Laser Applications in Surface Modification (Advanced Topics in Science and Technology in China, 65) 9811689210, 9789811689215

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Advanced Topics in Science and Technology in China 65

Jianhua Yao · Qunli Zhang · Rong Liu · Guolong Wu

Laser Applications in Surface Modification

Advanced Topics in Science and Technology in China Volume 65

Zhejiang University is one of the leading universities in China. In Advanced Topics in Science and Technology in China, Zhejiang University Press and Springer jointly publish monographs by Chinese scholars and professors, as well as invited authors and editors from abroad who are outstanding experts and scholars in their fields. This series will be of interest to researchers, lecturers, and graduate students alike. Advanced Topics in Science and Technology in China aims to present the latest and most cutting-edge theories, techniques, and methodologies in various research areas in China. It covers all disciplines in the fields of natural science and technology, including but not limited to, computer science, materials science, the life sciences, engineering, environmental sciences, mathematics, and physics. This book series is indexed by the SCOPUS database.

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Jianhua Yao · Qunli Zhang · Rong Liu · Guolong Wu

Laser Applications in Surface Modification

Jianhua Yao Institute of Laser Advanced Manufacturing Zhejiang University of Technology Hangzhou, Zhejiang, China

Qunli Zhang Institute of Laser Advanced Manufacturing Zhejiang University of Technology Hangzhou, Zhejiang, China

Rong Liu Department of Mechanical and Aerospace Engineering Carleton University Ottawa, ON, Canada

Guolong Wu Institute of Laser Advanced Manufacturing Zhejiang University of Technology Hangzhou, Zhejiang, China

ISSN 1995-6819 ISSN 1995-6827 (electronic) Advanced Topics in Science and Technology in China ISBN 978-981-16-8921-5 ISBN 978-981-16-8922-2 (eBook) https://doi.org/10.1007/978-981-16-8922-2 Jointly published with Zhejiang University Press, China The print edition is not for sale in China Mainland. Customers from China Mainland please order the print book from: Zhejiang University Press. © Zhejiang University Press 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publishers, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publishers nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publishers remain neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

In the service process of mechanical parts, key basic parts are prone to failure and cause equipment failure, which brings huge economic losses. Among them, more than 80% of the failures are caused by local surface wear and corrosion. Therefore, it is of great significance to adopt advanced surface modification technology to improve the surface performances of key basic parts. As one of the advanced surface engineering technologies, laser surface modification technology has the characteristics of high performances, low deformation, high bonding strength, green, and intelligence. It can achieve strengthening effects that traditional surface technology cannot meet and has been widely used in many areas of the national economy. Laser cladding of Stellite alloys and metal-ceramic composites were put forward in this book. Besides, nanomaterials including carbon nanotubes and Al2 O3 nanoparticles were introduced into laser processing to form high-temperature resistance, chemical stability, wear- and oxidation-resistant composite coatings. The contents of this book include recent research outcomes of the authors’ group, in various aspects such as theoretical analysis and experimental data, presented with the assistance of figures and tables, so as to help readers understand the advanced laser surface modification processes easier. From this book, the readers can get more knowledge about the basic principle and application of the two main laser-assisted surface modification technologies, i.e., laser cladding and laser surface hardening, and gain a deep insight into the process and characteristics of the nanomaterial-assisted laser surface enhancement. The book is aimed to provide the guidance and reference for the researchers, engineers, and students in the fields of mechanical engineering, laser processing, and material engineering. This book is written by Prof. Jianhua Yao, Prof. Qunli Zhang, Prof. Rong Liu, and Associated Prof. Guolong Wu, Zhejiang University of Technology, China. Prof. Yi Pan, Dr. Zhehe Yao, Dr. Liang Wang, Dr. Yuan Chen, Dr. Yinping Ding, and Dr. Jie Zhang have also contributed to the research and relevant experiments. For systematical integrity, this book has cited the research results published in book, journal, and conference articles from international and domestic experts in the related fields; hence, we wish to express sincere thanks to them. The research projects v

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covered in this book have been sponsored and supported by the National Key R&D Program of China (2018YFB0407300), the National Natural Science Foundation of China (52075495, 51975533), and the Key R&D Program of Zhejiang Province, China (2019C04004). Here, we all express our great appreciation. There are inevitably mistakes and errors in the book due to our oversight; please feel free to make comments and criticization. Hangzhou, China

Jianhua Yao Qunli Zhang Rong Liu Guolong Wu

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Hardfacing via Laser Cladding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 Materials for Hardfacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.2 Stellite Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.3 Hardfacing Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.4 Hardfacing from Laser Process . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Surface Hardening by Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Features and Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Applications on Steel and Cast Iron . . . . . . . . . . . . . . . . . . . . . 1.3 Laser Treatment of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Characteristics of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Beneficial Effects of Nanoparticles . . . . . . . . . . . . . . . . . . . . . 1.3.3 Nanoparticles in Surface Coatings . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 1 2 4 5 5 5 6 6 6 7 8 9

2 Laser Cladding of Stellite Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Stellite Alloy Hardfacing for Control Valves . . . . . . . . . . . . . . . . . . . . 2.1.1 Sealing Surface Enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2 Improvement of Stellite Alloy Hardfacings . . . . . . . . . . . . . . 2.2 Stellite 6 Hardfacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Problems in Amine Environment . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Fabrication of Hardfacings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Microstructure and Carbide Ratio . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Hardness and Wear Properties . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5 Corrosion Behavior Under Polarization Test . . . . . . . . . . . . . 2.2.6 Corrosion Behavior Under Failure Test . . . . . . . . . . . . . . . . . . 2.3 Stellite Alloy Mixture Hardfacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Laser Hardfacing Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Microstructure Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Hardness and Wear Resistance . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4 Cavitation-Erosion Performance . . . . . . . . . . . . . . . . . . . . . . . .

