Vibratory compaction utilizes a compacting press that has a mechanism for vibrating the die while powder is being compacted. This jostles the particles around producing more efficient packing thus increasing the apparent density. Isostatic pressing uses a flexible rubber or sheet metal mold instead of a die. The mold is evacuated, filled with powder, and then sealed. It is placed in a chamber which is then pressurized with a fluid (such as water, oil, or gas). The pressurized fluid causes the mold to squeeze the powder particles together thus compacting them. Because the work piece is subject to the same pressure over its entire surface area, isostatically compacted parts are much more page - 149

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uniform than mechanically compacted parts. Complex shapes and parts with high length-to-diameter ratios are often more easily compacted by isostatic pressing than by other methods. 4. Sintering - This is the step where a true metallurgical bond is formed in between the powder particles. The green compact is heated up to an elevated temperature (but well below the melting point) where the combination of internal pressure and heat causes bonding to take place. Note that there is no melting or fusing of the powder particles. Sintering is a complex process. There are many processes besides bonding that occur during sintering. These included densification, shrinkage, changes in the size and shape of pores, and even alloying where different metal powders have been blended together. The final density and strength of the sintered part are dependent on many factors including the density of the green compact, sintering time and temperature, and the composition of the powder. The density of the sintered part is typically 5-20% above that of the green compact. All conventional P/M parts will contain some pores. Although the volume of pores is reduced during sintering, it is impractical to completely eliminate them because of the excessive time and temperature it would take.

ADVANTAGES AND DISADVANTAGES OF CONVENTIONAL POWDER METALLURGY Powder metallurgy has many advantages over other metal fabricating processes. It can be used to produce complex parts such as gears that required little, if any, finish machining. Many refractory metals are impossible to forge and very difficult to cast yet are easily processed into a desired shape through powder metallurgy techniques. Virtually all tungsten carbide cutting tools, for example, are made using P/M techniques. The tungsten carbide seats and needle tips in our chokes are made using conventional powder metallurgy processes in which tungsten carbide particles are mixed with a cobalt alloy powder, compacted into the desired shape, and then sintered. The cobalt acts as a binder that holds the finished part together. A P/M part has no grain flow or cast microstructure consequently the properties are isotropic (the same in all directions). There is virtually no chemical segregation because the molten metal droplets are rapidly cooled after atomization during powder production and the subsequent sintering of the compacted powder takes place below its melting point. P/M parts can be intentionally processed to obtain interconnecting porosity by controlling the green density and sintering time and temperature. Fuel filters can be page - 150

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made this way. Permanently lubricated bearings are P/M parts that have oil (or some other lubricant) forced into the channels (interconnecting porosity) under pressure. Powder metallurgy preforms are often used for forging stock of difficult to forge materials. Powder metallurgy is used to distribute the bulk of the metal into the desired shape while the actual forging operation provides the finished dimensions and further densification. The disadvantages of conventional powder metallurgy include the cost of dies, parts must be shaped so that they can be ejected from the dies, some porosity will always be present, and mechanical properties are dependent on the amount of densification and sintering time and temperature. Although properties are isotropic; they may not be uniform throughout the cross section of a part. The powder near the surface of a large part will see the greatest pressure during compaction while the powder near the center will see the least. The sintered density of the surface material will thus be higher than that of the center. There is, of course, a limitation to the size of a part that can be produced through conventional powder metallurgy methods because of the available sizes of compaction presses.

HOT ISOSTATIC PRESSING (HIP’ing) Hot isostatic pressing is a relatively recent powder metallurgy process in which powder is simultaneously compacted and sintered inside an autoclave. We use HIP’ing as both a forming process and a means of cladding wellhead equipment. Let's look at each of these applications.

HIP’ing AS A FORMING PROCESS In order to produce billets, hollows, or shaped parts using hot isostatic pressing, we must first make a compaction container or "can" in which to process the powder. The can is generally made from a mild steel or stainless steel and may utilize a stamping, tubing, or spun sheet metal as the starting material depending on the configuration of the finished product. Starting material will be fabricated by welding into a gas tight container that is totally enclosed except for a small fill tube that is welded on and allows access to the inside of the finished can. The enclosed volume will have essentially the same shape as the desired part, but will be larger because of the shrinkage that occurs during the HIP’ing process. The purpose of the can is three-fold: 1. It protects the powder from the contamination during processing. 2. It acts as a "die" during the HIP’ing cycle that applies pressure to the powder within it. 3. It imparts the desired shape to the product.

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After fabrication, the can will be leak tested with helium to insure that it is gas tight and then thoroughly cleaned and degreased. If it is going to be stored for awhile before being used, it may be filled with dry nitrogen through the fill tube and the fill tube sealed with a cap. It is now ready to be filled with powder. The can will be attached to a vacuum pump via the fill tube and evacuated. The appropriate powder blend may be degassed by flowing it through a high vacuum. It is introduced into the can though the fill tube. As it is being filled, the can is vibrated. This jostles the particles around and insures the densest possible compaction. The "tap density" of the powder in the can is typically around 70% of the theoretical density of the metal from which the powder was made. Once the can is filled (including a portion of the fill tube), the fill tube will be closed off and the can detached from the vacuum/ loading equipment. There are many variations on how degassing is actually done. Some facilities degas with the can already filled with powder. The can may be heated during degassing to help drive off gases. Regardless of the technique actually utilized, the end result is a sealed can filled with a powder with a minimum amount of entrapped gas. The loaded can is now ready for HIP’ing. The HIP’ing cycle is done in a large autoclave. The filled cans are loaded into a detachable furnace and fixtured, if necessary, to prevent distortion. The furnace is then lowered into the autoclave vessel and the vessel sealed. The autoclave is evacuated and back-filled with argon. The temperature is raised up to the HIP’ing temperature. HIP’ing pressures may go up to 15,000 psi and temperature up to 2000 (F for Oil Patch materials. Total cycle time from heat up to cool down is typically twenty-four hours. Of course there are many variations in the HIP cycles used by different vendors. The combination of high temperature and pressure causes the can to collapse against the powder further compacting it. Individual powder particles diffusion bond to each other as well as to the can. Densification occurs as a result of the plastic deformation of the particles as well as through various mass transport mechanisms that will close up pores. The resulting HIP’ed material will have 100% of the theoretical density of the metal. After the HIP cycle is completed, the fill tube (which also contains HIP’ed powder) is cut off and evaluated. Typical quality control checks include an evaluation of grain size, inclusions, prior particle boundaries (PPB's), and thermally induced porosity (TIP). In a properly HIP’ed part, individual particles should no longer be discernable. If they are, it is an indication that some reactive contaminant found its way into the powder mixture. The presence of PPB's in a HIP’ed part can dramatically influence mechanical properties, notch toughness in particular. The TIP test consists of reheating a small section of the fill tube to a temperature approximately 100-200(F over the highest temperature the part will see in any subsequent heat treatment, cooling it, and then evaluating it for porosity. The presence of porosity would indicate that the powder was not properly degassed or that there was a leak in the can that allowed argon to enter during HIP’ing.

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Assuming that the quality checks are acceptable, the parts are ready for further processing. Can material can be removed by machining or acid etching. Parts may be heat treated to develop the required properties.

HIP’ing AS A CLADDING PROCESS HIP’ing as a means of applying a corrosion resistant cladding to low alloy steel wellhead equipment was developed by Cameron Iron Works in the early 1980's. The many advantages of HIP’ed cladding over weld cladding quickly became apparent as "CAM-CLAD®" (as Cameron referred to its HIP cladding process) wellhead equipment grew rapidly into Cameron's premier product line. The principles of cladding by HIP’ing are the same as forming by HIP’ing. The major difference, of course, is that instead of compacting the powder within a container that will later be removed, the HIP clad process will compact the powder directly onto the surface of the piece of equipment to be cladded. Let's look at this in detail for a gate valve body made out of 2¼CR-1Mo low alloy steel that we want to clad with nickel alloy 625. 1. The first step is to machine just those areas of the gate valve body forging that are to be cladded. Of course, the bore, seat pockets, cavity, etc., will be machined oversized to allow for the thickness of the cladding. Cans are fabricated out of sheet metal and tubing. 2. These cans are closed off on the ends that are inside the body, but are open on the other ends. A sheet metal collar is welded to each can near the open end. After cleaning and degreasing, the cans are inserted into the bore at both ends of the body and in the gate cavity. The collar extends from the can across the bonnet or flange face to just beyond the ring gasket groove where it is seal welded to the valve body. A fill tube is inserted into one of the collars and is welded in place. 3. The body with the welded can assembly is helium leak tested to insure the welds are gas tight. 4. The space in between the can assembly and the body is evacuated by connecting the fill tube to a vacuum pump and then filled with powder. The fill tube (which is partially filled with powder) is then crimped closed. As previously discussed for forming, there are many variations on how degassing is accomplished. 5. The powder filled assembly is loaded into an autoclave and undergoes a HIP’ing cycle identical to the one discussed for forming. The high temperature and pressure cause the can to collapse against the powder thus compacting the powder against the body (see Figure 9). Diffusion bonding takes place between individual powder particles as well as between the powder and the can and body materials. Again, page - 153

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densification of the 625 powder will occur because of plastic deformation as well as other processes and result in a clad layer that is 100% of the theoretical density.

Figure 9: HIP Cladding 6. After the HIP’ing cycle is completed, the fill tube is cut off and quality control checks are performed to evaluate grain size, inclusions, PPB's and thermally induced porosity. If acceptable, the body is released for further processing. 7. The clad body will undergo a standard austenitize, water quench, and temper heat treatment to develop the required mechanical properties in the 2¼Cr-1Mo low alloy steel. The 625 will be solution annealed and aged by this heat treatment. 8. The can material will be removed during final machining (Figure 9). 9. Cladding thickness and the bond at the 625/2¼CR-1Mo interface can be checked ultrasonically. Dye penetrant testing can be used to check the interface regions on the flange and bonnet faces.

ADVANTAGES OF FORMING BY HIP’ing HIP’ed parts obviously go through some fairly complex and expensive processing. Why does HIP’ing make sense for certain parts? For a variety of reasons some inherent to the HIP’ed structure and some that are part or alloy specific. Let's first examine some of the benefits of a HIP’ed structure.

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The HIP’ed structure is a fully wrought one. A properly HIP’ed part will be completely free of the porosity, chemical segregation, dendritic structure, and shrinkage voids that are so often characteristic of a cast structure. Because it is an isostatic process, compaction is uniform. Properties are isotropic because of the chemical homogeneity, uniform equiaxed grains, and the lack of direction in working. There is relatively little metal movement in HIP’ing - there is no grain flow as in conventional forging processes that could lead to anisotropic properties. HIP’ing can produce a microstructure with an extremely fine grain size (ASTM 12-14). The great care expended in preventing contamination of the powder results in an extraordinary clean microstructure. While the advantages of the HIP’ed structure are significant, they alone would not justify the expense of a HIP’ed product except for the most critical of applications. It is only when the advantages gained for a specific part or alloy are also taken into account, that HIP’ing really becomes a serious contender as a forming process for wellhead equipment. Some of these advantages are as follows: 1. Some materials cannot be conventionally forged. Cemented tungsten carbides are a good example of this. They are used in chokes and in other wellhead equipment in highly abrasive applications. They are typically fabricated using a powder metallurgy process. Only HIP’ing can form these into a desired shape and produce a microstructure totally free of porosity. 2. For a small run of parts, it may be significantly cheaper to HIP a desired shape rather than machine bar (with a lot of expensive material lost as chips) or to purchase closed die forging. 3. The availability of many conventionally wrought corrosion resistant alloys is limited. Suppose, for example, you need to make a single gate valve bonnet out of one of these alloys. A closed die forging would be out of the question. Bar stock with the required diameter would probably not be an off the shelf item. At best, you may find the right size bar, but would probably have to purchase the entire length. At worst, you may have to special order the bar stock and be subject to some minimum mill run. The bonnet can be readily HIP’ed without the necessity of purchasing unwanted material. Size is not a problem because the HIP’ing container is easily spun to the correct dimensions. 4. While HIP’ing cannot provide as intricate a shape as closed die forging, it can be used for near net shapes and is significantly better than conventional forging in its ability to form internal cavities in a part. 5. Some materials exhibit far superior properties in the HIP’ed condition as opposed to wrought or cast. Alloy 625, (UNS N06625), for example, is a widely used corrosion resistant alloy in the Oil Patch. Its outstanding corrosion resistance make it a natural choice for wells with high concentrations of H2S, CO2, and brine. Its main limitation as a page - 155

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wrought product is the relatively low strength that can be developed particularly in thick sections. It is difficult to consistently obtain 75 ksi yield strength (after an aging heat treatment) in anything over roughly two inches thick. HIP’ed 625, on the other hand, is frequently used for hangers at yield strengths of 105 ksi in the same heat treated condition. Why the HIP’ed 625 responds to the aging process so much better than the wrought is not clearly understood. The small grain size, cleanliness, and chemical homogeneity certainly play a role. Whatever the reason, HIP’ed 625 has found a great many more applications in wellhead equipment than the wrought.

ADVANTAGES OF CLADDING BY HIP’ing A cladding made by HIP’ing has many of the same advantages we discussed for forming. It is a wrought structure free from porosity, shrinkage, etc. Properties are isotropic and uniform throughout the cross section. Because the entire HIP’ing process takes place below the melting temperatures of the clad material and the substrate, it is not subject to the chemical segregation that occurs when a molten metal solidifies. The bond between the cladding and the substrate is metallurgical and very uniform. There is no dilution of the cladding by the base metal as may occur in weld overlays, although some diffusion of elements across the interface will occur. HIP cladding will have much less segregation than weld cladding. HIP cladding thus optimizes the mechanical properties and corrosion resistance of the cladding material. As a process, HIP cladding is considerably easier to control than weld overlaying. There are often many areas in wellhead equipment that may be exposed to well fluid, but are configured such that weld cladding is extremely difficult. Grease fitting ports are an example of this. The long, small diameter of the hole makes it hard to insure good tie-in with a high quality weld, but such holes are easily cladded by HIP’ing. Besides cladding with corrosion resistant alloys, HIP’ing is also being used to hardface gates. Some hardfacing materials, such as T-800®, are extremely prone to cracking when applied as a weld overlay, but can be readily HIP’ed onto a substrate.

LIMITATIONS OF HIP’ing HIP’ing is a complex and, consequently, an expensive process. While it may be economical to produce a HIP’ed 625 hanger, you certainly would not ever consider making a 4130 gate valve bonnet using HIP’ed material. HIP’ing as a forming process is limited to relatively simple shapes. While internal cavities can be made, they too must have a simple configuration (such as a throughbore). HIP cladding is a very complex and expensive process. Because a bi-metallic is formed, a great deal of development work is necessary in order to insure the materials are compatible in terms of strength level, heat treatment, thermal expansion, bond strength, etc. HIP cladding is a one time page - 156

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deal: screw up part of the cladding during finish machining and the only way it can be repaired is by welding. You can not reHIP a heat treated, semi-finished part.

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NOTES:

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NOTES:

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NOTES:

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NONDESTRUCTIVE EXAMINATION Looking For Trouble

A weld looks good on the outside. There are no cracks, undercuts, pinholes, or other obvious defects, but how can we tell what manner of evil is lurking in the murky depths of the weld? We could ask the welder who welded it, but he'd probably take the 5th or file a grievance. We could ask the Manufacturing types what they think, but they'd just tell us, "Don't go looking for trouble. You might find it and create more rework." Obviously no help. We won't even bother asking Production Control. They'll grow faint and nauseous at the mere suggestion of a weld defect: it could louse up their scheduling. We can't go to the Drafting Section for an answer because they have their hands full trying to update their drawings to match what the Shop is already doing. And as far as Industrial Engineering is concerned, they're too busy processing all the arbitrary Engineering Changes that have been issued by our Engineering Department to give us an answer. Looks like we're up the proverbial creek, right? Wrong! We can get one of those unsung heroes in the QC Department to tell us if the weld is good or bad. One of those magnificent inspectors in a crumpled shirt and dirty overalls who slogs it out in the trenches everyday, never asking for thanks, never receiving any. Yes, he can tell us if the weld is good. And the way our noble knight errant of the shop floor will do it is through our next topic: nondestructive examination. But first, a brief discussion of the legal profession is in order. "The first thing we do, let's kill all the lawyers." This immortal line from Shakespeare's Henry VI, Part 2, reflects the bard's opinion of that degenerate offshoot of the human race. Our society, like the Elizabethan, is a litigious one. Lawyers today reap handsome profits from the misfortunes of others just as their brethren did at the time of the Virgin Queen. Shakespeare's pronouncement undoubtedly struck a tender and sympathetic chord in an audience that would have been only too glad to help implement such a civic improvement. The opprobrious nature of ambulance chasers has changed very little in the last four centuries. Shakespeare is not alone: literature is replete with scurrilous diatribes hurled at members of the legal profession by the world's greatest authors. Metallurgists, on the other hand, have never been accused of being anything less than epitomes of excellence, paragons of virtue, pillars of society, and all around good Joes! Noteworthy is the fact that Shakespeare penned not one word disparaging the noble profession of metallurgy. Not given to showmanship, eschewing the lawyer's churlish behavior, shunning flamboyance, metallurgists maintain a quiet demeanor as they diligently strive for the betterment of mankind. Metallurgists find fulfillment in easing the burden of the common man. This too, the lawyer does, but only by lightening the common man's wallet. Altruistic by nature, metallurgists remain unblemished by the avarice that so deeply stains the legal profession. While the difference in moral principles between the two professions is manifest, the difference in appearance between practitioners of the two professions may not be. page - 161

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Both a lawyer and a metallurgist can blend in with the crowd. How can the average person such as yourself distinguish between the two with any degree of certainty? What test can you run that will separate the wheat from the chaff? The one sure way is to extract and assay their hearts. You will find the metallurgist's heart one of pure gold while the lawyer's is more akin to flint. This test, while definitive, has its drawbacks in that it destroys the usefulness of the metallurgist. It is consequently termed a destructive test. We've already discussed some of the destructive tests that are used to evaluate metals (tensile and impact testing, for example). Once tested, the test piece (as with our metallurgist) can no longer serve a useful function. Metallurgists are too rare and precious to wantonly waste by destructive testing. Is there a way to expose the shyster without causing society the grievous loss of a metallurgist? In other words, can we perform a test on two people to determine which is the lawyer and which is the metallurgist without impairing their usefulness after the test? Of course! Merely strap them both into chairs, bring an old widow with her seven hungry children into the room, and have the widow wave her last one dollar bill in front of the two subjects. The one that drools is the lawyer. (He will eventually work himself into a frenzy hence the straps.) This type of testing is called nondestructive: after the test the metallurgist can continue to serve mankind and the lawyer can do his damndest to get that dollar. Nondestructive testing is, as the name implies, a means of detecting, locating, and evaluating certain types of defects within metals without doing any physical damage to the parts being inspected. The types of nondestructive testing that we'll cover in this section are radiography, ultrasonic, magnetic particle, and liquid penetrant. We'll discuss the different techniques, the types of defects each method can detect, and the advantages and disadvantages of each.

RADIOGRAPHY (RT) RT - General Principles Visible light is a form of electromagnetic radiation (see, you should have stayed awake in high school physics) that has a wavelength on the order of 6000'. The "'" stands for angstroms. One angstrom (for you sleepers) is equal to 10-8 cm. If we tape a penny in between two sheets of white paper and then hold a bright light behind the paper, we can "see" the penny. The penny absorbs more light than the paper does thus we can locate the penny by the contrast in the transmitted light. This, in essence, is how radiography works.

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Radiography is a nondestructive means of examining the internal soundness of an object by evaluating the variation in intensity of radiation that has passed through the object. The types of radiation used to evaluate metals are gamma rays and x-rays. These are forms of electromagnetic radiation just like visible light, but have much shorter wavelengths (on the order of 0.003-300' for x-rays and 0.00003-3' for gamma rays). The shorter the wavelength, the higher the energy and the more penetrating the radiation. To radiograph a metal part, we must first direct a beam of x-rays or gamma rays at the part and then evaluate the intensity of the beam as it passes out the opposite side. Part of the beam will be absorbed by the metal and part of the beam will be scattered thus the radiation that actually makes it through the part may be considerably less than what you started with. The amount of radiation that gets absorbed is dependent on both the type and the thickness of the material through which it passes. Different materials exposed to the same radiation will absorb different amounts. For example, a two inch thick piece of steel will absorb more radiation than a two inch thick piece of plastic. Thus in our metal part, that portion of the beam that passes through an area containing a crack, void, inclusion, etc., will be absorbed differently than the portion of the beam that passes through solid metal. By evaluating the variation in the intensity of the out coming beam, we can detect the presence of the defect. We cannot observe the high energy beam directly because: 1) it's invisible and 2) we'd be fried or turned into mutants. We will instead place a piece of x-ray film adjacent to the part on the side opposite from the source just before we actually zap the part. The film is sensitive to electromagnetic radiation. When the exposed film is developed, a dark area will appear wherever electromagnetic radiation has impinged upon it. The greater the intensity of radiation, the darker the spot will be on the developed film. We thus have taken a "picture" of the inside of our metal part. Suppose our part had a large void in its center. The portion of the beam passing through the area containing the void will be absorbed less than if it had passed through the full thickness of metal. As a consequence, the intensity of that portion of the beam that traveled through the void is greater than the rest of the out coming beam. Our developed film will have a dark spot corresponding to where the portion of the beam that passed through the void impinged upon it. This process is illustrated in Figure 1.

RT - Radiation Sources We are obviously going to need a source of x-rays or gamma rays if we plan on making a radiograph. X-rays are produced wherever an electrically charged particle with sufficient kinetic energy is rapidly decelerated. X-rays for radiographic purposes are page - 163

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most commonly produced in an x-ray tube. A schematic of an x-ray tube is shown in Figure 2. The filament is heated up to incandescence and is the source of the electrically charged particles - electrons. It is also the cathode of the tube. The anode of the tube has a tungsten button on the surface. A high voltage is applied between the anode and the cathode and causes the electrons produced at the cathode (the filament) to be rapidly accelerated towards the anode. The focusing cup helps to concentrate this flow of electrons onto the tungsten target on the surface of the anode. All of this takes place under a vacuum inside the tube.

Figure 1: Radiography As the electrons impinge upon a small area of the target (known as the focal spot) they may collide with the orbital electrons of a tungsten atom or with its nucleus. The resulting interactions will produce x-rays. Naturally the tungsten target gets very hot from being bombarded with electrons. One of the reasons tungsten is used as the target material is that it has a high melting temperature. It is also very efficient in producing x-rays. The anode on which the tungsten target is mounted is usually made of copper. The good thermal conductivity of copper helps to draw heat away from the target. Some x-ray tubes have a circulating water or oil cooling system. Others have metal fins attached to the anode that are outside the tube, but inside the tube housing and are surrounded by oil. The fins draw heat away from the target by radiating it to the surrounding oil. The flow of electrons from the cathode to the anode is the tube current and is measured in milliamperes. As the current is increased, the number of x-rays that are produced increases. The shortest wavelength x-ray is produced when the fastest page - 164

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traveling electron smacks into the target and is stopped dead in its tracks. By increasing the voltage between the cathode and anode, we can increase the velocity of the electrons and thereby produce the shortest possible wavelength in the resulting x-rays. This is important because the shorter wavelength x-rays are the most penetrating. X-ray tubes are rated by the magnitude of the voltage that can be applied between the anode and cathode (a 200 kV or kilovolt tube, for example). The penetrating power and the intensity of the x-rays can be controlled by adjusting the tube current and the voltage.

Figure 2: X-ray Tube

Gamma rays are produced by the decay of radioactive isotopes. An atom, as those of you who did stay awake in high school physics remember, has electrons orbiting about a nucleus. The nucleus is made up of protons and neutrons. Each atom of a particular element will have the same number of protons, however, the number of neutrons may vary. Atoms that have the same number of protons, but a different number of neutrons are called isotopes. Radioactive isotopes are those that are unstable and will spontaneously decay or transform into one or more new, lighter nuclides. A "nuclide" is a fancy way of saying a nucleus with a given number of protons and neutrons in a particular energy state. This is all very interesting, but so what? The "so what" is that as radioactive isotopes decay they give off various particles and gamma rays and we can use gamma rays to make a radiograph. The two isotopes most commonly used in radiography are cobalt 60 and iridium 192 (the numbers refer to the total number of protons and neutrons found in each atom). Unlike the broad spectrum of wavelengths produced by an x-ray tube, radioactive decay produces gamma rays that have discrete wavelengths. Cobalt 60 produces page - 165

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gamma rays with two different wavelengths and iridium 192 produces gamma rays with 12. This means that the penetrating power of a gamma ray is established solely by the type of isotope used: you cannot adjust an isotope like you can an x-ray tube. The intensity of gamma radiation is dependent on the number of atoms that decay per second. A curie is equal to 3.7 x 1010 atoms decaying per second and is the unit used to measure source strength or activity. The specific activity of a source is its activity in curies divided by its weight in grams. Specific activity is important because the higher the specific activity is for a given source material, the smaller the source can be. The smaller the source dimensions, the sharper the radiograph will be and the shorter the exposure time. The activity of a radioactive source is not constant, but decreases over time. The rate of decrease is measured by the source's half-life. By definition, half-life is the time it takes for the intensity of a particular source material to decrease to one half of its starting value. The half-life of cobalt 60 is 5.3 years and iridium 192 is 70 days. A 40 curie source of iridium 192 for example will be reduced to 20 curies after 70 days, 10 curies after 140 days, 5 curies after 310 days, etc. Because exposure of the x-ray film will depend on the activity of the source, exposure parameters will have to vary over the life of the source. Eventually both cobalt 60 and iridium 192 sources will have to be replaced as their activity falls to such low levels that exposure times become far too long to be practical. The relative intensity of a gamma ray beam is dependent on the source activity and source dimensions. These are not characteristics that the radiographer can change unless he goes to an entirely new source. Virtually all gamma ray sources used in radiography are sealed. This means that they are stored in containers made out of lead or spent uranium that absorb gamma rays and make the sources safe to handle. When making a radiograph, the source is removed from the sealed container by remote control (the source is often mounted on the end of a cable that can be extended from or retracted into the container). X-ray tubes are generally used for relatively thin-walled parts. A 200 kV tube is capable of radiographing steel parts up to about 1" thick. They have the advantages of being easy to position, they can be safely handled when turned off, the current and voltage can be adjusted to optimize the x-ray intensity and penetrating power thus giving the sharpest possible radiograph, and the handling time to radiograph a thin part is minimized because you don't have to mess with moving and fixturing bulky cables like you do with radioactive sources. The big disadvantage of x-ray tubes is their lack of page - 166

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penetrating power. This limits their usefulness because only thin wall parts can be examined with reasonable exposure times. Even the largest commercial x-ray tubes are limited to steel parts no thicker than about 4". Other disadvantages include lack of portability, the tube itself is very fragile, and the size and complexity of the tube and associated equipment increases rapidly with kilovoltage. The largest commercially available x-ray tubes are rated around 500 kV. There are several types of x-ray sources that are capable of producing high energy x-rays that greatly extend the penetration of x-rays into steel. Two of the more common high energy sources are linear accelerators and betatrons. A linear accelerator basically consists of an electron gun, a radio-frequency power source, a wave guide (essentially a straight copper tube), and a target. Electrons from the electron gun are accelerated by the axial electrical field produced by a radio frequency oscillator power source. The accelerated electrons strike a transmission-type x-ray target at the end of the wave guide producing very high energy x-rays (up to about 25 MeV). A betatron uses magnetic induction to accelerate electrons in a doughnut- shape, evacuated, glass tube with a hot-cathode electron gun. The electrons are shot into the tube and magnetically guided in a circular path as they accelerate. When they have reached the desired energy level, they are magnetically guided to a x-ray target where x-rays up to 30 MeV are produced. Linear accelerators and betatrons can be used to examine steel parts up to 24" thick. The chief advantage of isotopes is their penetrating power. Iridium 192 is good for steel parts up to about 3" and cobalt 60 up to about 10". The sources are portable, relatively compact, and require no electricity which make them ideal for field work. Because the sources are radioactive, safety is a concern and requires that a protective enclosure be utilized when radiography is to be done near humans such as in a factory. There are many federal, state, and local laws governing the use of isotopes. The set up time for radiographing a part using an isotope may be longer than if an x-ray tube were utilized because of the positioning and fixturing that is involved. It is often impractical to get a radiograph that covers a 360 ( view of a hollow part (a circumferential butt weld on a pipe, for instance) utilizing an x-ray tube. Such radiographs are easily made, however, utilizing an isotope. The isotope can be placed inside the hollow because of its compact size and the film placed on the outside around the area to be radiographed.

X-ray Film X-ray film consists of a clear plastic film base that is coated (usually on both sides) with an emulsion of silver bromide mixed with gelatin. An adhesive undercoat page - 167

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helps to secure the emulsion to the film base. A top coat of tough gelatin is used on top of the emulsion to provide abrasion resistance. The silver bromide salts are very sensitive to electromagnetic radiation (including visible light) so consequently the film must be handled in light-proof containers called film holders. Of course the x-rays and gamma rays used in radiography can easily penetrate the plastic film holder and interact with the emulsion on the film. The radiation will produce a latent image on the film that is not visible until the film is chemically processed. After exposure, the film is removed from the holder in a darkroom and developed. The first step in developing the film is to treat it with a chemical solution called a developer that causes the exposed silver bromide in the latent image to convert into black, metallic silver. The metallic silver remains suspended in the gelatin and makes up the visible image. The next step is to remove the unexposed silver bromide. This is done by immersing the film in a chemical solution called a fixer. The fixer chemically converts the unexposed grains of silver bromide into a water soluble component which is then flushed away with water. The film is dried and can then be handled outside the darkroom. Visible images are formed by the black, metallic silver on both sides of the film (assuming of course that we're using film that has an emulsion on both sides). The images are separated by only a fraction of a millimeter so they appear as only one when viewed by eye. The density of a film is a measure of the degree of blackening that occurs. It is usually determined using an instrument called a densitometer which measures the intensity of light. The intensity of a light source is measured as the light strikes the surface of the film and then after it passes through the film. These numbers are plugged into the formula

D log

Io It

where D = density Io = the intensity of the light incident or the film It = the intensity of light transmitted through the film For example, if the intensity of the transmitted light is one half the intensity of the incident light, then the density equals 0.3.

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Exposure is a measure of the amount of radiation energy that reaches the film while the radiograph is being made and is equal to the intensity of radiation multiplied by the time. The greater the exposure for a particular area on the film, the greater the resulting density will be. Exposure can be specified in terms of absolute units such as ergs per cm2 (an erg is a unit of energy) or in relative terms where one specific exposure is used as a standard and all others are compared to it. Now that we know all about density and exposure, we can talk about characteristic curves for films. A characteristic curve for a particular film is a plot of the logarithm of relative exposure to density. A curve typical for the type of films we use in radiography is illustrated in Figure 3. A characteristic curve (also called and H and D curve) is generated by using a series of known exposures on the film and then measuring the densities after the film has been developed. There are three parameters used to characterize film: speed, contrast, and graininess. These parameters determine the performance of the film for a given set of exposure conditions and ultimately determine the quality of the radiograph. Film speed is inversely proportional to the total energy of radiation that produces a given density on the film. The faster the film, the less energy is needed to produce a given density, and the shorter the exposure time can be. Film speed is generally expressed in relative terms using a standard that is arbitrarily assigned some value. Assume film #1 is the standard so we will assign it a speed of 100. If film #2 requires twice the exposure of film #1 in order to produce the same density, it will have a relative speed of 50. In Figure 3, film A has a faster speed than film B. Film contrast (sometimes called gradient) is equal to the slope of the characteristic curve at a given density. It is a measure of the rate at which density changes as a function of exposure. High film contrast means that a small variation in relative exposure will produce a large variation in density. Thus defects that absorb only a small amount of radiation because of their size, orientation, etc., are more likely to be discernable on a high contrast film rather than a low. The contrast actually seen on a finished radiograph is known as radiographic contrast (big surprise, huh?). The variations in the amounts of radiation absorbed by the part being inspected (and subsequently the variations in radiation intensity impinging on the film) and film contrast determine radiographic contrast. Sometimes film contrast is expressed as an average value over a range of densities.

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Figure 3: Film Characteristic Curve

The silver bromide grains that are suspended in the gelatin (and together make up the emulsion) are extremely minute. They can be seen individually only through the use of an electron microscope. Although the emulsion is typically only about 1/2 mil thick, there will be a considerable number of silver bromide grains piled on top of each other due to their small size and vast numbers. This clumping of grains is discernable either visually or at low magnification on the finished radiograph and is known as graininess. Slower speed films generally have less graininess than higher speed films. Film latitude is the thickness range of the object being inspected that can be recorded with a single exposure. If the object has large variations in cross section over the area being radiographed, several exposures utilizing different set up conditions and exposure times may have to be made so that the density and radiographic contrast on the resulting radiographs are sufficient to allow meaningful interpretation. A film with low contrast generally has a wide latitude and thus can be used to radiograph the widest range in object thickness. Similarly, a high contrast film has narrow latitude so only a narrow range of object thickness can be radiographed and still produce the optimum density for interpretation. The number of exposures and exposure times can frequently be reduced by going to a faster film of lower contrast, but wider latitude. Unfortunately this also results in a reduction of the film's sensitivity: its ability to image small defects. The selection of any x-ray film involves compromises. Film speed, contrast, and graininess are all interrelated. As film speed increases, graininess increases and contrast decreases. The highest quality radiographs are produced using a fine grain, high contrast film. This type of film, however, has the slowest speed and thus requires higher radiation intensity or longer exposure times. Economic considerations will often page - 170

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dictate a compromise between the use of a low speed film (giving the highest quality) and a high speed film (allowing greater latitude and shorter exposure times).

RT - Imaging and Image Quality Radiation travels in a straight line. This is fortuitous as far as radiography is concerned because it means that an image that forms on the film will have a sharply defined outline. The image will never have the exact same size and shape as the object being radiographed because the image is really a projection of the area of the object that lies in a plane normal to the centerline of the beam. Just as your shadow can change lengths on a sunny day depending on the angle of the sun, and change shape depending on what profile you present to the sun's rays, radiographing an object from several different angles may result in totally different images on the film. It is important to understand how the image that forms on the film is related to the size and shape of the actual object being radiographed. Let's look at some of the more important factors affecting this. The image will always be larger than the actual object. Why this is so is illustrated in Figure 4. The amount of enlargement can be determined from the formula: L S M i i Lo So where, M Li Lo Si So

= = = = =

the amount of enlargement (or magnification) distance from the source to the image (film) distance from the source to object being radiographed size of the image size of the object being radiographed

By examining this formula and Figure 4, it can be seen that, other things being equal, the greater the distance between the film and the object, the larger the image will be. Similarly, the closer the source is to the object, the larger image. Because the film is generally placed adjacent to the object being radiographed for our type of equipment, the difference between Li and Lo is relatively small so enlargement is generally not a concern. The shape of the image will be distorted if the radiation beam is not directed perpendicular to both the film and the object being radiographed. This is illustrated in Figure 5. In the previous figures, I have always shown the radiation source as a single point from which all the radiation emanates. In the real world, it just ain't so. Most commercial x-ray tubes have a focal spot between 2 x 2 mm and 5 x 5 mm. Gamma ray sources come in many different sizes, but are almost always greater than 2.5 mm in page - 171

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diameter. The fact that we do not really have point sources can present problems because radiation is emanating from an area and consequently radiation beams strike the object at different angles (see Figure 6). The resulting images formed by the transmitted beams are not concentric, but will overlap. Instead of having a sharply defined outline, the overall image will have a fuzzy border. This is known as geometric unsharpness.

Figure 4: Enlargement

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Figure 5: Image Distortion

In Figure 6, the dark portion of the image formed by the overlap of radiation from all the points on the surface of the source is called the umbra. The lighter portion of the image extending beyond the umbra that formed by radiation from part, but not all of the source is called the penumbra. The geometric unsharpness is equal to width of the penumbra. It can be calculated from the following equation:

Ug

Ft Lo

where, Ug F t Lo

= = = =

geometric unsharpness the size of the focal spot or gamma ray source the object-to-image distance the source-to-object distance

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Figure 6: Geometric Unsharpness The only practical way to reduce the size of the penumbra is to lengthen the source-to-object distance. Of course this has its disadvantages in that it also reduces the intensity of radiation incident on the object. The intensity of radiation that penetrates the object being radiographed and produces the desired density on the film is a function of the energy and spectral quality of the incident radiation, source strength (as measured by the tube current for x-ray sources or activity for isotopes), the source-to-film distance, and the type and thickness of material being radiographed. The energy of the incident radiation (which depends primarily on tube voltage for x-ray sources and on the type of isotope for gamma ray sources) is selected so that the radiation is sufficiently penetrating for the type and thickness of the object to be radiographed. Once this is determined, the only parameters that can be adjusted by the radiographer to obtain a desired intensity of radiation are source strength and the source-to-film distance. The intensity of radiation and the amount of radiation incident on a unit area per unit time determines the exposure time for a given film. The x-rays or gamma rays emanating from a source diverge and cover an ever increasing area with lessened intensity as they travel away from the source (see Figure 7). The intensity of radiation will vary inversely with the square of the distance from the source according to the following equation:

IA IB page - 174



L B2 L A2

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Where, IA and IB are the intensity of radiation at distances LA and LB, respectively, from the source. This equation is known in physics as the inverse square law.

NOTE:

The same amount of radiation falls on film A and film B. The intensity of radiation on film B is only ¼ that on film A because it falls on 4 times the area. Figure 7: Inverse Square Law

As we previously mentioned, some of the radiation that impinges on an object will be absorbed, some will pass straight through, and some will be scattered. The scattered radiation can eventually strike the film from different angles and cause a haze to appear that may mask some details of the image. Radiation may be scattered from inside the part or off surrounding walls or even off the floor. Scattering can be reduced through the use of lead blocks or screens, masks and diaphragms, collinaters, or filters. Lead screens are thin sheets of lead that are placed in contact with both sides of the film holder and absorb the longer wavelength rays. Screens can also be made from other materials. Lead blocks can be used behind the film holder to absorb most of the radiation back scattered off the floor or walls. Sometimes a lead letter "B" is placed on the back of the holder to monitor back scatter. If the "B" shows up in the final radiograph, you know that you have a potential problem with back scatter. Masks and diaphragms are radiation absorbing materials in the form of sheet, blocks, pellets, etc., that are placed on top or around the object being radiographed such that only the area of interest is directly exposed to the radiation beam. A collinater is a device (usually made out of lead) that surrounds the radiation source and allows the primary radiation beam to exit only through a single small opening. It thus absorbs much of the radiation that would normally be scattered and helps keep the primary beam directed onto a localized area. Filters are thin sheets of copper or lead that are placed in the radiation beam in between the source and the object being radiographed. They can absorb some of the undesirable wavelengths of radiation that may contribute to scattering. page - 175

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If we don't see any defects showing up in a radiograph, does that mean that our part is defect-free? Obviously a loaded question and the answer is no. First of all if our procedure was done improperly, there may be insufficient radiographic contrast to make the image of the defect discernable on the film. Secondly, some defects because of their size or orientation may not make a discernable image regardless of whether or not the proper procedure was utilized. To verify that the parameters chosen for a given radiographic procedure are adequate for the desired sensitivity and that the procedure was properly followed, we will make use of a device called a penetrameter or image quality indicator. A penetrameter is nothing more than a small piece of material having a simple geometric shape that is radiographically similar to the object being radiographed. The thickness of the penetrameter is very important and is a specified proportion of the thickness of the area of the object being radiographed. The penetrameter is generally placed on the part to be radiographed such that it is in a place normal to the beam and near the outer cone of radiation. The appearance of the penetrameter in the completed radiograph is then evaluated to determine the quality of the image. We typically use ASTM E142 plaque penetrameters. These are flat sheets of metal cut into either the shape of a rectangle or circle depending on their thickness. Rectangles are used for metal penetrameters 0.005 - 0.160" thick and circles for 0.180" and over. Rectangular penetrameters have 3 holes of 1T, 2T, and 4T in diameter where T is the thickness of the penetrameter. Circular penetrameters have two holes of 1T and 2T in diameter. These holes will help to check the resolution capability of the radiographic procedure. Typically we will use a penetrameter that is 2% of the thickness of the area of the object being radiographed. The quality of the image on the resultant radiograph is expressed in terms of the appearance of the penetrameter. An image quality of 2-2T would mean that the thickness of the penetrameter is 2% of the thickness of the area being radiographed and that the 2T diameter hole is the smallest that can be detected. Another type of common penetrameter is the wire type. This penetrameter consists of a series of parallel wires encased in a plastic envelope. The diameter of each wire is different. Image quality is denoted by identifying the smallest diameter wire that can be discerned on the finished radiograph. The diameter of the wires used in a particular penetrameter will be dependent on the thickness of the part being radiographed. Wire penetrameters are frequently used in Europe and are usually made to ISO or DIN standards. Now that we know we have good image quality, can we rest assured that our part is defect-free? Nope. The part may be loaded with small defects. If our image quality is, for example, 2-4T, then there may be many defects that have a thickness less than 2% of the thickness of the part and therefore cannot be detected on the radiograph. Similarly, if the projected image of a defect is less than 4T in diameter, you may not be able to detect it on the radiograph. The orientation of a defect may determine whether or not it can be detected on a radiograph. For example, say our part has a cylindrically shape defect in it that has a page - 176

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small diameter, but is fairly long. If we radiograph our part such that the radiation is normal to the longitudinal axis of the defect, there might be so little difference in absorbed radiation that there would be insufficient contrast in the radiograph to make the defect discernable. However, if the radiograph is made such that the radiation travels parallel to the longitudinal axis of the defect, there's a good chance we will detect it in the radiograph. It is theoretically possible to radiograph a part that is completely broken in half and not be able to detect it in the radiograph. A radiograph of a 2" inch thick plate of steel will look exactly like a radiograph of two 1" thick plates of steel that are stacked together if the radiation is normal to the plates. All this illustrates the importance of radiographing a part from two or more directions whenever possible. As a minimum a second radiograph should be taken 90( to the first.

RT - Advantages and Disadvantages Many of the advantages and disadvantages of radiography as an NDE method should be apparent after the previous, marathon discussion. For those of you who didn't make the race or who quit after the first mile or two, we will briefly summarize them here. Advantages & & & &

It has good sensitivity (defects having a thickness of 2% or more of the test piece thickness can be detected). It is volumetric (internal flaws can be detected). It provides a permanent record of quality. Virtually all materials (within certain thickness limits) can be radiographed. Disadvantages

& & &

& & &

Equipment is expensive and sometimes bulky. Safety concerns necessitate special facilities, extensively trained personnel, and a great deal of regulations that must be complied with. Part configuration may be such that it is impossible to get a meaningful radiograph. This may be due to thickness, a complex geometry or an extremely irregular contour, or because it is impossible to place the film adjacent to the part directly opposite the radiation source. Set up and exposure times can be lengthy. Some defects may be missed because of orientation. The exact location of defects may be difficult to determine because a radiograph is only a two dimensional image: the depth of the defect is not easily found unless a second radiograph is made 90( to the first.

