Boiler Tube Failure Mechanisms: Case Studies 9819931290, 9789819931293

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Springer Tracts in Mechanical Engineering

Atanu Saha

Boiler Tube Failure Mechanisms Case Studies

Springer Tracts in Mechanical Engineering Series Editors Seung-Bok Choi, College of Engineering, Inha University, Incheon, Korea (Republic of) Haibin Duan, Beijing University of Aeronautics and Astronautics, Beijing, China Yili Fu, Harbin Institute of Technology, Harbin, China Carlos Guardiola, CMT-Motores Termicos, Polytechnic University of Valencia, Valencia, Spain Jian-Qiao Sun, University of California, Merced, CA, USA Young W. Kwon, Naval Postgraduate School, Monterey, CA, USA Francisco Cavas-Martínez , Departamento de Estructuras, Universidad Politécnica de Cartagena, Cartagena, Murcia, Spain Fakher Chaari, National School of Engineers of Sfax, Sfax, Tunisia Francesca di Mare, Institute of Energy Technology, Ruhr-Universität Bochum, Bochum, Nordrhein-Westfalen, Germany Hamid Reza Karimi, Department of Mechanical Engineering, Politecnico di Milano, Milan, Italy

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Atanu Saha

Boiler Tube Failure Mechanisms Case Studies

Atanu Saha Industrial Service and Research Group CSIR-Central Mechanical Engineering Research Institute Durgapur, West Bengal, India

ISSN 2195-9862 ISSN 2195-9870 (electronic) Springer Tracts in Mechanical Engineering ISBN 978-981-99-3129-3 ISBN 978-981-99-3130-9 (eBook) https://doi.org/10.1007/978-981-99-3130-9 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 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 translation, 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 publisher, 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 publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains 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

The fossil-fired boiler is the most critical component of thermal power plant, and tube failures in the fossil-fired boiler are one of the main causes of forced outages of power generating units. Almost every time, severe service environment in fossil-fired boilers is largely the reason for these failures. The high-temperature failure in combination with severe stress is very much common in critical components like superheater, reheater, waterwall and economizer tubes. So, one of the important tasks of the power utilities to reduce the forced outages of the critical component is to improve plant availability, safety and reliability. This can be achieved by investigative analysis of the boiler tubes to ascertain the causes of failure and thereby to suggest corrective action necessary to prevent the recurrence of similar failure in future. This book provides an in-depth examination of boiler tube materials failure in specific situations, a vital component in both developing and engineering new solutions. It covers materials for steam boiler, design life of components, different failure/ damage mechanism of boiler tube and variety of case studies of boiler tube failure. Different examination methodologies including true pictures and micrographs are incorporated in each analysis of every case. It covers analysis of materials failure in the areas of power industries, wherein the failure of a single component can result in devastating consequences and costs. Case studies related to short-term overheating, high-temperature creep, high-temperature failure, caustic corrosion, hydrogen damage, dissimilar weld failure, manufacturing defects and corrosion fatigue are elaborately depicted in this book. This book is an indispensable reference for engineers and scientists studying the mechanisms of failure in this field. The author thanks all the technical officials for excellent conducting of different tests of each case study. The author is also thankful to honorable Director, CSIRCMERI, Durgapur, India, Dr. Naresh Chandra Murmu, for his inspiration and encouragement of this work. I sincerely hope that the publication of this book will help people from industry particularly power industry and academia to get the maximum benefits from the experience contained in the published book. Durgapur, India

Dr. Atanu Saha

v

Acknowledgements

I wish to express my profound sense of gratitude to Head of Industrial Service and Research Group, CSIR-Central Mechanical Engineering Research Institute, Durgapur-713209, India, for his encouraging guidance, valuable suggestions and critical comments without which the present work could not have been carried out. I express my sincere thanks to all staff members of Industrial Service and Research Group, CSIR-Central Mechanical Engineering Research Institute, Durgapur-713209, India, for their advice and wholehearted support throughout my work. In this regard, I am particularly thankful to Dr. Himadri Roy, Mr. S. Chidambaram, Mr. Subrata Ray, Mr. Sanjib De, Mr. Abhijit Modal and Mr. Nasir Hussain. I express my gratitude to Head, Manufacturing Technology Group, CSIR-Central Mechanical Engineering Research Institute, Durgapur, India, for providing the machining facility of the tensile test specimens of this work. Furthermore, I am extremely grateful to Dr. Naresh Chandra Murmu, Director, CSIR-Central Mechanical Engineering Research Institute, Durgapur, India, for his constant encouragement and kind permission to publish this work.