11 11 11 13 14 14 14 16 18 19 27 29 29 36 36 39

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2.3.5 Electrochemical Corrosion Behavior . . . . . . . . . . . . . . . . . . . . 2.4 Molybdenum Stellite Alloy Hardfacings . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Laser Creation of Hardfacings . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Microstructural Characterization . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 Hardness and Wear Performance . . . . . . . . . . . . . . . . . . . . . . . 2.4.4 Corrosion Resistance of Hardfacings . . . . . . . . . . . . . . . . . . . . 2.5 Summary of Stellite Alloy Hardfacings . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

40 42 42 44 46 47 52 56

3 Laser Cladding of Metal-Ceramic Composites . . . . . . . . . . . . . . . . . . . . 3.1 Surface Enhancement with Composite Coatings . . . . . . . . . . . . . . . . . 3.1.1 Laser Deposition of Metal-Ceramic Composite Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2 Improvement of Metal-Ceramic Composite Coatings . . . . . . 3.2 Nickel Alloy Composite Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Laser Cladding of WC/NiCrMo Composite . . . . . . . . . . . . . . 3.2.2 Morphology of Coating Specimens . . . . . . . . . . . . . . . . . . . . . 3.2.3 Microstructure of Coating Specimens . . . . . . . . . . . . . . . . . . . 3.2.4 Elemental Dilution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.5 Coating Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.6 Coating Wear Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Steel Composite Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Laser Cladding of TiC/H13 Composite . . . . . . . . . . . . . . . . . . 3.3.2 Microstructure of TiC/H13 Composite Coating . . . . . . . . . . . 3.3.3 Tribological Behavior of TiC/H13 Composite Coating . . . . . 3.3.4 Thermal Stability of TiC/H13 Composite Coating . . . . . . . . . 3.4 Summary of Composite Coating via Laser Cladding . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59 59 59 61 61 61 63 65 70 72 72 74 74 75 76 77 78 80

4 Laser Surface Hardening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.1 Surface Hardening of Steels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.1.1 Methods of Surface Hardening . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.1.2 Surface Hardening via Laser Process . . . . . . . . . . . . . . . . . . . . 84 4.2 Surface Hardening of 17-4PH Stainless Steel . . . . . . . . . . . . . . . . . . . 85 4.2.1 Laser Hardening Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 4.2.2 Microstructure of Hardened Surface . . . . . . . . . . . . . . . . . . . . 86 4.2.3 Hardness of Hardened Surface . . . . . . . . . . . . . . . . . . . . . . . . . 87 4.2.4 Mechanical Properties of Hardened Surface . . . . . . . . . . . . . . 87 4.2.5 Cavitation-Erosion Resistance of Hardened Surface . . . . . . . 89 4.2.6 Extreme Value Analysis (EVA) Model . . . . . . . . . . . . . . . . . . 91 4.3 Surface Hardening of Cast Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 4.3.1 Laser Surface Remelting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 4.3.2 Microstructure of Hardened Layer . . . . . . . . . . . . . . . . . . . . . . 95 4.3.3 Hardness and Wear Resistance of Hardened Layer . . . . . . . . 98 4.4 Summary of Laser Surface Hardening . . . . . . . . . . . . . . . . . . . . . . . . . 100 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

Contents

5 Laser-Processed Coatings Involving Nanoparticles . . . . . . . . . . . . . . . . 5.1 Laser Process of Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1 Application of Nanoparticles in Material Development . . . . 5.1.2 Laser Process of Nanoparticle-Containing Materials . . . . . . 5.2 Carbon Nanotubes Containing Superalloy Coating . . . . . . . . . . . . . . 5.2.1 Strengthening of Inconel 718 Alloy . . . . . . . . . . . . . . . . . . . . . 5.2.2 Preparation of Coated Carbon Nanotubes (CNTs) . . . . . . . . . 5.2.3 Laser Cladding Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Morphology of Ni-Coated MWCNTs Powder . . . . . . . . . . . . 5.2.5 Microstructure of Cladded Specimens . . . . . . . . . . . . . . . . . . . 5.2.6 Laves Phase Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Coating Prepared Using Nano-Al2 O3 . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Preparation of Coating with Fine Grains . . . . . . . . . . . . . . . . . 5.3.2 Microstructures of Ni–P–Al2 O3 Plating and Coating . . . . . . 5.3.3 Microhardness of Plating and Coating . . . . . . . . . . . . . . . . . . . 5.4 Coatings Prepared with Different Laser Powers . . . . . . . . . . . . . . . . . 5.4.1 Coating Preparation Processes . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2 Coatings Prepared with Low Laser Power . . . . . . . . . . . . . . . 5.4.3 Coatings Prepared with High Laser Power . . . . . . . . . . . . . . . 5.4.4 Microhardness and Wear Resistance of Coatings . . . . . . . . . . 5.5 Cracking of Nano-Al2 O3 Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.1 Nano-Al2 O3 Coating Preparation . . . . . . . . . . . . . . . . . . . . . . . 5.5.2 Crack Examination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.3 Laser Process Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.4 Ratio of Nano-Al2 O3 Particle . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.5 Overlapping of Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Summary of Laser Cladding of Nanomaterials . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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103 103 103 104 106 106 106 108 108 110 114 117 117 119 122 124 124 125 126 128 131 131 131 133 134 136 139 141

Chapter 1

Introduction

Abstract Laser as a high energy source has been extensively utilized in various surface modifications such as hardfacing, hardening and coating. Laser cladding or hardfacing, which is a welding-related technique, is applied on surface enhancement by depositing various Stellite alloys, WC/NiCrMo composite and TiC/H13 composite on steels. Laser hardening is a heat treatment process and it is adopted to increase the wear resistance of 17-4PH stainless steel and infinite chilling medium NiCr cast iron. The microstructures and associated properties such as hardness and wear resistance of the fabricated coatings and hardened layers are characterized and evaluated. With the advance in nanotechnology, nanoparticles have been involved in forming surface coatings, because compared to large particles nano-scaled objects possess better surface properties due to large surface to volume ratio. These surface properties play a crucial role in the performance of the particles in different stages of production, processing and final application. One of the applications of nanoparticles is incorporating nickel-coated carbon nanotubes (CNTs) into the gas atomized spherical Inconel 718 superalloy powder to create a composite coating which has minimum brittle Laves phase in nickel-based Inconel 718 alloy during laser cladding. Preparation of nano SiC and SiO2 coatings on graphite is achieved via laser irradiation to protect the oxidation of graphite at high temperature. The effects of laser energy density on the microstructure, quality and high temperature oxidation resistance of the cladding layer are systematically studied. Additionally, nano Al2 O3 is used for preparing wear- and oxidation-resistant coatings with fine grains by codepositing Al2 O3 and Ni-P via electroless plating, combining with laser treatment on the Ni-P-Al2 O3 plating.