ULTRASONIC EXAMINATION (UT) page - 177

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UT - General Principles The time is July 28, 1991. Imagine that you're the skipper of the USS Cameron, a destroyer on escort duty in the Persian Gulf. You're accompanying a convoy of 5 merchant ships on their way to war-torn Kuwait. Their valuable cargo of Cameron blowout preventers and Christmas trees must get through so that oil production can begin again and keep the Free World's economies from collapsing. Suddenly your forward lookout yells, "Scope off the port beam!" You turn your head just in time to see a periscope disappear below the oil blackened water. Obviously one of our competitors' submarines trying to torpedo another one of our on-time deliveries. You order all hands to battle stations and all torpedoes armed. You grit your teeth and vow to do it to them before they do it to you. But how can you deep six what you can't even see? "Sonar, give me a ping," you bark. "Aye aye, Captain," comes the instant response. The sonar man pushes the button on his console. The transducer on the bottom of the hull emits a high energy, acoustic wave. The wave front races unimpeded through the murky depths in an ever increasing sphere. It quickly reaches your adversary. A portion of the wave is reflected off the submarine's metal hull back towards your ship. Receiving transducers on the bottom of your ship's hull pick up these reflected waves, convert them into electrical signals, and send the signals to the sonar scope. The sonar scope processes the signals and converts them into visual images. The sonar scope is filled with contacts - sound waves reflected off the ocean floor, oil production platforms, your five merchant ships, etc., but there's no mistaking the submarine's blip all by itself. The sound waves reflected off the bum aren't masked by any other reflections: the sub is in open water. "Contact bearing two niner zero, range 500 yards," your sonar man reports. "Got it, Captain!" reports your weapons officer. "Fire number two," you order. Number two torpedo tube spews its contents overboard into the undulating sea. Moments later a loud explosion confirms that the deadly, black, steel fish found home. "Another scumbag bagged and another on-time delivery made," you quietly announce. Your crew members stare at you in awe. Damn, you're good! Ultrasonic examination employs the same general principles that you just used to sink the competition. A transducer (in a probe that is acoustically coupled to the part to be examined) introduces ultrasonic waves into the part. These waves propagate through the part until they encounter a defect or the back surface of the part. A portion of the waves will be reflected back towards the transducer. The transducer will receive these reflected waves and convert them into electrical signals. These electrical signals are processed and then displayed visually on an oscilloscope screen.

UT - Physics of Wave Motion Remember the Slinky® that you played with when you were a kid? A Slinky® for those of you who forgot or had a deprived childhood is a flexible, coiled spring roughly 4" in diameter made from flat, rectangular wire. Free standing, all the coils are in page - 178

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contact with their neighbors and the length of the spring is about 4". It can be stretched out to a length of 10' or so without plastically deforming it. It makes an admirable tool with which to illustrate the physics of wave motion. Let's lay our Slinky® on its side on the floor, fasten one end to the floor, and then stretch it out along a straight line for the full 10'. If we now start shaking the free end of the spring from side to side, we will produce a wave motion that travels the length of the spring. At any instant in time, if we look down at the spring we will see a wave form similar to Figure 8.

Figure 8: A Slinky® With The Shakes Successive portions of the spring become displaced as the wave passes through. Obviously the displacement of one part of the spring cannot happen in isolation: the adjacent material gets dragged along. As successive portions of the spring become displaced, the wave propagates away from the source. After the wave has passed through a given portion of the spring, the elastic nature of the spring will cause that particular portion to return to its equilibrium position. It will oscillate about the spring's longitudinal axis with decreasing displacement until it finally comes to rest. Wave motion in a metal or other medium behaves the same way. Particles of the medium vibrate around their equilibrium positions as the energy wave passes through: the particles themselves do not move away from the wave source. The wave travels in a straight line within the homogeneous media. Energy waves traveling through air that can be detected by the human ear are referred to as sound. Humans can hear waves having a frequency (the number of vibrations or wave cycles per second) of about 20-20,000Hz (Hz stands for hertz which is a fancy way of saying cycles per second). Waves having a higher frequency are referred to as ultrasonic. Ultrasonic waves used in the inspection of metals typically have a frequency in the range of 1-25 MHz (the "M" means million). Ultrasonic rather than audible waves are used because their higher frequencies give them better sensitivity for finding small defects. The wave form of our Slinky® in Figure 8 has some nomenclature associated with it. The amplitude is the maximum deviation from the base line or equilibrium page - 179

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position and is a measure of the strength of the wave. The wavelength is the distance over which the wave form begins to repeat itself and is designated by the Greek letter . Wave length is related to frequency by the formula  = v/f where v is the velocity of the wave and f is the frequency. In most ultrasonic applications, the velocity of the ultrasonic wave remains constant so that as frequency increases, wave length decreases. A small group of waves occurring together and well separated from other groups is called a wave train or pulse. A pulse may have one of four different wave forms depending on the mode of vibration that generates the pulse. Waves may be longitudinal, transverse, surface, or plate. Longitudinal or compression waves have the particles that transmit the wave energy vibrating in a direction parallel to the wave motion direction. They consist of alternating compression and rarefaction zones. Audible sounds such as a ringing bell, a vibrating drum, or an exploding firecracker are all longitudinal waves. Longitudinal waves are the ones most commonly used in ultrasonic examination. The wave form produced in the experiment with our Slinky® is transverse. The particles transmitting the wave energy are displaced in a direction perpendicular to the wave motion direction. Transverse waves are sometimes called shear waves. A transverse wave travels at about half the speed of a longitudinal wave of the same frequency in the same material and consequently has a shorter wave length. It is more sensitive to small reflectors. Transverse waves are the next most commonly used waves in ultrasonic examination. Surface waves (sometimes called Rayleigh waves) are, not too surprisingly, a surface phenomenon. The waves essentially disappear at a depth of more than one wave length below the surface of the part being examined. They are often compared to ocean waves. If you were to take a cross section parallel to the wave motion of the deepest part of the ocean and observe the waves at the very top, you would have some idea of how Rayleigh waves propagate. If you throw a cork in the water and follow its path in your cross sectional view of things, you'd notice that the cork makes a circular path as it's pushed around by waves. The oscillation of the water particles is not back and forth along a straight line (like the particles in medium with longitudinal or shear waves), but circular in the plane of wave travel. Rayleigh waves are similar, but are propagated by particles that oscillate in an elliptical path in the plane of wave travel. Rayleigh waves can occur only when the surface of the metal is surrounded by a gas, such as air, that has very weak elastic forces between molecules. Rayleigh waves travel at about 90% of the velocity of a shear wave in the same material. They have an ability to travel around curves and follow surface contours, but are reflected by sharp corners or where an abrupt change in medium occurs such as at a surface crack. This, of course, makes Rayleigh waves useful for surface flaw detection. page - 180

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A B Symmetric Asymmetric Figure 9: Plate Waves Plate (or Lamb) waves occur when ultrasonic vibrations are introduced into relatively thin sheet of less than one wave length in thickness. The velocity of plate waves is dependent on the thickness of the sheet or plate, the frequency of the wave, and the type of particle movement. There are many possible modes of particle movement for plate wave propagation, but they can be broadly categorized as either symmetrical or asymmetrical. The symmetrical mode is illustrated in Figure 9(A). The passage of the wave through the sheet causes successive increases and decreases in the sheet thickness. The asymmetrical mode is shown in Figure 9(B). It can be thought of as a series of ripples traveling through the sheet. Plate waves are sometimes used for detecting delaminations in composites or for finding radial cracks in tubing. They are not used for inspecting our type of product. The acoustic impedance (designated by the letter Z) of a material is the product of its density multiplied by the velocity of sound in the material. When a beam of sound waves encounters the interface between two solids having different acoustic impedances, a portion of the wave energy will be reflected, but a portion will also pass into the second material. (If the interface is between a solid and a liquid or gas, virtually total reflection occurs.) The fraction of the wave energy that is reflected is dependent on the difference between the acoustic impedance for each material and the angle of incidence. For a beam traveling through solid A and encountering the interface with solid B at a 90( angle, the fraction reflected is equal to (ZA - ZB)2 divided by (ZA + ZB)2. The reflection of sound off an interface between materials is much like light reflecting off a mirror. The angle of incidence equals the angle of reflection (see Figure 10).

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Figure 10: Reflection

An ultrasonic wave can be reflected off the interface between metal and a defect (such as a crack, gas void, etc.) within the metal. The amount of energy that is reflected is very much dependent on the area of the surface on the defect that the wave is incident upon. For example, let's assume we have a thin, plate-shaped defect in the part we are ultrasonically examining. If the incident beam is parallel to the edge of the "plate", there may be so little reflection that the defect will go undetected. If the incident beam is normal to the face of the "plate", a great deal of energy will be reflected and we have a much better chance of finding the defect. Note that the optimum orientation for ultrasonic examination is opposite of that for radiography. Like radiography, ultrasonic examination should be done from two or more directions (preferably 90( to each other) to ensure no defects are missed because of orientation. If the angle of incidence on the interface of two materials of unequal velocity is anything other than 90(, the portion of the beam passing through the interface into the second material will be refracted (have its direction altered) as shown in Figure 11. In reference to Figure 11, the new direction of the refracted beam can be calculated from Snell's Law: vA sin  sin





vB

Where, vA = velocity of the ultrasonic wave in A vB = velocity of the ultrasonic wave in B At certain critical incident angles, the beam may be partially or completely transformed from one type of wave (for example, longitudinal) into other types (transverse and surface) besides being refracted. This is known as mode conversion and is illustrated in Figure 12. Mode conversion allows us to use a standard transducer that generates longitudinal waves to produce a different, desired wave form in the part being inspected. Having more than one type of wave in our part would lead to page - 182

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difficulties in evaluating defects because we wouldn't know which mode caused the reflection. As a consequence, we will have to adjust the angle of incidence so that only the desired wave form is actually transmitted into the part.

Figure 11: Refraction

Suppose we desire to examine a part using shear waves. Our transducer only generates longitudinal waves so we will have to use mode conversion. This is done by putting a plastic wedge underneath the transducer so that the longitudinal beam impinges on the part at an angle. A portion of the beam is converted into shear waves and is refracted at one angle, and a portion of the beam remains longitudinal and is refracted at a different angle. If we adjust the angle of incidence of the initial beam (by adjusting the slope of our plastic wedge) so that the angle of refraction of the longitudinal portion of the refracted beam is 90(, no longitudinal waves will be present in the material (see Figure 13). The angle of incidence at which this occurs is called the first critical angle. Referring to Figure 13 and using Snell's Law, we can calculate this angle:

sin sin



2

vA vB

or

sin  1

velocity of longitudinal wave in A (the plastic wedge) velocity of longitudinal wave in B (the part)

If we desire surface waves for testing, we will have the adjust the angle of incidence so that the shear wave portion of the beam is refracted at 90(. This is page - 183

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illustrated in Figure 14. The angle at which this occurs is the second critical angle. Referring to Figure 14 again and using Snell's Law, the second critical angle can be calculated as follows:

sin sin



1

vA vB

or

sin  1

velocity of longitudinal wave in A (the plastic wedge) velocity of transverse wave in B (the part)

Surface waves will be generated at angles equal to or greater than the second critical angle up to a maximum angle  where: sin



velocity of longitudinal wave in A (the plastic wedge) velocity of surface wave in B (the part)

Figure 12: Mode Conversion

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Figure 13: First Critical Angle

Figure 14: Second Critical Angle Attenuation is the loss of wave energy as the wave passes through a material. There are two primary causes for this. The first is absorption. Here energy is lost as heat as the wave excites individual particles in the medium. The other reason is scattering. Scattering is the loss of energy due to reflections and refractions of the page - 185

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beam as it passes through regions that are not homogeneous. Inclusions, pores, different microconstituents, contaminants, etc., may have completely different acoustic impedances than the base metal being tested and thus cause scattering. In addition to attenuation, beam intensity will decrease at increasing distances from the source (or at increasing distance from the focal point of focused transducers, see next section) because of divergence. Divergence means that the beam spreads out over an ever increasing area consequently the amount of energy transmitted through a unit area decreases.

UT - Transducers A transducer is a device that transforms one form of energy into another. In ultrasonics, the transducer converts an electrical signal into ultrasonic waves and vice versa. The transducer in ultrasonics has many aliases: search unit, probe, crystal, but they all refer to the same thing. Ultrasonic transducers are constructed out of piezoelectric materials. These are materials that will set up an electrical potential when they are mechanically deformed or will deform when exposed to an electrical potential. Commonly used piezoelectric materials in ultrasonics include single quartz crystals, lithium sulfate, and polarized ceramics. Let's look at a single quartz crystal and what makes it piezoelectric. Quartz has the chemical formula of SiO2. It has a complicated structure in which the silicon atoms lie on three interpenetrating hexagonal lattices which spiral relative to each other in the vertical direction. The oxygen atoms are grouped in tetrahedrons about the silicon atoms. Each unit cell has three silicon atoms. Quartz is a material held together by ionic bonds: bonds that result from the attraction between positive ions (Si+4) and negative ions (O-2). If we look at a small cross section of a quartz crystal it will look something like Figure 15(A). In the unstrained condition in Figure 15(A) the center of the negative charges lies at a point equidistant from the oxygen ions and coincides with the center of positive charges that is equidistant from the silicon ions. As a result, there is no net charge (the charges are not numerically balanced in Figure 15 because we stripped away some of the oxygen ions for clarity). If we apply a mechanical load to our quartz crystal, it will deform as shown in Figure 15(B). Notice that the center of positive charges and the center of negative charges no longer coincide. A potential difference now exists across the crystal that can be measured electronically. After the load is removed, the positive ions will repulse each other as do the negative ions and the crystal returns to page - 186

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equilibrium. Thus mechanical energy, such as a vibration initiated by a reflected sound wave, is converted by the crystal into an electrical signal. The product of the electronic charge and the distance between the centers of the positive and negative charges in Figure 15(B) is called the crystal's dipole moment . A crystal having a dipole moment is said to be polarized. If we artificially apply a potential across our crystal, the crystal will deform slightly (on the order of an atomic spacing) as the negative ions are attracted to the positive side and the positive ions to the negative side. When the potential is removed, ions with like charges repulse each other and them oscillate about their equilibrium positions with decreasing amplitude as the crystal returns to its original shape. We thus have utilized the crystal to convert electrical energy into mechanical vibration. The vibrating crystal will generate ultrasonic waves when brought into contact with the part to be inspected.

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A

B Figure 15: Piezoelectric Effect In Quartz

A

B

C

Figure 16: Quartz Crystal Slab Cuts

A quartz crystal is anisotropic. By making a transducer from a slab cut from a quartz crystal we can generate longitudinal, shear, or surface waves depending on which crystallographic directions the slab is cut from. Figure 16(A) shows an idealized quartz crystal with the crystal axis identified. An "X" cut crystal slab shown in Figure 16(B) will transmit longitudinal waves into the part. When a potential is applied, the slab will contract in the X-direction, but expand slightly in the Y-direction thus also generating transverse waves. These are soon lost as they are attenuated. A "Y" cut crystal shown in Figure 16(C) transmits transverse waves into the part and surface waves in the Xdirection. page - 188

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Some types of crystals may not have the center of positive charges and the center of negative charges coincide as they do in Figure 15(A). Instead, they may have a structure that has a dipole moment even in the unstrained state. Such crystals are said to be spontaneously polarized. An example of one is illustrated in Figure 17. Note the dipole moments that are created because of the asymmetric arrangement of charges. If we heat such a crystal structure, the interatomic spacings will increase, but not uniformly. The relative arrangement of charges will be altered and consequently so will the polarization or the potential difference. This change in polarization upon heating (or cooling) is known as the pyroelectric effect. The direction of polarization in some pyroelectric crystals can be permanently reversed by the application of a sufficiently intense external field. Such crystals are called ferroelectric. Due to imperfections in its lattice, a ferroelectric crystal actually consists of many different subregions. Each of these subregions, or domains, will be uniformly polarized within its boundaries, but the directions of polarization will differ from that of an adjacent ferroelectric domain. Some domains will reinforce each other while others with opposite polarization directions will tend to cancel each other out: the net effect depending on the volume and polarization direction of each domain. If a ferroelectric crystal is heated above a certain temperature, the ions in its lattice will eventually rearrange themselves into new equilibrium positions that result in a symmetric structure that is stable up to the melting point. This temperature is known as the curie temperature. Above the curie temperature, our crystal will no longer be ferroelectric because with the symmetric arrangement the centers of charges coincide. By now most of you have probably overdosed on the solid state physics of dielectric materials, but hang in there, the worst is over! Most ultrasonic transducers use ferroelectric ceramics as the source of the piezoelectric effect. Ceramics such as BaTiO3, CaTiO3, and PdTiO3 in powder form are mixed together in varying proportions depending on the application. The mixture can be molded, stamped, or even cast into the desired shape. The shape is then fired into a solid just like the chinaware on your table is made. Naturally the resulting solid contains a zillion small crystals that are randomly oriented and thus has no net polarization. If we reheat the now solid ceramic above its curie temperature and then allow it to cool while we apply a voltage across it, the domains will tend to align themselves in a common direction and be "frozen" in place. Our polycrystalline material is now polarized and will exhibit piezoelectric properties.

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A slab of polarized ceramic will deform when an electric field is applied to it just as the slab of quartz crystal that we previously discussed did. The slab direction in which this deformation takes place (and consequently the type of ultrasonic waves that the slab will produce) is dependent on the orientation of the aligned domains with the direction of the applied electric field. During the operation of a pulse echo ultrasonic inspection system (we will discuss this in detail in the next section), the probe is hit with a high voltage, short pulse. This causes the piezoelectric crystal (whatever the material) to vibrate or "ring". The vibrations are transmitted into the part being inspected as ultrasonic waves. Crystals tend to vibrate at certain frequencies that are dependent on their dimensions as well as their material properties. The lowest of these frequencies is known as the resonant frequency of the crystal. Eventually the ringing will die out as the vibrations are dampened by both the part and transducer materials.

Figure 17: Spontaneous Polarization Resolution is the ability of an ultrasonic system to separate signals occurring at different times. Resolution is improved if we minimize the time that the crystal rings after being hit with the pulse. A high frequency transducer will have better resolution that a low frequency one. The "Q" of the transducer is a measure of its ringing ability and is defined as the energy stored in a crystal divided by the energy lost for each cycle of page - 190

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vibration. The larger the value of Q for a particular transducer, the longer the crystal vibrates for a given pulse. A high value of Q means that the transducer is an efficient transmitter and a sensitive receiver, but has poor resolution. Common crystal materials used in ultrasonic transducers are quartz, lithium sulfate monohydrate, and polarized ceramics. As previously mentioned, polarized ceramics are by far the most commonly used. Table 1 shows some of the pros and cons of each. Table 1 Advantages

Disadvantages

Quartz

• • • • •

electrical and thermal stability high strength and wear resistance uniformity insoluble in most liquids very clear response signal because less mode conversion occurs

•

low electromechanical conversion efficiency

Lithium Sulfate

• •

easily dampened for good resolution intermediate electromechanical conversion efficiency very little mode conversion

•

cannot be used at temperature outside the range of 32 - 165(F

high electromechanical conversion efficiency high sensitivity easily manufactured

•

relatively high background noise in signal relatively high amount of mode conversion

• Polarized Ceramics

• • •

•

A typical transducer is illustrated in Figure 18. The same crystal is used to both generate ultrasonic waves and to receive their reflections. The transmitted pulse lasts 1-4 microseconds. The crystal then acts as a receiver for several thousand microseconds. This cycle is repeated 50 to 5,000 times or more per second. Another type of transducer utilizes two crystals in the same probe that are acoustically isolated from each other: one crystal is the transmitter and the other the receiver. This type of transducer is generally used for examining that portion of a part close to the surface. A pitch-catch system uses two separate transducers each having a single crystal. One transducer is the transmitter and the other the receiver. The beam may be continuous rather than pulsed. It is sometimes used on parts with a large grain size because the large grains can scatter the reflected beam in a different direction from which it came. It is also used on parts where the backside is not parallel to the surface being inspected. The sound beam will be reflected internally within the part. The receiving transducer, in either case, can be positioned at various locations on the part to best intercept the reflected beam.

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Figure 18: Typical Ultrasonic Transducer Although we can make a transducer that will produce shear or surface waves by selecting the appropriate crystal, in actual practice mode conversion of a longitudinal beam transducer is almost always used. As discussed in the previous section, a plastic wedge is made and fitted on the transducer so that the longitudinal beam will have an angle of incidence that will produce the desired mode in the refracted beam in the part. A straight beam search unit transmits the ultrasonic waves at a 90 ( angle to the probe housing. An angle beam search unit uses a plastic wedge (or "shoe") so the angle of incidence of the transmitted beam is something other than 90 (. Although both straight beam and angle beam search units can be designed to produce either longitudinal or transverse waves, in actual practice straight beam is the same as longitudinal wave and angle beam the same as transverse. The transducer that we have discussed so far is known as a contact unit because it physically touches the part being inspected during scanning. Immersion units, used in immersion testing (which we'll talk about in the next section), are similar to contact transducers except they are made water tight and the face of the crystal has a protective covering that provides a good impedance match with water. There are other types of search units for specialized applications that use various mechanical means to provide a couplant between the transducer and the part being inspected. A couplant is a liquid, grease, or paste that is used to provide a low resistance path for the ultrasonic waves leaving the transducer and entering the work piece. Air is a poor transmitter of ultrasonic waves and its acoustic impedance is so different than that of solids that a thin air layer will effectively cause total reflection of the beam. A couplant displaces the air between the transducer and work piece and fills in any gaps between the surfaces thus promoting better transmission. Its acoustic impedance is much closer to that of solids which also helps to improve transmission. Water is the couplant most commonly used for immersion testing. Various oils, silicone greases, wall paper paste, page - 192

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glycerine, and many proprietary pastes are all used as couplants for contact transducers with oil being the most common. Sound waves can be focused just like light. A sound beam transmitted between two different media is refracted. An acoustic lens can be designed so that all the sound is refracted through a small area known as the focal spot. Unlike light waves which use a convex lens, sound waves are focused by a concave lens or transducer crystal. Even flat transducers, which can be considered to be a concave lens having an infinite radius, will focus the sound beam to some extent. Focused transducers are used whenever high resolution and high flaw definition are required. They are also used for inspecting thin materials. Figure 19 shows a focused transducer. Note the beam quickly diverges past the focal spot thus focused transducers must be tailored for the specific thickness or depth of interest. Focusing may be accomplished by adding an acoustic lens to the transducer face or shaping the transducer crystal.

Figure 19: Focused Transducer Most transducers are relatively small (3/8", 1/2", 3/4" and 1" are standard diameters). A paint brush transducer is an exception. This type of transducer is typically 1/2" - 3/4" wide and up to 6" long. It is used for inspecting plate and sheet. In reality, a transducer is not a point source: the crystal can be thought of as a collection of point sources that each generate ultrasonic waves. As these individual waves move away from the transducer, they will overlap and interact with other. If the waves meet in phase they will reinforce each other, if they meet out of phase they will tend to cancel each other out. The maximum energy areas eventually converge toward a point in the center of the beam. Beyond this point of convergence, the beam will, in effect, have a single wave front. The area from the transducer to the point of convergence is called the Fresnel zone or near field. Because of the interaction of the multitude of waves, the reflection off a defect within the Fresnel zone can vary significantly with small changes in distance from the transducer. A single defect may also produce multiple reflections. This makes interpretation extremely difficult. Whenever possible, it's best to consider the Fresnel zone a "blind" spot and consider only the volume of material beyond the convergence point - the Fraunhofer or far field. Here there is no interference. The length of the Fresnel zone is given by the following formula: page - 193

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L

d2 x f 4 x v

Where, L f v d

= = = =

length of Fresnel zone frequency velocity of sound in particular media active diameter of transducer

The frequency of a transducer is important because it helps to determine penetrating power and the sensitivity for small defects. (In reality a transducer will produce a train of waves that cover a range of frequencies that extends on either side of the desired test frequency.) High frequency waves are more sensitive to small defects than low frequency waves. Low frequency waves, on the other hand, are more penetrating and are less affected by surface conditions.

UT - Equipment The basic set up for a pulse echo system, the most common in our business, is illustrated in Figure 20. Pulse echo means that the probe is utilized to pulse ultrasonic waves into the part and to receive the echoes, or reflections, of the waves.

Figure 20: Pulse Echo System Referring to Figure 20, the rate generator (sometimes called clock or timer) activates and synchronizes all the other circuits. It produces electrical pulses at a constant (but adjustable) rate that is typically 50-5000 pulses per second. These pulses trigger the pulser into producing an initial high voltage, short duration electrical pulse that excites the transducer in the probe. The transducer in the probe converts the high voltage pulse into a short train of ultrasonic waves that have a range of frequencies. The sweep circuit controls the display on the oscilloscope screen. A small beam of electrons is continuously "swept" across the fluorescent oscilloscope screen and creates the visual image just like in a television tube. The sweep circuit is also activated by the rate generator. page - 194

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The train of ultrasonic waves travels through the part until it encounters a different medium whereupon a portion will be reflected. The echoes are picked up by the transducer in the probe where they are converted into electrical pulses. These pulses are directed into the RF (radio frequency) amplifier where they are amplified; the amount of amplification being determined by the gain control. Pulse echo equipment is usually classified by the bandwidth of the RF amplifier. This is illustrated in Figure 21. At frequencies lower than fA or higher than f B, the RF amplifier will lower (rather than amplify) the signal strength. Only that portion of the signal that falls between fA and fB is amplified. This helps to eliminate signal noise. Band width is typically measured at a point equal to 1/2 the peak gain.

Figure 21: Bandwidth The peak gain of a wide band amplifier is usually less than that of a narrow band amplifier. As a consequence, the narrow band amplifier is more sensitive and has a higher signal to noise ratio. An ultrasonic pulse in reality consists of a number of frequencies superimposed upon each other. Small defects tend to reflect the higher frequencies of the pulse more efficiently than the lower. The wide band amplifier will amplify more of these high frequency signals than a narrow band amplifier and thus is often more sensitive to small defects. The resolution of a wide band amplifier is page - 195

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generally better because more of the frequency components are amplified resulting in less distortion or lengthening of the pulse. An auxiliary circuit in the amplifier may be used to increase the gain with the corresponding depth of the echo source (defect). The farther an echo has to travel before being picked up by the transducer the weaker its amplitude will be. The DAC (distance amplitude correction) circuit compensates for this loss of amplitude. After being processed by the RF amplifier, the signal is directed to the video amplifier where it is rectified, shaped, and further amplified before being displayed on the oscilloscope. The electric signals have both positive and negative voltages. By rectifying them, the video amplifier makes all the signals go in one direction relative to the base line so that the oscilloscope display will have peaks only above the horizontal line instead of above and below. This simplifies interpretation. The video amplifier will make the leading edge of the signal pulse nearly vertical. Filters may be used to control the smoothness of the signal displayed on the oscilloscope. A reject control in the video amplifier (or sometimes in the RF amplifier) may be used to eliminate those signals having less than a certain amplitude. This minimizes noise and leaves only the more important echoes. The signal is now ready for display on the cathode ray tube (CRT) of the oscilloscope. The timing of the events just described is critical and, as previously mentioned, is controlled by the rate generator. The proper sequence of events is shown in Figure 22.

Figure 22: Timing In Pulse Echo What does the display on the CRT look like and how do we correlate it to the quality of our part? This question has three answers because we can display the electrical signal three different ways. The first is A-scan. The A-scan display is the one typically used in the ultrasonic examination of metals. The horizontal line on the CRT represents elapsed time. The electron beam in the CRT sweeps from left to right. Each sweep cycle is controlled by the rate generator and is initiated after a given time period following each rate generator pulse (as shown in Figure 22). This time lag can be adjusted by the zero control on the instrument. The electron beam sweeps the CRT at a constant rate consequently it's position along the horizontal axis of the CRT display at any point in time is directly proportional to the distance the ultrasonic wave train has traveled through the metal. The vertical axis of page - 196

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the CRT display indicates the amplitude or strength of any echoes. The reflected sound waves off a defect, for example, will thus cause the electron beam to be deflected upward as the beam sweeps from left to right. The amount of this vertical deflection is proportional to the strength of the reflected waves. The distance between any two peaks on the horizontal scale of the display is directly proportional to the distance between the sources of the reflected waves within the part being inspected. We can calibrate the horizontal scale using the range control such that it corresponds to a unit of length in the part being inspected. The depth of a reflected wave source can then be read directly off the horizonal scale. The A-scan presentation thus gives the depth of a reflected wave source (horizontal axis) and the amplitude of the reflected wave (vertical axis) as illustrated in Figure 23.

Figure 23: A-Scan Display The B-scan display is seldom used for the ultrasonic examination of metal parts (it is frequently used in the medical field) so we will only briefly mention it. The horizontal axis on the CRT has the same significance as in A scan: it represents elapsed time or depth of the reflector. The vertical axis shows the position of the reflector along the scan path of the probe. The brightness of the display is a function of the reflected waves amplitude. At any given time, the B-scan display presents a cross sectional view of an object that lies in a plane perpendicular to the line of probe travel and perpendicular to the surface of the part. The C-scan display is a plan view of the object being inspected. The vertical and horizontal axis of the CRT display represent the axes of the plane of the surface being inspected. The probe is moved back and forth until the entire surface being inspected is covered. The signal from reflected waves will be displayed on the CRT in a position (relative to the CRT's vertical and horizontal axes) that correlates with the reflector's position relative to the axes of the part's surface. In other words, the CRT display gives you a two dimensional view of things just as if you were looking through the surface being scanned. In this regard it is just like a radiograph. The intensity of the display is proportional to the amplitude of the signals. Signals of waves reflected off the back surface of the part are blocked (or gated) out so that the image or the display is formed only by waves reflected off defects at the depth of interest. Naturally the probe cannot be everywhere on the surface at the same time so the image on the CRT may not be complete at any particular instant of time. This problem can be overcome by taking a photograph of the CRT display with the shutter of the camera remaining open until the scan is complete. C-scan display is illustrated in Figure 24. page - 197

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Figure 24: C-Scan Display

UT - Techniques Many things must be considered when selecting the appropriate ultrasonic technique for a part including size and shape of the part, base material, surface roughness, grain size, and the maximum allowable flaw size that can be tolerated in the part. Ultrasonic techniques may be broadly classified as either contact or immersion. There are many variations of these so we will examine only those methods commonly used on our type of equipment. Contact techniques are based upon the wave mode that is utilized and include straight beam, angle beam, and surface wave techniques. We've already talked extensively about the straight beam technique in which the transducer is mounted such that the beam is projected at a 90( onto the surface of the part. It is utilized for inspecting parts with relatively flat, parallel surfaces where the probe can be placed directly over the area of interest. It is often used for thickness measurements. The angle beam technique typically uses a plastic wedge on the transducer to convert longitudinal waves into shear waves as the beam enters the part. It is used to examine plate, pipe and other round parts, welds, and parts of irregular shape. Figure 25 illustrates the angle beam technique being used to examine pipe. The beam is repeatedly reflected off the walls of the pipe so it will travel around the entire circumference until it is attenuated or encounters a defect. To ensure complete coverage, the pipe or the transducer will have to be rotated as the pipe is scanned along its length.

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Figure 25: Angle Beam Technique On Pipe Figure 26 shows the angle beam technique being used to check a weld on a plate. Note that the weld can be checked without having to grind it flush as would be the case if a straight beam technique were utilized. Note also that in order to check the whole weld, the transducer must be moved in a zig-zag pattern. The same technique can be used to inspect a plate from an area near one edge. The size of the area that the transducer must zig-zag over in order to completely scan the plate is determined by the skip distance of the beam and the width of the plate.

Figure 26: Angle Beam Technique On Welded Plate

Figure 27 shows a contact transducer examining the surface of a part for discontinuities using surface waves. The obvious advantage of this technique is the ability of the surface waves to follow the contour of the part. As previously mentioned, surface waves essentially disappear just below the surface of the part so only the surface is examined.

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Figure 27: Surface Wave Inspection

Immersion testing is generally performed using one of two techniques. One is the bubbler technique in which a column of flowing water impinges on the work piece. A transducer, located within the column, uses the water as couplant to propagate the ultrasonic waves into the part. The second technique involves submerging the transducer and the part being tested into a tank full of water. This is the most common technique and when I hereafter refer to "immersion testing" it is this technique that I am referring to. Generally the tank is rectangular and has a manipulator on top that controls the position of the transducer within the tank. Immersion testing is easily automated and is especially suited for mass production where high speed, repetitive inspections are required. It is also frequently used in a manual mode for examining parts with rough surfaces, contours, etc., that would be difficult or impossible to completely examine using a contact method. Of course the water acts as a couplant and ensures good sound transmission into the part. The transducer does not come into contact with the parts. The angle of the transducer in relation to the surface of the part can be controlled by the manipulator so that either longitudinal or shear waves are produced. Both techniques are illustrated in Figure 28 and Figure 29.

Figure 28: Immersion Testing page - 200

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Figure 29: Bubbler Technique

UT - Calibration and Standard Reference Blocks Calibration blocks are metal blocks containing known, artificial defects that are used to standardize test conditions and to determine the operating characteristics of the ultrasonic system. Standard referenced blocks are metal blocks containing known, artificial defects that are used to help evaluate discontinuities found in the part being examined. The signal response from discontinuities in the part is compared against the signal responses from the artificial defects (with known sizes, shapes and depths) in the standard reference block so that it can be characterized. Many times the same set of blocks may serve both functions. We will refer to both sets of blocks as "reference" blocks for the balance of this section. There are many kinds of reference blocks available today. We'll discuss just one of the more commonly used types: ASTM E428 steel reference blocks. These blocks are made from material having the same composition, heat treat condition, grain size, surface finish, and method of manufacture as the part to be examined so that the acoustic response of the reference blocks is the same as that of the part. The blocks themselves are cylindrical and have a diameter of 2" for test distance up to 6" and a 21/2" diameter for test distances over 6". A flat bottom hole (FBH) is drilled in the center of the bottom face to a depth of 3/4". The diameter of the hole varies. Each block is marked with the alloy designation, hole diameter in 1/64" increments, and the distance in 00.00 inches that the sound beam must travel from the top face until it reaches the hole in the bottom. For example, a block marked 4130-5-0225 is made from AISI 4130 material, has a 5/64" diameter flat bottom hole, and the end of the hole lies 2.25" from the top face of the block. ASTM E428 reference blocks can be used to develop an area/amplitude response curve for our particular ultrasonic system and test conditions. As you recall, other things being equal, the amplitude of an echo is proportional to the area of the discontinuity that it reflected off. We'll use a set of blocks that have the same metal travel distance (the distance between the top face of the block where the sound first encounters the end of the hole), but have different FBH diameters. We'll start with the blocks having a FBH that is the middle of the range. We'll check its ultrasonic response page - 201

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and adjust our CRT display so that the signal is 30-40% of the full scale deflection (FSD) on the vertical axis. Without changing the test parameters, we will proceed to check the rest of the blocks and record the amplitude (as a percentage of FSD) of their signals. The resulting plot of amplitude versus reflector area is very useful. Suppose we now examine a part (again without changing our test parameters) and observe a defect. By noting where the amplitude lies on our area/amplitude curve, we can determine what size FBH produced a similar response. The area of the defect (which is an indication of its size) is at least as big as the corresponding FBH. The FBH is a perfect reflector while a defect may have an irregular surface in order to reflect a signal of the same amplitude as the FBH, the defect's surface area may need to be larger, consequently the corresponding FBH only sets the lower limit on area. Another useful curve that we can develop with our reference blocks is a distance/ amplitude response curve. Here we will use a set of blocks that have identical size flat bottom holes, but vary in metal travel distance. We'll start with the blocks that are the bottom quarter of the metal travel range and adjust the CRT display so that the signal is 70-80% of FSD on the vertical axis. The rest of the blocks are then checked (without changing any of the test parameters). A plot of amplitude (as a percentage of FSD) versus metal travel distance will characterize the attenuation and divergence losses of sound energy in a given material using a given test set-up. A typical curve will show increasing amplitude as metal travel distance increases up to a maximum amplitude at the point of convergence that separates the near field from the far field. Once the metal travel distance exceeds the length of the near field, the amplitude will continuously decrease as distance increases.

UT - Advantages and Disadvantages Another long winded section nearly finished! At the rate I'm going, this chapter on nondestructive examination is going to be longer than the rest of the course combined. No matter, it's all good stuff. Here, by the way, are the major pros and cons of ultrasonic examination: Advantages & & & & & & & & & &

page - 202

It's volumetric (longitudinal and shear wave methods). Large cross sections can be examined because of its high penetration power. It has high accuracy in the determination of flaw size and location. Only one surface needs to be accessible. Most units are portable. The fast response time allows rapid inspection. It can be easily automated for inspecting many types of products (such as plate, pipe, etc.). It has high sensitivity (a typical contact unit can detect a FBH as small as 3/64" in a 6" thick low alloy steel forging, for example). It can be used to make thickness measurements. It can be used to detect disbonding or delaminations of overlays.

CHAPTER VIII

NONDESTRUCTIVE EXAMINATION Disadvantages

& & & & & & & & &

&

For our type of product, it is primarily a manual technique. It requires highly skilled operators. Fresnel effect may prevent a meaningful examination of the volume of material near the surface. Surface preparation (typically grinding) may be necessary. There is no permanent record of inspection (usually!). Requires couplants that must eventually be removed. Must be calibrated using reference standards tailored for a specific material. Some defects may be missed because of orientation. Complex shapes may make ultrasonic examination very difficult or prevent complete coverage. Parts may have to be examined prior to finish machining. Some metallurgical considerations (large grain size or grain orientation in castings, for example) may prevent a meaningful examination.

MAGNETIC PARTICLE EXAMINATION (MT) MT - General Principles Remember when your fourth grade science teacher astounded your entire class by shaking some iron filings onto a sheet of paper held just above a bar magnet? Instead of falling onto the sheet of paper in a random pattern, the filings aligned themselves along a series lines that extended from one end of the underlying magnet to the other. Your teacher probably explained that these lines were called magnetic flux lines: magnetic lines of force that flow from the south pole through the magnet to the north pole and then loop back, outside of the magnet, to the south pole - and that the iron filings were attracted to them. Being a precocious 10 year old, you probably thought, "This is all very well, but such a phenomenon is obviously a mere laboratory curiosity having no practical import in the real world." Poor misguided youth! Little did you realize then that one day you would be attending a course such as this where the topic of magnetic particle examination would be discussed in detail, and that the key to understanding this nondestructive testing method is a firm grasp of those same magnetic flux lines that you so flippantly dismissed as insignificant when you were a kid. Intellectual arrogance is a phase of youth that soon dissipates as time and the school of hard knocks slaps one silly. Most of us who are graduates of this school have come to realize that instead of being "know-it-alls", we really don't know squat. Assuming you are also an alumnus, have recognized the errors of your misspent youth, don't know squat, and are now ready to pay greater attention to those "insignificant" flux lines, let us look at the basics of magnetic particles examination. Ferromagnetic materials are those that can be magnetized or are strongly attracted by a magnetic field. There are five elements that are ferromagnetic: nickel, cobalt, iron, gadolinium, and dysprosium. The alloys of these elements may also be ferromagnetic depending on composition as well as other factors. What makes some page - 203

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materials ferromagnetic, and others not, lies deep in the impenetrable fog of quantum mechanics into which we shall not venture. An electric current has a magnetic field associated with it. One of the ways that a part made from a ferromagnetic material may be magnetized is to pass a current through it. Another is to place the material into an existing magnetic field such as that created by a current flowing through a conductor (a coil of wire, for example). In either case, magnetic flux lines will be generated in the ferromagnetic material. The direction that these magnetic flux lines take can be determined using the right hand rule. Suppose we want to magnetize an iron bar by passing a current through it. If we grab the bar with our right hand such that the thumb points in the direction of the current, our curled fingers will indicate the direction of the magnetic flux lines. This is illustrated in Figure 30(A). If instead we magnetize our bar by wrapping a coil of wire around it, the direction of the magnetic flux lines induced in the bar can be found by grasping a segment of the wire in the right hand with the thumb pointed in the direction of current flow within the wire. The fingers will then point in the direction of the magnetic flux lines as shown in Figure 30(B).

A

B

Figure 30: Right Hand Rule Magnetic flux lines in a magnetized metal part are relatively smooth unless they happen to encounter an abrupt change in dimensions or a material discontinuity such as a crack or nonmetallic inclusion. These will cause the flux lines to become distorted from their original path. If this occurs at or near the surface, some of the magnetic flux lines may be distorted out of the metal path altogether. This condition is known as flux leakage. If we apply fine magnetic particles (such as the iron filings that your fourth grade teacher used) to the surface of a magnetized metal part that contains a surface discontinuity, the particles will be attracted to the part and especially to the leakage at the discontinuity. If the leakage is strong enough, the accumulation of magnetic particles will be a visible indication of the discontinuity. Although there are many variations in testing procedures, this is the fundamental principle behind all magnetic particle examination techniques. Magnetic particle examination is not volumetric like ultrasonic testing or radiography. It is capable of detecting discontinuities only on or very near the surface of the part. Because the accumulation of particles is dependent on the strength of the flux leakage, not all surface discontinuities may be detected. This may be due to the size and/or orientation of the discontinuities. For example, a surface crack that runs transverse to the direction of magnetic flux will interrupt the most flux lines and result in page - 204

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the greatest leakage. The same size crack running parallel to the direction of magnetic flux may interrupt so few flux lines that the resulting flux leakage may be insufficient to attract enough magnetic particles to make the crack visible. This is illustrated in Figure 31. As a consequence, a part being magnetic particle examined will be tested twice (whenever possible) with the direction of magnetic flux of the second test 90 ( to that of the first. This minimizes the possibility of a discontinuity escaping detection because of orientation. Naturally the smaller a discontinuity, the few magnetic flux lines interrupted and the smaller the resulting leakage. Depending on technique, discontinuities as small a 1/100" long can be detected about 45% of the time while discontinuities 3/64" long can be found virtually all the time.

Figure 31: Effect of Defect Orientation

Surface preparation of the part to be examined is generally minimal. Surfaces should be free of loose scale, rust, etc., as well as oil and grease or other contaminants that could impede the mobility of the magnetic particles or mask indications. Painted or plated parts can usually be magnetic particle examined provided the paint or plating is relatively thin (1 or 2 mils maximum thickness) and, for those magnetic particle techniques that actually pass a current through the part, electrical contact with the base metal can be made.

MT - Materials That Can Be Examined As discussed in the last section, there are five ferromagnetic elements. Of these gadolinium and dysprosium are not used in the manufacture of wellhead equipment (they are used in the manufacture of rare earth magnets and for other exotic applications). We of course do use many nickel, cobalt, and iron base alloys in our equipment. The alloys of nickel and cobalt that we typically use do not exhibit sufficient ferromagnetism to allow them to be magnetic particle examined. Iron base alloys may or may not exhibit sufficient ferromagnetism depending primarily on composition. If the page - 205

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composition is such that the alloy has a predominately austenitic structure, it cannot be magnetic particle examined. The following Table is a brief listing of which materials can and cannot be magnetic particle examined. Can Be Magnetic Particle Examined

Cannot Be Magnetic Particle Examined

•

Carbon Steels

• Aluminum Alloys

•

Low Alloy Steels

• Titanium Alloys

•

Martensitic Stainless Steels

• Austenitic Stainless Steels

•

Precipitation Hardening Stainless Steels with Martensitic Structures (17-4, 13-8, etc., after aging)

• Lead • Copper Alloys • Stellites® • Inconels®

MT - Types of Magnetic Particles The particles used in magnetic particle examination are considerably more complex than the iron filings used by your old science teacher. They are ferromagnetic materials carefully chosen for their magnetic properties and processed to the optimum size and shape. They have high magnetic permeability (the ratio of magnetic flux density to the strength of the magnetizing force) which means that the particles are easy to magnetize and are readily attracted to flux leakage at a discontinuity. The particles have low retentivity (the ability to retain a magnetic field after the magnetizing force has been turned off) so they do not attract each other and agglomerate. The particles are colored (generally red, sometimes black or another color) to improve their contrast with the surface of the part being examined. Magnetic particles (and consequently the examination techniques that utilize them) are broadly classified as either wet or dry. Wet particles are designed to be suspended in water or oil at a given concentration and then applied to the part by spraying, pouring, etc. They are most often used in wet, horizontal, magnetic particle examination equipment. This type of equipment (which we will later talk about in detail) recirculates the suspension: the suspension, or bath as it is usually called, is drawn from a reservoir, applied to the part being examined, and then drains back into the reservoir. Baths that are water based will also contain anti-foaming agents, corrosion inhibitors, wetting agents, etc. Dry particles, as the name suggests, are not used in a suspension. They are applied as a dry powder directly to the part either by spraying (often from an aerosol can) or by dusting. Dry particles are typically not recycled because of the possibility of the powder being contaminated by dirt, grease, etc., once it's been applied.