vii

Contents

Part I

Overview of Failure Mechanisms

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Material for Steam Boilers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Design Life of Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 5 8 13 13

2

Failure/Damage Mechanism of Boiler Tube . . . . . . . . . . . . . . . . . . . . . . 2.1 Short-Term Overheating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 High Temperature Creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Temperature Measurement Based on Steam Side Oxide Scale Thickness Measurement . . . . . . . . . . . . . . . . 2.3 Hydrogen Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Caustic Corrosion/Gouging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15 15 18

Part II 3

24 25 27 29

Case Studies

Case 1A: Short-Term Overheating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4 Mechanical Properties (Tensile Test) . . . . . . . . . . . . . . . . . 3.2.5 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . 3.2.6 Estimation of Peak Temperature . . . . . . . . . . . . . . . . . . . . 3.2.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.8 Conclusion for Case Study 1A . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

33 33 34 34 34 35 36 36 41 42 43 43

ix

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Contents

4

Case IB: Short-Term Overheating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . 4.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.6 Conclusion for Case Study IB . . . . . . . . . . . . . . . . . . . . . .

45 45 45 45 46 47 47 47 49

5

Case IIA: High-Temperature Creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Metallographic Examination and Hardness Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.5 Scale Thickness Measurement and Estimation of Tube Metal Temperature . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.7 Conclusion for Case Study IIA . . . . . . . . . . . . . . . . . . . . .

51 51 52 52 52 53

6

Case Study IIB: High-Temperature Creep . . . . . . . . . . . . . . . . . . . . . . . 6.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.4 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . 6.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.6 Conclusion for Case Study IIB . . . . . . . . . . . . . . . . . . . . . .

59 59 59 59 60 60 60 62 65

7

Case Study III: High-Temperature Failure . . . . . . . . . . . . . . . . . . . . . . 7.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.4 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . 7.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.6 Conclusion for Case Study III . . . . . . . . . . . . . . . . . . . . . .

67 67 68 68 69 69 69 69 71

53 54 56 57

Contents

xi

8

Case Study IV: Erosion Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.4 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . 8.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.6 Conclusion for Case Study IV . . . . . . . . . . . . . . . . . . . . . .

73 73 73 73 74 75 77 79 81

9

Case Study VA: Dissimilar Metal Weld . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.4 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . 9.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.6 Conclusion for Case Study VA . . . . . . . . . . . . . . . . . . . . . . 9.2.7 Suggestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

83 83 83 83 84 84 84 85 86 87

10 Case Study VB: Dissimilar Metal Weld . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Conclusion for Case Study VB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

91 92 92 93 93 94

11 Case Study VIA: Hydrogen Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.4 Metallographic Examination and Hardness Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.6 Conclusion for Case Study VIA . . . . . . . . . . . . . . . . . . . . .

95 95 95 95 96 96

12 Case Study VIB: Hydrogen Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

101 101 101 101 102 102

97 98 99

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Contents

12.2.4 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . 103 12.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 12.2.6 Conclusion for Case Study VIB . . . . . . . . . . . . . . . . . . . . . 106 13 Case Study VIIA: Fireside Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.4 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . 13.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.6 Conclusion for Case Study VIB . . . . . . . . . . . . . . . . . . . . .

107 107 107 107 109 109 110 110 112

14 Case Study VIIB: Fireside Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 14.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2.4 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . 14.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2.6 Conclusion for Case Study VIIB . . . . . . . . . . . . . . . . . . . .

115 115 116 116 116 117 118 120 121

15 Case Study VIII: Failure Due to Manufacturing Defect . . . . . . . . . . . 15.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.4 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . 15.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.6 Conclusion for Case Study VIII . . . . . . . . . . . . . . . . . . . . .