1.1 Hardfacing via Laser Cladding 1.1.1 Materials for Hardfacing Hardfacing is a metalworking process to apply harder or tougher materials to a base metal in order to increase its wear resistance. This technique can be employed on a new part during manufacturing production or be used to repair a worn-down surface [1]. In other words, hardfacing process is to deposit hard, wear-resistant materials © Zhejiang University Press 2022 J. Yao et al., Laser Applications in Surface Modification, Advanced Topics in Science and Technology in China 65, https://doi.org/10.1007/978-981-16-8922-2_1

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1 Introduction

on a worn or new component surface to achieve thick coatings for resisting severe wear in service [2]. There are many means to produce hardfacing layers, commonly including thermal spraying, spray–fuse, welding, and laser cladding [3]. A wide range of materials can be selected for depositing hardfacing layers, but the main categories of materials are hard metals or alloys, ceramics, and metal-ceramic composites. For hard alloys, there are three general groups: first, low-alloy iron-based alloys, which are composed of 12% alloy components in maximum, usually including chromium, molybdenum, and manganese [4]; second, high-alloy iron-based alloys, containing 12–50% alloy content; chromium is added in all iron-based hardfacing alloys, but nickel or cobalt is often added in some of these alloys [5]; and third, cobalt-based and nickel-based alloys, which contain relatively small amounts of iron (1.3 to 12.5%) [6]. Among these alloys, the most costly, but also the most versatile, are the cobalt–chromium–tungsten alloys [7]. Cobalt-based and nickel-based alloys possess excellent resistance to corrosion and oxidation and have low coefficients of friction, which make these alloys especially applicable in the environments involving metal-to-metal wear [8] at high temperatures above 600 °C [9]. Cobalt-based alloys can retain much of their room-temperature hardness at high temperatures up to 1200° [10]. Cemented tungsten carbide (WC) and tungsten carbide composites are deemed typical ceramic-based materials for hardfacings [11], but other ceramic materials such as alumina and silica are also often used. Tungsten carbide has several favorable features; in particular, it is one of the hardest materials available for industrial application; therefore, it is often referred to as a hard metal owing to a very high hardness value of 1600 HV, compared to mild steel which has the hardness in the region of 160 HV [12]. For the use for hardfacing, WC is crushed and employed in conjunction with a binding metal. Cemented carbides are composite materials consisting of individual tungsten carbide particles imbedded in a ductile metal binder matrix of either cobalt or nickel. Various properties of a particular grade of carbide, in particular, the physical and metallurgical, are governed by many factors, especially, the chemical composition of the carbide, the type and content of the binder metal, the hardfacing process, etc. [13]. Hard alloys can also be enhanced by adding a certain amount (usually less than 30 wt%) of WC to hard alloy powders, which creates metal-ceramic composite hardfacings. The hardness of the obtained composite coating highly depends on the amount of WC in the composite powder. For high-temperature wear applications, titanium carbide (TiC) with a high melting point (3065 °C), low density (4.94 g/cm3 ), and high hardness (3200 kg/mm2 ) is very popular as the reinforcement of composite hardfacings [14]. The binder materials are usually metallic elements such as Fe, Co, or Ni, present as a ductile second phase to improve the fracture toughness of the composites [15].

1.1.2 Stellite Alloys Stellite alloys are a group of cobalt-based alloys, mainly in two composition systems: Co–Cr–W–C and Co–Cr–Mo–C. With exceptional properties, these alloys

1.1 Hardfacing via Laser Cladding

3

are favorable hardfacing materials [16]. The designed compositions of Stellite alloys are primarily for wear resistance, but these alloys also display excellent hightemperature properties, good corrosion, and oxidation resistance [17]. In addition to Co, chromium (Cr) is the main alloying element of Stellite alloys, but they also contain tungsten (W) or molybdenum (Mo) and a small amount of carbon (C) [18]. The Stellite alloy family consists of a large number of various Stellite alloys with different proportions of cobalt, chromium, tungsten, molybdenum, nickel, iron, aluminum, boron, carbon, manganese, etc., but most alloys contain four to six of these elements [19]. The main identifications of current Stellite alloy grades are carbon and tungsten or molybdenum contents; hence, the amount and type of carbides are precipitated in the microstructures of the alloys during solidification [20]. The major difference among individual wear-resistant Stellite alloys is carbon content and thus carbide volume fraction in the materials. For Stellite 3 which has a very high carbon content of 2.4 wt%, the carbides constitute about 30 wt% of the material. These carbides are of the M7 C3 (chromium-rich primary) and M6 C (tungsten-rich eutectic) types, where M represents the metal components. For Stellite 6 with 1.2 wt% carbon, the carbides constitute approximately 16 wt% of the material, which are predominantly chromium-rich eutectic carbides of the M7 C3 type [18]. The main alloying element Cr in Stellite alloys plays several roles. First, it is one of the carbide formers; that is, there are Cr-rich carbides in Stellite alloys such as Cr7 C3 and Cr23 C6 . Second, it is also the strengthener of solid solution as a solute. Third, it provides corrosion and oxidation resistance by forming Cr-rich oxide films. Other alloying elements W and Mo have a key function in Stellite alloys; both strengthen the solid solution matrix due to large atomic size, which impedes dislocation flow when present as solute atoms [21], but they also create carbides in high-carbon Stellite alloys when added in large quantities (>5 wt%). These carbides are of M6 C and M3 C types where M represents metal [22], although the carbides in Stellite alloys are mainly Cr-rich such as Cr7 C3 and Cr23 C6 [18]. It is possible to tailor the properties of Stellite alloys by switching W content for Mo, but this change will not affect the strengthening effect to the solid solution. In the meanwhile, Mo can also improve the corrosion resistance of Stellite alloys in reducing environments [21] and increasing the Mo content influences the type of carbide formation in high-carbon Stellite alloys as well [23]. Differently, in low-carbon (1 wt%) Stellite alloys, where adding excessive Mo (>5 wt%) can promote the formation of Mo-rich carbides [25]. High-molybdenum Stellite alloys have been developed in recent years, which include low-carbon Stellite 22 and Stellite 728, and high-carbon Stellite 712 and Stellite 720. It was shown in previous studies that Stellite 22 and Stellite 728 differ from low-molybdenum, lowcarbon Stellite 21 in microstructure, mechanical properties, erosion, wear and corrosion performance [23–30]. The experiments revealed that increase of Mo content in

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a low-carbon Stellite alloy generated some new phases—intermetallic compounds (Co3 Mo and Co7 Mo6 ), which were brittle but can enhance the wear resistance of the alloy significantly.