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Both wet and dry particles may be coated with a fluorescent material that glows underneath a black (near-ultraviolet) light. Inspection is done in an area that excludes normal light. This of course greatly increases the contrast making discontinuities easier to find and speeds up the overall inspection time. Because of the greater contrast, fewer particles need be attracted to the flux leakage associated with a discontinuity before that discontinuity becomes visible. It is thus easier to detect a weak flux leakage field with fluorescent particles. As a matter of practice, fluorescent dry particles are seldom used because of cost and because dry particles are frequently used for testing in the field where examination under a black light is often impractical. Magnetic particle techniques that use dry particles have several advantages. The equipment used in "dry mag" is generally less complex, less expensive, and more portable than that used for "wet mag". Wet mag equipment is generally large and rather bulky so it is seldom used in the field. Dry mag can be done almost anywhere and under more extreme environmental conditions than wet mag. Dry particles can be applied by dusting the surface of the part using a shaker, rubber spray bulb, etc., or by spraying using a mechanical sprayer or aerosol can - all easily done in the field. Wet mag equipment requires a reservoir to hold the bath, agitators to help keep the particles in suspension, and a pump to circulate the bath. Dry mag techniques tend to be more sensitive to subsurface discontinuities than wet mag. They are also less messy and require less maintenance than wet mag techniques. Wet mag techniques have the advantage of being able to discern smaller and shallower surface defects than is possible using dry mag. The entire surface of the part being examined is quickly, easily, and uniformly covered by spraying or pouring the bath over it. Dry particles may be difficult to apply uniformly especially on threaded areas or parts with sharp contours. Wet particles are often fluorescent which improves sensitivity and inspection time as previously discussed. Wet mag techniques are generally much faster than dry mag techniques especially when large numbers of parts are involved. The bath in wet mag techniques must be carefully monitored to ensure the proper concentration of suspended particles and to avoid high levels of contaminants that may influence results. Wet fluorescent techniques require a special viewing area where a black light can be used and normal lights can be blocked out so that it doesn't lessen the contrast between the fluorescing particles and the part's surface. The intensity of the black light, which affects the intensity of the fluorescing particles, must be checked periodically.

MT - Magnetizing Currents An electrical current exists whenever an electrical charge (such as an electron) is moved from one point to another within a conductor. In order to get the charge to move, these must be a potential difference or voltage between the two points. The resistance of a conductor is that property that opposes the flow of a charge and causes the charge to give up energy as it moves through. Resistance is dependent on the size and type of conductor. Current in a conductor is related to voltage by Ohm's Law:

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Where, V = voltage in volts I = current in amperes R = resistance in ohms A current that always moves in the same direction through a conductor is called a direct current (DC). If the direction of the current reverses itself on a regular basis, the current is known as an alternating current (AC). If we make a plot of voltage as a function of time for DC we'll get something that looks like Figure 32. A similar plot for a single phase (one source) AC is shown in Figure 33. Note in Figure 33 how the voltage periodically changes signs and thus causes the current to alternate directions. A plot of current as a function of time would have a similarly shaped curve, but would not necessarily coincide with the voltage curve. These types of plots illustrate the waveform of a particular current.

Figure 32: Direct Current

Figure 33: Single Phase AC The wave form of a three phase alternating current is shown in Figure 34. It is really three separate waveforms superimposed upon each other. This type of current is generated by rotating a magnetic field within three equally spaced coils of wire. Referring to Figure 35, the rotor is either a permanent magnet or an electromagnet. As it rotates, the magnetic flux lines associated with the magnetic field are brought into proximity with a particular coil in the armature (or stator) and induces a current in that page - 208

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coil. As the rotor continues to rotate, the induced current in a particular coil will increase up to a peak value and then decrease. Before the induced current in a particular coil reaches zero, the rotor's magnetic field will already be interacting with the next coil. The three individual waveforms in Figure 34 are thus identical in size and shape, but there is a time lag between them.

Figure 34: 3 Phase AC

Figure 35: 3 Phase, 2 Pole AC Generator

Most magnetic particle equipment today uses one of three basic types of current for magnetization: alternating current (AC); single phase, half-wave rectified alternating current (HW); and three phase, full-wave rectified alternating current (FWDC). Direct current is seldom used nowadays having been replaced with rectified AC sources. Rectification means the conversion of AC to DC through the use of diodes. A diode is an electronic device that acts as sort of a check valve that permits the flow of electricity in one direction only. Half-wave rectification is like an electronic gate that allows only one half of the waveform of an alternating current through as shown in Figure 36. Fullwave rectification produces a waveform as shown in Figure 36. The polarity of one half of the waveform has been changed to match that of the other half. Magnetic particle examination techniques that utilize an alternating current for magnetization are very sensitive in detecting discontinuities such as fatigue cracks that are open to the surface. The magnetic field associated with this type of current is restricted to the very surface of the part so defects just below the surface may not be detected. Demagnetization of an inspected part is generally done with AC regardless of the type of current used to magnetize it.

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Figure 36: Rectification

Half-wave rectified alternating current is most often used with dry particle, portable techniques. The pulsing effect of the current causes the particles to jostle around thus increasing their mobility and the likelihood that they will accumulate at flux leakage. The combination of HW and dry particles is very sensitive to subsurface discontinuities. HW is also sometimes used with horizontal, wet particle equipment. Full-wave rectified alternating current is useful for the detection of both subsurface and surface defects. The AC power source allows high amperages to be attained which, in turn, increase the strength of the magnetic field in the part as well as allowing larger parts to be magnetized. The full-wave rectification increases the sensitivity for subsurface defects. FWDC inspection equipment, such as horizontal, wet particle units, have relatively large, bulky power sources and are generally not portable. Magnetic particle inspection techniques may be classified as either continuous or residual magnetization depending on the timing and the application of the magnetic particles and the magnetizing current. The continuous magnetization method has the magnetizing current turned on while the magnetic particles are applied. For wet, continuous mag techniques, the part is actually sprayed with the bath just before the current is applied (with some overlap possible). The current is typically applied for half a second. Dry continuous mag techniques require that the magnetizing current be on when the particles are applied because the particles lose much of their mobility upon contact with the surface of the part. The current will remain on until the application of the powder is completed and the excess blown off. The residual magnetization method has the magnetizing current applied prior to the application of the magnetic particles. The residual magnetic field must be strong enough to allow the particles to accumulate at discontinuities and be held there until inspection is complete. The residual method is page - 210

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sometimes used where large numbers of parts having high retentivity must be inspected or for some specialized inspections.

MT - Magnetization Techniques In this section we will discuss the various ways parts can be magnetized, describe the magnetic field produced by each technique, describe the type of equipment associated with each technique, and go over each technique's advantages and disadvantages. Magnetization techniques can be broadly classified into two groups: those techniques that require direct contact between the electrodes of the mag inspection unit and the part being examined, and those techniques that will induce a magnetic field in the part without physically touching the part. 1. Prods - Prods, as previously mentioned, are hand held electrodes on the end of the cables attached to the current source. They may have either solid copper or braided copper wire tips. The prods are pressed down onto the surface of the part being inspected (direct contact) and then the magnetizing current turned on. A circular magnetic field is created in the part around each of the prods (see Figure 37). Prods are generally used with dry magnetic particles and with half-wave rectified alternating current (HW). This combination gives excellent sensitivity for the detection of subsurface and surface defects. Units using prods are often portable.

Figure 37: Prod Technique The direction of magnetization is easily changed by altering the placement of the prods. The magnetic field direction can thus be readily adjusted to give the optimum sensitivity for specific areas of interest on parts. Normally after the area near the prods has been examined, the prods will be repositioned such that the magnetic field in the area of interest is perpendicular to the original direction and then the area reinspected. This minimizes the chance of a discontinuity escaping detection due to orientation. Prod spacing is limited by the magnetizing current. It is essentially a localized magnetizing technique page - 211

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and consequently it can be very time consuming to completely inspect large parts. Because it requires good electrical contact, the prod technique can only be done on bare metal parts with clean, dry surfaces. Poor contact may result in arcing between the tip of the prod and the surface of the part. Arcing must be avoided because it creates a very localized area with extremely high hardness. 2. Head/Tailstock Contact - This is a direct contact technique in which the part to be examined is clamped between two electrodes. The magnetizing current is then applied directly through the part thus the entire part becomes magnetized. This technique is usually associated with horizontal, wet magnetic particle units. These units generally have flat, vertical electrodes. One of the electrodes is usually stationary (the head) while the other (the tailstock) can be moved along the bed of the unit to accommodate different size parts much like the tailstock on a horizontal lathe (and consequently the name). The current passing through the part will establish a circular magnetic field as shown in Figure 38. Because the entire part is magnetized, this technique allows rapid inspection. The high amperage, magnetizing current develops a strong magnetic field in the part which provides good sensitivity for surface and subsurface discontinuities. Complex shaped parts that are difficult to examine using other techniques are easily inspected provided good contact can be made between the surface of the part and the head and tailstock.

Figure 38: Headstock/Tailstock Technique

The equipment used in the head/tailstock contact technique is bulky and consequently not portable. Arcing may occur if good contact with the electrodes is not made. The head and tailstock must contact bare metal. Of course the size of the unit will determine the maximum size of part that can be examined. Many parts can be rechecked using a magnetic field with a direction 90( to the original field merely by rotating them 90(. This is not practical for some parts (such as bar or tube) and other magnetizing techniques must be used. Hollow parts can only be checked on the outside surfaces. page - 212

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3. Coils - If the wire or cable that carries the magnetizing current is coiled around a bar, a longitudinal magnetic field will be induced in the bar. The coil does not need to touch the bar and is consequently an indirect method. The coil may be a permanent fixture consisting of loops of wire fastened together in the shape of a ring into which the part to be inspected is inserted or may be a cable that is wrapped around the part by the inspector. Horizontal, wet magnetic particle units often have a permanent ring shaped coil in addition to a head and tailstock. The coil allows bar, tube, and other long parts to be tested in a direction transverse to the magnetic field developed by the head and tailstock. The number of loops (or turns) of the coil multiplied by the amperage of the current is an important parameter in the technique because it determines the strength of the magnetic field induced in a given size part using a coil with a given radius. The magnetic field produced by a coil extends beyond either side of the coil by a distance roughly equal to the radius of the coil. For long parts that exceed this distance, the position of the coil relative to the part will have to be adjusted and multiple magnetizations made to ensure complete coverage. The sensitivity of the coil technique is lower at the ends of a part because of the general leakage field pattern. Long parts are easily magnetized in a longitudinal direction using coils, but may require another magnetizing technique to check 90( to this. Because coils are an indirect magnetizing method, there is no chance for arcing. 4. Yoke - Yokes, besides being what little Swedish children play on each other on April Fool's Day, are "C" shaped electromagnets that can be used to develop a longitudinal magnetic field in the part to be inspected. When a yoke (see Figure 39) is placed on the part and the yoke energized, the part completes the path of flux lines. Note that there is no electrical contact with the part being inspected. Yokes are used with dry magnetic particles. Either alternating current or half-wave rectified alternating current may be utilized. Yokes have the advantage of being very portable. The direction of the magnetic field is easily changed by repositioning the yoke. The primary disadvantage is that only a very localized area of the part is magnetized at one time so large parts required extensive inspection time.

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Figure 39: Yoke

5. Central Conductor - A central conductor technique uses a solid bar or a cable that carries the magnetizing current. It is placed inside and extends through a hollow part. The current that it carries will induce a circular magnetic field in the part. The magnetic field will be sufficiently strong to permit inspection of ID and OD surfaces, although for extremely heavy components the sensitivity on the OD may be considerably less than on the ID. It is an indirect technique in that the part does not have to be in contact with the central conductor. Horizontal, wet mag units will often use a solid copper bar for a central conductor. The hollow part being inspected, a hanger for example, is suspended by a sling from an overhead crane above the mag unit. The copper bar is placed through the hanger and the sling adjusted so that the copper bar can be clamped between the head and the tailstock. Light weight parts can actually be supported by the bar. Central conductors magnetize the entire length of the part, although if the ID of the part is much greater than the OD of the central conductor, then the part must be repeatedly magnetized. This is done by bringing the central conductor against the ID surface of the part, magnetizing, inspecting, and then rotating the part and repeating the process. Central conductors are the only practical way that ID surfaces of most hollow parts can be magnetized. Electrical contact is not required so arcing is eliminated. Inspection time is rapid because the whole part is magnetized. A high amperage current is required consequently the size of the conductor is important because that determines the amount of current that it can carry. It may not be feasible to magnetize a heavy wall part with a small ID using a central conductor because the ID may restrict the size of the conductor to one of insufficient current carrying capability.

MT - Odds and Ends page - 214

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The sensitivity of magnetic particle test methods is dependent on many factors including the size, shape, and composition of the part being inspected, the magnetization technique, the type of magnetic particles, and the strength of the magnetic field that can be developed. The strength of the magnetic field should be sufficient so that the desired size of discontinuities can be detected and it should be relatively uniform over the area being inspected. How is the strength of the magnetic field set and how is it verified? The magnetizing current is the chief test parameter that determines magnetic field strength. As the current increases so does the strength of the induced magnetic field. Techniques that magnetize the entire part (such as head and tailstock contact and central conductor) having the amperage determined by the thickness of the part. Naturally the thicker the part, the higher the amperage will have to be to develop a sufficiently strong field. The current required for a short bar is the same required for a long bar of the same diameter, however a higher voltage must be used when testing the long bar in order to produce the desired current. The current required for prods is determined by the thickness of the part being examined and by the spacing between the prods. The number of ampere-turns required for the coil technique is dependent on the length and diameter of the part being inspected and the diameter of the coils. The amperage of a yoke is adjusted so that the magnetic field strength is sufficient to lift a given size weight. The strength of the field may be checked during testing by using a magnetic field strength indicator. This is a small, hand held device made out of steel that contains known discontinuities. It is placed adjacent to the part being magnetized. The strength of the resulting magnetic field must be sufficient to be able to discern the discontinuities in the field strength indicator when the magnetic particles are applied. After examination, parts must be demagnetized. This operation, which removes any residual magnetization, is necessary because permanently magnetized parts could attract machine chips that could potentially cause damage or interfere with subsequent operations. The residual magnetism could cause problems with any subsequent arc welding operations by causing the arc path to be erratic. Residual magnetism may not be tolerable in the end product if it is to be used in an application near instrumentation that is sensitive to magnetic fields. The most common way of demagnetizing is to pass the part through a high intensity alternating current and then slowly remove the part from the field. An alternate method is to let the part remain in the magnetized field and slowly reduce the alternating current.

MT - Advantages and Disadvantages Well, we have another section of NDE under our belts. Those of you that have become totally saturated with NDE may consider it to be just another blow below the belt instead. Tough! An author has the prerogative to rant and rave about whatever topic he wants and for whatever length he wants. For those intelligent readers who possess inquisitive minds and delight in the intricacies of nondestructive examination, rejoice in the breadth and revel in the depth of this section! For those mutants who are regressing on the evolutionary scale and consider this section to be just another blow page - 215

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below the belt, buy a jock. Here, then, are the pros and cons of magnetic particle examination. Advantages & & & & &

Equipment is easy to operate and relatively inexpensive. It has high sensitivity for surface defects. Defects need not be open to the surface as is the case with liquid penetrant examination. Depending on the method, equipment is portable. Inspection time is relatively short. Results are easy to interpret in comparison to other NDE methods.

Disadvantages & & & & & &

It can detect surface or near surface discontinuities only (depending on technique, down to 1/4" or so from the surface under ideal conditions). It can only be used to inspect ferromagnetic materials. Demagnetization is necessary. Parts must be cleaned after examination. Arcing is a possibility with some methods. Some discontinuities may be missed because of orientation or limitations on the direction that the magnetic field can be generated in.

LIQUID PENETRANT EXAMINATION (PT) PT - General Principles General Principles was a hero of the Texas War of Independence and a man of varied accomplishments. While possessing many sterling attributes, his greatest renown was for his coolness under fire. General Principles' reputation for this was permanently established one night while entertaining a local belle in his quarters. He absentmindedly tried to extinguish his cheroot by immersing it into a sniffer of brandy that he was holding. The ensuing blaze caused him to drop the glass and in doing so, spread the fiery liquid over the front of his ruffled shirt and down onto his pantaloons. The General had no choice but to quickly doff all of his clothes. About that time two soldiers who had heard the commotion came running in only to find their commanding officer standing stark naked in front of his lady friend with a pile of smoldering clothes nearby. The soldiers were uneasy at the thought that they had interrupted the General's dalliance with his paramour during a moment of burning desire. The lady was embarrassed by the apparently compromising situation she found herself in. The General was chagrinned at being caught out of uniform with no outward evidence that would suggest his rank. It was a difficult moment that would have overwhelmed a lesser man. All were at a loss for words until the General, rising to the occasion, decided to break the tension by introducing the two soldiers to his companion. He turned and faced her, bowed ever so slightly, and then said, "Madam, allow me to present my privates." Such is the stuff that legends are made of. page - 216

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The General's influence on Texans was enormous during his lifetime and has not diminished with time. He is beyond a doubt the most cited authority in Texas on virtually any topic. Ask any Texan why Texas lags behind most other states in school finance, social services, mass transportation, police protection, mental and health care for the poor, or housing for the indigent. Ask any Texan why he continues to elect "good ol' boys" to a state legislature that has become the laughing stock of the nation or why the good ol' boys spend tens of thousands of dollars running for a state legislature job that pays only a fraction of that amount. Ask any Texan why he rants and raves about the revolving door policy of the Texas prison system that permits the early release of prisoners so that room can be made for more, when he's against the higher taxes to build new prisons and he won't spend a dime on trying to eliminate the poverty that breeds crime. Ask people from the other forty-nine states how many stars there are on the flag and they will answer fifty. Ask any Texan and he'll say one. Why? The answer is always the same: "General Principles". Despite his flamboyance and fascinating career, it is not this General Principles that we must now address. The general principles of liquid penetrant examination are as follows. Liquid penetrant examination is an NDE method used to detect open, surface discontinuities. A liquid that contains a dye or fluorescent material is applied to the part being inspected so that the entire surface under consideration is wetted. The liquid inspection medium, called the penetrant, seeps into the open nooks and crannies or is drawn into the tighter spaces by capillary action. It is allowed to remain on the part (the dwell time) for about five minutes in order to allow sufficient time for the tighter discontinuities to become filled with penetrant. The excess penetrant is then removed from the surface. A fine powder, called the developer, is dusted or sprayed on the surface until a thin layer covers the area of interest. This layer of developer acts like a sponge and absorbs the penetrant that was left in the discontinuities. The penetrant is drawn out of a discontinuity and spreads in the developer much like a drop of ink placed on a blotter. The bleed out of the penetrant covers a larger area then the actual exposed surface area of the discontinuity. Discontinuities can thus be found by the contrast between as applied developer and the developer that is stained by the penetrant drawn from the discontinuities. Liquid penetrant examination can be performed on any non-porous material. Proper cleaning is essential prior to examining a part. Obviously if surface discontinuities are plugged up with oil or grease then no penetrant can enter and the discontinuities will go undetected. Although as-cast and as-forged surfaces can often be examined successfully, it must be remembered that smaller discontinuities may be plugged with oxides that formed during casting or forging or during subsequent heat treatment and thus go undetected. The surface roughness of the part being inspected must be such that indications from discontinuities are not masked by background bleed out. Liquid penetrant examination techniques are classified by the types of penetrants and the methods used to removed them.

PT - Types of Penetrants page - 217

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Penetrants may be broadly classified as either visible or fluorescent. Visible penetrants are generally dyed red to provide the best contrast with the white developer powder. As the name implies, examination of any indications is done in visible light. Fluorescent penetrants require the use of a black (near-ultraviolet) light and a darkened area for viewing indications. Small particles of fluorescent material are suspended in the penetrant and glow brightly when excited by a black light giving them excellent contrast with their surroundings. Fluorescent penetrants are generally used in a process environment at a permanent work station because of the black light requirement and the fact that the methods that utilize fluorescent penetrants generally require more stringent controls in order to obtain satisfactory results. Fluorescent penetrants come in several different sensitivity levels. The sensitivity of the higher levels is considerably better than the best sensitivity of a visible penetrant. Fluorescent and visible penetrants may be further subdivided into groups based upon the method in which the excess penetrant is removed from the part after sufficient dwell time. Water-washable penetrants are removed by rinsing the part with water. They contain an emulsifier as part of their formulation. An emulsifier is a liquid that breaks up the oily portion of the penetrant and keeps the oil globules in suspension. The emulsified penetrant is easily rinsed off with water. Of course great care must be exercised when washing off the excess penetrant so that the penetrant within any open discontinuities is not also washed out. Water-washable penetrants are generally used in a process environment at a permanent work station where water and drain facilities are available, although they are sometimes used in kits for field testing. Their main advantages are convenience (a separate emulsifier does not need to be applied), excess penetrant is easily and rapidly removed which reduces inspection time, there are no health or safety concerns with using water as the removing agent, and their sensitivity is good. Post-emulsifiable penetrants are insoluble in water. After the proper dwell time, a separate emulsifying agent is applied onto the excess penetrant. This combines with the penetrant to form an emulsified mixture which then can be rinsed off with water. The emulsification time is critical (typically a few seconds to several minutes) because you want to ensure that the excess penetrant is easy to rinse off, but that the penetrant within any discontinuities does not become emulsified. If done properly, the penetrant within the discontinuities will retain its oily nature and not be subject to overwashing. The emulsifiers used in liquid penetrant examination are classified as either lipophilic or hydrophilic. Lipophilic emulsifiers are oil based and work by diffusing into the excess penetrant and forming a water-washable mixture. They are used undiluted. Hydrophilic emulsifiers are detergent based and work by displacing the oily penetrant from the surface thus allowing it to be water rinsed. They come in a concentrated form that is mixed with water prior to application. By varying the concentration and the emulsification time, the removal of excess penetrant can be closely controlled. Lipophilic emulsifiers work faster than hydrophilic, but are not as flexible and may not give as good a sensitivity. Post-emulsifiable penetrants are often fluorescent. They are generally used in a process environment at a work station where the procedure can be carefully controlled.

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Excess solvent-removable penetrants are removed from the surface of a part by first wiping as much as possible off the surface with a clean, lint free cloth. Then another clean, lint free cloth is lightly sprayed with a solvent (typically from an aerosol can) and is used to wipe off the last traces of penetrant. The solvent is never sprayed directly onto the part. Again, great care must be exercised to ensure that the penetrant within any discontinuities that may be present is not removed. Penetrants may be applied by spraying, brushing, dipping, etc. Solventremovable penetrants are often used in aerosol cans for field applications. The required dwell time varies by the type of material being inspected and the type and brand of the penetrant. For a casting, a dwell time of at least five minutes and no longer than sixty minutes is generally adequate. For a forging, dwell time is typically 10-60 minutes.

PT - Developers Developers, as previously discussed, draw the penetrant out of flaws and help it to spread thus increasing the size and visibility of the indication. There are four main types: dry powders, water soluble powders, water suspendable powders, and nonaqueous. Dry Powders are most often used in conjunction with fluorescent penetrants. They provide excellent sensitivity. The part is first dried to remove any water left over from the rinsing operation. The developer is applied by dusting with a hand held powder bulb, spraying, immersing the part into a container of dry powder, immersing the part into a fluid bed of dry powder, or by placing the part into a dust chamber in which powder particles from a controlled "dust cloud" eventually coat the part. Dry powders are easiest to apply. Water soluble powders are supplied as dry powder that must be dissolved in water before use. The developer is applied immediately after the excess penetrant has been removed and prior to the part being dried thus saving developing time. Application may be by immersion, spraying, flowing, etc., with immersion being the most common. After the part has been totally wetted by the developer, any excess developer is drained off and the part is allowed to dry. The dried developer will appear as a white coating on the part. Water soluble developers are typically used with fluorescent penetrants. They are seldom used with penetrants that are water washable because of the possibility of rinsing away some of the penetrant. They are the easiest developer to clean off a part. Water suspendable developers are similar to the water soluble developers just discussed (they may both be referred to as "aqueous" developers) except the powder does not dissolve in the water, but rather is kept in suspension. The proportion of powder to water is very important and specific gravity checks must frequently be made to ensure that the proper proportion is maintained. This type of developer is typically used with visible penetrants.

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Non-aqueous (or sometimes referred to as solvent) developers have the developer particles suspended in a solvent vehicle at the time of application. They come ready to use and are applied only after the part has been dried. Application is by spraying. The solvent vehicle quickly evaporates and leaves a white coating of powder on the surface of the part. It is easily applied using an aerosol can and is often used in kits for field work. Non-aqueous developers are usually considered to provide the best sensitivity. Development time depends on several factors. Typically the developer must be on the part for at least seven minutes before inspection begins. Developing time starts immediately after the application of a dry developer or when a non-aqueous or aqueous developer coating has dried. Development times may go as high as 30 minutes or longer as long as the bleed out does not alter the inspection results.

PT - Liquid Penetrant Examination PROCESSING FLOW DIAGRAMS Here we illustrate the use of various combinations of penetrant and developers in the inspection of a part. The flow diagrams are the same regardless of whether fluorescent or visible penetrants are used. 1. 2. 3. 4.

Water Washable Liquid Penetrant Examination - Figure 40 Post Emulsifiable (Lipophilic) Liquid Penetrant Examination - Figure 41 Post Emulsifiable (Hydrophilic) Liquid Penetrant Examination - Figure 42 Solvent Removable Liquid Penetrant Examination - Figure 43

PT - Advantages and Disadvantages This is it. The end of the line. Finis. No doubt you thought I would drivel on forever on the subject of nondestructive examination. You were almost correct. The broad subject of NDE cannot be encapsulated into just a few pages any more than the great epics of Homer, Virgil, or Milton. Yet this chapter, despite the number of pages, is a model of brevity and succinctness. If I had done full justice to the topic, Diderot's Encyclopedia or the seven volumes of Gibbon's treatise on the declination of the Roman Empire would be a mere billet-doux in comparison. Count your blessings. Advantages & & & &

page - 220

It is easy to use and in many cases is easily performed in the field. It can be performed on any non-porous material. Equipment is inexpensive in comparison to other NDE methods. Sensitivity is good (open discontinuities as small as 1/64" can be detected).

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&

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It is more sensitive to certain types of discontinuities than magnetic particle examination (porosity for example). Orientation of the discontinuity makes no difference.

Disadvantages & & &

&

Discontinuities must be open to the surface. Discontinuities such as stringers cannot be reliably detected. It is a surface technique only, defects below the surface are not detected. The surface condition of the part being examined must be such that there is not excessive background bleed out of the penetrant that would interfere with evaluation. Parts typically are machined before being inspected. Post-inspection cleaning is required.

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Figure 40: Water Washable Liquid Penetrant Examination

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Figure 41: Post Emulsifiable (Lipophilic) Liquid Penetrant Examination

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Figure 42: Post Emulsifiable (Hydrophilic) Liquid Penetrant Examination

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Figure 43: Solvent Removable Liquid Penetrant Examination

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NOTES:

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NOTES:

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CHAPTER IX

SPECIAL PROCESSES Hungarian Goulash

The armadillo is the unofficial mascot of Texas. This is only natural because armadillos and native-born Texans have a lot in common. Both are thick-skinned and somewhat nearsighted, and both are living relics from out of the past. Armadillos and Texans have both found that having a thick, protective hide over their soft "innards" has enabled them to survive in the harsh Texas climate. There are many environments where we are going to want a "thick, protective hide" over our base metal's "innards" in order to minimize the effects of corrosion, wear, abrasion, and galling. In this lesson we are going to examine the hodgepodge of different plating, coating, and surface hardening processes that we utilize to tan the hide of our metal components to make them more suitable for service. The purpose of the special processes that we are going to discuss in this section is to impart some desired property to a localized area of a part without altering the rest of the part. We may want to enhance corrosion resistance, lubricity, abrasion and wear resistance, or galling resistance. Why not use a metal that inherently has these properties? Why fool around with special processes? For several reasons. Very often the enhancement of one property by an alloy addition will be detrimental to another property. The alloying additions that make stainless steel "stainless" for example, can also make stainless steel difficult (in comparison to a low alloy steel) to form, machine, and weld as well as significantly increasing the cost. A special process such as plating may allow the use of a low alloy steel (with its good weldability, ease of fabrication, and low material cost) in lieu of a stainless steel in a corrosive environment. It is not always desirable to have completely homogeneous properties in a metal part. For example, a high surface hardness is desirable on a gear tooth to prevent wear and galling. We may be able to through harden a gear to the optimum hardness for wear and galling resistance, but in doing so significantly reduce its impact toughness. A special process such as carburizing or induction hardening can harden just the surface of the gear tooth while leaving the underlying material ductile and impact resistant. In essence it allows us to make a bimetallic material in which the properties of a specific area are tailored to a specific usage.

ELECTROPLATING Electroplating is a process used to deposit an adherent surface layer of a metal having some desired property onto a substrate that lacks that property. It may also be used to salvage undersize parts by building up the thickness. We use electroplating primarily for abrasion and wear resistance, corrosion protection, and to provide lubricity. The part to be plated is made the cathode of an electrochemical cell. The anode is usually made out of the metal to be plated on the part. Both the anode and the cathode are immersed in the plating solution or bath. The bath contains ions of the metal to be plated as well other ions that permit the flow of electricity. A solution of ions capable of carrying a current is sometimes referred to as an electrolyte. A rectifier supplies a direct current that causes the metal ions to lose their charge and plate out on the cathode. As page - 228

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current flows, the anode slowly dissolves and replenishes the ions in the bath. Some electroplating processes utilize a nonconsumable anode such as lead. Ions of the metal to be plated must be added to the bath by adding the appropriate chemicals and must be periodically replenished as the ions are plated out of solution. The amperage of the electroplating current divided by the surface area of the part is called the current density. It is a very important parameter because it has a strong influence on the deposition rate, plating adherence, and plating quality. Current density can vary over the surface of a part. Outside surfaces will have a higher current density than inside surfaces (bolt holes, valve bores, etc.) unless special provisions are made. Current density will vary as a result of surface profile. The current density at the crest of a thread, for example, is typically higher than that at the root. The higher the current density, the faster the deposition rate will be. Current density has a practical limit because if the deposition is too rapid, adhesion and plating quality will be poor. While most plating cells utilize a continuous direct current, the current in some high efficiency baths may be cycled on for 8-15 seconds and then off for 1-3 seconds until the desired plating thickness is obtained. In this way higher current densities can be used and still produce a quality deposit. Current is sometimes reversed when plating rough parts or when a bright finish is required. Reversing the current causes some of the plating to go back into solution. A high current density at a peak in the surface profile of a part will result in a greater deposit thickness than in a valley with a lower current density over a period of time with continuous current. By reversing the current, the high current density of the peak will cause more of the deposit to go back into solution while the deposit thickness in the valley is not significantly effected. This causes a leveling effect that eventually allows the valleys to be filled in without overplating the peaks. A strike is a special plating deposit that has high quality and good adherence to the substrate (the base metal). It is typically very thin (half a mil or less) and serves as a foundation for a subsequent plating process. The plating process for a strike uses a high cathode current density and a bath with low ion concentration. While producing a high quality, tightly adherent plating, the process is very slow so once the desired strike thickness is obtained, a switch is made to a more efficient plating process to build the deposit up to the final required thickness. Deposits of different metals may be used in combination for several different reasons. It may be desirable to plate one type of deposit onto a metal part to improve corrosion resistance, for example. But if the desired plating has inherently poor adhesion to the substrate metal, little corrosion protection will be afforded. We can solve this problem by first depositing a strike that is compatible with both the substrate and the plating desired for corrosion protection. As an illustration of this, electrolytic nickel has poor adhesion to zinc alloys. We can still plate a zinc alloy part with electrolytic nickel if we first plate the part with a copper strike. The copper plating has good adherence to zinc alloys and the electrolytic nickel, in turn, has good adherence to the copper. Two different metal platings are sometimes used in combination in order to combine two different desired properties. For example, one of our ball valve trims has the ball electroless nickel plated (which we will talk about later) and then electrolytic page - 229

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chromium plated. The electroless nickel plating provides excellent corrosion protection to the carbon steel ball while the hard chrome plating provides abrasion and wear resistance. Many plating baths use cyanides of the metal to be deposited as well as other cyanides (such as potassium cyanide) in their formulation. Free cyanides help to maintain a constant metal ion level in the bath by facilitating anode corrosion and also contribute to the conductivity of the bath. Other chemicals such as carbonates and phosphates may also be added to the bath to increase conductivity. Stop-offs are materials that can be applied to the substrate to mask off areas where plating is not desired. They merely prevent the bath from coming into contact with the masked off area. Common stop-offs include tape, foil, waxes, lacquers, etc. Hydrogen is a by-product of many plating processes. Some of the cleaning processes used to prepare parts for plating may also be sources of hydrogen. Hydrogen is a concern because monatomic hydrogen can interact with the microstructure of some susceptible metals and cause embrittlement. To prevent hydrogen embrittlement, many plating procedures require a post plating heat treatment or bake. A bake generally consists of heating the plated parts up to 375(F or higher and then holding for four or more hours. This allows the hydrogen to diffuse out of the metal. Cadmium plated parts require an extra long baking time (often over sixteen hours) because the diffusion of hydrogen through the plating itself is very sluggish. Baking is typically done immediately after plating, but in no case should it be delayed for more than four hours. In general, the higher the strength (or hardness) of a susceptible metal, the more prone it is to hydrogen embrittlement. Low alloy and carbon steels with a hardness range exceeding 35 HRC are especially suspectable. We will now look at some of the more common metal electroplatings used in the Oil Patch. 1. Chromium - Chromium plating is very hard and corrosion resistant. It is typically specified on parts requiring some degree of wear, galling, or abrasion resistance. The thickness of hard chrome plating may vary from a flash (about 0.01 mil) to 20 mils or more (particularly for salvage applications). The corrosion resistance of chromium plating is excellent except in reducing solutions, sulfuric acid, and halogen acids. Hydrochloric acid is of particular concern because it does the number on chromium plating and is frequently used in acidizing wells. Chromium is effective as a barrier coating (one that isolates the base metal from the environment) when it is plated within certain thickness ranges. When deposited in thicknesses exceeding approximately 0.00002", the internal stresses become high enough within the plating to cause hairline cracks. These cracks, if they extend all the way through the plating, naturally will expose the base metal to the environment and defeat the purpose of the plating. If we continue to plate until thickness page - 230

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exceeds several mils, eventually a point will be reached where the initial cracks become sealed off by the subsequent deposit. Of course, the subsequent deposit will also contain hairline cracks, but hopefully (as is often the case) there is no interconnecting path to the base metal. Where parts are critical and corrosion protection must be assured, electroless nickel would be a better choice for most environments. 2. Cadmium - Cadmium plating is widely used for corrosion protection on steel and cast iron parts in mildly corrosive environments. It is typically deposited about one half mil thick. It is used extensively on steel fasteners. The cadmium plating is thin enough so that it does not create a problem with thread clearances. It has a natural lubricity that helps to prevent galling between threads. The high purity cadmium anodes are typically ball shaped so that they present the maximum surface area. A cyanide bath is often used and contains cadmium oxide, sodium cyanide, as well as other chemicals. Noncyanide baths are becoming more common because of environmental restrictions on cyanides. These baths use bar-shaped anodes and produce much less hydrogen as a by-product than do the cyanide baths. Cadmium can cause embrittlement in certain metals when exposed to elevated temperatures. Cadmium plating should not be used on steel parts at service temperatures over roughly 400(F. Cadmium plating is often used with a chromate conversion coating. After baking, the plated part is immersed in a solution of chromic acid or other chromates and catalytic agents. The solution chemically reacts with the cadmium at the surface of the deposit to form a protective film containing complex chromium compounds that help to seal the plating and enhance corrosion resistance. Cadmium is a very toxic metal. Safety and environmental restrictions have cause many platers to drop cadmium altogether. Electroplated zinc is an acceptable substitute for cadmium for virtually all Oil Patch applications. 3. Silver - The primary use of electroplated silver in the Oil Patch is to provide lubricity on metal-to-metal seals during installation. It has excellent corrosion resistance, although it is rapidly attacked by H2S. Anodes are high purity silver bars. Silver plating baths typically contain silver cyanide as the source of metal ions. 4. Zinc - Like cadmium, electroplated zinc is used to provide corrosion protection for steel and cast iron parts in mildly corrosive environments. It is typically deposited 0.3-0.5 mils thick. As previously page - 231

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mentioned, it is rapidly replacing cadmium because of the problems associated with cadmium's toxicity. Both cadmium and zinc provide only mild corrosion protection. At the thicknesses plated on fasteners, they provide corrosion protection for long term storage and for short term atmospheric exposure in mild environments. Zinc plating is equal to or slightly better than cadmium in its atmospheric corrosion resistance. Cadmium is sometimes preferred for fasteners with extremely close tolerances because it has less of a build-up of corrosion products (particularly in marine environments). The anodes used in zinc plating are high purity zinc and may be spherical or barshaped. Zinc electroplating may be given a chromate conversion coating to enhance corrosion resistance.

ELECTROLESS NICKEL PLATING Electroless nickel plating, as the name implies, is not an electrochemical process like the other plating processes we just described. Instead, the nickel ions in solution in the plating bath are chemically reduced onto the activated surface of the part. The deposited metal is catalytic to the reduction reaction so the deposit will continue to grow in thickness as long as it is immersed in the plating solution. Electroless plating processes have the ability to form thick, uniform deposits. Areas that are difficult to plate electrolytically (blind holes, thread roots, etc.) are easily plated with an electroless process. There are several metals that can be plated electrolessly, but electroless nickel is the most common and the only one used in the Oil Patch. Electroless nickel is used for corrosion protection, wear and abrasion resistance, and for salvaging undersize parts. The composition of the deposit is not pure nickel, but may contain 4-12% phosphorous. This has a significant effect on the residual stresses and over-all mechanical properties of the plating. The optimum combination of strength, ductility, and low residual stresses is obtained when the deposit has 10-11% phosphorous and most commercial baths are formulated to provide this amount. Most commercial baths contain sodium hypophosphate as the reducing agent which provides the electrons to reduce the nickel ions. A nickel salt (such as nickel chloride or nickel sulfate) is the source of nickel ions. The bath contains other chemicals to control pH, the free nickel available for reaction, and the reduction process. The as-deposited plating is amorphous - it has no crystal structure. Electroless nickel in the as-plated condition is thus one of the few metallic glasses that has found widespread use. The as-plated condition offers the optimum corrosion resistance. Parts are sometimes baked after plating for hydrogen embrittlement relief or to increase hardness. Heat treating at approximately 430 (F and above causes nickel phosphides to precipitate out in the deposit. This increases the hardness of the deposit, but decreases corrosion resistance because it is accompanied by pore and hairline crack formation. Heat treating or baking will also cause the amorphous plating to crystallize. A 750(F bake for about one hour gives the maximum hardness (roughly 1050 HV), but results in the lowest ductility. A 400(F bake is sometimes used for hydrogen embrittlement relief page - 232

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because it is high enough to facilitate the diffusion of hydrogen, but low enough not to adversely affect corrosion resistance. Generally steel parts under 35 HRC do not need to be baked for hydrogen embrittlement relief after plating. Heat treating will also slightly improve the adhesion of the plating so baking is often specified regardless of hardness. Electroless nickel is usually plated 1-3 mils thick for most applications and often much thicker for salvage purposes. It is usually heat treated to increase hardness when used for abrasion and wear applications. It is used either as-plated or with a low temperature bake for optimum corrosion resistance.

THERMAL SPRAY COATINGS Thermal spray coating is a catch all phrase for many different processes in which the coating material is heated up and then propelled onto the surface of the workpiece. The molten or near molten particles of coating material will "splatter" on the surface just like snowballs thrown against a brick wall. The particles will adhere to the substrate as well as to each other through several different mechanisms depending on the particular process involved. Final bonding may be strictly mechanical as surface irregularities on the particles and the substrate interlock or may be metallurgical or a combination of the two. Like platings, thermal spray coatings are used primarily for providing corrosion resistance, wear and abrasion resistance, galling resistance, and for salvage. Many thermal spray coatings can be applied to finish machined parts without affecting the substrate's properties. Other processes may require that the coated part undergo a stress relief or a full heat treatment. Mechanical bonding plays an important role in most thermal spray processes either as the final bonding mechanism or as an intermediate bonding mechanism used to hold the coating particles in place until final, metallurgical bonding is obtained through further processing. Because of this, the surface of the part being coated must be thoroughly cleaned of substances that may interfere with bonding (rust, grease, oxides, etc.). Often the surface will be grit blasted to obtain a desired anchor pattern for the coating. Coated parts typically have a sandpaper-like or matte finish. If the coated surface is used in a sealing application or is a bearing surface, then a finishing operation such as grinding or lapping may be necessary to obtain the desired surface finish. Some of the softer coating materials are readily machined. Thermal spray coatings often contain porosity that is interconnecting. Sealers (such as waxes, silicones, resins, aluminum silicate, etc.) may be brushed on, sprayed on, or vacuum impregnated into the coating to plug the pores and keep the base metal from being exposed to the environment. Sealing must be done prior to any finishing operations. The thermal spray processes that are commonly used in industry are:

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1. Flame Spray (Powder) — Flame spray processes use a combustible gas to heat the coating material and to propel it towards the surface to be coated. The coating material is in the form of powder that is aspirated into the combustion flame in the nozzle of the spray gun. Oxygen and acetylene are the most commonly used combustion gases and generate flame temperatures over 4000(F. The high temperature rapidly heats the coating particles to a molten or semi-molten state. The expanding combustion gases propel the coating particles towards the surface of the part at typical velocities of 80-120 feet per second. The particles will splatter on the surface of the part and mechanically bond. The substrate will remain relatively cool (below 350(F) during the spraying process so coating may be done on finished parts without altering the base material properties. Flame powder spraying is inexpensive, easy to use, and has a moderate deposition rate. In comparison to other thermal spray methods, it has relatively poor adhesive and cohesive strengths. Flame powder spraying is frequently used to apply corrosion resistant coatings of zinc, aluminum, or other protective metal (this is often referred to as metallizing) and salvage work. Mechanical bonding is generally adequate for corrosion resistant coatings, but may not be for wear and abrasion coatings that are subjected to high shear stresses in service. Fortunately there are some flame spray coating materials that are designed so that they can be fused to the substrate for increased adhesion. After spraying, the inprocess coating (which adheres by mechanical bonding) is heated to typically 1850-2150(F by torch, induction heater, or by placing the part in a furnace or salt bath. At these temperatures, a true metallurgical bond will form between the coating particles and the base metal. The high fusing temperature is well into the austenitizing range for steels, so unlike other flame spray processes, parts with fused coatings are typically heat treated after coating in order to develop the base metal properties. Parts are often quenched in a salt bath (to minimize stresses) directly from the fusing temperature and then tempered. Examples of fused, flame spray coatings include the various type of Colmonoys® that we use to hardface gates, stab pins, etc. Fused coatings tend to have less porosity and, of course, much better adhesion than other flame spray techniques. 2. Flame Spray (Wire) — This process is similar to flame powder spray except that the coating feedstock is wire and while the combustion flame melts the wire, a stream of compressed air is used to atomize and propel the molten coating particles to the surface to be coated. It uses a higher combustion temperature (around 5000(F) than flame powder spraying. The impact velocity of the coating particles on the substrate is on the order of 100 feet/second. This gives it slightly better adhesive and cohesive strength than flame powder spraying. Like flame powder spraying, flame wire spraying is economical, easy to use, and readily available. It is also used primarily for applying corrosion resistant coatings.