123 123 124 124 125 125 125 126 127

16 Case Study IX: Corrosion Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.1 Scope of Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2 Examination Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2.1 Visual Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2.2 Dimensional Measurement . . . . . . . . . . . . . . . . . . . . . . . . . 16.2.3 Chemical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2.4 Metallographic Examination . . . . . . . . . . . . . . . . . . . . . . . 16.2.5 SEM/EDX Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2.7 Conclusion for Case Study IX . . . . . . . . . . . . . . . . . . . . . . 16.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

129 129 129 129 130 131 131 131 131 134 134

About the Author

Dr. Atanu Saha has over 25 years extensive experience in the field of residual life assessment study of power and process plant components and failure analysis of different engineering components. He has served numerous industries like power, petrochemical, steel etc. He has several research articles in reputed international SCI journals. Dr. Saha has authored and presented several technical papers relating to residual life assessment study and failure analysis of engineering components. He has conducted training programs on various aspects of residual life assessment and failure analysis techniques for practicing engineers. Dr. Saha is member of Central Boilers Board, Government of India (Ministry of Commerce and Industry) and Boilers and Pressure Vessels Sectional Committee, Bureau of Indian Standards (BIS), Government of India.

xiii

Part I

Overview of Failure Mechanisms

About 80% of national demand of electricity is met from thermal power plants which employ steam produced from burning fossil fuel in a boiler. The fuel employed in the boiler for producing steam may be coal, oil or natural gas. Out of these fuels, coal is the most abundant and hence most commonly used fuel for steam turbine plant. Boiler tube failures are the main causes of forced outages of power generating units. Most of the common failure occurs in water wall tubes, superheater/re-heater tubes and economizer tubes. More than 80% of unscheduled boiler shutdown is due to boiler tube failure [1]. Present study is aimed at identifying the cause/causes of in-service failure of boiler tubes in different sections of various capacity boilers and thereby to suggest corrective action necessary to prevent the recurrence of similar failure in future. Failure of boiler tubes of different sections of boiler may be due to short-term overheating, hightemperature creep, high-temperature failure, caustic corrosion, hydrogen damage, dissimilar weld failure, manufacturing defects, corrosion fatigue, etc. The study contains materials for steam boiler, design life of components, different failure/damage mechanism of boiler tube and variety of case studies of boiler tube failure. Different examination methodologies including true pictures and micrographs are incorporated in each analysis of every case. It covers analysis of materials failure in the areas of power industries wherein the failure of a single component can result in devastating consequences and costs.

Reference 1. A.F. Armor, Boiler tube failure, the number one availability problems for utilities, Failure and inspection of fossil fired boiler tube conference and workshop, EPRI, 1983

Chapter 1

Introduction

The function of a boiler is to convert water into superheated steam which is ultimately delivered into a steam turbine. The schematic illustration of a boiler is shown in Fig. 1.1. Coal, oil, natural gas, etc. with preheated air burned in the furnaces. The combustion gases flow up through the furnace and evaporate the water into steam inside the furnace water wall tubes. The mixture of water and steam is called the working fluid. At the roof of the furnace, the upward flowing gases pass through a bank of secondary superheater and re-heater tubes where the steam is superheated before putting it into turbine. The gases then turn downward and pass through primary superheater and economizer tubes. After passing the heating surfaces, the combustion products are cooled to a relatively low temperature and ejected from the boiler through a stack into the atmosphere. The superheated steam produced in a boiler is supplied into a steam turbine where its thermal energy is converted into mechanical work on the turbine shaft. The turbine shaft is connected to an electric generator in which the mechanical energy is transformed into electricity. Low-pressure steam from the turbine exit is processed into feed water via condenser, feed water heater and deaerators. In deaerator, the condensate is made to boil and is freed from oxygen and carbon dioxide that might cause corrosion of the equipment. The feed water is then pumped into the economizer, where it is heated and sent to furnace water wall tubes. From here, the high-pressure steam goes through the superheater into the turbine inlet and thus completes the cycle as shown in Fig. 1.2. Critical components of a power boiler such as superheater/re-heater tubes, headers and steam pipes operating in the creep range are designed for a finite life. Depending on the actual operational and environmental conditions, these high-temperature components degrade gradually during service due to various processes, including creep, fatigue, corrosion, erosion and embrittlement. The severe service environment in fossil fired boilers is largely the reason for the failures as the effect of stress, temperature, corrosion, erosion and vibration combine to produce degradation of the tubing steel. Corrosion by oxidation, by combustion products and by impure boiler water can significantly reduce the tube wall thickness and result in failure of a tube © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_1

3

4

1 Introduction

Fig. 1.1 Typical cross section of large drum-type utility boiler, showing major water and steamcooled tube circuits [1]