1.1.3 Hardfacing Process Hardfacing can be applied on a surface by a number of different welding processes. Depending on many factors such as nature of work to be hardfaced, function of the component, base metal composition, size and shape of component, accessibility of weld equipment, state of repair of worn components, number of same or similar items to be hardfaced, etc., the most suitable welding process can be selected for a given job. Various processes for hardfacing can be categorized as follows [31]: • Arc welding—shielded metal, flux-cored, and submerged arc welding • Gas welding—deposition by oxygen–acetylene gas welding • Combination of arc and gas—tungsten-inert gas welding and gas metal arc welding • Powder spraying—flame spraying, high velocity oxygen fuel process, electric arc spraying, and plasma transferred arc • Laser cladding. When minimal thermal distortion of the component and good process control are required, thermal spraying is a better selection for depositing a coating or hardfacing. Common materials deposited by thermal spraying for hardfacing can be cermets including WC–Co and alumina-based ceramics. The coatings from thermal spray process can achieve a thickness of about 0.3 mm [32]. Spray–fuse coatings, also known as self-fluxing overlay coatings, are first applied to the component surface using a flame spraying process and then subsequently fused using an oxyacetylene torch or a radio-frequency induction coil. A coating that is produced by fused deposition forms a metallurgical bond to the substrate and is free of porosity via wetting the substrate surface. Various types of alloys can be used with the spray–fuse process, but the most important group is from Ni–Cr–B–Si–C alloy system, which has a melting temperature in the range of 980–1200 °C [33], depending on the chemical composition. Weld hardfacing can achieve very thick (1–10 mm) dense coatings of wearresistant materials with high bond strength. Various welding techniques can be employed to produce hardfacings, including metal-inert gas (MIG), tungsten-inert gas (TIG), plasma transferred arc (PTA), submerged arc (SAW), and manual metal arc (MMA). A wide range of materials can be used for weld hardfacing, which include cobalt-based alloys (Stellite alloys), martensitic and high-speed steels, nickel alloys, and WC–Co cemented carbides. The component surface from weld hardfacing is often machined to achieve required finish for final use.

1.1 Hardfacing via Laser Cladding

5

1.1.4 Hardfacing from Laser Process Laser cladding or hardfacing is a welding-related technique which is mainly utilized to repair worn surfaces of used components by depositing a welded overlay to provide hardness, abrasion, erosion, galling, impact, corrosion, or heat resistance. The cladding layers perform better in a harsh environment for longer time and with less maintenance [2]. When applied to new parts, laser cladding protects the substrate material with a layer of a complex alloy coating which improves chemical, physical, and mechanical properties of the substrate. Laser hardfacing uses a high-power laser beam to fuse coating materials onto substrates; hence, a coating layer is produced with excellent wear resistance. This process is able to deposit hard materials onto engineering components of low-grade steels, thus extending the service life of the parts. This technique is complementary to or even substitutional to the manual gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), plasma arc welding (PAW), and other welding processes [34]. Laser hardfacing study has attracted much attention of researchers in recent years because of the advantage in saving strategic materials, compared to conventional coating processes. It can also provide better dimensional precision of the components owing to less thermal load on the bulk material. In this book, the studies of the microstructure, wear and corrosion performances of the hardfacings of various Stellite alloys, which are prepared via laser cladding, are presented. These Stellite alloys include conventional Stellite 6 and Stellite 21 and newly developed Stellite 22 and Stellite 728 as well as novel Stellite alloy mixture (70 wt% Stellite 3 and 30 wt% Stellite 21). Meanwhile, the WC/NiCrMo composite hardfacing and TiC/H13 steel composite hardfacing fabricated from laser process are studied, focusing on the comparison in microstructure, hardness, and wear resistance of the WC/NiCrMo composite hardfacing prepared using circular laser spot and wideband laser spot, respectively, and the microstructural nonuniformity of the TiC/H13 hardfacing due to the temperature gradient of the cladding layer during the laser process and the thermal stability of the TiC/H13 hardfacing at high temperature.

1.2 Surface Hardening by Laser 1.2.1 Features and Advantages Laser hardening is considered as a heat treatment process or surface hardening process in which the surface of a metal part is heated by a laser beam and then the surface is cooled down rapidly in surrounding air [35]. This process is used exclusively on ferrous materials which are able to be hardened, including steels and cast iron with a carbon content greater than 0.2 wt%. The parts from laser hardening process do not require much refinishing work, and this technique enables the work

6

1 Introduction

on irregular, three-dimensional workpieces. Laser hardening can improve the hardness and wear resistance of the workpiece, thus reducing abrasive wear. Another advantage of laser hardening technique is that it allows for absolute control on the surface hardness and texture. The process consists of rapidly heating the material surface by laser beam, short holding at the target temperature, and intensive cooling due to the high thermal conductivity of the material. During the cooldown period, a process called self-quenching takes place, resulting in fine-grained structure within the thin layer on the surface of the part [36]. The modified microstructure can alter the mechanical and tribological properties of the surface of the treated parts. Laser hardening is employed to locally improve the wear resistance and service life of parts for a wide variety of applications, from press-forming tools to oil-drilling equipment. This technology is especially suitable for the applications where minimal heat input into the surrounding material is critical. It offers many advantages over conventional heat treatment processes such as high throughput, reproducibility, and product quality. In many industrial applications, localized treatment with minimal heat input is required, which results in reduced distortion of the part; meanwhile, the rapid quench rate produces a fine microstructure.

1.2.2 Applications on Steel and Cast Iron In this book, the research in the laser surface treatment of 17-4PH stainless steel and infinite chilling medium NiCr cast iron is reported. The heat treatment applied on the surface of 17-4PH stainless steel via laser process was strengthening precipitation hardening, which included laser solid solution treatment of the steel surface, followed by aging the steel at 460 °C for 3 h, then cooling it by water. The microstructures, mechanical properties, and cavitation-erosion behavior of the laser-treated 17-4PH stainless steel were investigated. Laser surface remelting treatment was conducted on infinite chilling medium NiCr cast iron to modify surface properties. The lasermodified surface was microstructurally analyzed to identify three distinct zones: melting zone, transition zone, and heat-affected zone (HAZ). The hardness and wear resistance of the cast iron after the surface treatment were evaluated.