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3. Twin Wire Arc — In this thermal spraying process, two consumable wire electrodes made out of the desired coating material are fed into the spray gun. An arc is established between their tips as they come together causing them to melt. Compressed air is used to atomize the molten coating material and to propel it to the surface to be coated at velocities of around 800 feet/second. The deposition rate and the adhesive and cohesive strengths are much better than flame spraying. Like the flame spraying methods, twin wire arc in inexpensive, easy to use, and readily available. 4. Plasma — A plasma is a gas that has been heated to an extremely high temperature that causes each gas atom to become ionized (each atom loses one or more electrons depending on the material). A plasma-arc gun has a chamber in which the plasma is generated by introducing the primary arc gas (usually argon or nitrogen) and then ionizing it by electrical discharge. The resulting plasma consists of positive ions and free electrons and is thus capable of carrying a current. By passing an electric current through the plasma, the temperature can be raised through resistance heating. Secondary gases (such as nitrogen, helium, or hydrogen) are then added to increase the ionization potential of the gas mixture. Portions of the plasma may reach temperatures over 18,000(F. Coating material powder is introduced into the plasma. The plasma is used to accelerate the coating particles to speeds in excess of 4000 feet/second while heating them up to 6000(F. The water-cooled gun directs the stream of particles against the part causing them to interlock with themselves and the substrate to form a dense, tightly adherent coating. Bond strength is typically over 20,000 psi. The temperature of the part does not exceed 300(F during coating. Plasma-arc spray coatings are denser (usually less than 2% porosity) and have better bond strength than conventional flame spray techniques. Some types of coating materials can only be applied using plasma-arc spray techniques. The main disadvantage of plasma-arc (as well as detonation gun) techniques is the cost and complexity of the equipment. Sermatech’s GatorGard® process is an example of a plasma-arc spray process that we have used to hardface gates, etc. 5. Detonation gun coating is a special type of flame spray coating. Oxygen, acetylene, and particles of coating material are mixed together in the gun’s firing chamber. An electric spark is used to detonate the explosive mixture creating a hot, high speed gas stream. This instantly heats the coating particles and propels them at speeds up to 2500 feet/second. This process may occur five or more time per second. The temperature inside the combusting gases may reach 6000 (F. The part being coated remains below 350(F. Detonation gun coatings have excellent adhesion bond strength (over 20,000 psi) and less porosity than the conventional flame spray coatings. Union Carbide’s D-Gun Process® coatings are an example of the detonation

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gun process. We use the D-Gun Process® to apply Union Carbide’s LW-45® coating as a hardfacing to gates, etc. Hardfacing coatings, such as LW-45®, are a mixture of materials. Typically they utilize tungsten carbides (or some other extremely hard particles) for wear and abrasion resistance. These hard particles do not “splatter” very readily and consequently would have very poor adhesion if used by themselves. For this reason they are always used in a mixture with a soft, ductile material, such as cobalt or nickel alloy, that will form a continuous matrix in the coating that holds the hard particles in place. 6. High Velocity Oxyfuel (HVOF) — A fuel gas and oxygen enter a chamber in a spray gun where combustion takes place generating temperatures near 5500(F. Either powder or wire can be used as the coating feedstock. The high pressure combustion gases with the entrained coating particles exit through a small diameter barrel at velocities up to 3500 feet/second. HVOF coatings are extremely dense with excellent adhesive and cohesive strength. It is used extensively for both hardfacings, corrosion resistant coatings, and for salvage. We use it for hardfacing gates. Thermal spray coatings are being used more and more in the Oil Patch. Their most significant advantage is that they can be applied without screwing up the substrate’s properties. I have seen many engineers get starry-eyed when they first learn about thermal spray coatings. They immediately want to use thermal spray coatings as a panacea for all their problems. Thermal spray coatings are the answer to their prayers, a dream come true. Their imagination knows no bounds when it comes to repairing defects by thermal spraying. They are intoxicated by its endless possibilities. Unfortunately many thermal spray vendors contribute to the illusion that their processes are the ticket to the Promised Land. There’s only one way to handle an engineer that has gone ga-ga over thermal spraying: grab him by the collar and slap him silly until he comes to his senses. Depending on how deeply rooted the doctrine of deception is, rehabilitation may take years. Thermal spray coatings are not a cure-all for all of our problems. They have some inherent limitations that must be clearly understood before using them either for production or repair work. Thermal spray coatings are not a substitute for weld repair when it is necessary to build up a dimension that is below the minimum design allowable for structural integrity: the mechanical properties of the coatings are far below that of the substrate. Some thermal spray coatings (e.g. 625 nickel alloy) have very poor ductility making them prone to cracking if the substrate is flexed. Many tend to be quite brittle making them prone to chipping or cracking under an impact load. Adhesive strength varies by the type of application process. Porosity also varies by the type of process used to apply the coating. Thermal spray coatings can be applied to surfaces that are within “line of sight” of the spray gun — the coating particles travel in a straight line until they inpinge on the substrate. The part geometry thus heavily influences the feasibility of using thermal spray as a coating method — threads, blind holes, small bores, etc., can be very difficult or impossible to coat.

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As-deposited thermal spray coatings are relatively rough and very much like coarse sandpaper in appearance. For many applications (such as external corrosion protection) the coating can be left as-deposited. For other applications the coating may have to be machined, ground, or lapped to provide the required surface finish and finished dimensions. Except for hardfacings, most thermal spray coatings are each to machine, however, great care is required to insure that the machining process does not crack or chip the coating, or lift it off the substrate. Any Tom, Dick, or Harry can buy a thermal spray gun and setup a shop in his garage. He may even produce pretty decent looking coatings. But looks can be deceiving. Does the coating contain porosity? What are the mechanical properties of the coating? Are there any inclusions (e.g. grit blast media) in the coating? What is the adhesive strength of the coating? What is the cohesive strength of the coating? These and many other questions must be answered before putting a part that has been thermal sprayed into service. In most cases the answers to these questions can only be found by performing a destructive test on a sacrificial part or test panel: there is no NDE method to test a production part. As a consequence, it is vitally important that only qualified vendors using qualified procedures be used. Furthermore, it is vitally important that a given thermal spray process be qualified for each type of application before being specified. Depending on the criticality of the application, qualification of the thermal spray process, the vendor, and the vendor’s procedure may take several years or longer. Once qualified, vendors should be closely monitored to insure consistent quality. Thermal spray coatings have a bright future in our industry. Be sure you understand their advantages and limitations before using them.

PHOSPHATE COATING Phosphate coatings are used on steel parts for corrosion resistance, lubricity, or as a foundation for subsequent coating or painting. It is a type of conversion coating in which a dilute solution of a phosphoric acid chemically reacts with the surface of the part being coated to form a layer of insoluble, crystalline phosphates. The part may be sprayed with or immersed into the phosphating solution. The resulting phosphate layer is porous so for most applications that require corrosion resistance the coating will be treated with oil, sealers, etc. Phosphate and oil coatings (P&O) are frequently used for corrosion protection and for lubricity to prevent galling. Most phosphate coating is done as a surface preparation for painting and coating. The tightly adherent phosphate layer is an excellent base for painting and coating materials for two reasons. These materials will seep into the porosity and become mechanically interlocked after drying thus giving excellent adhesion. Secondly, the phosphate coating is a dielectric and will electrically isolate anodic and cathodic areas on the surface of the part from each other. This will minimize underfilm corrosion that takes place at the interface of the paint or coating and the substrate. There are three basic types of phosphate coatings: manganese, iron, and zinc. Iron phosphates are typically used as a base for paints or coatings. They are applied by page - 237

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immersion or by spraying and are generally used at coating weights of 20-80 mg/ft2. Zinc phosphates are used for rust proofing, lubricity (with oil), and as a paint/coating base. Coating weights are typically 100-1000 mg/ft2 when sprayed on or 150-4000 mg/ft2 when applied by immersion. Manganese phosphates are applied only by immersion which results in a typical coating weight of 500-3000 mg/ft2. They are used for both corrosion resistance and lubricity. The crystal structure as well as the weight of a phosphate coating significantly affects the performance of the coating. A tightly adherent fine grained ("microcrystalline") structure is generally the optimum for corrosion resistance or subsequent painting. For wear resistance, a coarse grain structure impregnated with oil may be the most desirable. Grain size and weight are controlled by selecting the appropriate phosphate solution, using various additives, and controlling bath temperature, concentration, and phosphating time. All phosphating solutions are based upon phosphoric acid. Phosphoric acid will attack the surface of the part being coated and cause some of the metal atoms to go into solution. This, in turn, neutralizes the acid solution within a thin zone along the metal-bath interface. The solubility of the metal phosphates is less in this neutralized zone than in the original acid solution and consequently they will precipitate out onto the surface of the metal. The crystalline precipitates are attracted to cathodic sites on the surface of the workpiece by electrostatic forces. Phosphating solutions contain divalent metal phosphates and chemicals known as "accelerators" in addition to phosphoric acid. The accelerators increase the rate of coating buildup by removing hydrogen (a byproduct of the reaction) from the surface of the metal. Hydrogen can blanket the surface, if not removed, and thus keep the solution from coming into contact with the metal. Other chemicals may be used in the phosphating solution to prevent the buildup of soluble metal compounds that would interfere with the coating process. A typical phosphating procedure consists of the following steps: surface clean, rinse, surface activation (depending on the process), phosphating, rinsing, neutralizing rinse (optional), drying, and the application of supplemental sealers, oil, etc. Phosphating solutions are generally kept at 120-200(F. Spraying builds up a given weight of coating much faster than immersion, but thick coatings are usually made by immersion. Phosphating time may vary from less than a minute to half an hour or longer depending on the process and the desired coating weight. There are many manufacturers of phosphate solutions. One that we frequently use on our products is Aerocote® #4. This is a microcrystalline nickel-manganese phosphate used for both wear and corrosion resistance (in conjunction with a suitable oil). It is frequently specified as an alternative to cadmium plating on threaded fasteners.

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Flame hardening is one way to perform a localized heat treatment on certain types of steels in order to increase the surface hardness. It involves heating the part with a high temperature oxygen-acetylene flame that is directed at the desired area by a nozzle. The surface of the part is heated by the flame until it is austenitized. A stream of water, diluted oil, polymer, or compressed air is then used to quench the heated area in order to form martensite thus hardening the surface. As always, the martensite must be tempered and this is usually done by directing another flame at the area. Flame hardening is often automated by rotating a part so that a particular area is first exposed to a flame nozzle for austenitizing, a quenchant nozzle for transforming to martensite, and another flame nozzle for tempering. Thus all three operations may be taking place simultaneously on the same part. Flame hardening is a fast inexpensive means of increasing the surface hardness of steel. Its main drawbacks are that only certain types of steel can be hardened effectively and that precise control over the area to be hardened is impossible. Carbon steels containing 0.35 - 0.55% carbon can be flame hardened as can many low alloy steels. The depth and the value of hardness that can be obtained is dependent on many variables including the composition of the metal, flame temperature, and the cycle time for each operation. One of the applications we have for flame hardening is surface hardening packer plates in annular BOP's.

INDUCTION HARDENING Like flame hardening, induction hardening is a localized heat treating technique for steel parts. A work coil or inductor is placed adjacent to the area of the part to be hardened. An alternating current is passed through the coil which induces a highly concentrated, alternating magnetic field within the coil. This magnetic field induces a current flow within the part. Resistance heating will raise the temperature into the austenitic region. After a suitable holding time, the part will be quenched in order to harden it. The pattern and depth of heating (and subsequent hardening) are controlled by the shape of the coil, the number of coil turns, operating frequency, and power input. The depth of heating increases as operating frequency decreases. High frequencies are thus used for surface hardening while low frequencies are used for through hardening. In order to restrict the hardening effect to the surface of a part, surface hardening processes utilize high power for a short duration. After quenching, the hardened part may be tempered using induction heating. The primary advantage of induction hardening over flame hardening is that it is much easier to control the depth, hardness, and area of the case (the hardened surface layer). The case is more uniform and is easily restricted to a particular area by altering the configuration and position of the coil. The hardness of the resulting case is dependent on the base metal composition as well as the process parameters. page - 239

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Disadvantages include high cost, coils must be designed for each application, and the fact that complex shapes may be difficult to heat in a prescribed area. Induction hardening is frequently used to surface harden shafts and gear teeth. We use it to harden the bore on pipe and variable rams.

NITRIDING Nitriding is a process for increasing the surface hardness of a suitable metal (we will limit our discussion to steels) by altering the surface composition. Nitrogen is diffused into the metal at an elevated temperature and combines with alloying elements such as aluminum, chromium, vanadium, tungsten, and molybdenum to form hard nitrides. Aluminum in particular is a strong nitride former and is intentionally added to some types of steels in the 0.85 - 1.50% range for just this reason. Stainless steels; tool steels containing 5% or more chromium; medium carbon; chromium containing low alloy steels; and aluminum containing low alloy steels can all be nitrided. Stainless steels will lose some of their corrosion resistance when nitrided, but will still be much more resistant than if they were carburized (which will be discussed later). Nitriding is done primarily to prevent galling, increase wear and abrasion resistance, and to improve fatigue life. Fatigue life is improved because nitriding causes a slight volumetric increase in the nitrided layer which, in turn, creates residual compressive stresses. There are three basic methods of nitriding: gas, ion (or plasma), and liquid. Gas nitriding uses a nitrogenous gas such as ammonia as the source of nitrogen. The ammonia dissociates at the surface of the hot steel parts in a nitriding furnace and releases nitrogen as one of the products. Liquid nitriding utilizes a molten salt bath containing cyanides and cyanates as the source of nitrogen. Ion nitriding uses nitrogen gas (N2) as a source of nitrogen. It is typically mixed with a carrier gas. Parts to be ion nitrided are placed in a vacuum chamber. The load is maintained at a high negative dc potential with respect to the chamber itself. When the process gas is introduced into the chamber, the nitrogen gas will dissociate and the nitrogen atoms ionize and accelerate towards the load. Close to the surface of the load, the nitrogen ion will gain an electron from the surface of the load and emit a photon thus producing a glow-discharge around the load. The kinetic energy of the nitrogen atom is converted into heat when it impacts the surface of the part. This heat is used (either by itself or in conjunction with an auxiliary heating source) to bring the load to nitriding temperature and allow the nitrogen atoms to diffuse into the parts. Nitriding is done on previously heat treated parts. Gas nitriding temperatures typically range from 925 to 1050(F depending on the type of steel being hardened. The tempering temperature of a steel part should be at least 50(F higher than the nitriding temperature to prevent over-tempering the part and to insure dimensional stability during nitriding. Because there is only a slight volumetric increase and because of the relatively low nitriding temperatures, finished parts can be nitrided. Various stop-offs

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can be used to restrict nitriding to specific areas on a part. We nitride many wellhead parts for wear and galling resistance. All three methods can give a satisfactory case. The advantages of gas nitriding include that it is the most readily available process, stop-offs are easily applied, and that gas nitriding can be done in large furnaces that can accommodate large parts. Liquid nitriding is probably the idiot-proof method particularly for stainless steel parts. The availability of liquid nitriding is becoming more restricted because of safety and environmental concerns with the cyanide salt baths. The size of the part that can be liquid nitrided is often limited by the size of the salt bath pot. Ion nitriding is probably the least available, but more expensive nitriding method. It offers the best control over the case depth and on the nature of the case. It can be done at substrate temperatures as low as 700(F (although typically done at higher temperatures). It nitrides with the least amount of distortion. The size of part is limited by the size of the vacuum chamber. Kolene Corporation's QPQ® Process is a special nitriding/oxidizing process that we use on some of our gates, stems, and other parts in standard service. It enhances corrosion and wear resistance in low alloy steel parts. Lubricity and fatigue life are also improved. The first step is to nitride the part in a molten salt bath at about 1075 (F for 10-80 minutes depending on the desired properties. The dissociation of cyanate (CNO-) in the bath releases nitrogen and some carbon which diffuses into the surface of the part. There they combine with iron to form a thin layer of a tough, ductile compound called epsilon iron nitride (Fe3N). Below this layer lies a much larger diffusion zone that contains nitrogen in solid solution. The part is removed from the nitriding bath and quenched into an oxidizing salt bath at about 750(F for 5-10 minutes and then air or water cooled to ambient. Salt residue is rinsed with water. Next, the part is mechanically polished by lapping, etc., to restore the original surface finish. It is then immersed again in the same salt quench bath for 20-30 minutes, removed and rinsed with water, and then dipped in oil. QPQ® is derived from the quench-polish-quench sequence of operations. The quench-polish-quench process following the liquid nitriding step introduces up to 6% oxygen into the epsilon iron nitride zone. In addition, a thin black layer (3-4 millionths of an inch thick) of Fe3O4 forms on the outer surface. It is this increase in the oxygen/oxide content of the epsilon iron nitride zone that gives the QPQ® processed carbon and low alloy steel parts significantly better corrosion resistance than conventionally nitrided parts.

CARBURIZING In the first chapter of this book we learned (hopefully!) that other things being equal, the higher the carbon content of a steel, the higher the hardness and strength and the lower ductility and toughness. Carburizing is a method for increasing the carbon content at the surface of a steel and thereby increasing its hardness and strength without lowering the toughness and ductility of the core. A hard surface with a tough page - 241

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core is desirable for many applications such as gears, shafts, cams, etc. There are three basic methods of carburizing: liquid, pack, and gas. They differ primarily in the source of the carbon. Gas carburizing is the most common method for our type of equipment so we will limit our discussion to this. The part to be carburized is machined close to finish dimensions to avoid having to remove part of the case after carburizing to bring it within dimensional tolerances. Parts must be thoroughly cleaned of rust, oxides, scale, etc., that might impede the diffusion of carbon into the surface. Stop-offs (such as electroplated copper, ceramic coatings, etc.) can be used to shield areas not to be carburized. Parts are loaded into a carburizing furnace so that the surfaces to be carburized are totally exposed to the furnace atmosphere. Gas carburizing uses a hydrocarbon gas as the source of carbon. Natural gas (consisting mostly of methane) or propane is frequently used. Hydrocarbon gases are rich in carbon so only relatively small amounts have to be used in the furnace atmosphere. The atmosphere must be circulated to insure a uniform case. To provide for a higher volume of gas to facilitate circulation, most carburizing operations will supplement the small volume of hydrocarbon gas with a carrier gas such as nitrogen, carbon monoxide, or hydrogen. The ratio of carrier to hydrocarbon gas is typically 8-to-1 to 30-to-1. Carbon from the hydrocarbon gas diffuses into the surface of the part. The amount of carbon in the case is consequently dependent on the furnace atmosphere and the carburizing time and temperature. Carburizing is usually done at about 1700(F. Carburizing time will vary depending on the desired hardness and case depth. The 1700(F carburizing temperature is within the austenitic range for steels. Carburized parts must be heat treated to develop the desired properties in both the case and the base metal. Parts may be air cooled from the carburizing temperature and then reaustenitized, quenched, and tempered. Sometimes parts are quenched directly from the carburizing temperature and then tempered. The carbon content of the case, the hardenability of the starting material, the required properties, and the possibility of distortion must all be taken into account when selecting the quenching medium and the tempering temperature. Many steels can be carburized. Although stainless steels can be carburized with a resulting increase in surface hardness, they are almost always nitrided instead of carburized. If carburized, the carbon atoms would combine with the chromium in a stainless steel to form the carbides that increase the surface hardness. The chromium is what gives stainless steels their corrosion resistance. If a stainless steel is carburized then the surface chromium is tied up as carbides and is unavailable to form the protective chromium oxide film. Corrosion resistance would thus be drastically reduced.

BORIDING Boriding is an elevated temperature process used to increase the surface hardness of a metal by diffusing boron into it. The boron will combine with the metallic page - 242

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elements to form extremely hard borides. Like carburizing and nitriding, the purpose of boriding is to enhance wear and abrasion resistance and to prevent galling while retaining a tough inner core in the part. The process that we use is Materials Development Corporation's Borofuse® Process. The Borofuse® Process is a proprietary process used to diffuse boron into the surface of a metal. Boron readily forms a variety of borides such as FeB, CrB2, M02B5, and W 2B5 with common alloy elements. In addition it can form a compound with carbon, B4C. As a consequence, a wide range of ferrous and nonferrous alloys can be surface hardened using the Borofuse® Process. Borides are extremely hard and the Borofuse® Process can produce a surface hardness significantly higher than those produced by carburizing, nitriding, or even hard chromium plating. The Borofuse® Process is done on finished, fully heat treated parts. We use the Borofuse® Process primarily to prevent galling on nickel base alloys stems, etc. Because the Borofuse® Process is done at or slightly above the aging temperature for many nickel alloys, we always run a test coupon with the parts being Borofused® and requalify the mechanical properties.

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Stick It To It Welding is a means of forming a metallurgical bond between two pieces of metal by the application of heat in order to join them together. The first weld was made when some unknown blacksmith heated two pieces of metal in a furnace, took them out and positioned them on an anvil, and then beat on them with a sledgehammer until they stuck together. Crude, but it worked. There are those who say welding has not progressed very far since our blacksmith took the first whack with the hammer, but I believe this to be slanderous. I personally know of three or four welds that were made in our shop that involved no repair. Let these detractors scoff if they must, but welding has proven to be invaluable as a localized casting technique used to hold forgings together; adding metal where it ain't, but ought to be; and for inlaying/overlaying base metals to enhance corrosion, wear, or galling resistance. In this chapter we will examine some of the more commonly used welding processes in the Oil Patch as well as go over welding terminology and metallurgy. Brazing will also be briefly discussed.

WELDING PROCESSES - GENERAL Most welding processes are broadly categorized by their source of heat. Individual processes may be further identified by other factors such as the filler metal, method of shielding, and flux. Filler metal, as the name implies, is the additional metal added to the base metal in order to make the weld. Shielding is the method used to protect the hot or molten metal from the atmosphere during welding. This is necessary because oxygen, nitrogen, and hydrogen in the air can react with the metal to form brittle compounds that will greatly reduce the ductility and toughness of the weld. Flux is a fusible material that dissolves oxides as well as other impurities. Flux facilitates the removal of impurities by combining with them to form a slag on the weld which is later chipped or knocked off. This slag may also provide shielding from the air. Virtually all the welding processes used in the manufacture or repair of wellhead equipment are fusion processes. Fusion processes are those that melt the base metal and the filler metal (if used) in order to make the weld. Arc welding and oxyfuel gas welding are the two most important groups of fusion welding processes and we will limit most of our discussion to these. Arc welding processes utilize a current that flows between the tip of an electrode and the part being welded through an ionized column of gas to provide the heat for welding. Oxyfuel gas processes use the combustion of a fuel gas with oxygen as the source of heat.

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ARC WELDING PROCESSES Arc welding processes are by far the most widely used welding processes in the Oil Patch. The "arc" refers to the current flowing from the tip of the electrode to the workpiece. Depending on the particular arc welding process, filler metal may or may not be used. A consumable electrode is one that slowly melts as welding progresses and provides the filler metal for the weld. The arc at the tip of the electrode causes it to melt and the droplets of molten metal are transferred across the arc and deposited on the workpiece. Non-consumable electrodes are used only for carrying the welding current and for sustaining the arc. Filler metal, if required, is provided using a separate rod or wire that is not part of welding circuit. Non-consumable electrodes are typically made out of tungsten or carbon. Arc welding processes are widely used because of their versatility, ease of operation, and relatively low cost. They are classified by the method of shielding employed to protect the weld from the atmosphere. Let's examine some of the commonly used arc welding processes. 1. Shielded Metal Arc Welding (SMAW) - "Stick" welding, as this process is often called, is a manual technique that utilizes a consumable electrode in the shape of a rod (typically 9-18" long) and with a flux covering. Figure 1 illustrates the basic set-up. The heat of the arc at the electrode tip causes the flux covering to decompose. The gases from the decomposition of the flux blanket the electrode tip, the arc, and the weld puddle protecting them from air (see Figure 2). The weld puddle refers to the pool of molten filler metal and base metal on the part being welded. Other components of the flux will combine with impurities in the weld puddle and then float to the surface to form a slag. The slag thus helps to purify as well as protect the weld puddle. The solidified slag must be removed after each welding pass before

Figure 1: Shielded Metal Arc Welding page - 248

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the next pass is made. This is easily done by chipping or grinding. SMAW has many advantages including short set-up time, inexpensive equipment, it is easy to use in the field, and welding can be done in all positions (the welded surface can be overhead, vertical, horizontal, etc.). Its major disadvantages are a low deposition rate, slag must be removed after each pass, and welding is temporarily interrupted after an electrode is consumed.

Figure 2: Shielded Metal Arc Welding Detail 2. Gas Metal Arc Welding (GMAW) - GMAW was once known as metal inert gas welding or MIG. In this process a bare, consumable, wire electrode is continuously fed through a hand held "gun". The electrode tip, arc, weld puddle and adjacent base metal are shielded by an externally supplied gas (such as an inert gas, CO2, or a mixture of an inert gas with another special purpose gas). The shielding gas is piped to the gun and comes out of the same orifice as the electrode (see Figure 3). Metal may be transferred across the arc by several different mechanisms depending on the electrode wire size, welding current and voltage, and the type of shielding gas. The short circuiting mode (sometimes called "short arc") is the one most frequently used in welding wellhead equipment. A molten drop of metal forms at the tip of the electrode and extends towards the weld puddle as the wire is continuously fed through the gun. When the molten metal comes into contact with the puddle, a short circuit occurs and the arc is extinguished. The molten drop of metal is pulled away from the tip of the electrode into the weld puddle by the weld puddle's surface tension. The resulting gap between the weld puddle and the tip of the electrode allows the arc to re-establish itself and the whole process is repeated (20-200 times per second). The short circuiting transfer mode is a relatively cool process so it is frequently used in making root passes where the weld metal must bridge over the gap at the bottom of a weld prep. The spray transfer mode has a high deposition rate. Fine, metal droplets are formed at the tip of the electrode and are page - 249

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carried across the arc to the workpiece. This mode of transfer requires a high current density and a mixture of argon with a small amount of oxygen as the shielding gas. The spray mode is generally limited to the flat position while the short circuiting mode can be used in any position.

Figure 3: Gas Metal Arc Welding GMAW has a high deposition rate because of the continuously fed electrode and because there is no slag to remove in between processes. Its major disadvantage is that because it is a relatively cool process, there is a concern that proper tie-in between the weld metal and base metal or between individual weld passes will not be obtained. "Tie-in" refers to fusion and subsequent metallurgical bonding. Equipment is relatively complex and expensive and is not practical for field use. 3. Flux-Cored Arc Welding (FCAW) - FCAW is an automatic or semiautomatic process that utilizes a continuously fed, tubular shaped electrode that is filled with flux (see Figure 4). The basic equipment setup is similar to GMAW. The arc is established between the tip of the electrode and the workpiece. As the tip melts, some of the flux ingredients inside the electrode will decompose and form a protective gas shield around the tip, the arc, and the weld puddle. Other flux ingredients are deposited in the weld puddle where they will act as deoxidizers, purifying agents, and ionizers. These ingredients combine with impurities and float to the top of the puddle and form a productive slag. Some fluxes will also contain alloying elements. Some set-ups will also use an externally supplied, shielding gas for additional protection for the air.

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Figure 4: Flux-cored Arc Welding Detail FCAW makes a fast, high quality weld. It is a relatively easy process for a welder to use because the arc is readily visible and the process can be used in all positions. The main disadvantages are the electrode is expensive compared to solid wire, slag must be removed between passes, and the equipment is relatively complex, expensive, and not portable. 4. Submerged Arc Welding (SAW) - SAW, or "sub arc" as it is commonly called, is an automatic or semi-automatic process that uses a bare, continuously fed, wire electrode. The arc is generated under a blanket of granular, fusible flux (see Figure 5). The weld puddle contains molten weld metal and molten flux. The flux contains deoxidizers and other ingredients to remove impurities from the weld metal. Some fluxes also contain alloying elements. The flux (as well as the impurities that have combined with it) eventually floats to the surface

Figure 5: Submerged Arc Welding page - 251

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and forms a protective slag. Flux is gravity-fed from a hopper onto the workpiece just ahead of the electrode (see Figure 6). The tip of the electrode is always below the blanket of flux hence the name "submerged" arc. The primary advantage of SAW is its high deposition rate. The high amperage current and the fact that the flux above the arc acts as an insulator that concentrates the heat in the welding zone ensure that each pass has good tie-in. Major disadvantages are that welding is generally restricted to the flat position, slag must be removed after each pass, the quality of the weld is somewhat more sensitive to the cleanliness of the base metal than other processes, and the equipment is relatively complex, expensive, and not portable.

Figure 6: Submerged Arc Welding Detail 5. Gas Tungsten Arc Welding (GTAW) - This process uses a nonconsumable, tungsten electrode. The arc is struck between the tip of the electrode and the workpiece while filler metal is added from a separate rod that leads the electrode (see Figure 7). The heat of the arc melts the surface of the workpiece as well as the tip of the filler metal rod thus forming a weld puddle. The puddle is protected from the atmosphere by a shielding gas coming out of the weld torch nozzle. Thin parts such as sheet may be welded together without the use of filler metal. Parts are butted tightly against each other. The arc follows the juncture and melts the base material on both sides of the joint thus forming a weld puddle that permanently joins the parts when the molten metal solidifies. page - 252

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Figure 7: Gas Tungsten Arc Welding Detail The major advantages of GTAW include the high quality of welds, there is no slag to be removed and no slag to form inclusions, welding can be done in all positions, the arc and weld are readily visible to the welder, and there is little weld spatter because the filler metal is not carried across the arc. Its major disadvantage is its low deposition rate. GTAW is sometimes referred to as TIG (tungsten inert gas). 6. Plasma Arc Welding (PAW) - Plasma arc welding is illustrated in Figure 8. There are two different modes: transferred arc and nontransferred arc. Transferred arc has a constricted arc that is established between the tungsten electrode and the workpiece. Nontransferred arc has the arc established between the tungsten electrode and a constricting nozzle that is part of the plasma arc torch. Argon gas flows between the electrode and the constricting nozzle where it is heated up by the arc until it becomes a plasma. The plasma exits the orifice of the nozzle in a well defined column that is directed towards the workpiece. The high temperature plasma causes the base metal to melt. It also provides shielding. Additional shielding gas (consisting of an inert gas or a mixture of gases) may also be used. Filler metal, if used, will be added in the form of rod or wire.

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Figure 8: Plasma Arc Welding Detail

The high temperature, well defined plasma column is a very efficient way to melt a localized area of base metal. While superficially similar to GTAW, PAW is capable of faster welding speeds and deeper penetration due to the higher heat transfer rate of the plasma. Often joints can be welded in a single pass that would take two or more with other arc welding processes. The plasma column is typically easier for a welder to control than the arc in GTAW. The HAZ is smaller than that produced by GTAW. The torch-to-workpiece distance is not as critical as in GTAW and gives the welder more freedom to position the torch to permit the best visibility during welding. The transferred arc mode is generally used for welding metals. The non-transferred arc mode is used in thermal spraying non-metal substrates. PAW is often used to manufacture welded tubing made from stainless steel, titanium, and other metals.

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OXYFUEL GAS WELDING (OFW) As previously mentioned, OFW uses the combustion of a fuel gas with oxygen as the source of heat for welding. Acetylene is the most commonly used fuel gas. The OFW process is illustrated in Figure 9. The temperature of an oxyacetylene flame can reach over 6000(F. By adjusting the oxygen-to-acetylene ratio, an oxidizing, neutral, or reducing flame can be obtained. Oxidizing flames have excess oxygen, reducing flame excess acetylene, and neutral flames have the correct proportion for complete combustion with no excess of either. Most welding is done with a neutral flame, but some metals may require an oxidizing flame (brass for example) while other need a reducing flame (such as aluminum). Depending on the thickness of the parts to be joined, filler metal may or may not be utilized. A flux may be required for some metals. OFW is inexpensive and portable. The equipment is versatile in that the flame can be adjusted for use as a source of heat for soldering, brazing, flame cutting, and preheating a localized area. The major disadvantages are it is a manual process, the quality of the weld is very dependent on the skill of the welder, and the deposition rate is extremely low.

Figure 9: Oxyfuel Welding Detail

ELECTRON BEAM WELDING (EBW) Electron beam welding is only occasionally used in the manufacture of Oil Patch equipment. It is a relatively recent development. Figure 10 is a schematic that illustrates the basic process. A high velocity beam of electrons is directed at the joint between the workpieces within a vacuum chamber. The impinging electrons give up their kinetic energy to the surface of the workpiece in the form of heat which melts the base metal. The beam of electrons is produced by an electron gun through the resistance heating of a filament. The electrons are accelerated and focused electrostatically. Welding must be done in a vacuum to avoid the scattering of electrons by collisions with air molecules (which would result in a large loss of power). The fact that welding is performed in a

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vacuum means that no shielding gas or protective slag is required. For most applications, no filler metal is used.

Figure 10: Electric Beam Welding The amount of heat generated by the impinging beam is dependent on many factors including beam current (the number of electrons per second striking the surface), the velocity of the electrons, the diameter of the beam at the point of impingement, and the travel speed of the beam as it moves along the joint. When the beam first strikes the workpiece, the surface heats up and eventually vaporizes allowing the beam to penetrate further into the metal. The beam's diameter is very small consequently joint fit-up is critical: any small misalignment could cause the beam to stray from the joint. Typically the beam is stationary and the workpieces moved in order to make the weld. At the start of welding, the beam melts the base metal through the entire thickness of the joint. As the pieces to be joined are moved relative to the beam, the beam creates a "keyhole" shaped penetration that progresses along the joint. The beam melts the base metal which then flows behind the beam and fills in the joint. A special optical sight allows the operator to ensure that the alignment is correct and to monitor the progress of the welding. EBW has many advantages. The depth-to-width ratio of an EB weld may be as high as 50-to-1. This, along with its high penetration power, allows thick joints to be page - 256

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welded in a single pass. It has a relatively high travel speed. It is often used to weld metals that are particularly reactive with air at high temperature. The heat input is small compared to other welding processes so the resulting heat affected zone is small. The major disadvantages are the cost of the equipment, welding must be done in a vacuum (which may limit the size of parts because the size of the available chamber or may require the construction of a special chamber), and very tight tolerances must be held on the joint fit-up. We have used EB welding in the past to weld titanium.

WELDMENT TERMINOLOGY A weldment is an assembly of individual parts that have been welded together. A weld joint describes how two pieces of metal are fitted up to each other within a weldment. Figure 11 illustrates the five basic types of weld joints. All major, pressure boundary welds in our equipment are different forms of butt joints. Notice in Figure 11 that the junction between the parts in a butt weld lies in the same plane as each of the parts. There are eight basic types of welds. These are illustrated in Figure 12. Of these, only the fillet, groove, back, and surface welds are commonly used for Oil Patch equipment. Fillet welds are often used for structural welding and are probably the most frequently made type of weld. They have the advantage of virtually no joint preparation. They do not have the strength of a groove weld, however, so are seldom used for pressure boundary welds. Groove welds are used for virtually all major, pressure boundary welds in our type of equipment. There are many types of groove welds. The common ones are illustrated in Figure 13. Some groove welds are full penetration while other are not. Full penetration means that the weld metal extends through the entire thickness of the joint (see Figure 14). Many codes prohibit the use of pressure boundary welds that are not full penetration because they have a built in "notch" that can act as a stress riser. Backing welds are often used in conjunction with a groove weld for several reasons. In order to make a full penetration groove weld with submerged arc welding, a backing

Figure 11: Types of Weld Joints page - 257

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weld is often first made. This bridges the gap so that the flux in the sub arc process does not just fall through the space between the two parts to be joined. Very often the bottom or root of a weld joint is difficult to weld because of restricted access and consequently the first pass may have the poorest quality. After the groove weld has been completed, the weldment is flipped over, the root pass ground out, and a backing pass made from the back side where access is good. Surface welds are done for salvaging undersize parts or for inlaying/overlaying with corrosion resistant and hardfacing weld metal. The terminology associated with fillet and groove welds is presented in Figure 15.

Figure 12: Types of Welds

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Figure 13: Groove Welds

Figure 14: Weld Penetration

Figure 15: Weldment Terminology page - 259

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WELD DEFECTS The possible sources for weld defects are legion and include problems associated with the joint design, welding procedure, base metal, and/or welder. The following is a brief survey of some of the more prominent welding defects and their possible causes. 1. Porosity - As we discussed for castings, porosity is the entrapment of small bubbles of gas in the solidifying metal. Gas may be generate during welding from the vaporization of volatile impurities such as oil, grease, paint, etc., on the surface of the base metal. Moisture on the surface of the metal is another common cause. Gas can be generated from volatile impurities in the base metal itself such as excessive sulfur. Damp electrodes are another culprit. Shielded metal arc electrodes must be stored in ovens once they have been removed from their protective containers because their flux coatings tend to absorb moisture from the air. Improper welding conditions (poor shielding gas coverage, damp flux, too low a welding current, no preheat, etc.) may also result in porosity. 2. Cracks - The presence of a crack in a weld tends to get people excited and for good reason. Cracks are effective stress risers and have the potential for rapid growth in service hence all the excitement. Cracks may appear on or below the surface. They may occur in the base metal, the weld metal, or both. They may be longitudinal (parallel to the face of the weld) or transverse (perpendicular to the face of the weld), or have no preferred orientation. The presence of cracks indicates that stresses induced during welding exceeded the strength level of the base and/or weld metal. These stresses may be due to an excessively fast cooling rate, improper joint preparation or design, a too highly restrained joint, the use of improper electrode, or poor workmanship. 3. Inclusions - Inclusions are contaminant particles that are trapped in the weld metal. Entrapped slag particles are the most common type of inclusions. Improperly cleaned surfaces may also contribute inclusions. Tungsten inclusions may result from small pieces of tungsten breaking off the tip of the electrode in GTAW.

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4. Incomplete Fusion - This defect describes the condition when weld metal does not completely fill in the joint: there is a space between the weld metal and base metal or between individual weld beads where there was lack of tie-in. Incomplete fusion may occur because of improper joint design, poor welding technique, improper welding current, or an excessively fast travel speed. 5. Undercut - An undercut is a groove melted into the base metal adjacent to the toe of a weld. It is detrimental in that it acts as a stress riser and may lead to subsequent cracking. 6. Incorrect Weld Profile - The weld profile is the shape of the cross section of the weld in relation to the weld joint. There are several defects related to weld profile. Overlap is excessive weld metal at the top of the weld that overlaps the top edge of the face of joint, but has not fused to the surface of the base metal. It acts as a stress riser. Underfill is where the weld metal does not completely fill up the joint. Typically an underfill condition will consist of a weld with a concave (rather than a convex) surface contour. Excessive reinforcement is weld metal that has been built up on the root or the face of the weld wall beyond the surface of the adjacent base metal. Excessive reinforcement can reduce the fatigue life of the weldment. 7. Arc Strikes - Arc strikes are hard spots in the base metal outside of the weld joint where the welder accidentally let the arc come into contact with the part. The arc melts a very localized area of base metal. The surrounding base metal quickly draws the heat away thus, in effect, quenching the area. In steels this forms hard, brittle martensite and may lead to cracking. 8. Crater Cracks - Crater cracks are associated with the tail end of a submerged arc weld. Sub arc weld deposits are relatively large. When the molten metal solidifies, it contracts. At the end of a submerged arc weld bead, this contraction may lead to a situation somewhat analogous to shrinkage in a casting. The outer skin of the bead solidifies first establishing a fixed volume. The solidification of the balance of the molten metal is insufficient to completely fill this volume. The contraction of the cooling metal will cause the top surface to be drawn down into the center of the bead forming a dimple or crater.

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There will often be a shrinkage crack in the center of the crater. These are easily ground out.

WELDING METALLURGY Welding metallurgy is an enormous and complex field of study because there are so many metallurgical processes going on during welding. The types of welding processes that we utilize all involve temperatures high enough to cause significant changes in the properties of the base metal. Filler metals are added which may or may not have the same composition as the base metal. Dissimilar metal welds can be made. Some mixing of the base metal and filler metal will always occur. This results in a zone of material having an intermediate composition that may have radically different properties than either the base or filler metal. Let's first look at some of the effects that the high temperature of welding has on the base metal. An arc struck between the tip of an electrode and the base metal will generate enough heat to melt a localized area of the base metal known as the fusion zone . Naturally this molten metal will intermix with the molten filler metal. The steep temperature gradient that exists between the fusion zone and the bulk of the surrounding base metal leads to severe thermal stresses. The base metal near the weld expands when it starts to heat up as the arc approaches and then contracts along with the newly deposited weld metal as it cools after the arc has passed by. This localized expansion and contraction of metal can lead to distortion of the weldment, cracking, and/or high residual stresses. The heat affected zone (HAZ) of a weldment is the region of base metal adjacent to the weld that was exposed to a high enough temperature during welding for a time sufficient to cause some alteration in microstructure. The temperature that the base metal sees varies from above the melting point in the fusion zone to ambient well away from the weld. Temperatures adjacent to the weld may be high enough to a variety of changes in the microstructure of the base metal including recovery (in cold worked metals), recrystallization (in cold worked metals), grain growth, and phase transformations. The heat affected zone where the changes occur is typically 1/8" - 1/2" wide in low alloy steels depending on the composition and size of the base metal and the heat input during welding. The changes that occur in the HAZ are dependent on the heat treatment of the base metal. For example, consider a weld made in an age hardenable metal. Adjacent to the fusion zone will be a solution annealed region where all the precipitates have been dissolved by the high temperature. Adjacent to this is an over aged region. There may be a region next to this that is actually harder than the starting base metal because of the additional aging time. Cold work metals will naturally have a loss of strength in page - 262

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the HAZ because of recrystallization and grain growth. Quenched and tempered steels have several things going on in the HAZ during welding. The metal adjacent to the fusion zone in a quenched and tempered, low alloy steel experiences a high enough temperature during welding to become austenitized. Grain growth may occur. Metal adjacent to this may become over-tempered by the heat and suffer a decrease in strength and hardness. As soon as the arc passes by, the metal will begin to cool. Depending on the size of the weldment, cooling can be very rapid as the bulk of the metal draws the heat away from the weld area. Cooling is often so rapid that the metal that was austenitized is, in effect, quenched and forms a hard, brittle region of martensite or bainite. This hard, brittle region in the HAZ can play havoc with the ductility and toughness of the weldment. The HAZ is often the weak link in the weldment chain. With careful attention to welding process details and weldment design, the effects of high temperature on the base metal can be minimized. Weldments in age hardenable alloys can sometimes be totally reheat treated depending on the specific alloy and the design of the weldment (distortion may occur). Sometimes just an additional aging cycle after welding can be tailored to raise the strength of the solution annealed region of the HAZ without over aging the bulk of the material. The best solution to mitigating the effects of decreased strength in the HAZ may be as simple as minimizing heat input during welding and allowing for the loss of strength by increasing the thickness of the weldment in the design. Cold worked materials are often welded. The decrease in strength due to recrystallization and grain growth in the HAZ is unavoidable and must be allowed for in the design. Of course heat input during welding should be kept no higher that what is necessary for a good weld. Quenched and tempered irons and steels, as well as many other metals, are usually preheated before welding. Preheating is the application of heat to the base metal just before welding. It is usually done at 200-600(F depending on the specific alloy and thickness of the parts. Preheating may be done by induction heating, electrical resistance heating, furnace heating, or by oxyacetylene torch. The entire workpiece may be preheated or just the weld joint and adjacent area. Preheating reduces the temperature gradient between the weld area and the bulk of the material and thus reduces the thermal stresses. Preheating also drives off moisture from the surface of the metal (which could cause porosity), lowers the required heat input, and facilitates the diffusion of absorbed gas (such as hydrogen) out of the base metal. A minimum and/or maximum interpass temperature is often specified during welding. The interpass temperature, as the name suggests, is the temperature of the base metal adjacent to the weld in between weld passes. A minimum interpass temperature is specified for essentially the same reasons as preheat and is often the page - 263

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same temperature. A maximum interpass temperature is specified to prevent heat from accumulating during multiple pass welding to the point where it becomes detrimental to base metal properties. Stress relieving is the application of heat after welding in order to remove residual stresses. In quenched and tempered steel weldments, stress relieving tempers any fresh martensite or bainite that may have formed in the HAZ. It is generally desirable to keep the stress relieving temperature 25-50(F less than the original tempering temperature so that the bulk of the metal does not become over-tempered. Stress relieving may be done in a furnace so the entire workpiece is heated or may employ induction or electrical resistance heaters, oxyacetylene torches, etc., to heat just a local area surrounding the weld. Filler metal composition is a prime factor in determining the end properties of a weld. The composition may be similar to the base metal or may be a totally different alloy (316 stainless overlaid on a 4130 low alloy steel part for example). Most weldment designs assume that the weld metal has the same minimum strength as the base metal. For some types of metals, this may require that the alloy content of the filler metal exceed that the base metal. This is particularly true when the weld is going to be used in the as-deposited or stress relieved condition. The higher alloy content of the weld may also help to lessen some of the detrimental effects of high temperature on the strength of the HAZ by providing alloying elements that mix with or diffuse into the base metal. Certain filler metals can be heat treated after welding in order to develop the required properties. Others are designed for use only in the as-deposited or stress relieved condition. It is critical to know which type was used in a particular weldment if the weldment has to undergo further heat treatment. Electrodes generally contain deoxidizers to minimize porosity and grain refiners to prevent the formation of the large grains associated with a cast structure. The microstructural changes occurring in the HAZ can affect other base metal properties besides mechanical. For instance, the corrosion resistance of certain stainless steels may be affected due to sensitization. As we discussed in Survey of Metals, sensitization is the precipitation of chromium carbides along grain boundaries in suspectable stainless steels and certain other alloys that leave a chromium depleted zone adjacent to the grain boundaries. Sensitization occurs when the suspectable alloy is held within a certain temperature range for a certain length of time - a combination which could easily occur in welding. Not all metals are weldable. Refractory metals have such high melting temperatures that welding is impractical. Some metals may be impossible to weld because of the ensuing degradation to properties. Some metals (such as free machining steels) have large amounts of sulfur or other low melting temperatures constituents that make them very prone to cracking and thus unweldable. The page - 264

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hardenability of some alloys may be so great that they are impossible to weld without cracking. High carbon steels, white cast irons, and many tool steels fall into this category.