Fig. 1.2 Simplified (3 cylinder type) steam power plant cycle

1.1 Material for Steam Boilers

5

many years before the expected end of its useful service life. The majority of forced outages of power boilers are due to premature failure of abovementioned components [2–5]. The cost penalty is estimated to be in excess of 5 billion dollars a year in replacement power charges and maintenance costs [3]. Contribution of total tube failure can be grouped as furnace water wall tubing 40%, superheater tube 30%, re-heater tube 15%, economizer tube 10% and burner tube 5% [6]. Exposure of tube steel to elevated temperature causes microstructural changes of the steel constituents [1, 7–9]. As for example, carbon steel will experience spheroidization in which iron carbide particles change from plate or rod shapes to more or less spherical ones. Plain carbon and carbon molybdenum steel will experience graphitization in which Fe3 C particles transform into free iron and graphite molecules. Austenite stainless steel will develop carbide precipitates intermetallic compounds. Microstructural changes occur even at design conditions with long-term service. Their formations are accelerated when tube steel is operated above design conditions. Operation above design conditions is called “overheating” and can lead to tube failure through a stress rupture failure mechanism. Erosion and vibration caused by ash-laden flue gas can result in boiler tube failures if operating conditions are different from design considerations. High gas flow velocities can cause tube to vibrate at high frequencies and result in fatigue cracking failures [6]. A large number of case studies are elaborately depicted in this book to understand the damage mechanisms of boiler tube [9]. Determination of the correct mechanism of a boiler tube failure is important for the prevention of future tube failures. Proper corrective measures can be undertaken to alleviate the root cause or causes for a failure once the correct mechanism is known. The plant’s personnel must provide the initial information on the failure and boiler conditions prior to failure so that pertinent data may be extracted.

1.1 Material for Steam Boilers Different sections of boiler work under different working parameters, so the designers select the material as per requirement at that particular section. The material of critical boiler elements like water wall tubes, drum, headers, superheater/re-heater tubes and steam pipelines operates under critical conditions. In steady regimes, it is subjected to stresses caused by internal pressure and the own mass of the element and to an elevated temperature. In unsteady regimes (start up and shut down), it is subjected to variable pressure and temperature. Besides those, as mentioned, some critical elements at high temperature can be acted upon by corrosive medium, flue gases, saturated and unsaturated steam, steam water mixture, etc. which can cause corrosion of the metal.

6

1 Introduction

All boiler elements operating at elevated temperature can be divided into two groups: (i) Those operated at temperatures below 350–400 °C, they include the drum, steam-generating tubes and their headers, tubes and headers of economizer and pipelines for water. (ii) Those operated at temperatures above 350–400 °C, they include steam superheaters and their headers, de-superheaters, superheated steam pipelines and fittings. Danger for the operation of critical boiler elements is the combined long-term effect of the internal pressure and high temperature (above 450 °C) of superheated steam in superheater tubes, headers and main steam pipelines. This may lead to a special kind of plastic deformation in which the strain increases slowly and continuously at a constant stress below the yield stress and the size of the element gradually increases and as the residual plastic deformation attains a definite limit, rupture of the metal occurs. Figure 1.3 shows creep curves of steel at various temperatures and constant stress (σ ). Here temperature t 1 < t 2 < t 3 . The creep curve at temperature t 1 can be divided into three portions as oa, ab and bc. Portion oa (the portion of alternating creep) corresponds to a short initial period I when the metal is even strengthened slightly (primary creep). Then follows a longer period II of steady-state (secondary) creep (portion ab) during which an element still can operate reliably without rupture. The rate of rupture during that period is constant θ = Δl/T = tan α.

(1.1)

In the final critical period III of accelerated creep (portion bc), a high plastic deformation occurs in an element (for instance, “inflation” of superheater header), after which rupture takes place at point c. Reliable operation of element is only possible within the period II of steady-state creep. At a higher temperature (t 2 or t 3 ), Fig. 1.3 Creep curves of steel at various temperatures and constant stress (σ )

1.1 Material for Steam Boilers

7

the process of creep occurs in a similar manner, but more quickly the steady-state creep increases and rupture takes place earlier. The stress at which the rate of creep during period II does not exceed the specified value or the stress that causes the total plastic deformation during a specified time of operation not above a safe limit is called the creep stress or creep strength σ cr . For most steel grades, the allowable total plastic deformation of 1% after 100,000 h of operation is allowed [10]. The strength of the metal in operation under creep conditions can be characterized by the long-term strength. As the metal is stressed under creep conditions, the time of its reliable operation up to rupture depends on the stress applied. The stress which causes rupture of the metal in creep during a specified period is called the long-term strength. The relationship between stress (σ ) and time to rupture (T p ) at constant temperature (t) is shown below (Figure 1.4). Figures 1.5 and 1.6 show the experimental curves relating stress versus time to rupture and stress versus creep rate respectively for 2.25 Cr-1 Mo steel. Materials for fossil boiler construction can vary within the boiler depending on duty requirements, economics and the availability of the component in the sizes required. Materials that are mostly used for fossil fuel boiler construction are listed below. Property requirements and materials for construction are given in Tables 1.1 and 1.2 [4]. Maximum tube metal temperatures permitted by ASME code for boiler manufacture are given in Table 1.3 [11]. Fig. 1.4 Relationship between long-term strength and time to rupture