1.3 Laser Treatment of Nanomaterials 1.3.1 Characteristics of Nanomaterials Nanoparticles are defined as ultrafine particles sized between 1 and 100 nm in diameter. In recent decades, considerable scientific research has been conducted in various uses of nanoparticles in many fields such as construction, electronics, manufacturing, cosmetics, and medicine. The advantages of adding nanoparticles in structure

1.3 Laser Treatment of Nanomaterials

7

materials are immense, including promising extraordinary physical, chemical, and mechanical properties [37]. Among the many different types of nanoparticles, titanium dioxide, carbon nanotubes, silica, copper, and alumina are the most popular in mechanical applications. Nanoparticles and nanostructured materials represent an active research area spreading and expanding in many application fields. These materials have gained prominence in technological advancements due to their tunable physicochemical characteristics such as melting point, wettability, electrical and thermal conductivity, catalytic activity, light absorption, and scattering, resulting in enhanced performance over their bulk counterparts [38]. There are four material-based categories of nanoparticles and nanostructured materials. (1) Carbon-based nanomaterials contain carbon and are found in geometries such as hollow tubes, ellipsoids, or spheres. The main production methods of these materials are laser ablation, arc discharge, and chemical vapor deposition (CVD) [39]. (2) Inorganic-based nanomaterials include metal and metal oxide nanoparticles. For metals, Au or Ag nanoparticles are popular, and for metal oxides, TiO2 and ZnO nanoparticles are commonly used. There are also semiconductors such as silicon and ceramics [40]. (3) Organic-based nanomaterials are made mostly from organic matters, which find applications mainly in biomaterials [41]. (4) Compositebased nanomaterials are multi-phase nanoparticles and nanostructured materials with one phase on the nanoscale dimension. These nanomaterials can form by combining nanoparticles with other nanoparticles or nanoparticles combined with larger or bulktype materials or even more complicated structures [42]. The composite types are in a wide range, which can be any combinations of carbon-based, metal-based, or organic-based nanomaterials with any form of metal, ceramic, or polymer bulk materials. Nanomaterials are synthesized in various shapes and geometries depending on the application requirements.

1.3.2 Beneficial Effects of Nanoparticles Particle size determines the ratio of surface area to volume. Powders of small particle size possess high specific surface areas. Therefore, the specific surface area of a powder particle is considered a dimensional property. In many applications, high specific surface area is attractive and desired. In general, nanomaterials used for surface coatings are to selectively change or influence distinct particle properties. Compared to large particles, nanoscaled objects possess better surface properties, in particular, if they are smaller than 10 nm, because of large surface-to-volume ratio. These surface properties are crucial for the behavior of the particles during different stages of production, processing, and final application. Nanotechnology is beneficial because it possibly tailors the structures of materials at extremely small scales so as to achieve specific properties. The materials made using nanotechnology can be much stronger, lighter, more durable, more reactive, etc. For instance, nanostructured ceramic coatings exhibit much higher fracture toughness

8

1 Introduction

than conventional wear-resistant coatings on machine parts. It has also been reported that nanotechnology-enabled lubricants and engine oils significantly reduce wear and tear, which definitely extends the service lives of moving components in various applications, for example, power tools, joint parts, and industrial machinery [43]. Regarding lightweight issues on many mechanical systems such as cars, trucks, airplanes, boats, and spacecraft, nanoscale additives in polymer composite materials can result in significant fuel savings. Other applications of nanoparticle-based composites include baseball bats, tennis rackets, bicycles, motorcycle helmets, automobile parts, and power tool housings, which are of lightweight, high stiffness, good endurance, and resilience [44].

1.3.3 Nanoparticles in Surface Coatings The progressive deterioration of metallic surfaces due to corrosion and wear is crucial for many components during service, in particular, in major industrial plants, because it can cause great loss of plant efficiency and at worst a shutdown. Using wear and corrosion-resistant coatings is one of the most effective means for metal surface protection. Additives containing nanoscale materials in various coatings have long been recognized to be promising for surface enhancement. Nanomaterial-containing coatings offer much better material and processing properties than conventional coatings, for instance, increased indentation and cracking resistance, improved elasticity and plasticity, etc. [45]. It has also been found that nanomaterial-containing coatings can improve the scratch resistance of a surface and increase hardness [46]. The presence of nanoparticles in coatings represents a highly innovative application of nanotechnologies, in line with the requirements of surface protection. Adding nanoparticles into galvanic coatings allows the replacement of the chrome surface passivation, preserving the features of resistance to corrosion and wear, owing to the decrease of the friction coefficient [47]. According to various studies reported in literature, nanocoatings certainly show undisputed advantages, compared to conventional coatings, among which the most interesting is higher homogeneity versus microscale, low thermal conductivity, enhanced hardness and fracture toughness, the independence between toughness and hardness, improved resistance to wear and corrosion. In this book, the study in minimizing the formation of brittle Laves phase in nickel-based Inconel 718 alloy during laser cladding by incorporating nickel-coated carbon nanotubes (CNTs) into the gas atomized spherical Inconel 718 superalloy powder to create a composite coating is reported. It was found that the addition of nickel-coated CNTs can suppress the effect on the element segregation and Laves phase formation of laser-clad IN718 superalloy significantly. The studies of nanoAl2 O3 used for preparing wear- and oxidation-resistant coatings with fine grains by co-depositing Al2 O3 and Ni–P via electroless plating, combining with laser treatment on the Ni–P–Al2 O3 plating, are also detailed in this book. Minimization of cracking in the cladding layer of Ni-coated nano-Al2 O3 is delineated.