CORROSION RESISTANT WELD CLADDING PROCESSES Cameron utilizes many different processes for cladding the surfaces of steel parts for corrosion protection. Cladding is a complex, time consuming, expensive process so why do we do it? The alloying elements that make a metal corrosion resistant are typically expensive and make the metal much more difficult to form and machine. By cladding we can minimize the amount of expensive, corrosion resistant alloy actually used and we can retain the benefits of a steel substrate (i.e. good forgeability, machinability, weldability, and availability as well as low cost). The alloy that we most commonly use to clad with is Inconel® 625. This alloy has outstanding corrosion resistance in virtually all well environments. A maximum iron content is often specified in conjunction with a cladding process. This refers to the maximum iron content of the as-deposited cladding material at a location within the weld metal specified by the applicable code or standard. The 625 weld metal can pickup iron as it fuses with the steel substrate. Too much iron dilution of the 625 can affect the corrosion resistance of the cladding. Typical maximum iron contents frequently specified by customers are 5%, 10%, and 12%. The 5% limitation matches the maximum amount of iron allowed in the Unified Numbering System (UNS) for wrought 625. In general, as the iron dilution increases beyond 5%, the corrosion resistance decreases. Why not then always specify 5%? Because to obtain a 5% maximum requires tighter controls on the weld procedure. It may require that two passes be made rather than one (putting down a single thick pass requires a greater heat input then two thinner passes built up to the same overall thicknesses consequently iron dilution may be considerably more in the single thick pass). It is thus more costly and time consuming to work to a 5% maximum rather than a 10% or 12% maximum. A 5% maximum should only be specified for a high temperature, highly aggressive environment (e.g. Mobile Bay). For most other applications a 10% maximum will work fine. The welding processes used by Cameron for cladding are as follows: 1. Tungsten Inert Gas (GTAW or TIG) This process was probably the first process to be evaluated for weld cladding. Although the basic TIG process produces a good metallurgical bond, it is relatively slow and generates a significant amount of heat at the weld page - 265

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interface. This increases the dilution and usually results in iron contents above the 5% maximum. To correct this problem, multiple layers are normally required. Typically, the deposits are made with the bore axis in the vertical position. The basic TIG process is more applicable to both small and large parts with small bores. 2. “Slammer” (GTAW or TIG) This process is currently a variation of the basic TIG process. It utilizes an oscillating “hot wire” technique with the deposits being made with the bore axis of the part in the horizontal position. The “hot wire” technique involves passing a current through the weld wire causing it to be preheated which in turn results in an increase in the deposition rate. The oscillating technique simply deposits the increased volume of hot metal over a larger area. Since both of these techniques have a positive effect on the production time and the 5% iron limitation, a one layer deposit is generally sufficient. This process is more cost effective on larger parts particularly those of heavy wall thickness and bore greater than five inches in diameter. 3. Metal Inert Gas (GMAW or MIG) A patented variation of this process called "Tuff Trim" is being used at Missouri City. This particular process requires that the metal deposition be made with the bore axis at a 45 degree angle. This feature was incorporated into the procedure to help dissipate the heat input and to provide the desired clad thickness. This process also has a high deposition rate and normally requires only one layer to meet the 5% iron maximum. The process is best suited for small and medium sized parts where the clad areas are either flat or cylindrical. 4. Pulse Metal Inert Gas (GMAW or MIG) This is also a variation of the MIG process. The beneficial aspects of this variation is that it utilizes a non-oscillating "Pulse" or interrupted current technique. This allows for better control of the current which has a direct influence on the heat input and the deposition rate. Controlling these two variables allows for optimum metal deposition with a minimum heat input. The metal deposition is made with the bore axis of the part at a 45 degree angle. It too is a one layer process and is currently limited to small and medium size parts with bores greater than four inches in diameter. Any of the above welding processes can produce acceptable cladding. Different plants may elect to go with different processes depending on their product mix, the standard they have to work to, etc.

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HIP CLADDING VS WELD CLADDING Unlike the welding processes just described, hip cladding does not involve the fusion (melting) of either the steel substrate or the clad material. This gives the hip cladding corrosion resistance that is superior to that of weld cladding for several reasons. First, that there is very little iron dilution from the steel substrate. Some iron will diffuse into the clad layer during HIP’ing, but the amount is insignificant compared to the amount picked up in the fusion zone in weldments as molten base and weld metal mix together. The second reason is that HIP’ed clad material is chemically more homogeneous than a weld deposit. Unlike a pure metal, an alloy in the molten state will freeze (solidify) over a range of temperatures as it is cooled. The alloying elements that may be completely homogenous throughout a molten metal, may become segregated in the solid metal (rich in some areas, dilute in others). The alloying composition of molten metal changes as certain elements are lost to the solidifying metal. Thus the composition of the metal that first solidifies can be considerably different from that of the last. This micro-segregation can reduce corrosion resistance. Because fusing does not occur during HIP’ing, the HIP’ed powder retains its chemical homogeneity. HIP cladding is a very expensive, long lead process. Cameron does not have any in-house HIP’ing facilities. It’s use is generally restricted to the most severely corrosive environments such as primary wellhead equipment in Mobile Bay. HIP’ing is a one shot deal: it is not possible to hip repair damaged cladding or to build up areas with insufficient cladding. Weld repair of the cladding may be possible provided there is sufficient cladding below the area being repaired such that the heat affected zone (HAZ) of the weld lies wholly within the 625 and does not extend into the steel substrate. This restriction is necessary because if the HAZ extends into the steel, the part must be given a stress relief. The stress relief may cause additional aging to occur in the 625 cladding thus potentially increasing the hardness of the cladding adjacent to the weld to above NACE limits. When the HAZ is wholly within the 625, no stress relieving is necessary.

BRAZING Brazing is not a welding process, but I stuck it in this chapter because it did not seem to fit anywhere else. We use brazing primarily to join tungsten carbide tips to choke needles and to join tungsten carbide inserts to choke seats so a brief discussion about brazing is in order. Brazing is a means of joining two pieces of metal together by heating the surfaces to be joined up to a suitable temperature and allowing molten filler metal to flow into the joint by capillary action. Brazing temperatures are below the melting temperatures of the metals to be joined. By definition, brazing is done above 840(F (soldering, by definition, is done below 840(F).

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Close tolerances are required for brazed joints in order for the capillary action to take place. Both butt joints and lap joints are used in brazing. Butt joints will not be as strong as the base metal because of the relatively low strength of the filler metal. Lap joints are frequently used because with sufficient overlap of the parts the brazed area is large enough to have a joint strength equal or exceeding the strength of the base metals. Joint clearances are generally 0.001 - 0.010". Filler metal comes in many forms including powder, paste, sheet, wire, rod, or is precladded on the surface of the part to be brazed. Filler metals are classified by their composition. Common filler metals include copper, copper-zinc, copper-phosphorus, copper-gold, magnesium, aluminum-silicon, heat resisting materials, and silver. There are many different ways to apply the filler metal to the joint. It may be prepositioned in the form of sheets, washers, etc., before heat is applied. Filler metal may be applied manually after the parts to be joined have been brought up to brazing temperature. Typically when filler metal is applied manually to lap joints, it is done so from one end only so that the filler metal does not entrap any gas in the joint. A gas pocket, of course, would keep the filler metal from completely filling the joint. Filler metals melt over a range of temperatures unless they happen to have a eutectic composition. As we discussed in Chapter I, melting for a noneutectic alloy begins at the solidus or eutectic isotherm (depending on composition) line and continues up to the liquidus line above which melting is complete. The range of temperatures from incipient melting to the completion of melting has a profound influence on the fluidity of the filler metal. Naturally the smaller the range for a given alloy type of filler metal, the less time it takes for the filler metal to solidify to the point where capillary action can no longer draw the "slushy" filler metal into the joint. Flux is always used with brazing except for a few special processes that are done under a special atmosphere. Fluxes are in the form of liquids, powders, or most commonly pastes that are "buttered" over the surfaces to be joined. Fluxes are necessary in order to remove existing surface oxides and to form a protective liquid film at brazing temperatures to prevent the formation of new oxides. Very often brazing fluxes are highly corrosive so all traces of flux should be removed after brazing is complete. A typical brazing operation has the following steps. The surfaces to be joined are thoroughly cleaned. Flux is brushed or sprayed on the area to be brazed on each part. The parts are fitted up so that the correct joint clearances are established. The joint area is then heated up to the brazing temperature. Filler metal is added at the edge of the joint. The high temperature melts the filler metal and capillary action draws the molten metal into the joint. The capillary attraction between the base metal and the filler metal is much greater than cohesive forces between the flux and the base metal consequently the filler metal will displace the flux in the joint. As the assembly cools, a

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true metallurgical bond forms and holds the surfaces together. The last step is a cleaning process to remove all vestiges of flux. Brazing processes are classified by their heat source: torch brazing, furnace brazing, induction brazing, infra-red brazing, resistance brazing, etc. The entire assembly may be brought up to brazing temperature (such as in furnace brazing) or just the joint area (such as infra-red or torch brazing).

BRAZING METALLURGY Although brazing temperatures are below the melting temperatures of the base metals, they are still sufficiently high enough to affect many base metal properties. Heat treated parts may become over-aged or over-tempered. Cold worked metals may recrystallize and consequently soften. It is sometimes possible to incorporate the brazing process into the heat treatment of the base metals. Brazing, for example, may be done during the aging cycle of a precipitation hardening alloy. When this is not possible, then the lowest melting temperature filler metal that still gives the required joint strength should be utilized so that the overall heat input is minimized. Many brazes, such as for our choke parts, require that the joint be at temperature for only minutes. There is thus no detrimental impact to properties because of the short time involved. The bonding that takes place in a brazed joint is complex. Adhesion and mechanical bonding play some role, however, the primary bonding mechanism is metallurgical. Metallurgical bonding takes place as the molten filler metal dissolves (not melts) atoms of the base metal off the surface and together form an alloy. This alloying is sometimes referred to as wetting. The ability of a particular filler metal to form an alloy with the surface of the base metal is called its wettability. Filler metals are formulated such that they have good wettability for a particular base metal and so that they do not diffuse excessively into the base metal, cause base metal erosion, or form brittle compounds with the base metal. The alloy that results from the interaction between the filler metal and the base metal has a higher strength than the pure filler metal. It is thus important to use only enough filler metal to complete the joint. Excess filler metal will actually weaken the joint because it dilutes the alloying effect. The joint strength will approach the strength of the filler metal if too large a clearance is used. Some metals cannot be brazed because there are no suitable filler metals as well as for other reasons. Sometimes metals that cannot be brazed because of the poor wettability of available filler metals can be brazed if they are first plated with electrolytic

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copper, electroless nickel, etc. The plating provides a readily wettable surface for many brazing filler metals.

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NOTES:

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CORROSION The following is a paid, political advertisement

The dentist says your youngest needs braces. The principal has declared your kid in high school to be a menace to society and wants him committed. And after footing the bill for nearly four years, your college senior has decided that Mechanical Engineering isn't really relevant and is going to drop out of school in order to join a commune that raises organic vegetables. The wife just ran away with the mailman and now your girlfriend says she is thinking about becoming a nun. Even Rover has stopped wagging his tail when you get home at night. But what do you care? You have your bowling league on Tuesdays, Monday night football, and your Lone Star Beer so why get upset? Then the phone rings at 8:01 Monday morning. You hold the receiver a foot away from your ear and hear an extremely pissed-off customer screaming that he used the gate valve trim that you recommended to him at 4:59 one Friday afternoon two months ago and now the body looks like a sieve and the gate a piece of Swiss cheese. He wants to know if he can borrow your head for awhile so he can stuff it into the valve bore to stem the flood. After 20 minutes of casting aspersions on your intellect, character, and parentage, he slams the receiver down: an eerie silence ensues. Ten minutes later you come out of shock and hang up your receiver. Now you are upset. On that fateful Friday afternoon two months ago, the customer described the line fluid as a mixture of crude, brine, and gas containing 7% chlorides, 30% CO2, and 14% H2S with the whole mess at 350(F. Instantly your bear trap-like mind suspected that this could be corrosive. Your keen intuition, honed by seven years of intensive study while working towards your BS degree in Mechanical Engineering at Texas A&M, told you to play your hunch and so you specified a corrosion resistant trim consisting of a 410 stainless steel body and gate. You slept well that night. And now you've got problems. "410 should have worked because 410 is stainless, right? Where did I go wrong?", you mutter to yourself. "Can't anybody fill me in on the evil ways of this miscreant called Corrosion?" "Things look grim!", you say? Not so! Because in this world of madness called Cameron, there remains one department that stands as a pillar of strength amid weakness, a beacon of light shining through the darkness made all the more brilliant by surrounding mediocrity. A department with both feet firmly planted on the ground and head held high in the air, ready for the next challenge, ready to routinely accomplish the "impossible". There is but one. And that one is the Cameron Metallurgy Department. Has there ever been such a department? The department is a winning hand packed full of Aces! It sparkles from having so many Gems! Like the cosmos, it is full of Stars! They can put the finger on that felon Corrosion. They can put the collar on that brutal beast and nail it to the wall. They can also answer the phone. Where did you go wrong? YOU DUMMY! You didn't phone and ask the good ol' boys in the Met Department for their two cents worth before you recommended the trim. The phone call is free. Next time, give them a ring: you might learn something and save yourself a lot of grief. page - 273

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Corrosion is the deterioration and loss of metal due to chemical or electrochemical attack. Obviously a brief refresher course in chemistry is now in order.

THE BASICS Atoms are made up of three major types of particles: protons, neutrons, and electrons. Protons are positively charged, electrons are negatively charged, and neutrons, as the name implies, have no charge. Protons and neutrons inhabit the center of the atom known as the nucleus. Electrons, on the other hand, orbit around the outside of the nucleus. For every proton in the nucleus, there is an electron zinging in orbit around the nucleus. Thus an atom has an equal number of positively and negatively charged particles and therefore has no net charge. Generally whenever an atom is involved in a chemical reaction it will either gain or lose one or more electrons. If an atom loses an electron it is said to have been oxidized. If an electron is gained, the atom has been reduced. An atom, or a group of atoms that behaves as a unit in a chemical reaction, that has a net charge because it gained or lost one or more electrons is known at an ion. A positively charged ion is called a cation, and a negatively charged ion is called an anion. If we take a teaspoonful of salt and stir it into a glass of water, the salt will dissolve (go into solution). Salt is NaCl and when we dissolve NaCl in water, we cause the salt to dissociate into Na+ and Cl— ions. The + and — superscripts indicate the charge that these ions have as a result of gaining or losing a negatively charged electron. We can write the reaction that occurred as NaClNa++Cl—. If we continue to stir more salt into our glass of water, we will eventually reach a point where no more salt will dissolve: any additional salt will just fall to the bottom of the glass. The water has become saturated with the salt. It doesn’t matter how much we stir the saturated water, we won’t get any more of the salt to go into solution. But that doesn’t mean that there aren’t chemical reactions occurring, there are! We just don’t notice them because the amount of salt at the bottom of the glass stays the same. NaCl is still dissociating into Na+ and Cl—, but somewhere else in the solution Na + and Cl — are combining to form NaCl so that there is no net change. Our salt/water system is in equilibrium. We can write the reaction occurring at equilibrium as NaCl:Na+ and Cl— where the double arrows show that the reaction occurs in both directions. At the equilibrium point, there is no net change in the quantity of either reactants or products. Let’s consider a solution containing A, B, C, and D where A+B:C+D. At equilibrium, A and B are combining to form C and D at the same rate that C and D are dissociating to form A and B. If we mix a solution of pure A with a solution of pure B, they will react to form C and D. However, not all the A and B will react: the reaction will only proceed to the equilibrium point at which time there will no net change in the amounts of A, B, C, and D that are in the solution. Le Chatelier’s Principle states that when a system in equilibrium is subjected to a stress, the system will adjust its equilibrium conditions to minimize the stress. The page - 274

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stress may be a change in pressure, temperature, or, our primary concern, in the concentration of one of the reactants. I call it the teetertotter principle. A familiarity with Le Chatelier’s Principle will enable us to understand the behavior or metals in corrosive environments without resorting to the painful topics of physical chemistry, thermodynamics, and the Nernst equation that corrosion engineers love to expound on. Let’s see how it works. Assume A+B:C+D. At equilibrium we can visualize the products and reactants being in balance on a teetertotter (or seesaw, if you prefer) as shown in Figure 1.

Figure 1: Equilibrium

Suppose we add more of substance A to our system. Things are now out of kilter. Our system is no longer in equilibrium and our teetertotter is out of balance (Figure 2).

Figure 2: Excess Amount of “A” — Nonequilibrium Our system in equilibrium has been subjected to a stress. Le Chatelier’s Principle says that our system will now change its equilibrium conditions to minimize the stress. To get things back into balance in Figure 2, it’s obvious that some additional A and B will now have to react to form additional C and D. This is indeed what happens. The system will remove some of the excess A by forming more C and D. The reaction will continue to produce more C and D until a new equilibrium point is reached and things are back in balance (Figure 3). Note in Figure 3 that the amount of A has been reduced from Figure 2, but is still more than in Figure 1. The amounts of C and D in page - 275

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Figure 3 are greater than in the original equilibrium system in Figure 1, while the amount of B is less.

Figure 3: New Equilibrium Conditions What would happen in Figure 1 if instead of adding extra A to our system in equilibrium we removed some? Again we’re now out of balance (Figure 4).

Figure 4: Portion of “A” Removed — nonequilibrium To get things back into balance in Figure 4, it’s obvious that the system will have to form more A and B at the expense of C and D. C and D will continue to react producing more A and B until a new equilibrium point is reached and things are back in balance (Figure 5).

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Figure 5: New Equilibrium Conditions The amount of A in Figure 5 will be more than in Figure 4, but less than in Figure 1. The amount of B has increased from Figure 4, while the amounts of C and D are less. To summarize what we’ve learned about the teetertotter principle (or Le Chatelier’s Principle for you purists) if we add more reactants to one side of a chemical reaction in equilibrium, the equilibrium point will shift to favor the formation of the products on the other side. If we remove some of the products, the system will compensate by adjusting its equilibrium point to allow more of the reactants to react to makeup for some of the loss of products. We have one more topic of basic chemistry to discuss: water. Water has the well known formula of H2O. Water also contains trace amounts of H+ and OH— ions that come from the dissociation of the H2O molecule. In equilibrium this can be shown as H2O:H++OH—. At room temperature, the concentration of H+ ion in water is approximately 10—7 mole/liter. (A mole, for those of you who slept through college chemistry as I did, is that amount of substance containing the same number of atoms as 12 grams of carbon 12. It is equal to Avogadro’s number: 6.023 X 1023.) Although the total amount of H+ ion in water is very small, it can have a profound effect on the corrosion of metals. We will be constantly referring to the concentration of H+ ions in water or in various solutions because of the role it plays in corrosion. Rather than describing the concentration in terms of moles per liter, we’ll talk in terms of pH. The pH of a solution is defined as follows:

pH log

1 [H ]

— log [H ]

where [H+] is the concentration of H+ in moles per liter

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Note that the lower the pH number, the greater the concentration of H+ ions. The pH of pure water at room temperature can be found by plugging 10—7 into the above formula.

pH of water at room temperature — log [10—7] 7 By convention water is considered to have a neutral pH and serves as a reference point for solutions having higher or lower H+ concentrations. An acid solution has a higher H+ concentration than water (its pH is less than 7). An alkaline or basic solution has a lower H+ concentration than water (its pH is greater than 7).

ELECTROCHEMISTRY The corrosion of metals in aqueous (water based) solutions is electrochemical in nature. This means that the rate and mechanism of corrosion is dependent on the relationships between electrical energy and chemical change. We will look at the corrosion of iron in water to develop an understanding of electrochemistry. If we put a small chunk of pure iron into a bucket of water it will corrode (rust). The overall chemical equation for this reaction is: 4Fe + 302 +6H2O  4Fe (OH)3 or rust Actually rust is quite a bit more complicated than just Fe(OH)3, but for the purpose of our discussion this simple formula will admirably serve to illustrate all the important principles. The above equation tells us that oxygen and water must both be present for iron to rust. It shows us what the reactants are (the left side) and what the product is (the right side): it does not show us the intermediate steps of the reaction. These intermediates are of considerable importance to an understanding of corrosion so we will examine them in detail. Let’s first look at a simplified case where iron is immersed in deaerated water (water containing no oxygen). Some of the iron (a very minute amount) will dissolve just as sugar or salt would. The iron will continue to dissolve until the water surrounding the iron becomes saturated. At this point there is an equal number of iron atoms going into and out of solution: the iron is in equilibrium with the water. When iron dissolves in water, the iron atoms are oxidized and enter the solution as either Fe+2 or Fe+3 ions. This can be shown by the equations: Fe  Fe+2 + 2e— Fe  Fe+3 + 3e— The +2 and +3 superscripts refer to the net charges on the iron ions, and the 2e— and 3e— to the number of electrons lost during oxidation. The Fe+2 ion is called the ferrous ion and the Fe+3 is the ferric ion. The production of positively charged ions and page - 278

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negatively charged electrons as the iron goes into solution builds up an electrical potential. The magnitude of the electrical potential is a reflection of how easy it is for a metal to oxidize and form ions in a solution (corrode). Unfortunately this potential cannot be measured directly. However, we can measure the potential difference, or voltage, between the metal of interest and a standard (or reference) electrode in the same solution. This voltage is known as the metal of interest’s electrode potential. The hydrogen electrode (see Figure 6) is frequently used as the standard. Hydrogen oxidizes according to the equation H22H++2e—. A negative voltage (electrode potential) indicates that a metal is more easily oxidized than hydrogen while a positive voltage indicates the reverse.

Figure 6: hydrogen Electrode The electromotive force series (see Table 1) is a tabulation of the potential differences that have been measured between various metals and the standard hydrogen electrode. The series is arranged so that the higher up a metal is in the table, the more negative the electrode potential and the easier it is to oxidize. The hydrogen electrode is not very practical or convenient for making field measurements so other, more rugged, reference electrodes are typically used. The silver/silver chloride reference electrode is the most common one for measuring electrode potentials in marine structures.

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ELECTRODE REACTION Li = Li+ + e

-3.05

+

K = K + e

-2.93

+2

Ca = Ca + 2e +

STANDARD ELECTRODE POTENTIAL, E( (V), 25(C

-

Na = Na + e +2

Mg = Mg + 2e +2

-2.87 -2.71 -2.37

Be = Be + 2e

-1.85

+3

Al = Al + 3e

-1.66

+2

-1.63

Ti = Ti + 2e +4

Zr = Zr + 4e +2

Mn = Mn + 2e +3

Cb = Cb + 3e +2

Zn = Zn + 2e +3

Cr = Cr + 3e +2

Fe = Fe + 2e

-1.53 -1.18 -1.1 -0.763 -0.74 -0.440

+2

-0.403

+2

Co = Co + 2e

-0.277

+2

-0.250

Cd = Cd + 2e

Ni = Ni + 2e +3

Mo = Mo + 3e

-0.2

+2

-0.136

+2

-0.126

+3

-0.045

+

-0.000

Sn = Sn + 2e Pb = Pb + 2e Fe = Fe + 3e H2 = 2H + 2e +2

+0.337

+

+0.521

+

+0.800

+2

+0.987

Cu = Cu + 2e Cu = Cu + e Ag = Ag + e Pd = Pd + 2e +2

Pt = Pt + 2e +3

Au = Au + 3e

+1.2 +1.50

Figure 7 illustrates one example of a galvanic cell. A galvanic cell consists of two dissimilar electrodes (in this case one of iron and one of silver) that are in electrical contact with each other and are immersed in an electrolyte. An electrolyte is a solution of ions capable of carrying an electrical current.

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ELECTROMOTIVE FORCE SERIES Metals have different electrode potentials in different electrolytes. In a galvanic cell, the electrode where oxidation takes place and where electrons are provided to the external circuit is called the anode while the electrode that receives the electrons from the external circuit and where reduction takes place is call the cathode. Corrosion takes place predominately at the anode.

Figure 7: Fe-Ag Galvanic Cell

What’s going to happen in our Fe-Ag cell? There are four possibilities: 1) the iron could oxidize, 2) the silver could oxidize, 3) both could oxidize, and 4) neither could oxidize. Looking at Table 1, we see that both the Fe Fe+2+2e— and FeFe+3+3e— reactions have much more negative electrode potentials than AgAg++e— and therefore oxidize easier. The more negative electrode potential of the iron will cause the iron to corrode. Iron will go into solution as Fe+2 and Fe+3 while the produced electrons will flow from the iron electrode through the external circuit to the silver electrode. What about the silver? According to Table 1, silver could go into solution according to the reaction AgAg++e—. Is this what actually happens? No. To understand why, we need to apply the teetertotter principle (alright, Le Chatelier’s Principle). A silver electrode by itself in an electrolyte will go into solution until the solution surrounding the silver becomes saturated with Ag+ ions and equilibrium is attained. This can be represented as follows:

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Figure 8: Equilibrium If we now electrically connect the silver electrode to an iron electrode in the same electrolyte, the corroding iron will produce an abundance of electrons that will go through the external circuit to the silver electrode and upset the equilibrium conditions in Figure 8. This is shown in Figure 9. Le Chatelier’s Principle says that the system will adjust its equilibrium point to minimize the stress caused by the excess electrons. It will get things back in balance by favoring the formation of Ag (the excess electrons will combine with the Ag+ ions and cause metallic silver to plate out on the silver electrode). Because the tendency of iron to go into solution is much greater than the tendency of the silver, there will be more than enough electrons to reduce any Ag + ion in solution. The reaction Ag Ag++e— will be totally suppressed. In our Fe-Ag galvanic cell, the iron electrode will corrode, the silver will not. The iron electrode is thus the anode and the silver electrode the cathode.

Figure 9: Nonequilibrium after Connecting Silver Electrode to Iron Electrode If we look closely at the silver electrode, we can see bubbles forming on its surface. These bubbles are hydrogen gas. The H + ions present in water from the reaction H2OH++OH— combine with the excess electrons at the silver electrode (the cathode) in the reactions: page - 282

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1) H++e—H (monatomic hydrogen) 2) H+HH2 (hydrogen gas) The H+ ion is reduced (gains an electron) in the first reaction. The second reaction is relatively slow and consequently is the rate controlling step. If the electrolyte solution in our Fe-Ag cell is deaerated (contains little or no oxygen), the corrosion of the iron electrode will be very slow. Notice in Table 1 that the electrode potential for the reaction FeFe+2+2e— is –0.440V while that for FeFe+3+3e— is –0.045V. As a consequence, the iron that goes into solution will do so primarily in the form of the ferrous ion (Fe +2). An iron electrode by itself in a deaerated electrolyte will continue to go into solution until equilibrium is reached as illustrated below:

Figure 10: Equilibrium If we electrically connect the iron electrode to a silver electrode in the same deaerated electrolyte, some of the electrons shown in Figure 10 will be lost as they combine with H+ and Ag+ ions at the cathode. Things are now out of balance (see Figure 11). Le Chatelier’s Principle tells us more iron must corrode to make up the electrons that are lost in reducing H+ and Ag+ ions. The amount of Ag + ions in solution is insignificant because the electrode potential of silver is so low. The reduction of the H+ thus accounts for the greatest loss of electrons. As previously mentioned the amount of H+ ion in pure water is very small (10—7 mole/liter at room temperature) so the overall loss of electrons due to the reduction of H+ ions is small. The hydrogen gas that forms at the cathode is lost to the atmosphere thus removing some of the H+ ions from our cell. Applying Le Chatelier’s Principle to H2O:H++OH—, we see that by removing some of the H+ ions we cause more H2O to dissociate to make up the loss. The corrosion of iron in deaerated solutions is very slow because only a small amount of iron must go into solution to replace the electrons lost in the production of hydrogen gas at the cathode.

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Figure 11: Nonequilibrium As we use up H+ ions at the cathode by generating hydrogen gas, more H2O must dissociate. This dissociation increases the OH— ion concentration at the cathode. The OH— ions in solution will combine with the Fe+2 ions at the anode to form Fe(OH)2 . What Fe+3 ions there are in the solution may diffuse to the cathode where they will combine with OH— ions and precipitate out as Fe(OH)3. The formation of Fe(OH)2 and Fe(OH)3 effectively reduces the concentration of Fe+2 and Fe+3 ions in the solution. Again applying Le Chatlelier’s Principle, more iron must go into the solution to make up the loss of the ferrous and ferric ions, and more water must dissociate to make up the loss of OH— ions. Let’s look at what happens to our cell if we now introduce oxygen. Oxygen will react with the H+ ions near the cathode to form water in the reaction O2+4H++4e—2H2O. This greatly reduces the amount of H+ ions available for hydrogen gas formation. As we discussed earlier, hydrogen gas formation is the rate controlling reaction for the corrosion of iron in deaerated water and is the reason why the corrosion of iron in deaerated water is so slow. The presence of oxygen rapidly removes H+ ions from solution as water is formed thus avoiding the slow, hydrogen gas formation reaction. Oxygen can also combine with water at the cathode to form more OH— ions in the reaction O2+2H2O+4e—4OH—. The additional OH— ions will combine with Fe+2 ions at the anode. More iron will now have to go into the solution to replenish the supply of Fe+2 ions lost to Fe(OH)2 formation as well as to supply the additional electrons needed for the oxygen reactions at the cathode. The bottom line of all this is that the presence of oxygen will greatly accelerate the corrosion of the iron electrode. The Fe(OH)2 at the anode may be converted into Fe(OH)3 by the reaction 4Fe(OH)2+2H2O+O2Fe(OH)3. The primary reactions taking place in our cell are shown in Figure 12. Although we have limited our discussion of galvanic cells to iron and silver, the principles are the same for other combinations of metals as well. The electrode having the more negative electrode potential will become the anode while the other electrode will be the cathode. Oxidation takes place at the anode and reduction takes place at the cathode. Corrosion takes place primarily at the anode (some corrosion may take place at the cathode for those metals that are attacked by strong concentrations of OH—). The electrons produced during oxidation travel from the anode through the external circuit to page - 284

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the cathode. The difference between the electrode potentials of the anode and cathode is a measure of the driving force for the reaction. Corrosion resulting from the formation of a galvanic cell is called (not too surprisingly) galvanic corrosion. It is not necessary to have two separate pieces of metals as the anode and the cathode in a galvanic cell. A galvanic cell can be established on the surface of a metal part where variations in composition, internal stress, temperature, environment, etc., can cause localized areas to be anodic to other areas.

Iron Electrode (Anode)

Fe  Fe 2 2e Fe 2  20H  Fe (OH )2 4Fe(OH)2

 2H2O  O2  4Fe(OH)3

Silver Electrode (Cathode) 2H   2e  H2 4H 

 O2  4e  2H2O

O2  2H2O  4e  4OH

Figure 12: Fe-Ag Cell Reactions

TYPES OF GALVANIC CELLS There are four basic types of galvanic cells: 1) composition cells, 2) stress cells, 3) concentration cells, and 4) differential temperature cells. We will briefly discuss each type.

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1. COMPOSITION CELLS A composition cell is established whenever two different types of metals are electrically connected and immersed in an electrolyte. The metal with the more negative electrode potential will be the anode and corrode. By coupling the two metals together, we may accelerate the corrosion of the anodic material over what its corrosion rate would be if it were electrically isolated in the same environment. By coupling the two metals together, we may completely suppress the corrosion that would have taken place in the cathodic material had it been electrically isolated in the same environment. Corrosion resulting from a composition cell established between two different types of metals is often referred to as dissimilar metal corrosion. There are myriad examples: the Fe-Ag cell we just talked about, stainless steel in contact with low alloy steel, aluminum in contact with steel, etc. The anode and cathode in a composition cell do not have to be separate pieces of metal. Composition cells can be established on the surface of a single piece of metal due to local variations in chemistry. For example, the ferrite phase in a duplex stainless steel may preferentially corrode (become the anode of a composition cell) in certain electrolytes because it does not have the same alloying composition as the austenite phase. In general, the farther apart the electrode potentials are for two metals, the greater the driving force will be for corrosion to occur in a composition cell. Conversely, if the difference in electrode potentials is small, then corrosion may be insignificant in a particular environment and the metals are said to be compatible in that environment. The greater the cathode-to-anode area ratio, the more accelerated the corrosion will be at the anode. For example, in the Fe-Ag cell we previously talked about, the corrosion of the iron anode is the source of electrons for cathodic reactions including the formation of H2, H2O, and OH— as well as the reduction of Ag+ at the silver cathode. By increasing the surface area of the cathode, we increase the number of sites where these reactions can occur. This creates a greater demand for electrons that can only be met by the further corrosion of the iron electrode. 2. STRESS CELLS A particular metal under stress has a higher electrode potential then the same metal unstressed. As a consequence, in a metal part that has received varying amounts of cold work, the areas with the greatest amounts of cold work, (and thus having the greatest internal stresses) will be anodic to the lower stressed areas. Similarly, a heavily cold worked part in electrical contact with an unstressed part made out of the same material in a electrolyte will become the anode of a galvanic cell. You can see the effects of a stress cell by looking at a steel nail or a threaded pipe that’s been outside for awhile. The head and pointed end of the nail, page - 286

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which have been severely cold worked, will typically be rusted more than the shank of the nail. The threads of the steel pipe, which are severely cold worked during machining, will typically be more rusted than the remaining surface. 3. CONCENTRATION CELLS Electrodes of the same metal immersed in different concentrations of electrolyte will have different electrode potentials. The electrode in the weaker solution will be anodic to the electrode in the stronger solution. Let’s look at an example. Figure 13 shows two copper electrodes immersed in a solution of copper sulfate (CuSO4). The left side of the cell has a stronger solution of CuSO4 than does the right. Copper sulfate goes into solution as Cu+2 and SO4—2 ions. The left side of our cell consequently has more Cu+2 ions than the right. Copper will enter into solution (corrode) by the reaction CuCu+2+2e—. The excess Cu+2 ions already in solution in the left side of the cell will tend to suppress this reaction according to Le Chatelier’s Principle.

Figure 13: Copper/Copper Sulfate Concentration Cell

The electrode in the dilute CuSO4 solution must corrode in order to provide more Cu+2 ions to bring the system into equilibrium and thus becomes the anode. The electrode in the concentrated solution where corrosion is page - 287

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suppressed will be the cathode. The Cu+2 ions will be reduced (gain electrons) at the cathode and plate out of the solution as metallic copper onto the cathode. A very important type of concentration cell is the oxygen concentration cell. Suppose we immerse two iron electrodes into a solution of NaCl. On one side of the cell we will bubble oxygen into the solution (see Figure 14). The side with the least amount of oxygen will become the anode and corrode. We’ve already discussed how oxygen promotes the corrosion of iron in water so how come the electrode exposed to the high concentration isn’t the anode? Remember one of the reactions that oxygen is involved with at the cathode of a galvanic cell is the formation of OH — according to the reaction O2+2H2O+4e—4OH—. Electrons are used up as they react with oxygen and water at the cathode. More electrons must consequently be supplied to the cathode by the oxidation of metal in areas having lower amounts of oxygen. The electrode in the side having the oxygen bubbled though it will thus be the cathode and the electrode in the side with the lowest oxygen concentration the anode. Oxygen concentration cell formation explains why corrosion often occurs under scale, sediment, in crevices, etc.: areas where oxygen is depleted by corrosion and then cannot be easily replenished. It also explains why partially filled tanks will often corrode just below the surface level of the liquid. Figure 15 shows some of the different forms that oxygen concentration cells may take. 4. DIFFERENTIAL TEMPERATURE CELLS The last type of cell we will talk about is the differential temperature cell. This type of cell consists of two electrodes of the same type of metal with each electrode immersed in an electrolyte at the same initial concentration, but at a different temperature. A potential difference will exist between the two electrodes so one will become the anode and corrode. For copper, the electrode at the lower temperature is the anode, but for silver, the electrode at the higher temperature is the anode. Not much is known about the corrosion mechanism of this type of cell. It can contribute to corrosion in boilers, superheaters, heat exchangers, etc.

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Figure 14: Oxygen Concentration Cell

Figure 15: Oxygen Concentration Cells

POLARIZATION As corrosion proceeds in a galvanic cell, the whole system will tend towards equilibrium. For example, in our copper sulfate concentration cell the dilute side page - 289

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becomes increasingly rich in Cu+2 ions as the anode corrodes while the concentrated side becomes poorer as Cu+2 ions plate out onto the cathode. The two sides slowly approach each other in potential. The difference in their potentials is a measure of the driving force behind the corrosion. As the difference becomes less, the tendency to corrode becomes smaller. This voltage drop that occurs as corrosion products build up at the anode and/or cathode is known as polarization. Polarization is generally more influenced by the cathodic reactions than by those taking place at the anode. The formation of hydrogen gas at the cathode of a Fe-Ag cell with deaerated saltwater as the electrolyte is an example.

PASSIVITY Certain types of metals have the ability to form corrosion products that tightly adhere to their surfaces in some environments. If these corrosion products form a continuous, impenetrable barrier over the anode, then the anode will be isolated from the corroding media and corrosion will be greatly reduced. The surface is said to have been passivated. There are many examples of passivation in metals. Lead immersed in H2SO4 will be passivated by a protective layer of lead sulfate on its surface. The most important protective films, however, are oxides. Aluminum, nickel, and titanium alloys as well as stainless steels have superior corrosion resistance in many environments because of a tenacious oxide film that isolates the surface of the metal. Stainless steels are steels that contain approximately 10.5% or more chromium. Table 1 indicates that chromium has a more negative electrode potential than iron so we can expect the chromium in stainless steel to oxidize in water. This indeed happens, but only to a limited extent. Chromium will react with oxygen to form a protective oxide layer. This layer will be continuous over the surface provided the amount of chromium is 10.5% or more by weight. Stainless steels are "stainless" or passive in certain environments such as nitric acid, many organic acids, the atmosphere, alkalies, etc., that help the oxide layer to form. In these types of environments, if the oxide layer is scratched or otherwise mechanically damaged, it will immediately reform and continue to provide protection. There are some environments, however, that tend to break down the protective oxide layer. Examples include dilute or concentrated HCl, HBr, or HF acids, sea water, oxidizing chlorides (NaOCl, CuCl2, FeCl3, etc.) and others. If the oxide layer is broken down to the point where it can no longer protect the underlying base metal, then the surface is said to be active. Stainless steels (as well as other metals) may be passive or active in certain chemical environments depending on concentration. 316 stainless steel, for example, can be exposed at room temperature to sulfuric acid at concentrations less than 20% and more than 85% without extensive corrosion taking place. At sulfuric acid concentrations in between these levels, the oxide layer is attacked and subsequent corrosion becomes rapid.