Fig. 1.5 Stress versus time to rupture relationship for 2.25 Cr-1 Mo steel [4]

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1 Introduction

Fig. 1.6 Stress versus creep rate for 2.25 Cr−1 Mo steel [4]

Table 1.1 Property requirements and materials for construction Component

Major property requirement

Water wall tubes

Corrosion resistance, tensile properties and weldability Carbon and C-Mo steel

Drum

Tensile strength, corrosion resistance, weldability, corrosion fatigue strength

C, C-Mo and C-Mn steels

Superheater/ re-heater tubes

Weldability, creep strength, oxidation resistance, low coefficient of thermal expansion

C-Mo, austenitic stainless steel

Steam piping

Weldability, creep strength, oxidation resistance, low coefficient of thermal expansion

C-Mo, austenitic stainless steel

Typical materials

Boiler tubes are seamless, extruded tubes and can vary all the way from carbon steel in the low temperature water wall section to austenitic stainless steel in the finishing stages of the superheater. Selection of material depends on their creep strength and resistance to oxidation at high temperature. The ASME, boiler and pressure vessel code of section 1, paragraph A-150 states the criteria for determining the maximum permissible stress. Table 1.4 shows maximum tube metal temperature permitted by ASME code and other boiler manufacturers. Figures 1.7 and 1.8 show the graphical relationship on the effect of temperature on ASME boiler and pressure vessel code allowable stress for different grades of steel tubing.

1.2 Design Life of Components Components which operate at low temperature below the creep regime are generally designed on the basis of yield strength, ultimate tensile strength and fatigue strength by applying suitable safety factor to these values. Because deformation and fracture

Product form

SA-335

Ferritic alloys

SA-213

SA-213

9 Cr.1 Mo

Tube

21 /4 Cr.1 Mo Tube

1 Cr. 1 /2 Mo

SA-106

High strength Pipe

Pipe

SA-210

SA-192

ASME/ AS TM No (ksi)

Tube

Intermediate strength

Low strength

Carbon steels Tube

Alloy

T9

T22

P12

C

A1

–

Grade

60

60

60

70

60

47

Min Tensile strength (ksi)

30

30

30

40

37

26

Min.yield strength

0.15

0.15

0.15

0.35

0.27

0.06–0.18

C

Composition

Table 1.2 Material used in boiler construction as per ASME/ASTM code

0.30–0.60

0.30–0.60

0.30–0.60

0.29–1.06

0.93

0.27–0.63

Mn

0.030

0.030

0.045

0.048

0.048

0.048

P

0.030

0.030

0.045

0.058

0.058

0.058

S

0.25–1.00

0.50

0.50

0.10

0.10

0.25

Si

–

–

–

Ni

8.00–10.00

1.90–2.60

0.80–1.25

–

–

–

Cr

0.90–1.10

0.87–1.13

0.440.65

–

–

–

Mo

1.2 Design Life of Components 9

10

1 Introduction

Table 1.3 Maximum tube metal temperatures permitted by ASME code for boiler manufacture Tube steel type

ASME specification

Maximum temperature (° C)

Carbon steel

SA 178 C

538

Carbon steel

SA 192

538

Carbon steel

SA 210 A1

538

C-Mo steel

SA 209 T1

538

Cr-Mo steel

SA 213 T11

649

Cr-Mo steel

SA 213 T22

649

Stainless steel

SA 213, 321H

816

Table 1.4 Maximum tube metal temperatures permitted by ASME code and boiler manufacturers [4] Tube steel type

ASME specification No

ASME ° F (°C)

Babcock and Wilcox ° F (° C)

Combustion engineering ° F (°C)

Riley stoker ° F (°C)

Carbon steel

SA-178 C

1000 (538)

950 (510)

850 (454)

850 (454)

Carbon steel

SA-192

1000 (538)

950 (510)

850 (454)

–

Carbon steel

SA-210 A1

1000 (538)