References

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References 1. Gualco A, Svoboda HG, Surian ES, de Vedia LA (2010) Effect of welding procedure on wear behaviour of a modified martensitic tool steel hardfacing deposit. Mater Des 31(9):4165–4173 2. Kwok CT, Man HC, Cheng FT (2001) Cavitation erosion–corrosion behaviour of laser surface alloyed AISI 1050 steel using NiCrSiB. Mater Sci Eng A 303(1–2):250–261 3. Buchely MF, Gutierrez JC, León LM, Toro A (2005) The effect of microstructure on abrasive wear of hardfacing alloys. Wear 259(1–6):52–61 4. Berns H, Fischer A (1997) Microstructure of Fe-Cr-C hardfacing alloys with additions of Nb, Ti and B. Mater Charac 39:499–527 5. Fouilland L, El Mansori M, Massaq A (2009) Friction-induced work hardening of cobalt-base hardfacing deposits for hot forging tools. J Mater Process Technol 209(7):3366–3373 6. El Mansori M, Nouari M (2007) Dry machinability of nickel-based weld-hardfacing layers for hot tooling. Int JMach Tool Manuf 47(11):1715–1727 7. Riddihough M (1970) Stellite (Cobalt-Chromium-Tungsten alloy) as a wear-resistant material. Tribology 3(4):211–215 8. D’Oliveira ASCM, Vilar R, Feder CG (2002) High temperature behaviour of plasma transferred arc and laser Co-based alloy coatings. Appl Surf Sci 201(1–4):154–160 9. Drapier JM, Davin A, Magnee A, Coutsouradis D, Habraken L (1975) Abrasion and corrosion resistant cobalt base alloys for hardfacing. Wear 33(2):271–282 10. Yaedu AE, D’Oliveira ASCM (2005) Cobalt based alloy PTA hardfacing on different substrate steels. Mater Sci Technol 21(4):459–466 11. Blombery RI, Perrott CM (1974) Wear of sprayed tungsten carbide hardfacing deposits. Wear 29(1):95–109 12. Jankauskas V, Antonov M, Varnauskas V, Skirkus R, Goljandin D (2015) Effect of WC grain size and content on low stress abrasive wear of manual arc welded hardfacings with low-carbon or stainless steel matrix. Wear 328:378–390 13. Nagentrau M, Mohd Tobi AL, Kamdi Z, Ismail MI, Sambu M (2017) A study on wear failure analysis of tungsten carbide hardfacing on carbon steel blade in a digester tank. J Fail Analy Preven 17:861–870 14. Wang XH, Song SL, Zou ZD, Qu SY (2006) Fabricating TiC particles reinforced Fe-based composite coatings produced by GTAW multi-layers melting process. Mater Sci Eng A 441(1– 2):60–67 15. Wang XH, Zou ZD, Qu SY, Song SL (2005) Microstructure and wear properties of Fe-based hardfacing coating reinforced by TiC particles. J Mater Process Technol 168(1):89–94 16. Wu YX, Bousser E, Schmitt T, Tarfa N, Khelfaoui F, Rene R, Klemberg-Sapieha JE, Brochu M (2019) Thermal stability of a Stellite/steel hardfacing interface during long-term aging. Mater Charac 154:181–192 17. Ahmed R, De Villiers HL (2017) Friction and wear of cobalt-base alloys. Frict Lubr Wear Technol 18:487–501 18. Davis JR (2000) Cobalt-base alloys, in Nickel, Cobalt, and their alloys. ASM International, Materials Park, p 362–406 19. Boeck BA, Sanders TH Jr, Anand V, Hickl AJ, Kumar P (1985) Relationships between processing, microstructure, and tensile properties of a Co-Cr-Mo alloy. Powd Metall 28(2):1–10 20. Kapoor S, Liu R, Wu XJ, Yao MX (2013) Microstructure and wear resistance relations of Stellite alloys. Int J Adv Mater Sci 4(3):231–248 21. Liu R, Yao MX (2012) High-temperature wear/corrosion resistant Stellite alloys and Tribality alloys. In: CRC handbook on aerospace and aeronautical materials. CRC Press, Taylor & Francis, p 151–235 22. Jiang WH, Yao XD, Hu ZQ (1999) Secondary M6 C precipitation in a cobalt-base superalloy. J Mater Sci Lett 18(4):303–305 23. Kamal K, Ding YP, Liu R, Yao JH, Yao MX (2019) Corrosion performance of 700 series Stellite alloys in various media. J Mater Eng Perform 28(9):5605–5615

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24. Huang P, Liu R, Wu XJ, Yao MX (2007) Effects of molybdenum content and heat treatment on mechanical and tribological properties of a low-carbon Stellite alloy. J Eng Mater Technol 129(4):523–529 25. Liu R, Yao JH, Zhang QL, Yao MX, Collier R (2015) Microstructures and hardness/wear performance of high-carbon Stellite alloys containing molybdenum. Metall Mater Trans A 46(12):5504–5513 26. Liu R, Yao JH, Zhang QL, Yao MX, Collier R (2015) Effects of molybdenum content on the wear/erosion and corrosion performance of low-carbon Stellite alloys. Mater Des 78:95–106 27. Yao JH, Ding YP, Liu R, Zhang QL, Wang L (2018) Wear and corrosion performance of laser-clad low-carbon high molybdenum Stellite alloys. Opt Laser Technol 107:32–45 28. Collier R, Liu R, Wu XJ, Zhang XZ, Yao MX (2020) Dry-sliding wear performance of molybdenum-containing Stellite alloys. Wear 29:1384–1399 29. Nsoesie S, Liu R, Chen KY, Yao MX (2013) Erosion resistance of Stellite alloys under solidparticle impact. J Mater Sci Eng B 3(9):555–566 30. Nsoesie S, Liu R, Chen KY, Yao MX (2014) Analytical modeling of solid-particle erosion of Stellite alloys in combination with experimental investigation. Wear 309(1–2):226–232 31. Pradeep GRC, Ramesh A, Prasad BD (2010) A review paper on hardfacing processes and materials. Inter J Eng Sci Technol 2(11):6507–6510 32. Tucker RC Jr (1994) Thermal spray coatings. In: ASM handbook, surface engineering, vol 5. ASM International, USA, p 497–509 33. Valean PC, Kazamer N, Muntean R, Pascal DT, Marginean G, Serban VA (2018) Investigation on the characteristics of thermally sprayed NiCrBSi coatings fused by flame and inductive processing. IOP Conf Series: Mater Sci Eng 416:1–5 34. Ming Q, Lim LC, Chen ZD (1998) Laser cladding of nickel-based hardfacing alloys. Surf Coat Technol 106(2–3):174–182 35. Babu PD, Balasubramanian KR, Buvanashekaran G (2011) Laser surface hardfacing: a review. Int J Surf Sci Eng 5(2–3):131–151 36. Moradi M, Arabi H, Nasab SJ, Benyounis KY (2019) A comparative study of laser surface hardfacing of AISI 410 and 420 martensitic stainless steels by using diode laser. Opt Laser Technol 111:347–357 37. Khan I, Saeed K, Khan I (2019) Nanoparticles: properties, applications and toxicities. Arabian J Chem 12(7):908–931 38. Jeevanandam J, Barhoum A, Chan YS, Dufresne A, Danquah MK (2018) Review on nanoparticles and nanostructured materials: history, sources, toxicity and regulations. Beilstein J Nanotechnol 9:1050–1074 39. Aqel A, Abou El-Nour KM, Ammar RA, Al-Warthan A (2012) Carbon nanotubes, science and technology part (I) structure, synthesis and characterization. Arabian J Chem 5:1–23 40. Li JH, Hong RY, Li MY, Li HZ, Zheng Y, Ding J (2009) Effects of ZnO nanoparticles on the mechanical and antibacterial properties of polyurethane coatings. Prog Org Coat 64:504–509 41. Virlan MJR, Miricescu D, Radulescu R, Sabliov CM, Totan A, Calenic B, Greabu M (2016) Organic nanomaterials and their applications in the treatment of oral diseases. Molecules 21(2):207–229 42. Tjong SC (2007) Novel nanoparticle-reinforced metal matrix composites with enhanced mechanical properties. Adv Eng Mater 9:639–652 43. Zhou JF, Wu ZS, Zhang ZJ, Liu WM, Xue QJ (2000) Tribological behavior and lubricating mechanism of Cu nanoparticles in oil. Tribo Lett 8:213–218 44. Gupta M, Wong WLE (2015) Magnesium-based nanocomposites: lightweight materials of the future. Mater Charac 105:30–46 45. Singh SK, Chattopadhyaya S, Pramanik A, Kumar S, Gupta N (2018) Influence of nanoparticle on the wear behavior of thin film coatings: a review. Inter J Appl Eng Res 13(6):4053–4058 46. Li XH, Cao Z, Zhang ZJ, Dang HX (2006) Surface-modification in situ of nano-SiO2 and its structure and tribological properties. Appl Surf Sci 252(22):7856–7861 47. Wang HJ, Chen T, Cong WL, Liu DF (2019) Laser cladding of Ti-based ceramic coatings on Ti6Al4V alloy: effects of CeO2 nanoparticles additive on wear performance. Coatings 9:109–125