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Table 1 does not take passivity into account and therefore has limited value in predicting what actual corrosion rates will be. The galvanic series was developed to overcome this problem. It lists metals in order of their tendency to corrode in a particular environment (see Table 2). Note in Table 2 how some metals appear twice - their position depending on whether or not they are in the active or passive state. TABLE 2 GALVANIC SERIES GALVANIC SERIES OF METALS IN SEAWATER AT AMBIENT TEMPERATURES Anodic

Magnesium Magnesium alloys Zinc Aluminum Cadmium Carbon steel Copper steel Cast iron 12 to 14% Cr steel (active) Ni-resist 8% Ni, 18% Cr steel (active) Lead Tin Naval Brass Nickel (active) Inconel (active) Copper Silicon Bronze Nickel (passive) Inconel (passive) Monel 12 to 14 Cr steel (passive) 8% Ni, 18% Cr steel (passive) Silver

Cathodic

Graphite

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TYPES OF CORROSION DAMAGE Corrosion damage is frequently classified by its appearance into six basic categories: uniform attack, pitting, crevice corrosion, intergranular attack, selective leaching, and cracking. We will describe each of these and examine why they occur. 1. UNIFORM ATTACK Uniform attack, as the name implies, means that the surfaces of a metal part that were exposed to a corrosive medium corroded at essentially the same rate. Uniform attack is often quantitatively described in inches of metal lost per year or IPY. It is the simplest and the most common type of corrosion. Uniform attack may consist of heavy weight loss corrosion (e.g. carbon steel in saltwater) or a light tarnish (e.g. silver exposed to air). All metals are subject to uniform attack in certain environments. Uniform attack in aqueous solutions results from galvanic corrosion, either on a microscopic or macroscopic scale, that occurs evenly and consistently over the surface of the metal. 2. PITTING Pitting is a type of localized corrosion that occurs when a small area of metal is anodic to the surrounding metal. As the name implies, a pit is essentially a depression or hole in the surface of a part. There may be little general corrosion apparent. Pitting can be initiated by inhomogeneities on the metal surface, a localized loss of passivity, galvanic corrosion from a relatively distant cathode, or by the formation of a concentration cell under a solid deposit on the surface of the metal. The bottom of the pit, of course, is anodic to the rest of the metal. The pit will often grow in depth without a significant increase in diameter. This is because of the concentration cells (particularly oxygen concentration cells) that are set up inside the pit. The area at the bottom of the pit will be depleted in oxygen while metal at the part’s original surface that is exposed to fresh electrolyte will be oxygen rich. The build up of corrosion products within the pit can drastically alter the chemistry of the electrolyte in the pit as well as prevent oxygen from outside the pit from reaching the bottom of the pit. The bottom of the pit will corrode as a result of the oxygen concentration cell that is established. Pitting can be very serious because a component can fail due to a perforation and not show any signs of general corrosion. Pits are frequently masked by the products of other forms of corrosion. All metals will pit in certain environments. Some stainless steels, for example, are particularly vulnerable in chloride bearing solutions which can destroy localized areas of the protective oxide film. 3. CREVICE CORROSION page - 292

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Crevice corrosion not too surprisingly occurs in a crevice. If the crevice is tight enough to prevent the electrolyte in the crevice from being constantly replenished, then the stagnant electrolyte will become depleted in oxygen as well as other corrodants. Thus concentration cells are established with the surfaces inside the crevice being anodic to those just outside the crevice. There are many mating and overlapping surfaces in the design of our equipment so crevice corrosion is often encountered. 4. INTERGRANULAR ATTACK (IGA) In this type of corrosion, a metal's grain boundaries are preferentially attacked by the corrosive medium resulting in a substantial decrease in the metal's ductility and strength. This type of corrosion can be quite rapid because the limited area of the grain boundaries acts as a small anode surrounded by a large cathode. The larger the cathode-to-anode ratio, the faster a metal will corrode. Intergranular attack can result from two possible causes. Grain boundaries may have a different composition than the adjacent areas and consequently form a composition cell. A good example of this is the carbide precipitation that occurs along the grain boundaries in sensitized stainless steels. The second possible cause is the fact that grain boundaries have inherently higher energy levels than the interiors of grains with their nice, neat, orderly arrays of atoms. As a consequence, a stress cell is established. 5. SELECTIVE LEACHING Selective leaching, or dealloying, is the removal of one or more individual elements from an alloy through corrosion. Dezincification of brasses in water is a typical example. Brass is an alloy of copper and zinc. The zinc is preferentially attacked and goes into solution leaving a residue of copper and corrosion products behind. The part that has been corroded may keep its shape and appear undamaged, but will suffer a severe degradation of tensile strength and ductility. 6. CRACKING There are several different corrosion mechanisms that may result in cracking. Among the most important are stress corrosion cracking, corrosion fatigue, and hydrogen damage. Stress corrosion cracking (SCC) is a brittle failure mode in which a susceptible alloy under a tensile load cracks as a result of interaction with a specific environment. Failure may occur at tensile loads well under the yield strength. SCC will only occur with certain combinations of alloys and environments. The alloy must be in a susceptible condition (strength level, hardness, heat treatment, etc.). For example, a low alloy steel bolt loaded in tension in seawater may crack if its hardness is much over 35 HRC, but is relatively immune to SCC at lower hardnesses.

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There are many factors that determine whether or not a particular environment will foster SCC in a particular alloy. Of these, temperature and concentration of the specific chemicals or ions that cause SCC are the most important. For example, austenitic stainless steels such as 316 may fail by SCC in chloride solutions at temperatures above approximately 150(F, but are relatively immune to SCC at lower temperatures. Parts that fail by SCC may or may not show evidence of other types of corrosion. SCC of steel parts in seawater is often initiated from corrosion pits that act as stress risers. It is not possible to predict when a susceptible part will fail in a particular environment. For a part to fail by SCC it must be loaded in tension. There is a threshold stress below which it will not fail. Corrosion fatigue describes the failure of a metal part by cyclic loading in a specific environment in a lower number of cycles or at a lower stress than would have been required in air at room temperature. Many metals are susceptible to corrosion fatigue in seawater including brasses, aluminum, and some types of steels. The fatigue cracks that form are typically transgranular and are often branched. There is no endurance limit for metals susceptible to corrosion fatigue in specific environment: all susceptible metals will eventually fail at a particular alternating load given enough cycles. Hydrogen damage may be the cause of cracks through several different processes including hydrogen embrittlement, hydrogen attack, and by hydride formation. Hydrogen embrittlement is the loss of ductility and toughness of a susceptible metal as a result of interaction with hydrogen. There are several proposed mechanisms for why this occurs, but nobody knows for sure. One theory is that monatomic hydrogen diffuses into a susceptible metal and accumulates around internal defects such as inclusions, porosity, etc. Here the monatomic hydrogen combines to form hydrogen gas. The hydrogen gas molecule is too large to diffuse out so an increase in pressure occurs as more and more gas forms. Eventually a point is reached where the pressure is sufficient to cause microcracking. Another theory holds the hydrogen reduces the interatomic cohesive forces of the base metal. Still another theory holds that somehow hydrogen facilitates the plastic deformation of the material at the tip of a microcrack causing rapid propagation. Whatever the mechanism, hydrogen embrittlement is a serious concern for many common engineering materials. It may cause a rapid fracture at stresses well below the yield strength of the metal. There are many potential sources of hydrogen. It is a byproduct of corrosion in an aqueous environment, of pickling operations, cathodic protection, and of electroplating. Certain impurities in a metal such as sulfur, phosphorous, arsenic, and the cyanide ion (CN ) can interfere with hydrogen gas formation at the surface of the metal in these processes thus enabling more monatomic hydrogen to diffuse into the metal.

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Many alloys are subject to hydrogen embrittlement. Steels have received the most attention because they are often in a susceptible condition and because they are often exposed to hydrogen during processing or service. Generally, high strength steels are more susceptible to hydrogen embrittlement than low strength steels. The more residual stress a metal has, the greater its susceptibility because cracks are then easier to form and because there are more internal defects for hydrogen to interact with. Steels are most susceptible to hydrogen embrittlement at and slightly above room temperature. At lower temperatures, the diffusion of hydrogen is so slow that not enough can accumulate in the steel to cause a problem. At temperatures over about 250(F, the diffusion is so rapid that hydrogen cannot build up in the steel: there is no localized accumulation necessary for embrittlement to occur. Hydrogen attack of susceptible steels takes place at temperatures over 400(F in the presence of high pressure hydrogen. Long term exposure may result in decarburization at the surface, blister formation on the surface, and cracking. Monatomic hydrogen diffuses into the steel and reacts with the carbon in iron carbides to form methane gas (CH4). The methane molecule is too large to diffuse out consequently pressure will build up at discontinuities in the microstructure as more and more methane forms. This may cause cracks to initiate or, if it occurs near the surface, a blister to form. Hydrogen attack is primarily a concern for carbon steels. It can be avoided by using low alloy steels containing carbide stabilizing elements such as chromium and molybdenum. These tie-up the carbon atoms so that they are unavailable to form methane. Nelson curves are a series of curves (named after the guy who developed them) that show what maximum temperature and partial pressure of hydrogen a particular type of steel can be exposed to without suffering hydrogen attack. Hydride formation is another embrittling mechanism caused by hydrogen. Certain alloys of magnesium, tantalum, columbium, vanadium, zirconium, and especially titanium can combine with hydrogen at elevated temperatures to form extremely brittle hydrides. When a stress is applied, the hydride layer can crack and thus expose more metal at the crack tip to the hydrogen thus rapid crack propagation can occur. Susceptible alloys must be shielded from hydrogen in the air during welding, heat treatment, and even during melting.

SULFIDE STRESS CRACKING (SSC) Sulfide stress cracking is a special type of stress corrosion cracking. It is of great concern to us because H2S is commonly encountered in oil fields. Many different alloys are susceptible to SSC. The H2S is a source of hydrogen. H 2S can react with the iron on the surface of a steel part, for example, to produce monatomic hydrogen according to the reaction Fe + H2S  FeS + 2H. The hydrogen can then diffuse into the part. The page - 295

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presence of sulfur tends to accelerate the absorption of the hydrogen. As we learned in the last section, the absorption of hydrogen can create many problems. "Sour" gas or crude contains enough H2S gas to make H2S embrittlement a concern. The National Association of Corrosion Engineers (NACE) has issued a standard, NACE MR0175, that gives specific guidelines for using materials in sour service. All of our sour trims must comply with this standard. NACE MR0175 defines the amount of H2S that must be present in order for a given environment to be considered sour. MR0175 imposes restrictions on the composition, processing, maximum hardness, and heat treatment of metals that may be used in sour service. Carbon and low alloy steels with high strength and hardness are particularly vulnerable to SSC. MR0175 imposes a maximum hardness of 22 Rockwell C (or 237 Brinell) on these materials. Low alloy steels with more than 1% nickel (such as 4340) also appear to be very susceptible to SSC consequently MR0175 prohibits their use. MR0175 imposes heat treat restrictions on some materials while others may be used in any condition. For example 410 stainless steel must be double tempered, 17-4 precipitation hardening stainless steels must be given one of two specified heat treatments, while nickel base alloy 625 can be used in any heat treated condition. The allowable hardness may vary by heat treat condition. Alloy 718, for example, may be used up to 40 HRC in the solution annealed and aged condition, but only up to 35 HRC in other conditions. Residual stress can accelerate H2S cracking. Heavily cold worked steels are particularly susceptible and NACE greatly restricts their use. This of course limits the use of many austenitic stainless steels (which cannot be strengthened by heat treatment) in oilfield equipment.

BACTERIA AND CORROSION Some types of bacteria can greatly accelerate the corrosion of ferrous alloys in water or soils that are poor in oxygen. No, they don't munch their way through the metal, but they are capable of creating an environment that is highly damaging to buried or immersed metal parts such as oil well casings, water well pipe, buried pipeline, heat exchangers, cooling towers, etc. Anaerobic bacteria are typically the guilty parties. Anaerobic means that this group of bacteria live in an environment with little or no oxygen present. Troublesome bacteria include sulfate reducing bacteria (SRB), sulfur/sulfide-oxidizing bacteria, iron/manganese oxidizing bacteria, and methane producing bacteria. SRB have caused the most damage. One proposed mechanism for how these miscreants do their number on steel parts involves the following reactions: Anode: 4Fe

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 4Fe 2  8e

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Cathode: 8H20

8H

 8e  8H  8 0H

 Na2S04 bacteria › 4H20  Na2S

Na2S  2H2CO3  2NaHCO3  H2S Overall Reaction: 4Fe  2H2O  Na2SO4

 2H2CO3  3Fe(OH)2  FeS  2NaHCO3

SRB thus reduce inorganic sulfates to sulfides and, in doing so, consume large amounts of hydrogen (depolarizing cathode areas). The cathode reactions use up a great deal of electrons which can only be replaced by the further corrosion of the anode. Other types of anaerobic bacteria can cause corrosion because they produce various acids as by-products of their metabolism. Aerobic bacteria (those that live in oxygen rich environments) can indirectly cause corrosion by forming mats of slime. The mat of slime may restrict the access of the base metal to oxygen in the environment hence establishing an oxygen concentration cell. The slime may exclude oxygen to the extent that a layer of anaerobic bacteria may coexist beneath it.

CO2 CORROSION Carbon dioxide gas dissolved in water forms carbonic acid in accordance with the reaction CO2+H2O:H2CO3. Although relatively weak as far as acids go, carbonic acid can cause severe pitting in many steels. CO2 corrosion is characterized by numerous, broad, relatively shallow (in relation to their diameters) pits. API 6A has established the following guidelines for CO2 corrosion based on partial pressure of the CO2 (the partial pressure can be found by multiplying the shut-in pressure of the well by the volume or mole% CO2): Partial Pressure CO2

Relative Corrosivity

1.

< 7 psi

noncorrosive

2.

7 to 30 psi

slightly corrosive

3.

> 30 psi

moderately to highly corrosive

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In general, carbon and low alloy steel components are suitable when the CO2 partial pressure is less than 7 psi. For CO2 partial pressures of 7 to 30 psi, carbon and low alloy steels are often used for body components and CRA’s (corrosion resistant alloys) or stainless steels for valve bore sealing mechanisms. For CO2 partial pressures over 30 psi, CRA’s and stainless steels are used for all wetted components. It must be emphasized that these are guidelines only. CO2 may act in concert with other corrodants (particularly H2S and high chloride formation water) to produce an environment far more corrosive than what would have been predicted by totaling the individual effects of the corrodants. This is known as the synergistic effect. CO2 cannot be considered in isolation: the presence of other corrodants, the temperature, the amount of produce water, etc., can have a profound effect on CO2 corrosion. In wells producing mixed phase fluids including crude, if the water cut is less than roughly 30% and the flow is greater than 1m/s, then CO2 corrosion is generally not a problem. The water will be entrained in the crude and consequently metal exposure to carbonic acid will be slight. The crude will also form a protective layer of oil on the surface of metal parts. Of course trouble may be encountered during lengthy shut-ins where the oil and water tend to separate out.

CORROSION EROSION Erosion is the loss of metal by mechanical impingement or abrasion on the surface of a part exposed to a moving fluid. It is greatly accelerated when the moving fluid has particulate matter (such as sand) entrained in it. Corrosion erosion is the loss of metal due to the combined action of erosion and corrosion at a rate exceeding what would have been predicted from the sum of the effects of corrosion or erosion acting by themselves. In most cases corrosion erosion occurs because a protective film (such as the oxide layer on stainless steels) or a tightly adherent, protective layer of corrosion products is continuously stripped away by mechanical abrasion thus exposing the underlying metal to the corrosive environment.

CHEMICAL ADDITIVES USED IN PETROLEUM PRODUCTION As we have learned, well fluids can be highly corrosive. But it’s not just what comes out of a well that can cause us grief. Many chemical additives are used to enhance or control well production. Some of these additives can be highly corrosive. Of course some additives (such as inhibitors) may be beneficial from a corrosion standpoint — we’ll discuss these in a later section on corrosion prevention. Let’s focus our attention for now on the bad actors. Wells in limestone, silicate-rich, or carbonate bearing sandstone are often acidized to increase (“stimulate”) production. A mixture of one or more acids is pumped down the production tubing and into the formation. There the high pressure acid mixture will react with the formation and increase its permeability by enlarging/unplugging pores, fissures, etc. The “live” acid (the new acid pumped down the well) is frequently a page - 298

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15% solution of HCl. A mixture of 12% HCl and 1.5% HF is also commonly used. Depending on the formation, one or two “flushes” (acid injections) may be required. Different acid mixtures may be used for each flush. Live acidizing mixtures can be highly corrosive to metals. However, the exposure time during the injection of the acid into the formation is very brief so corrosion is minimal. In addition, acidizing mixtures typically contain corrosion inhibitors that will reduce the rate of attack. After the acid has done its job and the treatment is complete, the “spent acid” must be flushed out of the formation. This “after flush” is typically done using EGMBE (Ethylene Glycol Monobutyl Ether) or methanol. The composition of the spent acid is of course different than the live because it has dissolved rock, etc. Typically spent acid is still strongly acidic and contains a great deal of silt. It will have a high chloride concentration. Corrosion damage due to acidizing is generally the result of the spent acid rather then the live. The “after flush” operation may take six or more hours so the exposure to the spent acid is considerably longer than the live acid exposure. The types of corrosion damage that could occur include general (weight loss) corrosion, pitting, crevice, and stress corrosion cracking (due to the very high levels of chlorides). Although corrosion inhibitors are generally included in the acidizing mixture formulation, they are not always effective. The amount of inhibitors is purposely kept low to avoid damaging the formation. Some CRA materials are very difficult to adequately inhibit during acidizing. Stainless steels such as 410 are readily attacked by HCl. They are depassivated as the hydrochloric acid strips off the protective oxide film. Duplex stainless steels can be severely attacked because once the protective oxide film is removed, there are countless galvanic cells setup between the ferrite grains and the austenite grains (the austenite grains are richer in alloying elements that prevent corrosion and thus are more noble than the ferrite grains). Acidizing can be performed through Cameron trees and wellhead equipment made out of any of our standard trims providing 1) good acidizing practice is followed, and 2) the inhibitor in the acidizing solution is appropriate for the equipment trim. It is essential that the spent acid be flushed out as rapidly and completely as possible after the acidizing treatment is complete to minimize exposure time. The inhibitor must be tailored to the specific metals, acidizing mixture, and well conditions. Packer fluids are weighted fluids put in the annulus of the well (the area between the ID of the casing and the OD of the production tubing) on top of the packer to counterbalance the pressure exerted by the produced fluid thus helping the packer maintain a seal. A workover or completion fluid is a weighted fluid used to maintain control of the well during workover and completions. Drilling muds, inverted emulsions, and clear brines have all been used in the past. Clear brines are rapidly becoming the packer/workover/completion fluid of choice because they are easily handled and, unlike the other two types, do not have particulate matter that can settle out and cause page - 299

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operational problems. A heavy brine may be made up on one or more salts including KCl, KBr, NaCl, NaBr, CaCl2, and ZnBr2. At low temperatures heavy brines are generally not too corrosive. But at high temperatures or when contaminated by CO2, H2S, or oxygen, corrosion can be severe. Down hole temperatures can be quite high. Many alloys that are normally resistant to corrosion in oxygen-free brine solutions are readily attacked at the elevated temperatures found near the bottom of a well. The form of attack may be rapid general corrosion, pitting or crevice corrosion, or catastrophic failure due to stress corrosion cracking. Inhibitors are generally added to weighted fluids, but many inhibitors lose their effectiveness at high temperatures. Corrosion is more of a concern for packer fluids because of the long term exposure. Exposure to workover and completion fluids is temporary, but can still cause problems. Material selection for downhole equipment is complex because of the high temperatures involved and because of potential exposure to corrosive packer/workover/completion fluids. We’ve only talked about a couple of chemicals used to enhance or control production on a well. There are many others — scale inhibitors, hydrate preventors, biocides, etc. It is very easy to shoot yourself in the foot with any of them from a corrosion standpoint. It’s just as important to know what a customer plans to put down a well as it is to know what the production fluid is coming out when selecting an appropriate trim.

MARINE CORROSION Close your eyes for a minute and think of the sea. What pictures do you see in your mind? Sailboats racing before the wind? Waves gently lapping white, sandy beaches? Sea gulls hovering over shrimp boats screeching their pleas for handouts? Exuberant dolphins chasing each other in a game celebrating their joy of life? Palm trees swaying in a gentle sea breeze while scantily clad maidens in grass skirts pulsate to the hypnotic rhythms of a Hawaiian steel guitar? Time out! Time for a reality check. If these are the kind of visions your mind has conjured up, you’re clearly not a metallurgist or a corrosion engineer. It’s time to pop your bubble, to lose your innocence, to find out what the sea really is. It is not our friend. It is a hungry, rapacious monster covering over 70% of the earth’s surface just waiting to devour our expensive production equipment. Much of the earth’s remaining large oil and gas reserves lie subsea and the sea guards them zealously. Resentful of intruders trying to rob its treasures, the sea defends its domain by attacking the plundering production equipment through corrosion. This attack may be rapid and catastrophic or slow and insidious. We must be ever vigilant because time doesn’t matter to the sea.

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We are going to expend a great deal of time, effort, and expense in developing subsea oil and gas fields. We must have the utmost confidence that our equipment will remain intact and function over its design life. We cannot tolerate unplanned interruptions to production, safety hazards, or threats to the environment as a result of corrosion. What can we do to protect our investment from the relentless onslaught of the sea? That’s what this section is all about. Now close your eyes again and think of the sea. What do you see? Public enemy number one? A wild beast to be caged? A tiger to be tamed? Good. You’re already making progress!

THE MARINE ENVIRONMENT Over 70% of the earth’s surface is covered with a very efficient electrolyte; seawater. Bring steel oilfield productive equipment into contact with this electrolyte and one or more types of galvanic cells will establish themselves causing the steel to corrode unless protective measures are taken. The rate of corrosion is dependent on many factors including the chemical makeup of the seawater, temperature, wave action, etc. As a consequence, the corrosion rate of unprotected steel in a marine environment will vary by geographical location, the time of year, and by its position of exposure relative to the seawater. The positions of exposure can be divided into the following five categories. 1. 2. 3. 4. 5.

Marine Atmosphere Splash Zone Tidal Zone Immersion Zone Mud Zone

Metals exposed to a marine atmosphere are located above the water level and above tidal and wave action. Corrosion is a function of the amount of salt or seawater mist that collects on the metals’ surface. Corrosion may be particularly severe in crevices or low spots where mist or run-off can collect. The concentration of salts in these areas may be much higher than in seawater due to evaporation. Oxygen is readily available to accelerate corrosion. Rainfall may increase or reduce the rate of corrosion depending on circumstances. Heavy rainfall may reduce corrosive attack by rinsing off salt residues on metal surfaces. On the other hand, rainfall may accelerate corrosion by providing the moisture necessary for salts on the metal’s surface to stay in an aqueous solution. The splash zone, as the name implies, is that area that is above sea level, but is wetted by waves. For carbon and low alloy steels, this zone is the most corrosive. The seawater is well oxygenated. The impingement of the waves may mechanically remove corrosion products from the steel’s surface thereby exposing fresh metal to further attack. Metals such as stainless steels and nickel base alloys that rely on a thin oxide surface layer for their corrosion resistance fare much better in the splash zone than page - 301

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carbon and low alloy steels because the well aerated seawater immediately supplies oxygen to repair any damage done to the oxide film. There is no marine fouling in the splash zone. Steel corrosion is often relatively low in the tidal zone - the area between high and low tides - because of the formation of oxygen concentration cells. Metal surfaces in the tidal zone are exposed to well aerated seawater while adjacent metal surfaces that are submerged are exposed to less oxygen (especially if covered with marine growth). This establishes an oxygen concentration cell with the submerged metal being anodic to the metal in the tidal zone. The submerged metal just below the tidal zone will corrode, but in doing so suppresses corrosion of the metal in the tidal zone (the cathode of the cell). Small, isolated panels of steel in the tidal zone will, however, be rapidly attacked by the highly oxygenated seawater. Marine fouling may be present in the tidal zone. Metal in the immersion zone is continuously in contact with seawater. The rate of corrosion is primarily dependent on the rate at which oxygen can diffuse through rust, other surface deposits, marine growth, etc., to reach and react with the metal’s surface. Water near the surface of the ocean is usually saturated with oxygen. The oxygen content at other depths varies by region. In the Pacific Ocean, the oxygen content at great depths is much lower than at the surface, while in the Atlantic there is little difference between the two. Marine fouling may be heavy in shallow depths. At continental shelf depths, there is no plant fouling and greatly reduced animal fouling. The mud zone refers to the bottom sediments in the ocean. Here there are a number of competing processes that make the rate of corrosion difficult to predict. Oxygen concentration cells may establish themselves at the mud-bottom water interface. Anaerobic bacteria in or on the mud may cause corrosion. The reduction in the corrosion rate of steel well below the mud line is attributable to the lower oxygen content and to the fact that any protective films that form on the steel’s surface are protected in turn by the surrounding mud.

SEAWATER All seawater is not the same. There may be significant differences in composition due to local factors. For instance, seawater taken from a bay that has a large freshwater river emptying into it will have lower salinity than seawater taken far offshore. Dissolved gas content may vary by the time of year because of variations in wave roughness and biological activity. In this section we’ll examine some of the parameters of seawater that can influence its corrosivity. These parameters cannot be considered in isolation: they can and do influence each other. The salinity of seawater is defined as the total weight of dissolved solids in grams per 1000 grams of water. Thus a salinity of 33 means 33 grams of dissolved solids for every 1000 grams (or 1 liter) of water. This equates to 33,000 ppm (often abbreviated as 33 ‰). Table 3 shows the major components of seawater.

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GRAMS/KILOGRAM OF WATER

Chloride

19.353

Sodium

10.76

Sulfate

2.712

Magnesium

1.294

Calcium

0.413

Potassium

0.387

Bicarbonate

0.142

Bromide

0.067

Strontium

0.008

Boron

0.004

Fluoride

0.001

There is not a great deal of difference in the open water composition of the world’s large oceans. Most tend to fall within 35 ±3 ‰. This variation in terms of corrosivity is not significant. In localized areas of the world’s oceans or in isolated seas the salinity can go all over the map. The Caspian Sea, for instance, has a salinity of 130 ‰ while that of the Baltic is only 8 ‰. Depending on the ingress of freshwater from rivers, bays can vary widely in salinity. The effects of salinity on corrosivity are complex because salinity may affect other factors that influence corrosion. High salinity tends to be more aggressive than low salinity because it is a more efficient electrolyte, however, this is partially offset by the lower solubility of oxygen in high salinity seawater. Steels can form a protective carbonate-type scale under certain conditions in seawater that greatly reduces corrosive attack. This protective scale is less likely to form in low salinity water. Marine growth is often very sensitive to salinity variations. In diluted seawater, such as in a bay, a protective bio-fouling layer may not form. The pH of seawater at depths below approximately 1500 feet is a fairly uniform 7.4. As you go towards the surface the pH tends to increase because photosynthesis by plant life consumes carbon dioxide (carbon dioxide acidifies water through the formation of carbonic acid). The pH of surface water may vary from daylight hours to night as a result of the difference in photosynthetic activity. Surface pH can be as high as 8.2. The variation in seawater’s pH is not sufficiently great to have much direct influence on corrosion reactions. It can, however, significantly affect corrosion rate because it influences whether or not a protective carbonate film can form on the surface of a steel part. The lower the pH, the less likely the film formation.

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Oxygen, as we have previously discussed, can greatly increase the rate of corrosion of steels because it acts as a depolarizer at the cathode of a galvanic cell. The oxygen content of seawater is thus an important factor in determining corrosion rates. There are many things that influence the oxygen content of seawater. Among them are wave action, temperature, photosynthesis of plant life, decomposing organic matter, and ocean currents. Seawater may have up to 12 ppm oxygen. The surface will always be well oxygenated because of wave action and photosynthesis of marine plants. In the Pacific Ocean there is a rapid fall-off in oxygen content as you go to deeper depths. In the Atlantic Ocean there is little variation in oxygen content with depth of water. In either of the world’s two great oceans, there is sufficient oxygen even at great depths to cause some corrosion of steel to occur. Seawater low in oxygen may actually cause greater corrosion of steel than seawater high in oxygen. The important factor in determining the corrosion rate is the amount of oxygen delivered to the surface of the metal. Flowing seawater low in oxygen may provide a greater amount of oxygen than stagnant seawater rich in oxygen. The velocity of flowing seawater influences corrosion in several ways. As we just discussed flowing seawater can bring more oxygen to a metal’s surface. This will tend to increase the corrosion rate of carbon and low alloy steels, but will decrease the corrosion rate of passive metals such as nickel base alloys that rely on a protective oxide film. Pitting will be less in flowing seawater than in stagnant seawater. The flowing seawater will constantly sweep the surface of the metal thereby preventing concentration cells from becoming established. Flowing seawater may cause mechanical damage to metals through erosion-corrosion, impingement attack, and cavitation. These forms of corrosion will be discussed in detail in the next section. Temperature effects on seawater corrosion are a mixed bag. Increased temperature generally speeds up chemical reactions, but this does not necessarily equate to increased corrosion. The solubility of oxygen in seawater decreases with increasing temperature. Biological activity will generally increase with increasing temperatures. The tendency for protective scale formation increases as temperature increases. The overall effect of temperature on corrosion will be dependent on which of these factors predominate. Biological activity can influence the corrosivity through several different mechanisms. The oxygen content of seawater can be altered by biological activity; increasing by photosynthesis or decreasing by the decay of organic matter. Marine growth on steel components may provide a protective mat that isolates the underlying metal from the corrosive environment. It will act as a diffusion barrier to oxygen. Anaerobic bacteria can play an active role in seawater corrosion. They may be found in the mud zone or, under unusual circumstances, in open water (e.g. deep water in the Black Sea). Aerobic bacteria may form slimes on the surface of metals. As they consume oxygen, they may create an oxygen-free environment underneath the slime that allows anaerobic bacteria to flourish and cause corrosion. Sessile (permanently attached) organisms such as barnacles, mollusks, sponges, etc., can cause pitting underneath their point of attachment through the formation of oxygen concentration cells. page - 304

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FORMS OF MARINE CORROSION We’ve talked about the various forms of corrosion and we’ve talked about the marine environment. Let’s put them together and see what the most common forms of corrosion are in the marine environment. Galvanic corrosion heads our list. With over 70% of the world covered by a highly efficient electrolyte, you’d expect one or more types of galvanic cells to establish themselves on production equipment used offshore. Material selection for production equipment is seldom clear cut: it general involves a series of trade-offs. Cost, strength, toughness, availability, machinability, weldability, galling resistance, abrasion resistance, hardness, and corrosion resistance are all factors that must be weighed before selecting the optimum material for a given application. The relative importance of these material factors will vary part by part depending on the end application of the part. As a consequence product equipment is manufactured from a multitude of different alloys. No one alloy will satisfy all the different requirements of all the different components. Different alloys have different electrode potentials so its a safe bet that dissimilar metal corrosion will be a problem unless we take precautions. In a marine atmosphere environment, such as topside equipment on a platform, dissimilar metal corrosion is generally limited to within a fraction of an inch of the junction of the two metals. The electrolyte may be salt laden mist or spray from the ocean, or rain or dew that resolutionizes salt deposits left on the equipment after seawater films have evaporated. Because the thin layer of electrolyte is localized, corrosion is limited to near where the two different metals come into contact. Subsea is a different story. Dissimilar metal corrosion may cause problems with parts that are electrically connected, but are a hundred or more feet apart. Both parts are completely surrounded by and are in intimate contact with a very efficient electrolyte that readily facilitates corrosion current flow. Dissimilar metal corrosion may appear as a general wastage of the more anodic metal or may result in accelerated attack in a localized area in the more anodic metal near the junction of the two metals. The rate of attack depends on many factors. Among the more important are the difference in electrode potentials between the metals (the greater the difference, the greater the driving force for corrosion), and the ratio of the cathode area to anode area (the greater this ratio, the greater the driving force for corrosion). If the two metal parts are in electrical contact, but are physically separated, then the distance between then will affect the corrosion rate. Oxygen concentration cells can establish themselves under deposits such as mill scale, marine growth, sand, etc., causing pits to form. Crevice corrosion results from oxygen concentration cells that form in narrow crevices that restrict the ingress of fresh seawater. Oxygen is consumed from the stagnant seawater in the crevice as the metal corrodes: the narrow opening prevents fresh, oxygenated seawater from replenishing the oxygen supply other than by the slow process of diffusion. The bottom of the oxygen depleted crevice will then become anodic to the surface of the part (outside the page - 305

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crevice) exposed to flowing seawater. Many alloys that rely on protective oxide films for their corrosion resistance, will perform admirably in flowing seawater, but will rapidly pit and crevice corrode in calm or stagnant seawater. The chloride ions in seawater tend to breakdown the protective oxide film in localized areas. The film can rapidly repair itself as flowing seawater continuously brings fresh oxygen to the damaged areas and also sweeps away corrosion products. Production equipment typically has many built-in “crevices” in the form of ring gasket joints, bolting threads, washers, etc., where crevice corrosion can be initiated. Dealloying may occur in certain copper alloys in seawater. Brasses containing 15% or more zinc are particularly prone to “dezincification.” Zinc rich phases in the copper matrix will preferentially corrode leaving behind a weakened, porous copper shell. Aluminum may be preferentially attacked in aluminum bronzes in seawater. Fortunately dealloying can be avoided or minimized by appropriate alloy selection and/or heat treatment so that many copper alloys find widespread marine service. Stress corrosion cracking is of great importance to engineers specifying materials for marine service. Many of the commonly used metals in production equipment can be susceptible to SCC under certain conditions. Carbon and low alloy steels are best kept below 35 HRC in hardness to avoid chloride stress corrosion cracking. Selected steels can be used at higher hardnesses provided they have sufficiently high fracture toughness in seawater (KIscc) for the particular application. Highly loaded steel parts or those with built-in stress risers (such as capscrews) may require even lower maximum hardnesses to avoid SCC. Austenitic stainless steels are widely used in marine environments. They are virtually immune to SCC at ambient conditions, but become susceptible to chloride SCC at elevated temperatures (over roughly 150(F). 17-4PH® stainless steel is susceptible to chloride SCC at high strength levels (e.g. H900 condition). High strength aluminum alloys are also susceptible to chloride SCC. High strength carbon and low alloy steels, titanium, martensitic stainless steels, duplex stainless steels, Monel K-500®, 17-4PH® stainless steels, Inconel X-750®, and many other alloys are susceptible to hydrogen embrittlement in marine environments. There are two primary sources for the hydrogen. As we have discussed, hydrogen forms at the cathode of a galvanic cell. A dissimilar metal couple in seawater may generate enough hydrogen to cause embrittlement. The second source of hydrogen is from cathodic protection systems. We’ll discuss the details of cathodic protection in the next section, but for now it’s important to know that hydrogen is generated from seawater at the surfaces of parts being cathodically protected. This hydrogen can lead to embrittlement in susceptible alloys. There are many applications calling for high strength materials in marine production equipment. Bolting, springs, snap rings, etc., are often designed with high specified minimum yield strengths. The temptation is great to specify high strength levels because thinner cross sections are then required, fewer bolts are necessary, smaller springs can be used, a great deal of cost and weight can be saved. But the road to hell is paved with good intentions. More than one young design engineer has page - 306

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heard the siren’s song and succumbed to this temptation only to rue the day he did when the heads popped off his capscrews and his springs broke into individual coils because of stress corrosion cracking. Specifying high strength materials in marine environments is playing with fire. Know what the limitations are so you don’t get burned. Waves, tides, and currents are among the various factors that conspire to cyclically load equipment in marine environments and make corrosion fatigue a concern. Most commonly used metals in marine environments including steels, copper alloys, nickel alloys, and aluminum will have a degradation in fatigue properties when immersed in seawater. A typical S/N curve for a low alloy steel is show in Figure 16. There are several important things to note in Figure 16. First, there is an endurance limit - a threshold stress below which failure will not occur regardless of the number of cycles - when the steel is tested in air. There is no such limit when the steel is tested in seawater. As a consequence no matter how small the cyclic loading, a steel part will eventually fail by fatigue in seawater given enough cycles (unless cathodically protected, see next section). The second thing to notice is that for a given stress, steel will always be able to endure a greater number of cycles in air than in seawater. Figure 16 graphically illustrates why it’s important not to use published fatigue data for an alloy tested in air when you’re designing for subsea applications.

Figure 16: Typical S/N Curve for Low Alloy Steel

There are several seawater variables that can influence corrosion fatigue. Oxygen content plays a strong role in corrosion fatigue. In deaerated seawater the fatigue properties of steel are the same as in air. At oxygen levels below 0.01 cc/liter, page - 307

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fatigue properties will vary with oxygen content; approaching those in air at extremely low oxygen levels. From 0.01 cc/liter up to saturation at 1.98 cc/liter, the fatigue properties are relatively independent of oxygen content and are substantially less than those in air. The oxygen levels normally found in seawater are well above 0.01 cc/liter so fatigue properties of steel will always be degraded in seawater unless protective measures are taken. Temperature can influence low cycle fatigue properties of steel in seawater. As temperature increases up to a certain point, low cycle fatigue resistance will decrease. Fatigue properties in seawater are affected by pH values above 10 or below 4. These values are well outside the normal pH of seawater thus need not concern us. Hydrostatic pressures of 1000-2000 psi have been shown to degrade corrosion fatigue properties when compared to results of tests at ambient pressure. This may be due to the increased chemical reaction kinetics at the higher pressures. This degradation may be an important consideration when designing deep water structures. Corrosion erosion can be a problem in marine environments. The amount of damage is dependent on the velocity of the seawater, the amount of particulate matter entrained in the flowing seawater, and the angle of impingement. Most commonly used marine materials can suffer corrosion erosion under the right (or wrong, depending on how you look at it) circumstances. Air bubbles can become entrained in turbulent seawater. When high velocity seawater with entrained air bubbles impinges against the surface of a metal, protective films may be damaged thus permitting localized corrosion to take place. Surface deposits may promote the required turbulence. Not too surprisingly this form of corrosion is known as impingement attack. Impingement attack may take place on external metal surfaces being swept by turbulent, high velocity seawater. It may also cause internal damage to seawater piping systems where sharp changes in the direction of flow or abrupt changes in the pipe size can create turbulence. The last form of seawater corrosion that we’ll discuss is cavitation damage. Seawater will boil at ambient temperatures if ambient pressure is reduced to the vapor pressure of the seawater. Localized boiling may occur when seawater passes over the surface of a propeller blade or a pump impeller blade at high velocity. Sudden changes in cross section of the blade (such as at the tip) will cause very low pressure zones to develop. Vapor bubbles can form and then collapse downstream. The repetitive hammering caused by the collapse of these bubbles can eventually lead to local compressive failures on the metal’s surface in the form of flakes. These flakes will be swept away by the flowing seawater exposing fresh metal to further attack.

PROTECTIVE MEASURES Things look bleak, you say? After reviewing all the potential problems that our equipment may encounter in seawater, are you ready to concede that the ocean reigns supreme? Get a grip, man! Grow a backbone! All is not lost. Did you think that you page - 308

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would have to emulate Desmothenes and challenge the sea with only mere words? Did you think that we’d send you out to battle the sea totally unarmed? Of course not! We’re going to make a whole arsenal of protective measures available to you to stave off the forays of Neptune’s realm. We cannot defeat the sea; we don’t have to. All we have to do is hold the sea at bay for the design life of our equipment while we reap the sea’s riches. It’s time to choose your weapon! The protective measures available to fight the sea or corrosive wellbore fluids can be conveniently grouped into the following general categories: 1) cathodic protection, 2) isolating equipment from the environment, 3) selecting inherently corrosion resistant materials, 4) adding a corrosion allowance, and 5) altering the environment.

CATHODIC PROTECTION Remember our Ag-Fe galvanic cell? The iron corroded as FeFe+2+2e—. The electrons that were produced as the iron went into solution traveled from the iron anode through the external circuit to the silver cathode where they totally suppressed the reaction AgAg++e—. Suppose now that we set up a galvanic cell with iron as the cathode. If we select a material with a much more negative electrode potential than iron for the anode, we would expect the anode to supply an abundance of electrodes to the cathode via the external circuit. The iron will try to corrode as FeFe+2+2e—. If we now continuously dump a large number of electrons (provided by the corroding anode) on the right side of this reaction and apply Le Chatelier’s Principle (there’s that damn Frenchman again), we will drive the reaction to the left. The excess electrons will reduce any Fe+2 ions that form at the surface of the cathode and thus keep the cathode from corroding. This is the basis for cathodic protection. Cathodic protection is our most efficient and cost effective means of providing corrosion protection to offshore equipment and structures. Because of its importance, we’ll look at it in detail. How does one go about making an entire steel structure sitting in the ocean the cathode of a galvanic cell? The first (and by far the most common for offshore structures) is by sacrificial anode cathodic protection. The other method is impressed current cathodic protection. The underlying principle of how these two methods protect steel is the same: they differ primarily in the source of electrons (or protective current). If we place a bare piece of low alloy steel in seawater it will corrode due to the many localized galvanic cells that become established on its surface. Let us supply an external current to the corroding steel as shown in Figure 17. The current leaves the auxiliary anode, passes through the electrolyte (seawater), and enters both the cathodic and anodic areas of the localized corrosion cells before returning to the d-c power source. When the cathodic areas are polarized by the external current to the opencircuit potential of the anodic areas, all the steel’s surface will be at the same potential. There will then be no driving force for localized corrosion currents and corrosion will cease. Figure 17 illustrates an impressed current system which uses a d-c power source (typically rectified a-c). In sacrificial anode (sometimes referred to as galvanic) cathodic page - 309

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protection, no power source is used. The auxiliary anode in Figure 17 is replaced with a metal having a much more negative electrode potential than the steel we are trying to protect. As the anode corrodes, it provides the current necessary to polarize the cathodic areas on the steel’s surface. A glance at Table 2 shows three candidate materials for our anode: magnesium, zinc, and aluminum. All of these have been used as anode material to cathodically protect steel structures offshore, but aluminum alloys are now by far the most commonly used.

Figure 17: Cathodic Protection How do we know once our cathodic protection system is in place that we are achieving the required degree of protection? We’ll have to measure the potential of the structure we are trying to protect. Optimum cathodic protection is achieved when the structure being protected is polarized to the open-circuit anode potential of localized galvanic cells on its surface. This potential for steel in aerated seawater has been empirically determined to be -0.80V relative to the silver/silver chloride/seawater reference electrode. This number is of fundamental importance. If we polarize the structure to a less negative number, some corrosion will take place. On the other hand we don’t want to overprotect by polarizing to a much more negative number because of the risk of hydrogen embrittlement (remember that hydrogen forms at the cathode of a galvanic cell), because of possible damage to coatings on the structure, and because of the increased expense. We can measure the potential of the structure as shown in Figure 18. Sacrificial anode systems are generally used only when the current demands for protection are relatively small and where the resistivity of the water is low. A properly designed sacrificial anode system requires a minimum of maintenance and monitoring. It is not subject to human operating error, power outages, etc., that may plague page - 310

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impressed current (IC) systems. Sacrificial anode system are simpler and more rugged than IC systems. There is no complex power sources or electronic equipment to go haywire. Although the anodes used in both systems can be mechanically damaged by passing objects that strike them, the anodes of IC systems require leads and have electrical connections that are easily damaged. Of course where there is no source of electricity, only sacrificial anode systems can be used.

Figure 18: Measuring Structure Potential

IC systems obviously must have some advantages over sacrificial anode systems or else they’d never be used. IC systems have a great flexibility: the current can be adjusted to meet changing requirements. IC systems utilize smaller and fewer anodes than sacrificial anode systems thus saving considerable weight and reducing drag. IC systems can be used in applications requiring higher currents than what can be practically provided by sacrificial anode systems. For long term protection of large structures (10 or more years), IC systems are often the most economical. 1. Anodes in Cathodic Protection The anodes used in sacrificial anode systems are typically aluminum, zinc, or magnesium alloys with aluminum alloys being the most common for offshore structures. The anodes come in a variety of shapes and sizes. They can be as small as a pound or two, or weigh over 1,500 pounds. Most anodes used on offshore structures will be long bars having either a cylindrical or trapezoidal cross section. They are typically cast on a steel core that extends beyond the ends of the anode. The steel core is used to mount the page - 311

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anode by welding the core to the protected structure. Some anodes may be attached by mechanical fasteners. Common anode shapes are shown in Figure 19.

Figure 19: Common Anode Shapes

Sacrificial anodes used on large structures are typically mounted with one or two feet “standoff” distance from the surface of the structure. This improves the current distribution to the structure. The steel core extensions as well as the area of the structure adjacent to the anode will often be coated to further enhance current distribution and consequently reducing the number of required anodes (see Figure 20). The size (weight), shape, and alloy type are important anode parameters that must be carefully chosen to insure that the anode will provide the required current over its design life for the given environmental conditions. The electrochemical efficiency of an anode is defined as the number of amp hours/kilogram of weight that the anode can provide. Aluminum alloys have relatively high electrochemical efficiencies which is why they are the type of anode material most frequently specified for long term protection of offshore structures. An anode’s utilization factor is the fraction of an anode’s weight that may be considered in design. Once an anode has corroded beyond its utilization factor, its performance becomes unpredictable. Proper selection of the number of anodes and the page - 312

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location of the anodes is of paramount importance in providing adequate coverage to an entire structure. There are several design standards for cathodic protection systems that address these issues. The most widely used one is DNV RP B401, Cathodic Protection Design.

Figure 20: Anode Attachment

Anodes for IC systems also come in many different shapes and sizes. The most common materials used for offshore structures are platinum on a substrate of titanium, columbium, or tantalum (applied by cladding, tack welding, or plating). On most large offshore structures, the IC anodes will be attached to that portion of the structure that is subsea in an evenly dispersed, geometric pattern. The attachment is made with nonconducting fittings because the anodes must be electrically isolated from the structure. A typical installation will have a transformer/rectifier control unit. The anodes are connected in parallel to a heavy cable which is attached to the positive terminal of the rectifier. The negative terminal of the rectifier is connected to the structure itself by a cable. Cables are always encased in steel pipe for protection. To obtain the optimum current distribution, dielectric shields will be installed on the structure beneath and around the anodes. These are typically glass reinforced polyester or epoxy resins. 2. Cathodic Protection Current Density page - 313

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Current density refers to the amount of cathodic protection current per unit surface area. The current density (given in amps/meter2) required to protect a steel surface will change over a period of time. The initial design current density, as defined by DNV RP B401, will be high for a bare steel surface. We want to rapidly polarize the surface to minimize corrosion. As you recall, hydrogen gas is produced at the cathode of a galvanic cell. As hydrogen is lost to the atmosphere, more water must dissociate to make up the loss. This produces an abundance of OH— ions adjacent to the cathode’s surface resulting in very high pH levels. The high pH reduces the solubility calcium carbonate-type (CaC02 ) minerals as well other minerals in seawater and causes them to deposit on the cathode’s surface. The calcareous deposit, even though often quite thin, is a very effective barrier coating that will isolate the underlying metal from the corrosive environment. As a consequence, as the calcareous deposit is forming, the current density requirement decreases. The final design current density will be substantially less than the initial design current density. It is the current density required to maintain corrosion protection of metal with established calcareous deposits and marine growth. Both the initial and final design current densities are based upon the current density requirements at a potential of -0.80V relative to the Ag/AgCl/seawater reference electrode during dynamic polarization from a more positive potential (i.e. nonprotective) towards the protective potential range. The average design current density is the current density required for protection after the system has reached its steady-state protection potential. The required driving voltage will be less consequently the average design current density will be less than either the initial or final. The average design current density is used to determine the minimum weight of anodes required to cathodically protect a structure over its design life. The current density required for protection will vary by the corrosivity of the seawater which in turn is dependent on temperature, salinity, oxygen content, etc. As an example DNV RP B401 specifies an initial design current density of 0.150 amp/m2 for tropical waters (>20(C) less than 30 meters deep. The final and average design current densities are specified as 0.090 and 0.070 amp/m2, respectively. The corresponding values specified for arctic waters (50%) and tin alloy with small additions of other elements. Bronze now also refers to a group of binary (two primary components) copper alloys such as aluminum bronze, tellurium bronze, silicon bronze, etc. Brush Plating - A portable means of selectively electrodepositing a metal plating. The workpiece is the cathode of a cell. An absorbent pad (the anode) is soaked in a solution containing the dissolved salts of the metal to be deposited and then is "brushed" over the area of the starting material. "Bubba" - A common and affectionate appellation given to the flower of Southern manhood: the quintessential red neck name. In the North, "Bubba" (along with "Fido" and "Rover") is generally restricted to dogs. Bubbler Technique (UT) - A type of localized immersion testing. Bulk Forming Processes - Those shaping processes that significantly change the cross sectional area of the starting material. Bullion - A semirefined alloy that contains enough metal to make recovery profitable. Also bulk, refined gold or silver. Burnishing - A method of smoothing the surface of a metal by frictional contact between the workpiece and some hard material. Burr - A turned over edge on a machined part resulting from an unclean cut. Burst - A forging or extrusion defect in which the center of the workpiece has ruptured from the build up of hydrostatic pressure during working. C-Scan Display (UT) - An oscilloscope display presentation that gives a two dimensional view of reflected wave sources from the surface being inspected. The position along the page - 346

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horizontal and vertical axes corresponds to the position of the reflector relative to the axes of the surface being inspected. Cadmium Electroplating - An electrodeposited layer of metallic cadmium used primarily for mild corrosion protection. Calcareous Deposit - A deposit of calcium carbonate (CaCO2) type minerals that forms on the surface of bare metals being cathodically protected. Calcium Treating - Adding calcium to the molten metal in ladle in order to control the shape of sulfide inclusions and thereby improve toughness. Calibration Blocks (UT) - Metal blocks containing known, artificial defects used to standardize test conditions and to determine the operating characteristics of a system. Capped Steel - A slightly deoxidized steel similar to a rimmed steel except that the rimming action was stopped before it was complete by placing a heavy metal cap on the ingot mold. Carbide - A compound consisting of one or more metallic elements with carbon. Carbon Equivalent - A number derived from an empirical formula that provides a summation of the hardenability effects that several elements have on steel in terms of the hardenability effect of carbon. One commonly used formula (there are many) is:

C.E. C% 

Mn%  Cr  Mo  V%  Ni  Cu% 6 5 15

Carbon Steel - A steel having a maximum of 1.65% Mn, 0.60% Si, and 0.60% Cu. No other intentionally added elements are permitted except when deoxidizers are specified or for certain specific grades that allow the addition of boron (for hardenability) or sulfur, phosphorous, or lead (for machinability). Carbonyl Process - A refining process used in the production of pure, metallic nickel. Carburizing - A surface hardening process for ferrous metals in which carbon is diffused into the workpiece at an elevated temperature. Case - The hardened surface layer in carburized, nitrided, etc., steels. Cast Iron - An alloy of iron and carbon as well as other alloying elements. Carbon content is always greater than 2%. Casting - A metal part made by pouring molten metal into a mold having a cavity of the desired configuration and allowing it to solidify. Catalyst - A substance that accelerates the rate of reaction between other substances, but does not itself undergo any net change.

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Cathode - The electrode in an electrolytic cell where reduction takes place. It receives electrons from the external circuit in a galvanic cell. Cathodic Protection - A means of providing corrosion protection by impressing a direct current on a metal part thus making it cathodic to it surroundings. Cation - An ion with a positive charge. Cavitation Damage - Corrosion that occurs as the result of the formation and collapse of vapor bubbles against a metal’s surface in relative motion to water. Cementite -An intermetallic compound of iron and carbon that is one of the chief hardening agents in steels and many cast irons. It has the stoichiometric formula of Fe3C. Centerless Grinding - Grinding the inside or outside of a workpiece that is mounted on rollers rather than centers. Central Conductor (MT) - A magnetizing technique for hollow parts that utilizes a solid bar or cable passing through the workpiece to carry the magnetizing current and induce a magnetic field in the workpiece. Centrifugal Casting - A casting process in which the cylindrically shaped mold is rotated about its longitudinal axis as the molten metal solidifies. Ceramic Cementation - A process in which one or more elements are diffused into the surface of a metal and form a protective ceramic layer. Chalking - In coatings, a defect in which an organic binder disintegrates due to exposure to ultraviolet rays in sunlight allowing the pigments to remain on the surface as powder or “chalk.” Chamfer - A beveled edge used to eliminate the stresses associated with a sharp corner. Chaplets - Metal clips used to hold cores in place in a mold during casting. Characteristic Curve (RT) - A plot of the logarithm of relative exposure to density for a particular film. Charge - The sum total of all liquids and solids fed into a furnace during one cycle of operation. Charpy V-notch Impact Test - A mechanical test used to determine the notch toughness of a metal. Chatter - A wavy surface on a machined or ground surface caused by vibration in the workpiece or the tool or grinding wheel. Checking - In coatings, a defect consisting of many small, nonlinear breaks in the surface of the coating that do not extend all the way down to the substrate.

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Chemical Vapor Deposition (CVD) - A coating process in which the workpiece is exposed to several gases at an elevated temperature in a closed chamber. The reaction of the gases deposits a thin layer of metal on the surface. Chili - A delicious concoction of ground beef, tomatoes, onions, spices, and chili powder often found on Texas tables. Its popularity is well deserved, however, over indulgence may lead to flatulence. One of the principal reasons why so many Texans are full of hot air. Chromate Conversion Coating - A coating process in which the workpiece is immersed into an acid solution containing chromium salts. The solution reacts with the surface of the metal forming a protective film containing complex chromium compounds. Chromium Electroplating - An electrodeposited layer of chromium used primarily for corrosion and wear resistance. Chuck - An adjustable device for gripping a workpiece and thus holding it in position on a machine. Cleavage Fracture - Fracture that has occurred along certain crystallographic planes. It is often associated with brittle fracture. Closed Dies - Forging dies in which the metal to be shaped is completely enclosed within the die cavity as the two halves of the die come together. CO2 Corrosion - Corrosion resulting from the formation of carbonic acid when CO2 is dissolved in water. Cobalt 60 - A radioactive isotope used in radiography. Cogging - Reducing the cross section of a cast ingot or a billet by open die forging in partial steps along the length of the workpiece. Coherent - Describes the condition where the lattice of a precipitate is continuous with the surrounding matrix. Coils (MT) - A magnetizing technique primarily for inducing a longitudinal magnetic field in a long part (such as a bar) by surrounding the workpiece with a coiled conductor that carries the magnetizing current. Coining - A closed die forging process in which all the surfaces of the workpiece are confined within the die cavity in order to reproduce close details and hold tight tolerances. Cold Shut - A defect in a casting where two streams of molten metal met within a mold cavity, but did not metallurgically bond. Cold Work - Deformation of a metal below its recrystallization temperature. Collinater (RT) - A device, usually of lead, that surrounds the radiation source and allows the primary radiation beam to exit only through a small opening.

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Colmonoy® - A family of nickel based, wear resistant alloys generally applied as thermal spray coatings. Completion Fluids - Weighted fluids (typically heavy brines) used to maintain control over a well during workovers or completion. Composite - Metallic or nonmetallic material consisting of two or more separate components, each having some desired property and each retaining its own identity, that have been combined so that they act in concert and complement each other. Composition Cell - A type of galvanic cell in which two dissimilar metals are coupled to each other in an electrolyte. Compression Wave (UT) - See longitudinal wave. Concentration Cell - A type of galvanic cell in which electrodes of the same metal are coupled to each other and each is immersed in a different concentration of electrolyte. Contact Technique (UT) - An ultrasonic technique that uses a transducer that comes into physical contact with the part being inspected. Continuous Casting - A casting technique used to make tubes, plates, and other products having a uniform cross section of any length by continuously adding molten metal to the mold as the solidified metal product is slowly withdrawn from it. Continuous Circulation Degassing - Circulating the molten metal in a ladle through a vacuum chamber and then back to the ladle in order to degas it. Continuous Cooling Transformation Curve - A diagram that shows the transformation products of a given metal as it is continuously cooled from an elevated temperature over a period of time. Continuous Magnetization (MT) - A magnetization technique in which the magnetizing current is turned on while the magnetic particles are applied. Converter - A furnace in which impurities are removed from molten metal by oxidizing them with forced air. Cope - The top half of a mold used in casting. Core - A preform made of sand, plaster, or other material placed in a casting mold to make an internal cavity or surface. Corrosion - The chemical or electrochemical attack of a metal by its environment. Corrosion Fatigue - The fatigue failure of a metal in a specific environment in a fewer number of cycles or under a smaller load than would have been required in the absence of that environment. Counterboring - Boring or drilling a flat bottom hole concentric with an existing longer, but smaller diameter hole. page - 350

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Countersinking - Flaring the top of a hole in order to receive the head of a fastener or a machine center or for deburring. Couplant (UT) - A liquid, grease, or paste used to provide a low resistance path for ultrasonic waves leaving the transducer and entering the workpiece and vice versa. Cracking - In coatings, a defect in which a break extends all the way through the coating to the substrate. Crater Crack - A crack in the depression at the end of a weld bead in arc welding caused by the top surface of the bead collapsing inward as the molten interior shrinks as it solidifies. Cratering - Small water-like depressions in a coating randomly dispersed over the surface due to contamination of the atomizing air, improper solvent mixture, etc. Creep - The slow, plastic deformation of a metal under a constant load. Stresses may be well below the yield strength. It is a function of time, temperature, and load. Creep Strength - The amount of constant stress that will cause a specified amount of plastic deformation in a given time period. Crevice Corrosion - A form of corrosion in which the local environmental conditions inside a crevice differ from those outside the crevice thus establishing a concentration cell with the metal surfaces within the crevice being anodic. Critical Cooling Rate - The slowest rate at which a metal can be cooled from an elevated temperature and still obtain some desired transformation or to avoid an undesirable one. Critical Stress Intensity Factor (Kc) - The value of the stress intensity factor (K) above which unstable crack growth occurs. Crystal - A group of atoms in a solid that has a particular arrangement that it repetitive over all three dimensions. Crystal Lattice - The arrangement of atoms in the unit cell of a crystal. Cupola - A vertical, cylindrically shaped furnace charged with iron and coke used in the production of gray iron. Curie (RT) - A measure of activity of a radioactive isotope that is equal to 3.7 x 1010 atoms decaying per second. Curie Temperature (UT) - The temperature above which a crystal is no longer ferroelectric. Cyaniding - A surface hardening process in which both carbon and nitrogen are diffused into a ferrous metal at an elevated temperature. D-Gun Process® - A detonation gun process developed by Union Carbide to apply hardfacing and corrosion resistant materials on to a metal substrate. DAC Circuit (UT) - See distance amplitude correction circuit. page - 351

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Decarburization - The loss of carbon at the surface of a metal as the carbon reacts with oxygen at an elevated temperature. Deep Drawing - A forming process in which a shaped punch presses a sheet metal blank through the opening of a die. Delta Iron - The stable form of iron from 2541(F to 2800(F. It has a body centered cubic structure (BCC). Demagnetization (MT) - Processing the workpiece to remove all traces of residual magnetism. Density (RT) - A measure of the degree of blackening that occurs on an x-ray film. Deoxidizers - Substances added to a molten metal (generally in a ladle) that remove oxygen by combining it with it to form oxides that subsequently float to the surface and form a slag. Detonation Gun Process - A thermal spray process that uses the explosive combustion of oxygen and a fuel gas to heat the coating particles up and propel them towards the workpiece. Developer (PT) - A fine powder applied to the workpiece after the excess penetrant has been removed. The developer draws the remaining penetrant out of any open discontinuities. The contrast between the developer and the penetrant indicates the presence of a discontinuity. Developer (RT) - A chemical solution used to convert the exposed silver bromide in the latent image on an x-ray film into black, metallic silver thus making the latent image visible. Diamond Pyramid Hardness Test - See Vickers hardness test. Diaphragm (RT) - Radiation absorbing material in the form of sheet, blocks, pellets, etc., that are placed on top or around the object being radiographed such that only the area of interest is directly exposed to the radiation beam. Die - Tooling used in forging, extrusion, drawing, etc., that imparts a desired shape to the starting material as it is forced into a cavity or through a hole within the tooling. Die Casting - A casting technique in which molten metal is injected under pressure into the mold (or die). Differential Temperature Cell - A type of galvanic cell in which electrodes of the same metal are both immersed in the same electrolyte, but are at different temperatures. Diffusion - The spontaneous movements of atoms or molecules within a material from regions of high concentration to regions of low concentration. Dimpled Rupture - See fibrous tearing. Dipole Moment (UT) - The product of electronic charge and the distance between the centers of the positive and negative charges in a crystal.

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Direct Current (MT) - Current that always flows in the same direction in a conductor. Dislocation - A linear defect in a crystal structure. Dispersion Strengthening - The strengthening of a soft metal matrix by the formation of a hard, finely distributed, second phase. Distance Amplitude Correction Circuit (UT) - An auxiliary circuit in an amplifier that increases the gain with the corresponding depth of the echo source. It compensates for the loss of signal strength as the signal travels through greater distances in the workpiece. Domain (UT) - In a ferroelectric material, the subregion in a crystal which is uniformly polarized. Drag - The bottom half of a mold used in casting. Drawing - A process for making small diameter bar, wire, rod, and tube by pulling the starting stock through a die which imparts the desired cross section. Drawn Over Mandrel (DOM) - A process for making tubulars by pulling the hollow starting stock simultaneously through a die and over a mandrel. The die imparts the desired OD while the mandrel imparts the desired ID to the tube. Drop Hammer - A forging hammer that develops it's striking force through gravity during free fall. Drop Weight Testing - A test method used to determine the nil-ductility transition temperature (NDTT) of a metal. The test consists of a free falling weight that strikes test specimens of precracked plates at various temperatures. Dry Film Thickness - The final thickness of a coating after curing. Dry Mag (MT) - A magnetic particle examination technique in which the magnetic particles are dry (as opposed to being suspended in a liquid). Ductile Cast Irons - A family of cast irons in which the excess carbon is in the form of graphite spheres thus improving ductility. Ductility - The ability of a metal to plastically deform without fracturing. Dwell Time (PT) - The time that penetrant is allowed to remain on the workpiece before the excess is removed. Edge Dislocation - A linear defect in a crystal structure consisting of an extra plane of atoms extending part way through the crystal. 885(F Embrittlement - The loss of ductility that occurs in some martensitic and ferritic stainless steels when held at 700-950(F for a period of time. Elastic Deformation - Temporary deformation that occurs when a metal is subjected to a relatively small stress. It disappears when the load is removed.

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Elastic Limit - That point on a stress-strain curve beyond which increasing stress will result in plastic deformation. Electric Arc Spray - A thermal spray process in which the coating material is in the form of two wire electrodes. An arc struck between the electrode causes the tips to melt and then compressed gas is used to atomize and propel the molten metal at the workpiece. Electric Furnace Process - A steelmaking process in which the furnace charge is melted by an arc between three carbon or graphite electrodes and the charge itself. Electrode Potential - The electrical potential of an electrode in an electrolyte when compared to a reference electrode. Electroless Nickel Plating - A layer of metallic nickel deposited on a metal substrate by the chemical reduction of nickel ions. Electrolyte - A substance that when dissolved in a suitable solvent becomes an ionic conductor. Electromotive Force Series - A tabulation of the potential differences that have been measured between different metals and the standard hydrogen electrode. Electron Beam Welding (EBW) - A welding process that uses the impingement of a high velocity stream of electrons as the source of heat to melt the base metal during welding. Electroplating - A deposition process for metal coatings in which the part to be plated is made the cathode of an electrolytic cell and is immersed in an electrolyte containing ions of the metal to be plated. Electroslag Remelt (ESR) Process - A refining process used to improve the cleanliness of a metal. The metal is first cast into a consumable electrode. The tip of the electrode is immersed into a special slag in the ESR furnace. The tip of the electrode melts as an arc is struck between the tip and the bottom of the furnace. Refining takes place as the molten metal droplets interact with the slag. Elgiloy® - A cobalt base alloy that is strengthened by a combination of cold work and precipitation hardening. It has excellent corrosion resistance. It is frequently specified for high strength springs. Eliot, T.S. - An American-born English poet, critic, and playwright (1888-1965). His most famous poem is all about Texas. Elongation - See percent elongation. Embrittlement - The loss of ductility in a metal due to a chemical or physical change. Emulsifier (PT) - A liquid that breaks up the oily portion of a penetrant and keeps the oil globules in suspension. Endurance Limit - In fatigue, the stress below which failure will not occur regardless of the number of cycles.

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Engineering Strain - In tensile testing, the increase in the specimen's gage length at any point in time of fracture divided by the original gage length. Engineering Stress - In tensile testing, the load at any given point in time divided by the original cross section of the specimen's gage length. Equilibrium - A state of a system in which there is no net change. There may be many reactions going on, but the rate of reaction of the reactants equals the rate of decomposition of the products so there is no net change. Equilibrium Diagram - See phase diagram. EQUOTIP® Hardness Tester - A portable hardness tester. The hollow test probe contains a free falling indenter that has a spherical tip. A permanent magnet is incorporated into the indenter. The probe is placed on the surface of the part to be tested. A button on the top of the probe is pushed thereby releasing a compressed spring that propels the indenter down the probe tube towards the part. The indenter strikes the surface of the part and rebounds back up the probe. The velocities of the indenter are determined at the time of impact and rebound by measuring the induced voltages created in a coil (surrounding the path of the indenter) as the permanent magnet in the indenter passes through. The induced voltage is proportional to the indenter velocity. The harder the metal, the greater the rebound velocity. The probe is electrically connected to a small console where the signal is processed and the hardness displayed. The indentation made by the EQUOTIP® is barely discernible thus making hardness testing of sealing surfaces possible. Erosion - The gradual wearing away of a metal in contact with a moving fluid. ESR - See electroslag remelt process. Etch - A laboratory method for revealing the grain structure of a metal by the preferential attacks of certain microstructural features (such as grain boundaries) by an acid or by using an electrochemical process. Eutectic - An isothermal reaction in which a liquid is transformed into two separate, but intimately mixed solid phases upon cooling. Also the alloy that is formed by a eutectic reaction. Eutectic Isotherm - The horizontal line on a phase diagram that contains the eutectic point. Eutectic Point - That point on a phase diagram where a liquid isothermally transforms into two different solid phases. Eutectoid - An isothermal reaction in which one type of solid phase transforms into two different solid phases upon cooling. Also the alloy that is formed by a eutectoid reaction. Eutectoid Isotherm - The horizontal line on a phase diagram that contains the eutectoid point. Eutectoid Point - That point on a phase diagram where a single solid phase isothermally transforms upon cooling into two different solid phases.

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Excessive Reinforcement - A welding defect in which weld metal is built up on the face or root of a weld so that it is much higher than the adjacent base metal. Exfoliation - A form of corrosion damage in which the corrosive attack proceeds laterally near the surface of a metal. Explosive Forming - A forming technique that uses the high pressure of an explosive charge to force a metal blank into a die cavity. Exposure (RT) - The intensity of radiation multiplied by the time that an x-ray film is exposed to the radiation while making a radiograph. The total amount of energy impinging on the film. Extrusion - A forming process in which a metal billet, pipe, etc., is forced through the orifice in a die in order to make a product having a uniform cross section of some desired shape. Face - The top surface of a weld. Face Centered Cubic (FCC) - One of the basic crystal structures of metals. Facing - Generating a surface on a rotating workpiece in a machine by the traverse of a tool perpendicular to the axis of rotation. Far Field (UT) - That area beyond the Fresnel zone. Fatigue - Failure of a metal under a cyclic load. Fatigue Limit - See endurance limit. Ferralium® 255 - A duplex stainless having a microstructure consisting of approximately equal amounts of austenitic "islands" in a ferrite matrix. Ferrite - A phase of steel or cast iron consisting of a solid solution of carbon atoms in alpha iron. It has a body centered cubic (BCC) structure. Ferritic Stainless Steel - A stainless steel with a stable BCC structure. Ferroelectric Crystal (UT) - A pyroelectric crystal in which the direction of polarization can be permanently reversed by the application of a sufficiently intense external field. Ferromagnetic Material (MT) - Materials that are strongly attracted to a magnetic field. Fibrous Fracture - A fracture characterized by a dull surface and the elongation and tearing of grains. It is associated with ductile failures. Fibrous Tearing - The nucleation and growth of voids (especially around precipitates or impurities) as a metal is stressed in tension. The voids eventually grow to the point where they coalesce and the metal breaks in two leaving a dimpled surface on the fracture faces. Filler Metal - The extra metal added to a weld joint during welding. page - 356

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Fillet Weld - One of the eight basic types of welds. It has a triangular cross section. Film Contrast (RT) - The slope of the characteristic curve at a given density for a particular xray film. Film Latitude (RT) - The thickness range of the object being radiographed that can be recorded with a single exposure on a particular film. Film Speed (RT) - A characteristic of an x-ray film that is inversely proportional to the total energy of radiation that produces a given density. Fixer (RT) - A chemical solution used to convert unexposed grains of silver bromide into a water soluble product on an x-ray film after it has been developed. This prevents the film from darkening when exposed to light. Flaking - A coating defect where small pieces of coating are easily removed. Flame Hardening - A localized heat treating technique in which a high temperature flame is utilized to heat up the area to be hardened on the workpiece. Flame Spray - A thermal spray process in which the combustion of a fuel gas is used to heat up the coating material and propel it to the workpiece. Flash - The excess metal in a closed die forging that extends beyond the past cavity along the parting line. Flash Butt Welding - A resistance welding process in which current is applied to the parts to be joined as they are mechanically forced together. Flask - An open metal frame placed around a pattern into which sand is rammed during the manufacture of a sand casting mold. Flotation Separation - A mineral beneficiation technique used to enrich ore. Fluorescent Mag (MT) - A magnetic particle examination technique that uses magnetic particles that fluoresce under a black light. Fluorescent Penetrants (PT) - Those penetrants that must be evaluated under a black light where they fluoresce. Flux - A fusible material used in welding to dissolve oxides and other imparities and to facilitate their removal. It may also provide shielding of the weld puddle. Flux Cored Arc Welding (FCAW) - An arc welding process that utilizes a hollow electrode filled with flux that is continuously fed through a hand held gun. Focal Spot (UT) - The localized area through which an acoustic lens refracts all the sound waves. Forging - Plastically deforming metal into a desired shape using a compressive force (generally at an elevated temperature). page - 357

APPENDIX A

GLOSSARY

Forming Process - A shaping process that plastically deforms a metal between dies. Foundry - A facility for the production of castings. Fracture Mechanics - A quantitative method of characterizing fracture behavior in terms of toughness, flaw size, environment, and stress level. Fraunhofer Field (UT) - See far field. Frequency (RT & UT) - The number of cycles per second. Fresnel Zone (UT) - The area from the point of convergence of the maximum energy areas of a multi-source ultrasonic waves to the transducer. Fretting - The surface damage resulting from two pieces of metal rubbing against each other particularly in a corrosive environment. Friction Welding - A welding process in which one of the pieces to be joined is rapidly rotated relative to the other piece and then both pieces mechanically forced together. The friction between their surfaces develops enough heat to weld the surfaces together. Full Anneal - A heat treatment of steels and cast irons in which the metal is heated up into the fully austenitic region and then slowly cooled in order to homogenize, stress relief, remove gases, etc. Full Penetration - A weld that extends all the way through a weld joint. Full-Wave Rectification (MT) - The conversion of alternating current to direct current such that the polarity of each half of the AC waveform is made the same. Fusion Welding - a family of welding processes that melt the base metal and filler metal (if used) in order to make the weld. Fusion Zone - That portion of the base metal adjacent to a weld that melts during welding. FWDC (MT) - See full-wave rectification. Gage Length - The reference length on a tensile specimen used in calculating strain, elongation, etc. Gain (UT) - The amount of amplification that a signal undergoes in an amplifier. Galling - Surface damage that occurs when two metals are rubbed together such that the friction that is generated is sufficient to cause the localized welding of high spots on the metal's surface. This may cause the surfaces to roughen, spall, or even seize up. Galvanic Cell - An electrochemical cell in which two electrodes each having a different potential are in electrical contact and are immersed in an electrolyte. Galvanic Corrosion - The electrochemical attack of metal due to the formation of a galvanic cell. page - 358

APPENDIX A

GLOSSARY

Galvanic Series - A tabulation of metals arranged in order of their electrode potentials in a given environment. Galvanizing - A process for coating steel for corrosion protection by immersing it into a bath of molten zinc. Gamma Iron - The stable form of iron in the 1674(F to 2541(F range. It has a face centered cubic (FCC) structure. Gamma Prime - The precipitate in age hardenable nickel base alloys that is the primary strengthening agent. It is an intermetallic compound having a base composition of Ni3 (Al,Ti). Gamma Ray - A type of electromagnetic radiation used in radiography that is produced during the decay of radioactive isotopes. Gangue - The trash rock, silt, clay, etc., that is mixed with the mineral of interest in an ore. Gap - The opening at the root of a weld. Gas Metal Arc Welding (GMAW) - An arc welding process in which a bare, consumable electrode is continuously fed through a hand-held gun. A shielding gas is used and comes out of the same orifice in the gun as the electrode. Gas Tungsten Arc Welding (GTAW) - An arc welding process in which a nonconsumable tungsten electrode is used. Filler metal may be added using a separate rod. Gate - Where the molten metal enters the part cavity in a mold during casting. Gator-Gard® - A plasma arc spray process used to apply hardfacing and corrosion resistant materials. Geometric Unsharpness (RT) - The fuzziness in a radiograph resulting from the overlapping images formed by the impinging radiation beams on the film. Grain - An individual crystal in a polycrystalline substance. Grain Boundary - The junction between two crystals in a metal. Grain Refinement - The processing of metal to obtain a smaller grain size. Grain Size Number (ASTM) - The exponent "n" in the formula N=2n-1, where N is the number of grains per square inch when the metal is viewed under a microscope at a linear magnification of 100X. Gray Cast Iron - A cast iron in which the excess carbon is in the form of graphite flakes. Green Compact - A powder metallurgy term for an inprocess part made out of compacted powder, but not yet sintered.

page - 359

APPENDIX A

GLOSSARY

Grinding - Removal of surface metal from a part by the use of an abrasive wheel moving at a relatively high speed past the part. Grits - A ghastly substance made from coarse ground corn. In Texas it is a staple food often appearing as a side dish at breakfast, lunch, and dinner. In the North it is used as wallpaper paste and slops for the hogs. Groove Weld - One of the eight basic types of welds. H2S Cracking - A form of stress corrosion cracking that takes place when a susceptible alloy is loaded in tension in the presence of H2S. Half-Wave Rectification (MT) - The conversion of alternating current to direct current by allowing only one half the waveform through an electronic gate. Halflife (RT) - The time it takes for a radioactive source to decay to an intensity that is 1/2 its original intensity. Hammer Forging - Forging in which the workpiece is roughly shaped by rapid and repeated blows of a hammer. Hard Facing - Depositing weld metal on the surface of a part for the purpose of increasing erosion, wear, and abrasion resistance. Hardenability - A measure of an alloy's ability to be hardened to a desired level at a particular location through heat treatment. Hardness - A measure of a metal's ability to resist penetration of its surface. Hastelloy® - A family of nickel base alloys that contain substantial amounts of molybdenum and chromium used in applications requiring a high degree of corrosion resistance. Head/Tailstock Contact (MT) - A direct contact technique used to magnetize the entire workpiece. The workpiece is clamped between the electrodes (the head and tailstock). Heat Affected Zone (HAZ) - The region of base metal adjacent to a weld that is exposed to a temperature high enough to cause some alteration in microstructure. Heat Treatment - Heating and cooling a metal at controlled times, temperatures, and rates in order to develop the desired properties. Heavy Media Separation - A method of mineral beneficiation used to enrich ore. Heliarc - Another name for gas tungsten arc welding. Hematite - An iron-bearing mineral (Fe2O3) that is one of the principal sources of iron in iron ores. Hexagonal Close Packed (HCP) - One of the basic crystal structures of metals.

page - 360

APPENDIX A

GLOSSARY

High Velocity Oxyfuel (HVOF) Coating - A thermal spray coating in which high pressure combustion gases are used to melt and propel the coating materials. HIP - An acronym for hot isostatic pressing. A powder metallurgy technique in which powder is consolidated in a flexible mold at high temperature and pressure. Holiday - In coatings, an application defect where a small area of substrate is left uncoated. Honing - Smoothing the surface of a metal by rubbing a tool or fixture having bonded abrasives against it. Horse - In Texas, a cowboy's best friend. I prefer a voluptuous blonde. This does not speak highly of Texas cowboys. Hot Isostatic Pressing - See HIP. Hot Shortness - Embrittlement in a metal in the hot working range caused by low melting impurities that segregate out along grain boundaries. Hot Tear - A casting defect resulting form the stresses induced in the cooling metal as the metal shrinks. Hot Work - Deformation of a metal above its recrystallization temperature. HW (MT) - See half-wave rectification. Hydrogen Attack - The blistering, decarburization, and /or cracking that may occur in certain alloys when exposed to hydrogen at high temperatures and pressures. Hydrogen Embrittlement - The loss of toughness and ductility that occurs in some alloys when exposed to hydrogen. Hydrophilic Emulsifiers (PT) - Detergent based emulsifiers the displace the excess penetrant from the surface of the part. Hypereutectic - An alloy having a composition lying to the right of the eutectic point in a phase diagram. Hypereutectoid - An alloy having a composition lying to the right of the eutectoid point in a phase diagram. Hypoeutectic - An alloy having a composition lying to the left of the eutectic point in a phase diagram. Hypoeutectoid - An alloy having a composition lying to the left of the eutectoid point in a phase diagram. Immersion Technique (UT) - An ultrasonic examination technique in which the transducer and the workpiece are immersed in a couplant. The transducer does not come into contact with the part.

page - 361

APPENDIX A

GLOSSARY

Immersion Zone - The area of exposure in a marine environment that is continuously wetted by seawater. Impingement Attack - Corrosion that takes place when high velocity seawater containing entrained bubbles impinges against the surface of a metal. Impressed Current Cathodic Protection - Cathodic protection in which the protecting current is provided by an external power source (a battery, rectified A-C, etc.). Inclusion - A foreign particle trapped in solidified metal. Incoherent - Describes the condition where the lattice of a precipitate is not continuous with the surrounding matrix. Incomplete Fusion - A weld defect in which the weld metal does not completely fill the prepared joint. Inconel® - A family of nickel base alloys containing substantial amounts of iron and chromium used primarily for their corrosion resistance. Induction Hardening - A localized or through hardening technique in which the resistance of an induced current in the workpiece is the source of heat. Ingot - A casting suitable for subsequent mechanical working or remelting. Inhibitor - A chemical that reduces the corrosion of a metal without significantly reacting with the environment. Intercoat Delamination - Loss of adherence between two coating layers. Intergranular Attack (IGA) - A type of corrosion damage in which the grain boundaries of a metal are preferentially attacked. Intermediate Coats - In a multicoat system, those coating layers between the primer coat and the top coat. They provide strength, flexibility, and corrosion resistance to the system. Interpass Temperature - The temperature of the weld area in between welding passes. Interstitial Solid Solution - A solid solution in which the solute atoms occupy a position in between the solvent atoms in the solvent's crystal structure. Investment Casting - A casting technique used to make relatively small, finely detailed castings. Ion - An atom or group of atoms that behave as a unit in a chemical reaction and has a net charge because of the gain or loss of electrons. Iridium 192 (RT) - A radioactive isotope used in radiography. Isostatic Pressing - A powder metallurgy technique that uses a flexible mold subjected to a high pressure to compact the powder. page - 362

APPENDIX A

GLOSSARY

Isotherm - A horizontal line in a phase diagram that separates the stable regions of two or more phases. Isothermal - Taking place at a constant temperature. Isotopes - Atoms that have the same number of protons, but different numbers of neutrons. Isotropic - Having identical properties in all directions. Jalapeno Pepper - A fiery condiment with a reputation for causing severe burns of the throat, esophagus, stomach, and lower regions. The hot temperament of many native Texans is attributable to the heavy use of this condiment over several generations. K - See stress intensity factor. KC - See critical stress intensity factor. KEE - The equivalent energy fracture toughness of a material. KI - The stress intensity factor that the describes the magnitude of the elastic stress field ahead of a sharp crack in mode I loading. KIc - The critical value of KI above which unstable crack growth will occur. Killed Steels - Steels that have been deoxidized to the extent that very little gas is evolved during solidification. King® Portable Hardness Tester - A portable Brinell hardness tester. It is clamped onto the part being tested. Hydraulic pressure is used to apply the necessary load to the standard Brinell indenter. The hydraulic pressure is obtained through the use of a hand pump integral with the tester. The pump's lever is stroked until the required load is indicated on a gage mounted on the tester. The pressure is then released, the tester taken off the part, and the diameter of the indentation measured. Knoop Hardness Testing - A type of microhardness that employs a diamond indenter ground into a pyramidal form that makes a diamond shaped indentation having roughly a 7-to-1 ratio between the long and short diagonals. Ladle-To-Ladle Degassing - See stream degassing. Lap - A defect occurring when metal is mechanically worked in which metal has been folded over on itself, but not welded together. Lapping - Smoothing the surface of a metal part by rubbing it against a tool (such as a wheel or cylinder) that has loose abrasive on it. Lateral Expansion - The increase in width of a Charpy V-notch impact specimen resulting from the flaring that occurs when the hammer strikes the specimen. Le Chatelier's Principle - When a system in equilibrium is subjected to a stress, the system will adjust its equilibrium conditions to minimize the stress. page - 363

APPENDIX A

GLOSSARY

Leaching - A type of corrosion damage in which one or more elements in an alloy are selectively attacked. Leg - When looking at the profile of a fillet weld, the length that each side of the weld is in contact with the base metal. Lifting - In coating, the softening and wrinkling of an undercoat when a topcoat is applied. Linear Accelerator - A source of high energy X-rays (up to 25 MeV) used in radiography to examine steel parts up to 24 inches thick. Lipophilic Emulsifiers (PT) - Oil based emulsifiers that diffuse into the excess penetrant and form a water-washable mixture. Liquid Penetrant Examination - A nondestructive inspection technique used to detect open discontinuities on the surface of a part. Liquidus - That line in a phase diagram that separates a two phase region consisting of a solid and a liquid phase from an entirely liquid phase region. It marks the temperatures above which melting is complete for various compositions. Longitudinal Direction - The direction of greatest working (and consequently grain flow) in a metal. Longitudinal Wave (UT) - A wave in which the particles of the transmission medium vibrate in a direction parallel to the direction of wave motion. Lost Wax Casting - See investment casting. Lower Bainite - Bainite that is formed below 660(F. LW-45® - A tungsten carbide hard facing coating applied by a detonation gun process. Macroscopic - Visible with the naked eye or at a magnification no greater than 10X. Magnetic Field Strength Indicator (MT) - A small hand held device containing discontinuities of a known size used to verify the adequacy of the magnetic field on the surface of the workpiece. It is held adjacent to the magnetized workpiece. After magnetic particles are applied, the discontinuities in the indicator must be discernable. Magnetic Flux Lines (MT) - Magnetic lines of force. Magnetic Particle Examination - A non destructive examination technique used to detect surface and near-surface discontinuities in ferromagnetic materials. Magnetic Permeability (MT) - The ratio of magnetic flux density to the strength of a magnetizing force. Magnetic Separation - A mineral beneficiation technique used to enrich the content of ferromagnetic minerals in iron ore.

page - 364

APPENDIX A

GLOSSARY

Magnetite - An iron-bearing mineral (Fe304) that is one of the principal sources of iron in iron ore. Malleability - The ability of a metal to be deformed in compression without fracture. Malleable Cast Iron - White cast iron that has been annealed until the cementite is transformed into graphite. Marine Atmosphere - The area of exposure in a marine environment above the water level and above tidal and wave action. Martempering - A heat treatment for steels and cast irons in which the austenitized part is quenched to a temperature just above the martensite start temperature, held until the part has attained a uniform temperature, and then air cooled. This reduces the chances of cracking or distortion. Martensite - A metastable phase of ferrous alloys formed by austenitizing and then cooling at a suitable rate. It is a supersaturated solution of carbon in a body centered tetragonal iron crystal lattice. It is one of the principal strengthening agents in ferrous alloys. Martensitic Stainless Steels - Stainless steels that develop a martensitic structure after austenitizing and quenching thus are hardenable by heat treatment. Mask (RT) - Radiation absorbing material in the form of sheet, blocks, pellets, etc., that is placed on top or around the object being radiographed so that only the area of interest is directly exposed to the radiation beam. Matte - A mixture of metallic sulfides produced by melting the roasted product during the smelting of nickel, copper, and lead. Mechanical Engineer - A well intentioned design engineer that knows just enough metallurgy to be extremely dangerous. A loose cannon. Mechanical Properties - The characteristics of a metal that describe its response to an applied stress. Metallizing - Another name for flame spraying. Metallurgist - An engineer that has answered a higher calling. A benefactor of mankind. Metastable Phase - A phase that exists in a nonequilibrium condition, but is none the less stable under some conditions. Microconstituent - An identifiable portion of a metal's microstructure consisting of one or more phases. Microstructure - The size, shape, and orientation of grains and the types and relative amounts of phases present in a metal as revealed by polishing, and etching and then viewing it under a microscope at a magnification of at least 10X. Microvoid Coalescence - See fibrous tearing. page - 365

APPENDIX A

GLOSSARY

MIG Welding - See gas metal arc welding. Milling - A metal cutting process in which a multi-toothed rotary cutter passes across the workpiece. Mineral Dressing - See beneficiation. Mismatch - In forging or casting, a gross dimensional defect caused by the upper and lower halves of the die or mold being misaligned with each other. Mist Coat - A very thin intermediate coat applied directly to a primer before heavier layers are applied in order to prevent blistering from gas evolution. Mode Conversion (UT) - The transformation of one type of wave into another. Modulus of Elasticity - The ratio of stress to strain on the linear portion of a stress-strain curve. It is a measure of a metal's rigidity. Monel® - A family of nickel base alloys containing roughly 2/3 nickel and 1/3 copper as well as minor additions of other elements. MP35N® - A nickel/cobalt base alloy that is strengthened by a combination of cold work and precipitation hardening. It has excellent corrosion resistance. It is frequently specified for high strength springs. Mud-cracking - A coating defect in which extensive, interconnecting cracking gives the coating the appearance of cracks in sun-baked mud. Mud Zone - The area of exposure in a marine environment at the bottom sediments of the ocean. Multiplicative Effect - The phenomenon whereby alloying elements used in combination with each other often produce an effect on some property of the alloy that is much greater than would be expected from the sum of each individual element's contribution. Muntz Metal - An alloy of copper containing approximately 40% zinc. NACE MR0175 - A standard published by the National Association of Corrosion Engineers that establishes the conditions that must be complied with in order to use a particular metal in H2S service. "Native Texan" - A bumper sticker that permits you to legally park your car in a space reserved for the handicapped. Naval Brass - A copper alloy containing approximately 40% zinc and 1% tin. Near Field (UT) - See Fresnel zone. Necking - The nonuniform reduction in cross section of the test specimen that occurs during tensile testing due to localized strains.

page - 366

APPENDIX A

GLOSSARY

Nelson Curves - A series of curves that show the operating limits (in terms of hydrogen partial pressure and temperature) for various grades of steels that may be used in service requiring exposure to hydrogen. NewAge® Rockwell Hardness Tester - A portable hardness tester used primarily for determining Rockwell hardness. The tester is positioned on the part to be tested. It is slowly pushed down by the person performing the test using two handles on either side of the tester. The tester automatically applies the minor lead followed by the major load on the indenter as the tester is being pushed down. After the tester has bottomed out, the hardness is read on a dial. Nickel Electroplating - An electrodeposited layer of metallic nickel used for corrosion protection or for decorative purposes. Nil-Ductility Transition Temperature (NDTT) - In drop weight testing, the highest temperature at which a standard drop weight specimen breaks. It is a measure of the dependency of a metal's toughness on temperature. Nitriding - A surface hardening process for some metals that diffuses nitrogen into the surface at an elevated temperature where it forms hard nitrides. Nitronic® - A family of austenitic stainless steels containing substantial amounts of chromium and manganese that are strengthened by nitrogen. Normalizing - A heat treatment for ferrous alloys in which the metal is heated up into the austenitic region and then air cooled. Open Die Forging - Forging utilizing dies that offer little or no restraint on the lateral movement of the workpiece. Open Hearth Steelmaking - A steelmaking process in which iron ore, scrap, and flux are charged into a shallow basin in a refractory lined furnace and then swept by flames. Orange Peel - A coating defect where the surface of the coating has a bumpy appearance. Overlap - A weld defect in which the top of the weld extends beyond the joint, but has not fused to the top surface of the base metal. Oxidation - The loss of one or more electrons by a substance in a chemical reaction. Oxy Fuel Gas Welding (OFW) - A welding process which uses the combustion of a fuel gas with oxygen as the source of heat. Oxygen Concentration Cell - A concentration cell in which the electrode exposed to the least amount of oxygen is anodic. Packer Fluids - Weighted fluids (typically heavy brines) put into the annulus of a well on top of a packer to counterbalance the pressure exerted by the produced fluid thus helping the packer maintain a seal.