Chapter 2

Laser Cladding of Stellite Alloys

Abstract Metal components often operate in a corrosion and wear combined environment, which results in surface-initiated failure. Therefore, any approach that improves surface performance can extend the service life of the components. Laser cladding (LC) as an advanced surface modification technique has been widely employed in surface engineering. Stellite alloys are cobalt-based superalloys, displaying exceptional properties such as high-temperature strength, corrosion/oxidation, and wear/erosion resistance. They have a wide range of applications, for example, Stellite 6 a common material for the seat surface enhancement of various control valves, and Stellite 21 is often used for valve trims under high-pressure steam. In this chapter, the wear and corrosion performances of conventional and novel Stellite alloy hardfacings, which are prepared via laser cladding, are studied. The microstructures of the hardfacings are characterized using scanning electron microscopy (SEM) with an EDAX energy-dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD). The wear properties of the hardfacings are evaluated on a pin-on-disk tribometer in dry-sliding mode. The corrosion performance of the hardfacings is investigated under electrochemical tests in various corrosive media such as morpholine solution with pH 9.5, to simulate the amine environment of boiler feedwater in power generation industry, 3.5 wt% NaCl solution, which is a common corrosive solution used to rank materials for corrosion resistance, and Green Death solution, representing the typical industry corrosive environment. It is demonstrated that novel Stellite alloy mixture (70 wt% Stellite 3 and 30 wt% Stellite 21) hardfacings, Stellite 22 and Stellite 728, are all superior to corresponding conventional Stellite 6 and Stellite 21 hardfacings, respectively, with respect to hardness, wear, and corrosion resistance.

2.1 Stellite Alloy Hardfacing for Control Valves 2.1.1 Sealing Surface Enhancement The sealing is a crucial issue for a control valve because it is directly related to the reliability of the valve operation. Therefore, the sealing surface is required to © Zhejiang University Press 2022 J. Yao et al., Laser Applications in Surface Modification, Advanced Topics in Science and Technology in China 65, https://doi.org/10.1007/978-981-16-8922-2_2

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2 Laser Cladding of Stellite Alloys

possess sufficient resistance to wear, in order to work along with cyclic metal-tometal contact operation. In coal power plants, the seat and trim faces of control valves must be capable of withstanding very high-contact stress without cracking, severe wear, corrosion, and cavitation without damage. Consequently, the major consideration in the valve design is to minimize the sealing surface damage in the working condition of high-impact loads and severe environments have. To devise a means to cope with this thorny problem, a hardfacing layer is usually promising, which can be applied on the sealing surfaces via various processes. Thus, the service life of a control valve relies significantly on the extent of sealing surface damage and the impact toughness of the hardfacing layer. Stellite 6 alloy (29% Cr, 4.5% W, 1.5% Mo, 1.2% C, in weight) and Stellite 21 alloy (27% Cr, 5.5% Mo, 0.25% C, Co balance, in weight) have been popularly employed as the hardfacing materials for the seat surfaces and trim surfaces of various control valves, respectively, due to their unique chemical compositions and excellent combined mechanical, anti-corrosion, anti-wear, and high temperature properties [1–3]. They are frequently deposited via conventional welding processes, such as tungsten-inert gas and plasma transferred arc (PTA) welding methods. Laser cladding utilizes laser technology, combined with the welding process to create a wear-resistant surface layer. It is an advanced hardfacing technique that can deposit hard materials on a tough metal surface with controlled thickness in the selected area of the metal substrate. The process requires a low energy input, thus causing less heat-induced distortion of the component as compared to conventional welding methods [4–6]. Many studies in Stellite 6 hardfacing of valve seats have been reported [4–11]. For example, according to the laser beam interaction time, the dendrite structure formation in laser cladding of Stellite 6 on mild steel and CrNi base materials was investigated, with the focus on the influence of various laser process parameters such as heat input, beam interaction time, scanning frequency, and transverse speed on the microstructure development [4]. The friction behavior at the interface between the valve disk and the valve body seat of a gate valve during operation, whose surfaces were hardfaced with Stellite 6, was also studied analytically. A more accurate estimate of the friction coefficient between the disk and seat was needed to predict the valve thrust accurately [7]. The microstructure, composition distribution, and mechanical properties of Stellite 6 hardfacing on austenitic engine valve via PTA were studied via various means such as optical microscope, scanning electron microcopy (SEM), field emission electron probe analyzer (EPMA) and nanoindentation [11]. The microstructure and mechanical properties of laser-clad joint of Stellite 21 on AISI 316L stainless steel were investigated concerning the gradient in chemical composition across the substrate/clad interface. It was found that the interdendritic carbides generated low-energy fracture path in the laser cladding, and the Stellite 21 coating exhibited superior fatigue strength to the AISI 316L substrate [12]. The fatigue resistance of Stellite 21 cladding on S355 structural steel substrate was evaluated using four-point bending and torsion fatigue tests. The results revealed that the fatigue life of S355 structural steel was not improved at all the applied loads with the application of Stellite 21 cladding [13]. This was attributed to significant Fe

2.1 Stellite Alloy Hardfacing for Control Valves

13

dilution from the S355 steel substrate during the laser cladding process, which had a detrimental influence on the fatigue strength of Stellite 21 cladding. There was an obvious thin Fe-rich layer in the Stellite 21 coating close to the fusion line, which was the main cause of the fatigue fracture of the Stellite 21 cladding in intergranular rupture mechanism [14].