page - 367

APPENDIX A

GLOSSARY

Paint Brush Transducer (UT) - A large transducer used in the ultrasonic inspection of plate and sheet. Parting Line - The line on a forging or casting that marks where the two halves of the die or mold came together. Passive - The condition of a metal when a surface layer of corrosion products (such as an oxide film) significantly reduces further corrosion making it much more noble than the active state. Pattern - A form around which molding material is packed in order to make a cavity in a casting mold. Pearlite - A microconstituent of steel and cast iron consisting of alternating layers of cementite and ferrite. Penetrameter (RT) - An image quality indicator used in radiography. Penetrant (PT) - A liquid that is used in liquid penetrant examination that flows into open surface discontinuities. The excess is removed and the residual penetrant drawn out of the discontinuities by the developer. The contrast between the developer and the penetrant indicates the presence of the discontinuity. Penumbra (RT) - The lighter portion of the image on a radiograph extending beyond the umbra that is formed by some, but not all of the radiation from the source. Percent Ductile Fracture - In Charpy toughness testing, the percent of the fracture face area exhibiting ductile fracture. Percent Elongation - The percent increase in a tensile specimen's original gage length at the end of the test. Percent Reduction of Area - The percent decrease in a tensile specimen's original cross section (within the gage length) at the end of the test. Percent Shear - See percent ductile fracture. Phase - A physically and chemically homogeneous and distinct portion of a metal. Phase Diagram - A diagram that shows the equilibrium phases of an alloy as a function of temperature and composition. Phase Transformation - The change that occurs when one phase change into another as a result of a change temperature, composition, pressure, etc. Phosphate Coating - A coating process in which a phosphate film is formed on a metal for corrosion protection by immersing the metal into a dilute solution of phosphoric acid and other chemicals. Pickling - Removing oxides on a metal's surface by chemical or electrochemical reaction.

page - 368

APPENDIX A

GLOSSARY

Piezoelectric (UT) - Materials that will set up an electrical potential when they are mechanically deformed or will deform when exposed to an electrical potential. Pig Iron - The iron produced by a blast furnace. Pigments - In coatings, solids in coating materials that impart strength and may be used to enhance corrosion resistance or provide color. Pinholes - A coating defect consisting of small holes in the coating resulting from poor application. Pitting - A form of localized corrosion damage in which small, open cavities are formed on the metal's surface. Plane Strain - A condition characteristic of thick or brittle parts in which the state of stress next to a crack tip is triaxial tension. Plasma Arc Spray - A thermal spray process in which a plasma is generated in a gas by electrical discharge which is then used to heat and propel the coating material towards the workpiece. Plasma Arc Welding (PAW) - An arc welding process in which the arc is used to heat up an inert gas until it becomes a plasma. The plasma is directed at the workpiece and is the primary source of heat for melting the base metal. Plastic Deformation - Permanent deformation that remains after the load has been removed from a metal. Plate Wave (UT) - A wave that propagates through thin material that is less than one wavelength thick. Plug Weld - One of the eight basic types of welds. Poisson Effect - The lateral contraction or expansion that occurs when a metal is loaded in tension or compression. Poisson's Ratio - In a Tensile Test, the ratio of the strain occurring transverse to the applied load to the strain occurring in the direction of the applied load at any point below the proportional limit. Polarization - The voltage drop between electrodes in a galvanic cell that occurs over a period of time as the corrosion products build up. Polarized Crystal (UT) - A crystal with a dipole moment. Polycrystalline - Having many crystals as opposed to just one. Polymer Quench - A quench using a synthetic polymer that can be tailored to give a wide range of cooling rates. Porosity - Small holes or pores that result from the evolution of gas as metal solidifies. page - 369

APPENDIX A

GLOSSARY

Post-Emulsifiable Penetrants (PT) - Penetrants that require the application of a separate emulsifier on the workpiece in order to remove the excess penetrant. Powder Metallurgy - The various processes used to make metal parts through the compaction and sintering of metal powder. PPB's - A powder metallurgy term that stands for prior particle boundaries. It refers to the appearance of a HIP’ed structure in which individual powder particles are identifiable because of surface contamination during processing. Precipitate - A new solid phase that forms out of either a liquid or solid solution because of a change in pressure, composition, temperature, etc. Precipitation Hardening - See age hardening. Precipitation Hardening Stainless Steel - An age hardenable stainless steel with a martensitic, austenitic, or semi-austenitic matrix. Preferred Orientation - The alignment of crystal lattices in a metal in a common direction. Preheating - In welding, applying heat to the base metal before welding is actually begun in order to reduce the thermal gradient between the weld area and the rest of the base metal, to remove moisture, to facilitate degassing, etc. Press Brake Forming - A sheet metal forming process in which a punch presses the workpiece into an open ended die. Press Forming - A forming process in which a shaped punch presses a sheet metal blank into a die cavity. Primary Mill - A rolling mill used for the initial, heavy reductions of ingot during the production of rolled plate, bar, etc. Primer Coat - The first coat of a coating system that is in contact with the substrate. It’s main purpose is to insure good adhesion of the coating system. It may also enhance corrosion resistance. Process Anneal - See subcritical anneal. Prods (MT) - Hand held electrodes used to magnetize a local area. Proeutectic - Describes a phase that forms above the eutectic isotherm in a phase diagram as opposed to the same phase which forms below the isotherm. Proeutectoid - Describes a phase that forms above the eutectoid isotherm in a phase diagram as opposed to the same phase which form below the isotherm. Projection Welding - An electrical resistance welding technique similar to spot welding except one of the pieces to be joined has a dimple (projection) where the weld is to be made. This concentrates the pressure at the weld.

page - 370

APPENDIX A

GLOSSARY

Proportional Limit - The point on a stress-strain curve in tensile testing where the curve deviates from linearity thus the ratio of stress to stain is no longer constant. Pulse Echo System (UT) - An ultrasonic system in which the same transducer is used to both pulse the ultrasonic waves into the workpiece and to receive their echoes. Pyroelectric Effect (UT) - A change in the polarization of a crystal due to heating or cooling. QPQ® - A nitriding/oxidizing process used to increase surface hardness and enhance fatigue and corrosion resistance in steels. Quenching - The rapid cooling of a metal in an agitated fluid. It is done, depending on the alloy, to obtain some desired phase transformation or to prevent the precipitation of undesirable phases. Radial Forging - A forging process that uses two sets of opposing hammers (within the same plane) to deform the workpiece. The workpiece is fed into the forge perpendicular to the plane of the hammers. Radioactive Isotope (RT) - An isotope that spontaneously decays. Radiography - A volumetric nondestructive examination method in which discontinuities are detected by the amount of radiation that is attenuated as it passes through the part being examined. Rayleigh Wave (UT) - See surface wave. Recovery - The reduction or removal of strain hardening by holding the metal at an elevated temperature for a sufficient length of time, but not so long as to cause recrystallization or grain growth. Recrystallization - The formation of new, strain-free grains within the existing grain structure of a cold worked material. Recrystallization Temperature - The temperature at which recrystallization occurs upon heating a cold worked metal. Rectification (MT) - The conversion of alternating current to direct current. Red Brass - A copper alloy containing approximately 15% Zn. Red River - The southern limit of civilization on the American plains. Reduction - A chemical reaction in which the substance being reduced gains one or more electrons. Reduction of Area - See percent reduction of area. Reference Blocks (UT) - Metal blocks with known, artificial defects used to help evaluate discontinuities found in the part being inspected.

page - 371

APPENDIX A

GLOSSARY

Refining - Purifying or improving the clear lines of molten metal. Refraction (UT) - The change in direction that occurs when a beam of sound enters a medium in which it has a different velocity. Refractory - Any material having a high melting point. Residual Magnetization (MT) - A magnetization technique in which the magnetizing current is turned on while the magnetic particles are applied. Residual Stress - Stress in a metal that is free from external stresses or pressure gradients. Resistance Welding - A group of welding processes that utilize the heat generated by an electrical current that is passed through the parts to be joined, and pressure to make the weld. It is limited to relatively thin parts. Resolution - The ability to distinguish between two closely occurring signals. Resonant Frequency (UT) - The lowest frequency at which a crystal tends to vibrate at. Retained Austenite - Nonequilibrium austenite that failed to transform during quenching because of local variations in chemical composition. Retentivity (MT) - The ability to retain a magnetic field after the magnetizing force has been turned off. Reverberatory Furnace - A refractory lined furnace with a shallow hearth. Furnace flames are directed at the roof which then radiates the heat towards the surface of the change in the hearth. Rile - To irritate or perturb. In Texas, what trains run on. Rimmed Steels - Steels that have been only slightly deoxidized in the ingot mold. They have a decarburized surface or "rim" as a result of deoxidation. Ring Rolling - A forming process for making seamless rings out of cylindrically shaped preforms. The preform is deformed in between two or more rollers as the rollers are gradually brought closer together. Riser - A reservoir for molten metal within a casting mold. Roasting - Heating an ore to produce some chemical change that will make subsequent melting operations easier. Rockwell Hardness Test - A hardness test method in which various combinations of loads and indenters are used, depending on the scale. Hardness is measured by the difference in the depth of penetrations made by the indenter with a small preload and then with the final load. Roll Forging - A forging process that passes the workpiece through two counter rotating, contoured rollers. page - 372

APPENDIX A

GLOSSARY

Rolling - Changing or reducing the cross section of a metal by passing it between two or more rollers. Root - The bottom portion of a weld. Runner - The channel in a casting mold that directs the molten metal from the sprue to the cavity. Rust - The corrosion of ferrous alloys or the corrosion products themselves. S/N Curve - A plot of stress versus the number of cycles to failure for a metal subjected to cyclic loading. It characterizes the fatigue properties of the metal. Sacrificial Anode Cathodic Protection - Cathodic protection in which the protecting current is provided by the planned corrosion of a metal more active (the sacrificial anode) than the metal being protected. Sand Casting - A casting process that utilizes a mold made out of sand and binders. Scale - The oxide film that forms on the surface of a metal when exposed to a high temperature. Scholar - An erudite person having a reputation for expertise in a particular field of the humanities. In Texas, a high school graduate who can count to twenty with his shoes and socks on. Screens (RT) - Sheets of material (usually lead) placed on either side of the film holder that absorb the longer wavelength radiation. Screw Dislocation - A linear defect in a crystal that has the crystal lattice spiraling around it. Seam Weld - One of the eight basic types of welds. Seam Welding - A resistance welding process that utilizes cylindrically shaped electrodes. The parts to be joined are rolled through the electrodes as current is applied thus producing a continuous weld. Season Cracking - The stress corrosion cracking of certain copper alloys in the presence of ammonia or amines. Segregation - The nonuniform distribution of alloying elements, impurities, or phases that can result during solidification. Selective Leading - Also called dealloying, the preferential corrosive attack of one or more individual elements in an alloy. Semi-Killed Steels - A steel that is not fully deoxidized. It still contains sufficient dissolved oxygen to offset any shrinkage during solidification as the oxygen combines with carbon to form carbon monoxide gas which is trapped in the frozen metal. Sensitivity - The ability to detect small defects.

page - 373

APPENDIX A

GLOSSARY

Shear Forces - Two equal, but oppositely directed forces in different planes acting upon a body. Shear Fracture - A fracture characterized by its dull surface indicating that crystals were separated by a combination of sliding and tearing action. Shear Pin Hardness Tester - A portable hardness tester for making hardness measurements (see Figure 1). The tester consists of an indenter, a calibrated shear pin, and the housing. The tester is placed on the surface of the part to be tested and then sharply struck with a hammer (some models may use a "c" clamp type housing which applies the load by tightening the clamp). The indenter is forced into the surface of the part being tested only up to the point where the shear pin breaks. The shear pin is made such that it will break after the necessary load has been applied to the indenter. Excessive force is absorbed by the upward movement of the indenter into the housing after the shear pin has broken. The diameter of the impression is then measured.

Figure 1: Shear Pin Hardness Tester Shear Wave (UT) - See transverse wave. Sheet Forming Process - Forming processes for sheet or plate that do not significantly change the thickness of the starting blank. Shell Casting - A casting process that utilizes a mold made by compacting a thin layer of thermosetting resin/sand mixture onto a heated metal pattern. Shielded Metal Arc Welding (SMAW) - A manual welding process in which an arc is struck between the tips of a flux covered, consumable electrode and the workpiece. Shielding - The method by which the molten and heated areas of a weld are isolated from air. Short Arc - The short circuit transfer mode in GMAW. Shrinkage - The contraction of metal as it solidifies and cools. Also a casting defect that is a void in roughly the center of a wall resulting form metal shrinkage.

page - 374

APPENDIX A

GLOSSARY

Sigma Phase Embrittlement - Embrittlement occurring in some ferritic and duplex stainless steels when held at 1050-1800(F for a period of time as a result of the formation of sigma phase. Sinter - A powder metallurgy process in which the green compact is heated up to a temperature well below its melting point and then held for a period sufficiently long to permit metallurgical bonds to form between the particles. Slab - An intermediate product of a rolling mill which has a rectangular cross section. Slag - A nonmetallic material that is the product of the reaction of nonmetallic impurities in a molten metal with flux. Slip - The movement of dislocations along close packed planes in a crystal. Smelting - Any of a number of thermal processes used to produce molten metal from an enriched ore. It usually involves the reduction of the metal-bearing minerals. Snell's Law (UT) - A formula for calculating the angle of refraction as sound waves travel from one medium into another based upon the angle of incidence and the velocity of sound in the different media. Soldering - A joining process in which the parts are fitted up, the joint area heated to the proper temperature, and then filler metal applied. The filler metal melts and is then distributed by capillary action throughout the joint. Parts are bonded after the filler metal solidifies. By definition soldering takes place below 840(F. Solid Solution - A single, solid crystalline phase made up of two or more elements. Solidus - The curved line on a phase diagram that separates a single, solid phase region from a region at higher temperatures containing the same solid phase and liquid. It marks the onset of melting as the solid phase is heated. Solute - The least abundant element in a solid solution. It is dissolved in the solvent. Solvent - The most abundant elements in a solid solution. It makes up the continuous matrix. Solvents - In coating materials, the liquids (usually volatile) that are used to transport the binders and pigments to the substrate. Solvus - The curved line on a phase diagram that separates a two phase region consisting of two solids from a single phase region consisting of only one of the solids. It represents the maximum solubility of one of the elements that make up the phase diagram in the crystal structure of the other. Sour Service - An environment containing sufficient H2S (as defined by NACE MR0175) to make sulfide stress corrosion cracking a concern. Spalling - The cracking and flaking of particles out of a metals' surface as it is rubbed against another metal.

page - 375

APPENDIX A

GLOSSARY

Spheroidizing Anneal - A heat treatment for ferrous alloys that causes iron carbide to agglomerate into spheres thus improving the metals' ductility and toughness. Spinning - A forming process used to make seamless parts that are symmetric about their longitudinal axis out of sheet metal. A blunt tool is used to contour the metal over a rotating mandrel. Spirit of Texas, The - Tequila. Splash Zone - The area of exposure in a marine environment that is above sea level, but is wetted by waves. Spot Weld - A resistance welding process in which two electrodes directly opposite of each other press the two pieces of metal to be joined together as current passes through thus producing a circular shaped weld. Spray Transfer - One of the metal transfer modes in GMAW. Spring Back - The change in dimensions that occurs when a metal elastically recovers after being removed from a die. Sprue - A vertical channel in a casting mold that connects the pouring basin with its runners. Stainless Steel - A corrosion resistant steel containing at least 10.5% chromium. Steel - An alloy of iron and carbon (up to a maximum of approximately 2%) with other alloying elements present to improve corrosion resistance, mechanical properties, etc., depending on the type of steel. Stellite® - A family of cobalt based, wear resistant alloys. Stick Welding - See shielded metal arc welding. Straight Beam Search Unit (UT) - An ultrasonic transducer that transmits the beam at a 90( angle to the surface of the part being inspected. Strain Hardening - See cold work. Stream Degassing - A degassing method in which the molten metal is degassed as it is poured from one ladle on top of a vacuum chamber into another ladle inside the chamber. Stress Cell - A type of galvanic cell where high stress regions of a metal exposed to an electrolyte are anodic to the low stress regions. Stress Corrosion Cracking - The embrittlement and subsequent cracking of a susceptible metal under a tensile stress in a particular environment. Stress Intensity Factor (K) - In fracture mechanics, the single parameter that describes the stress field ahead of a sharp crack.

page - 376

APPENDIX A

GLOSSARY

Stress Relieving - A heat treatment to remove residual stresses after welding and to control hardness in the HAZ. Stress Rupture Test - A test in which a standard test specimen is subjected to a constant tensile load at an elevated temperature until it fails. The time to failure and load are recorded. Strike - A high quality, tightly adherent electroplating used as a foundation for other platings. Subcritical Anneal - A heat treatment for ferrous alloys in which the metal is heated to a temperature just below the beginning of the ferrite to austenite transformation and held until the desired degree of softening has been obtained. Submerged Arc Welding (SAW) - An arc welding process in which a continuous, bare, consumable electrode is used. The tip of the electrode is "submerged" under a blanket of flux. Substitutional Solid Solution - A solid solution in which the solute atoms have replaced some of the solvent atoms in the solvent's crystal structure. Sulfidation - The formation of sulfur compounds on or beneath the surface of a metal exposed to a high temperature gas containing sulfur. Surface Profile - In coating applications, the maximum average peak to valley depth of a roughened surface prepared for coating. Surface Wave (UT) - A wave in which the particles of the transmission medium vibrate in an elliptical path in the plane of wave travel. It is limited to the surface of the medium. Surface Weld - One of the eight basic types of welds. Swaging - A forging process in which two or more hammers (dies) strike radial blows on tubing, bars, etc., to reduce their cross section. Taconite - Iron ore from the Lake Superior District. Tap - A tool that cuts internal threads. Telebrineller® - A portable hardness tester (see (2)). A soft rubber head that can be slid along the test bar holds an anvil in place on the top surface of the bar and a hardened indenter ball in position on the opposite side directly underneath the anvil. The Telebrineller® is placed on the part to be tested. The indenter ball is in contact with the bottom surface of the test bar and the top surface of the part. The anvil is stuck sharply with a hammer simultaneously creating indentations in the test bar and the part. Because the impact force used to make both indentations is the same, the size of the indentations are strictly a function of hardness. The test bar has a known hardness. The diameters of both impressions are measured and then the hardness of the part can be determined by the formula:

page - 377

APPENDIX A

GLOSSARY

BHN of part

dia. of test bar impression dia. of part impression

X (BHN of test bar)

Figure 2: Telebrineller

Temper Embrittlement - Embrittlement that occurs in some types of steels when held at 7001500(F for a length of time. Tempered Martensite Embrittlement - Embrittlement occurring in some high strength, low alloy steels having a bainitic or martensitic structure when held at 400-700(F for a length of time. Tempering - A heat treatment for ferrous alloys in which the austenitized and quenched metal is heated to a temperature below the austenitic range and holding it until the desired degree of softening has occurred. Ten-gallon Hat - An oversized felt hat much used in Texas because it readily accommodates inflated heads. Tensile Properties - The tensile strength, yield strength, percent reduction of area, and the percent elongation of a metal as determined by a tensile test. Tensile Strength - The maximum load that a tensile specimen can withstand before breaking divided by the original cross action of the gage length. Texan - A person who is part armadillo, part mule, part 'gator, part hound dawg, and all hot air. Texas - Only the second largest state in the union, however, it does have the largest accumulation of red necks in the world. Texas is derived from the Indian word "Tejas" meaning "coconut head": a reference to the early settlers who had an opportunity to live anywhere they wanted, but settled in what is now Texas. The aborigines found that it took a great deal of effort utilizing their crude stone tomahawks to crack open the extremely hard heads of these early Texans and, like coconuts, once opened there just wasn't much inside. page - 378

APPENDIX A

GLOSSARY

Texture - Preferred orientation of grains. Thermal Spray Coatings - A group of coating processes in which the coating material is heated and then propelled onto the workpiece as a spray. Three Roll Forming - A forming process that rolls plate, bar, sheet, etc., between three rollers to bend them into some desired curve. Throat - The distance from the inside corner of the weldment to the midpoint on the surface of the face of a fillet weld. Time-Temperature-Transformation (T-T-T) Diagram - A diagram that shows the transformations that occur when a phase that is stable at a high temperature is rapidly quenched to a temperature of interest and then is held at that temperature for various lengths of time. Tie Coat - An intermediate coat selected for its good adhesion to the primer and for its compatibility with subsequent intermediate coats. TIP Test - A test for thermally induced porosity in powder metallurgy parts. Toe - The juncture between weld metal and base metal on the surface of a weldment. Top Coat - The upper most layer of a multicoat system that is exposed to the environment. It is selected for its resistance to the environment and for color. Toughness - The ability of a metal to absorb energy and plastically deform prior to fracturing. Transducer (UT) - A device that transforms ultrasonic waves into electrical signals or vice versa. Transverse Wave (UT) - A wave in which the particles of the transmission medium vibrate a direction perpendicular to the wave motion direction. Tribaloy® T-800 - A cobalt based, wear resistant alloy. Turning - A metal cutting process used to generate a surface of revolution. Twin Wire Arc Coating - A thermal spray coating in which two consumable wire electrodes made out of the desired coating material are fed into the spray gun. An arc is established between their tips. Compressed air is used to atomize and propel the molten droplets. Ultimet® - A cobalt base alloy used for its excellent corrosion and galling resistance. Ultrasonic (UT) - Waves that have a frequency greater than what the human ear can detect. Ultrasonic Examination - A volumetric nondestructive examination method that uses ultrasonic sound waves to detect internal defects in a metal. Umbra (RT) - The dark portion of the image on a radiograph resulting from radiation from all the points on the source.

page - 379

APPENDIX A

GLOSSARY

Undercut - A welding defect where a groove has been melted into the base metal adjacent to the toe of a weld. Undercutting - Corrosion that takes place under the edge of the coating at pinholes, holidays, breaks, etc. Underfill - A forging or casting defect where metal has not completely filled the die or mold cavity. A welding defect in which the weld metal does not completely fill the joint all the way to the top. Unified Numbering System - A numbering system that identifies commercial alloys based upon some controlling limits that have been established in government or industry specifications (usually composition). It is administered by ASTM and SAE. Uniform Attack - A form of corrosion damage in which the entire surface of a metal corrodes at approximately the same rate. Unit Cell - The smallest repetitive volume of a given crystal structure. Universal Mill - A rolling mill that has additional vertical rollers on either side that roll the edge of plates to control width and finish. Upper Bainite - Bainite that is formed above roughly 660(F and below the knee of the applicable T-T-T diagram. Upsetting - A forging operation used to increase the cross sectional area of bar or billet by striking the ends so that the middle barrels out. Vacuum Arc Degassing (VAD) - A degassing process that utilizes a vacuum chamber with electrodes extending through the top into the chamber. Molten metal is charged into a ladle along with fluxes, grain refiners, etc. The ladle is placed into the chamber. An argon purge and the arcing action of the electrodes mix the contents of the ladle. A vacuum is then pulled to degas. Vacuum Arc Remelting (VAR) - A degassing process in which the metal to be degassed is first cast in the form of an electrode. This cast electrode is then remelted in a vacuum furnace by an arc generated between the tip of the electrode and the bottom of the furnace. Vacuum Coating - A coating process in which metals or metal compounds are deposited from a vapor onto the surface of the workpiece inside a vacuum chamber. Vacuum Induction Melting (VIM) - Melting a charge of metal inside a vacuum furnace through the resistance heating of induced currents in the metal. VAR - See Vacuum Arc Remelting. Versitron® Hardness Tester - A type of Rockwell hardness tester that utilizes a spring to provide the load to the indenter instead of the usual weights and lever arrangement. This allows the head of the test machine to be moved relative to the part which greatly facilitates the testing of large parts.

page - 380

APPENDIX A

GLOSSARY

Vibratory Compaction - A powder metallurgy process used to mechanically compact the powder. The die is vibrated as the powder is poured in allowing the powder particles to rotate, etc., to better fill in any empty spaces. Vickers Hardness Test - A hardness test that uses a square base pyramid indenter and various loads to make the indentation. Hardness is determined by measuring the length of the diagonal of the indentation. VIM - See Vacuum Induction Melting. Visible Penetrants (PT) - Those penetrants that can be evaluated under visible light. War - An armed conflict. In Texas, an interrogative used to ascertain "to what place" as in: War you goin' to? Water-Washable Penetrants (PT) - Those penetrants that are formulated with an emulsifier so that the excess can be removed by a gentle stream of water. Wavelength (UT & RT) - The shortest distance over which the waveform of a particular wave begins to repeat itself. Weld Joint - The way two pieces of metal to be welded (or have been welded) together are fitted up. Welding - A process for forming a metallurgical bond between two pieces of metal, with or without the use of filler metal, by the application of heat. Weldment - An assembly of parts that have been joined together by welding. Wet Film Thickness - The thickness of a coating immediately after application before the solvents have had time to evaporate. Wet Mag (MT) - A magnetic particle examination technique that uses magnetic particles that are suspended in oil or water. Wetting - The spreading and adherence of the filler metal to the surfaces of the metals to be joined in brazing and soldering. White Cast Iron - A cast iron that has its excess carbon in the form of large precipitates of cementite. Wrinkling - Furrows and ridges in a coating. Wrought - A metal mechanically worked so that the starting ingot's cast structure is completely broken up. X-ray (RT) - A type of electromagnetic radiation used in radiography produced by an x-ray tube. Yankee - An altruistic person of great integrity, renowned for his business acumen, ingenuity, entrepreneurship, technical expertise, and work ethic. Tens of thousands of Yankees have been imported into Texas over the last twenty years in an effort to stem the growing page - 381

APPENDIX A

GLOSSARY

malaise of a Texas economy too dependent on a native population that has been overbaked by the southern heat. Sadly, it will be years before the beneficial effects are realized. A missionary that teaches manners, culture, and English to the denizens of the southern climes of the United States. Yellow Brass - A copper alloy containing approximately 30% zinc. Yield Strength - The stress at which a specified amount of permanent strain (usually 0.2%) occurs in a tensile specimen. Yoke (MT) - A magnetizing technique that uses a "C" shaped electromagnet to induce a longitudinal magnetic field in the workpiece between the legs of the yoke. Young's Modulus - See modulus of elasticity. Zinc Electroplating - An electrodeposited layer of metallic zinc used primarily for mild corrosion protection.

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APPENDIX B

   " '

 )

AC Ac1

& & & & & & & & & & & &

Ac3

&

Ae1

&

Ae3

&

Aei

&

Ag AISI Al API Ar Ar1

& & & & & &

Ar3

&

ASME ASTM Au B BCC Be Bi Br C Ca Cb CCT Cd Cl Co COD Cr

& & & & & & & & & & & & & & & & &

%EL %RA

page - 384

ABBREVIATIONS & SYMBOLS Percent elongation Percent reduction of area Alpha, BCC iron, HCP titanium Beta, BCVC titanium Gamma, FCC nickel matrix Gamma double prime, Ni3Cb Gamma prime, Ni3 (Al, Ti) Delta, BCC iron Lambda, wavelength Sigma, stress Alternating current Lower critical temperature at which the transformation to austenite begins for a given ferrous alloy upon heating Upper critical temperature at which the transformation to austenite begins for a given ferrous alloy upon heating Lower critical temperature where the transformation to austenite begins in the Fe-C equilibrium diagram Upper critical temperature where the transformation to austenite is complete in the Fe-C equilibrium diagram Lower critical temperature where austenite transformation begins upon heating Silver American Iron and Steel Institute Aluminum American Petroleum Institute Argon The temperature at which the transformation to austenite is complete on cooling for a given ferrous alloy The temperature at which the transformation to austenite begins on cooling for a given ferrous alloy American Society of Mechanical Engineers American Society for Testing and Materials Gold Boron Body centered cubic Beryllium Bismuth Bromine Carbon, Carbide Calcium Columbium Continuous Cooling Transformation Curve Cadmium Chlorine Cobalt Crack Opening Displacement Chromium

APPENDIX B Cs CTOD Cu CVN DC DPH e EBW F FCC Fe g.s. GMAW GTAW H HAZ HB HCP He Hf Hg HK HRB HRC HT HT HV I In Ir K K KEE KI KIc ksi M MF MF Mg MIG Mn Mo Ms Ms MT N

& & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & &

ABBREVIATIONS & SYMBOLS Cesium Crack Tip Opening Displacement Copper Charpy V-notch Direct current Diamond pyramid hardness (Vicker's) Electron Electron beam welding Fluorine Face centered cubic Iron grain size Gas metal arc welding Gas tungsten arc welding Hydrogen Heat affected zone Hardness Brinell Hexagonal close packed Helium Hafnium Mercury Hardness Knoop Hardness Rockwell "B" scale Hardness Rockwell "C" scale Heat treat Heat treatment Hardness Vickers Iodine Indium Iridium Potassium Stress intensity factor Equivalent energy stress intensity factor Stress intensity factor in mode I loading Critical stress intensity factor in mode I loading thousand pounds (force) per square inch Metallic element Martensite finish temperature Martensite finish temperature Magnesium Metal inert gas welding Manganese Molybdenum Martensite start temperature Martensite start temperature Magnetic particle testing Nitrogen page - 385

APPENDIX B

Na NACE NDT Ni O OFW P P P/M PAW Pb PPB psi PT Ra RT S SAE SAW Sb Si SMAW Sn Ta Ti TIG TTT U UT UTS v V W YS Zn Zr

'  (C (F (K (R

page - 386

& & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & &

ABBREVIATIONS & SYMBOLS

Sodium National Association of Corrosion Engineers Nondestructive testing Nickel Oxygen Oxyfuel welding Pressure Phosphorus Powder metallurgy Plasma arc welding Lead Prior particle boundary pounds (force) per square inch Liquid penetrant testing Radium Radiographic testing Sulfur Society of Automotive Engineers Submerged arc welding Antimony Silicon Shielded metal arc welding Tin Tantalum Titanium Tungsten inert gas welding Time-Temperature-Transformation Curve Uranium Ultrasonic testing Ultimate tensile strength Velocity Volume, voltage Tungsten Yield strength Zinc Zirconium Angstrom, 10 8 centimeter Epsilon Degrees Celsius Degrees Fahrenheit Degrees Kelvin Degrees Ranksine

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APPENDIX C

REGISTERED PRODUCTS REFERENCE

17-4PH® - Armco Advanced Materials Corporation Custom Age 626 Plus® - Carpenter Technology Corporation Aerocote® - Aerocote Corporation Beta-C® - RMI Titanium Company Borofuse® - Materials Development Corporation Camclad® - Cooper Industries Colmonoy® - Wall Colmonoy Corporation D-Gun Process® - Union Carbide Corporation Elgiloy® - Elgiloy Company EQUOTIP® Hardness Tester - Proceq Ferralium® - Langley Alloys, Ltd. Gator-Gard® - Sermatech Corporation Hastelloy® - Haynes International, Inc. Haynes® - Haynes International, Inc. Inconel® - INCO Family of Companies King® Portable Hardness Tester - King Tester Corporation LW-45® - Union Carbide Corporation Monel® - INCO Family of Companies MP35N® - SPS Technologies, Inc. NewAge® Portable Hardness Tester - NewAge Industries NITRONIC® - Armco Advanced Materials Corporation QPQ® Process - Kolene Corporation Stellite® - Deloro Stellite, Inc. Slinky® - James Industry & Company Telebrineller® - Teleweld Ultimet® - Haynes International, Inc. Tribaloy® - Deloro Stellite, Inc. Versitron® Hardness Tester - NewAge Industries

page - 388

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APPENDIX D

BIBLIOGRAPHY

The following publications were utilized in writing this text. American Society for Metals, Atlas of Isothermal Transformation and Cooling Transformation Diagrams, 1977, American Society for Metals American Society for Metals, Metals Handbook, Ninth Edition, American Society for Metals American Society for Metals, Metals Handbook, Tenth Edition, American Society for Metals American Welding Society, Welding Handbook, Seventh Edition, American Welding Society Atkins, M., Atlas of Continuous Cooling Transformation Diagrams for Engineering Steels, 1980, American Society for Metals Azaroff, Leonid V. and Brophy, James J., Electronic Processes In Materials, 1963, McGraw-Hill Book Company Bethlehem Steel Corporation, Modern Steels and Their Properties, Handbook 3310, Bethlehem Steel Corporation Boyd, W.K., and Fink, F.W., Corrosion of Metals in Marine Environments , 1975, Materials and Ceramics Information Center, Battelle’s Columbus Laboratories, Report MCIC-75-245R Cary, Howard B., Modern Welding Technology, 1979, Prentice-Hall Dieter, George E., Mechanical Metallurgy, Second Edition, 1976, McGraw-Hill Book Company Halliday, David, and Resnick, Robert, Physics, 1966, John Wiley & Sons Heine, Richard W., Loper, Carl R., and Rosenthal, Philip C., Principles of Metal Casting, Second Edition, 1967, McGraw-Hill Book Company Hirschhorn, Joel S., Introduction to Powder Metallurgy , 1969, American Powder Metallurgy Institute Jaske, C.E., Payner, J.H., and Balint, V.S., Corrosion Fatigue of Metals in Marine Environments, 1981, Materials and Ceramics Information Center, Battelle’s Columbus Laboratories, Report MCIC-81-42 Jenson, Jon E., ed., Forging Industry Handbook, 1966, Forging Industry Association page - 390

page - 390

APPENDIX D

BIBLIOGRAPHY

Kittel, C., Introduction to Solid State Physics, Third Edition, 1968, John Wiley & Sons McGannon, Harold E., ed., The Making, Shaping, and Treating of Steel, Ninth Edition, United States Steel McMaster, Robert C., ed., Nondestructive Testing Handbook, 1959, The Ronald Press Company NACE, Corrosion Control in Petroleum Production, 1979 Newton, Joseph, Extractive Metallurgy, 1959, John Wiley & Sons Pludek, V. Roger, Design and Corrosion Control, 1977, John Wiley & Sons Reed-Hill, Robert E., Physical Metallurgy Principles, Second Edition, 1973, D. Van Nostrand Company Uhlig, Herbert H., Corrosion And Corrosion Control, Second Edition, Herbert H. Uhlig, 1971, John Wiley & Sons

page - 391

APPENDIX E

A CHECKLIST OF FACTORS TO BE CONSIDERED WHEN SELECTING A MATERIAL FOR MARINE ENVIRONMENTS

All the factors in the following checklist may or may not be important for a given application, but it’s always a good idea to run through them all just be sure that you didn’t overlook anything! A. PHYSICAL CHARACTERISTICS • Weight or density limitations • Electrical conductivity • Thermal conductivity • Thermal expansion coefficient • Magnetic properties • Composition limitations B. MECHANICAL PROPERTIES • Yield and Tensile strength • Hardness • Ductility (% elongation & % reduction of area) • Fatigue resistance • Toughness (notched impact resistance • Fracture toughness • Galling resistance • Abrasion resistance C. DESIGN RESTRICTIONS • Size & thickness • Temperature • Form • Type of loading (static, dynamic, impact, alternating, cyclic) • Cleanliness • Surface finish • Protection method

• • • • • •

Hardenability Fire resistance Toxicity Biofouling resistance Position within EMF table Weldability

• • • • • • • •

• • • • • •

Wear resistance Modulus of elasticity Bearing strength Temperature effects on mechanical properties Compressive strength Poisson’s ratio Shear modulus Effect of the direction of work on mechanical properties

Maintainability Velocity Contents or exposure to chemicals Depth Design life Applicable standards

D. CORROSION RESISTANCE Consider the following for the particular zone in the marine environment of interest (i.e. marine atmosphere, splash zone, mud zone, immersion, etc.). • General corrosion resistance • Pitting and crevice corrosion resistance • Corrosion resistance to contained fluids (e.g. H2S, CO2, formation water, drilling muds, completion fluids, acidizing chemicals, crude, etc.) • Corrosion fatigue • Corrosion erosion • Seawater velocity effects • Temperature effects on corrosion • Susceptibility to stress corrosion cracking page - 392

page - 392

APPENDIX E • • • • • • • •

A CHECKLIST OF FACTORS TO BE CONSIDERED WHEN SELECTING A MATERIAL FOR MARINE ENVIRONMENTS

Susceptibility to hydrogen embrittlement (due to hydrogen from corrosion or cathodic protection) Fretting resistance Galvanic compatibility with mating parts Selective attack Bacterial corrosion Cavitation resistance Contamination of contents by corrosion products Ease of protecting or compatibility with planned protective measures

E. PROCESSING • Melting practice • Grain size • Heat treatment • Minimum hot work • Casting process • Forging process • Joining method

F. QUALITY • Mechanical property qualification requirements • Nondestructive examination (PT, UT, RT, MT) • Dimensional inspection • Certification requirements

• • • • • •

• • • • • •

G. COMMERCIAL • Cost • Availability in required size, form, condition, quantity

• • •

Effect of stress relieving on properties Repairability Formability Machinability Surface finish Compatibility with special processes (e.g. nitriding, plating, flame hardening, etc.)

Types of defects and acceptance criteria Traceability Weld procedure qualification Pipe bending procedure qualification Witness hold points Compliance to standards

Delivery and schedule requirements Restrictions on country of origin Approved vendors list

page - 393

APPENDIX F A. ENVIRONMENTAL FACTORS • Temperature • Chemicals • Fresh/Seawater • Erosion • Wave impacts B. DESIGN FACTORS • Design life of equipment • Cathodic protection • Fabrication and installation of equipment • Equipment configuration — crevices, skip welds, etc. C. COATING MATERIALS FACTOR • Resistance to corrosive media • Hardness • Resilience • Moisture transfer rate • Maximum dry film thickness per coat • Compatibility with substrate/other coats • Surface preparation requirements • Curing requirements

FACTORS TO CONSIDER WHEN SELECTING A COATING SYSTEM • • • •

Sunlight (UV) resistance Immersion, splash, fumes exposure Biological attack Marine fouling

•

Accessibility of equipment for repairs after installation Storage conditions prior to installation Maximum coating thickness allowed by design

• •

• • • • • • •

Repairability Compatibility with cathodic protection Cost Availability Number of coats required Weatherability Inhibitive or galvanic protective primers

D. SUBSTRATE FACTORS • Type of metal • Required surface preparation • Cure temperature of coating versus tempering temperature of substrate • Rigid of flexible E. APPLICATIONS FACTORS • Type of equipment required/available at application site • State/EPA restrictions on VOC’s and OSHA restrictions on blasting at application site • Ease of application • Environmental (temperature/humidity) restrictions during application F. MISCELLANEOUS FACTORS • Customer requirements • Regulatory requirements • Inspection requirements

page - 394

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APPENDIX G

TRIM SELECTION

INTRODUCTION There is perhaps no other subject in oilfield metallurgy that is as emotional, that causes so many difficulties, and is the source of such bitter arguments as the selection of materials for a particular corrosive environment. A trim that is too conservative can add a great deal to the initial cost of the equipment. A trim that is not conservative enough may cost a great deal more in the long run due to equipment repair, downtime, and environmental and safety problems. Trim selection is a minefield that can safely be navigated only if you have a thorough knowledge of the well environment, the available materials, the equipment, and customer requirements. Trim selection is not for the faint of heart, the untrained, or the unwary. There are a multitude of factors that must be considered in trim selection. Appendix F lists some of the more common ones. Each of these factors must be evaluated and prioritized for a given application in a particular environment. The selection of a material always involves trade-offs: no one material will have all the desired characteristics or properties. For example, corrosion resistance in steels can be improved by adding certain alloying elements, but these same alloying elements may significantly increase the cost of the steel as well as making it more difficult to machine and more prone to galling. Which factor then becomes more important? It depends on the circumstances. A customer with a land well having a seven year design life may be willing to settle for a less corrosion resistant trim (and consequently less expensive) than a customer with a subsea well having a 30 year design life even though the fluid analyses of both wells is identical. Trim selection involves selecting materials appropriate for a corrosive environment. Our selection is not limitless: we are restricted to choosing amongst only those materials that have suitable engineering properties (strength, toughness, galling resistance, etc.). The allowable materials choices for a particular type of component may vary by product line. A gate valve stem in a Cameron valve may require a different material than a WKM valve used in the same service. Product Engineering and Metallurgy have worked together to setup bills of materials for components that are designed for certain categories of service based upon well environment. Trim selection for Cameron Sales personnel consists of categorizing the well environment of interest in terms of one of our standard service categories and then determining the applicable API 6A material class. Once the API 6A material class has been determined, we can use our family group code system to find the part number of an assembly suitable for that service. The corrosivity of a particular well is influenced by many factors. Some of the more important ones are listed in Table 1.

page - 396

page - 396

APPENDIX G

TRIM SELECTION & & & & & & & & & & & & & & &

TABLE 1 Temperature Pressure Production Rate CO2 content H2S content Composition and amounts of hydrocarbons Composition and amounts of produced water and condensate pH Sand production Presence of sulfate reducing bacteria Use of packer/completion fluids Acidizing Inhibition Chemical injection for hydrate control, paraffin control, etc. Enhanced recovery techniques

It is readily apparent why API 6A states that Choosing material classes is the ultimate responsibility of the user. The user has first hand knowledge of the well composition, operating conditions, the history of the field, etc. The user may have valuable experience with various materials in a particular field so knows what does and doesn’t work. As a manufacturer, Cameron is not in a position to state definitively what material will work in a given field. We have no control over how equipment is used or abused in the field. We have not control over reliability of inhibition systems, acidizing practice, chemical injection, etc. Very often the well analysis in an RFQ is an estimate if the field is new. Well fluid composition can change as a field matures. The bottom line is that Cameron has no crystal ball in which to gaze and foresee how our equipment will hold up in a given application. Cameron is more than willing to review a well analysis at the request of a customer and make a trim recommendation. Our recommendation will be based upon our experience with equipment in similar environments, on laboratory tests, and on data from published literature. We’ll be happy to discuss how we arrived at our recommendation with the customer. We, however, cannot assume the ultimate responsibility for trim selection: it lies with the customer.

TRIMS FOR PRODUCTION WELLS Table 2 shows how to determine that applicable API 6A material class for a given standard service in terms of H2S, CO2, and chloride contents. This table applies only to production wells. Note that the table utilizes the concept of partial pressure for H2S and CO2 contents. The total pressure of a mixture of gases is equal to the sum of the partial pressures of the components. The partial pressure of a component of the mixture is the pressure that the component would exert if it alone occupied the same volume as the mixture under the same conditions. In other words, the partial pressure of a component page - 397

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of a gas mixture is that portion of the total pressure that is contributed by the component gas. To find the partial pressure of either H2S or CO2 use the following formulas: — If given mol%, Partial Pressure =mol% X shut-in pressure of well 100 — If given volume%, Partial Pressure = volume% x shut-in pressure of well 100 — If given ppm (parts per million), Partial Pressure = ppm x shut-in pressure of well 1,000,000

Table 2 should only be used when: & There is no inhibition program. & There is little or no sand production. & Some produced water is expected. & There is little or no crude oil being produced. & There are no special customer requirements. & Acidizing will not be used. If any of the above statements is not true, then you should contact Oil Tool Metallurgy for a trim recommendation. If the well analysis of interest lies outside the H2S, CO2, and chloride limits prescribed in Table 2, again you should contact Oil Tool Metallurgy when requesting a trim (you can use the items in Table 1 as a guideline). The guidelines for trim selection in Table 2 are just that - guidelines - and not hard and fast rules. Metallurgy may very well recommend a material class for a given analysis that exceeds the limits of H2S, CO2, or chlorides for that material class in Table 2 if there are mitigating circumstances. If you’re not sure how to use Table 2 for a given analysis, don’t guess! Ask Oil Tool Metallurgy for assistance.

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TRIM SELECTION

TABLE 2 TRIMS (MATERIAL CLASSES) FOR PRODUCTION WELLS MATERIALS CLASS

H2S

CO2

CHLORIDES

MAXIMUM TEMPERATURE

AA (alloy steel) Non-corrosive general service

0.05 psi