2.1.2 Improvement of Stellite Alloy Hardfacings Stellite 6 is the most popular in Stellite alloy family, which finds application in various fields of industry. However, it was reported from the valve industry recently that in the working conditions involving metal-to-metal sealing of the valve Stellite 6 hardfacing was found deficient when subjected to high loading and high-cycle operating due to insufficient hardness and wear resistance. This would cause leaking of liquid from the valve after operation for a period of time. For such severe service conditions, the hardness of the sealing hardfacing is required to be at least HRC50, while the hardness of Stellite 6 alloy is only about HRC40. Therefore, pure Stellite 6 hardfacing definitely cannot meet the requirements of the valves, which pushed material engineers to find a harder and more wear-resistant Stellite alloy for such applications. Stellite 3 (30.5% Cr, 12.5% W, 2.4% C, Co balance, in weight) with very high carbon content is a good candidate because of its high hardness (about HRC52) and excellent wear resistance [3, 15]. The wear tests demonstrated that the sliding wear resistance of Stellite 3 is more than three times better than that of Stellite 6 [1], but this alloy was very sensitive to cracking during the deposition process of hardfacing due to large volume fraction of carbides (about 33%) present. Therefore, a more effective approach to solving this problem is necessary. Since Stellite 21 is the most applicable low-carbon Stellite alloy, it was proposed that adding a small amount of Stellite 21 powder into Stellite 3 powder may create a novel alloy mixture hardfacing, which would be superior both to Stellite 6 in hardness and wear resistance and to Stellite 3 in prevention of cracking during hardfacing [16]. In the Stellite alloy family, Stellite 21 has unique properties such as creep resistance and mechanical strength at elevated temperatures, good corrosion resistance, and strain or work hardening capacity [15–20], which make this alloy popular material for valve trims of high-pressure steam, oil and petrochemical processes, forging or hot stamping dies, medical implants, and prosthetics. It has also been recognized that for valve trims where enhanced wear resistance is required while good corrosion resistance must be maintained, Stellite 21 is found deficient due to small amount of carbides precipitated in the alloy. However, the increase in hardness by raising carbon content in Stellite 21 may deteriorate anti-corrosion properties, because the carbides in Stellite 21 are Cr-rich. The more the carbides in Stellite 21, the less the Cr in the solid solution is. Thus in the Cr-rich regions, Cr depletion will occur. Therefore, two new alloys which are modified Stellite 21 alloys, designated as Stellite 22 and Stellite 728, have been proposed and created with

14

2 Laser Cladding of Stellite Alloys

increased Mo content (from 5.5 to 11 wt%) [19–22]. This special chemical composition results in large amounts of Co3 Mo intermetallic compound precipitated in the alloys. The experimental results confirmed that Co3 Mo intermetallic compound has a similar function to the carbides in Stellite alloys in enhancing the hardness and wear resistance. Due to large atom size, molybdenum in Stellite alloys provides additional strength to the solid solution matrix when present as a solute atom, which impedes dislocation flow [23]. On the other hand, when added to low-carbon Stellite alloys in large quantities, molybdenum participates in the formation of intermetallic compounds (Co3 Mo or/and Co7 Mo6 ) [19, 20].

2.2 Stellite 6 Hardfacing 2.2.1 Problems in Amine Environment As mentioned above, Stellite 6 is usually overlay-welded on valve seats as a hardfacing layer for the valve disk and body, owing to its excellent combined mechanical, corrosion, wear, and high temperature properties, via welding or laser cladding processes. However, the problem in the practical application of Stellite 6 hardfacing has been reported from the valve industry [24], when boiler feedwater is treated with hydrazine (N2 H4 ) and other types of amine derivatives, which says that Stellite 6 hardfacing could fail in such corrosive environment with wear also involved, characterized by the cracking of the hardfacing. Although various comments on this issue had been noticed, it was still not understood why this failure occurred on Stellite alloy, because it contains high chromium and can form an oxide film on its surface to protect corrosion of the substrate. To help the industry solve this problem, the corrosion performance of Stellite 6 hardfacing prepared via laser cladding in an amine environment, saline (3.5 wt% NaCl) and Green Death solution, was investigated. The last two solutions are a common corrosive medium used to rank materials for corrosion resistance and the typical industry corrosive environment, respectively [25], which were used in this study for comparison with an amine solution. Cast Stellite 6 was also tested in parallel for comparison with hardfaced Stellite 6. Meanwhile, the hardness and wear resistance of Stellite 6 prepared via the two different processes were evaluated to response the real operation scenario of the alloy in the valve industry where wear and corrosion are synergetic.

2.2.2 Fabrication of Hardfacings The cast Stellite 6 specimens and Stellite 6 powder used in this research were all supplied by Kennametal Stellite Inc., with the chemical compositions given in Table

2.2 Stellite 6 Hardfacing

15

2.1. The morphology of the powder was examined with SEM, which shows a perfectly spherical shape, and the powder sizes vary in the range of 45–150 µm (Fig. 2.1). The substrate was a stainless steel 316 plate having the dimensions of 100 × 60 × 10 mm. Before the hardfacing process, the stainless steel 316 plate was grit-blasted using 24-mesh alumina and ultrasonically cleaned in absolute ethyl alcohol. Stellite 6 hardfacing was processed using a LDF400-2000 high-power flexible fiber-coupled diode laser, as schematically shown in Fig. 2.2. The laser beam profile, showing energy distribution across the beam in the system, is given in Fig. 2.3. It is apparent that the energy density of the laser beam is uniformly distributed, which meets the requirement for producing hardfacing. The system parameters of the laser device are the output power of 2 kW, the output wavelength of 900–1070 nm, and the IRB2400/16 robot motion device with six degrees of freedom. Stellite 6 powder was fed by a powder feeder (GTV-PF-Twin2/2), which was supplied by GTV GmbH, Germany. The laser cladding process was performed under argon gas shielding. The process parameters were selected as the laser energy density of 56.25 J/mm2 and the feeding rate of 13 g/min. In order to obtain optimal process parameters, which would be used to prepare the Stellite 6 hardfacing specimens, single-track specimens were prepared first with different process parameters and then laid for examining the physical appearance of the clad layers. Laser tracks were laid at ~50% overlapping to achieve a uniform clad thickness. The powder aligning with the laser beam was focused on the surface of the specimen by using a coaxial cladding head. Table 2.1 Chemical compositions of Stellite 6 materials (wt%, Co in balance) Element

Cr

W

Mo

C

Ni

Mn

Si

Fe

Cast Stellite 6

28.87

4.31