950 (510)

850 (454)

850 (454)

C-Mo steel

SA-209 T1

1000 (538)

–

900 (482)

900 (482)

C-Mo steel

SA-209 T1a

1000 (538)

975 (524)

–

–

Cr–Mo steel

SA-213 T11 SA-213 T22

1200 (649) 1200 (649)

1050 (566) 1115 (602)

1025 (552) 1075 (580)

1025 (552) 1075 (580)

Stainless steel

SA-213 321H

1500 (816)

1400 (760)

–

1500 (816)

Stainless steel

SA 213 347H

1500 (816)

–

1300 (704)

–

Stainless steel

SA-213 304H

1500 (816)

1400 (760)

1300 (704)

–

are not time dependent under these conditions, there is no specific value of design life associated with them. In principle, as long as the applied stresses do not exceed the design stresses, these components should last indefinitely, although in practice, various factors cause reduction in life. In case of high-temperature components operating in the creep regime, both deformation and fracture are time dependent. They are, therefore, designed with respect to a target life usually based on a specified amount of allowable strain in 100,000 h. A further factor of safety is applied in selecting the stress which translates into an expected life of 30–40 years. Many metallurgical and operational factors can extend the actual life of the component beyond the design life. Alternatively, if these factors are adverse, actual life can be reduced. Some of the many favorable and unfavorable factors that hold the balance between design life and actual life are illustrated schematically below [12].

1.2 Design Life of Components Fig. 1.7 Effect of temperature on ASME boiler and pressure vessel code allowable stress for several grades of steel tubing [4]

Fig. 1.8 Use of boiler and pressure vessel code criteria to establish the allowable stress for a 21 /4 Cr.-1 Mo steel [4]

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12

1 Introduction

Actual Life • • • • •

Use of minimum properties in design Factor of safety in design Conservative operation of unit Inaccuracies of data extrapolation Overestimation of oxidation effects

Design Life

Actual Life

• Unanticipated stressese.g., residual, systems stresses • Operation outside design limitse.g., excessive temperature, load cycling of base load units • Operation and environmental effects • Degradation of material properties in servicee.g., softening, temper embrittlement etc.

• • • • • • •

Use of minimum properties in design. Factor of safety in design. Conservative operation of unit. Inaccuracies of data extrapolation. Overestimation of oxidation effects. Unanticipated stresses-e.g., residual, systems stresses. Operation outside design limits- e.g., excessive temperature, load cycling of base load units. • Operation and environmental effects. • Degradation of material properties in service- e.g., softening, temper embrittlement. etc. If the actual material of construction exceeds these expectation, the actual can then be far exceed the design life. A number of factors can also lead to premature failure of components. Stresses in components frequently exceed the design stresses as a result of hidden residual stresses, system stresses and local stress concentration. Operating temperatures in boilers invariably exceed design temperature at least over short duration reducing component life. Unanticipated start up and shut down cycle lead to fatigue damage not originally provided in the design criteria. Unanticipated environmental effects leading to corrosion pitting, stress corrosion, etc. leading to major reduction in life. Pre-existing fabricated defect may cause crack initiation and growth of crack during service and lead to premature failures. Last but not least important factor adversely affecting the component life is the in-service degradation of components due to various microstructural changes and embrittlement phenomena.

References

13

1.3 Conclusion In spite of the best efforts of design engineers and material scientists, engineering components fail in service. In some cases, failure may lead to serious consequences like huge financial loss, environmental contamination and even loss of life. The failure of industrial boilers has been a prominent feature in fossil fuel fired power plants. The contribution of several factors appears to be responsible for failures, culminating in the partial or complete shutdown of the plant. There are several mechanisms of boiler tube failure, including short term overheating, high temperature creep, high temperature failure, caustic corrosion, hydrogen damage, dissimilar weld failure, manufacturing defects, corrosion fatigue etc. These mechanisms can lead to various types of tube failures, such as fish mouth type rupture, small fracture opening at the apex of bulge, thick edged fracture, thin edged fracture, window type opening, thickedge split type fracture, cracks and pin hole type leaks, which can cause catastrophic incidents. In the event of a failure it is therefore essential to investigate the root cause of failure in terms of design, quality of material and fabrication procedure. A failure analyst must have an open mind and be ready to examine and evaluate the views of other involved in operation of power plant. There are innumerable case studies have been reported and case studies reported indicated that actual mechanism of boiler tube failure can be identified through systematic metallurgical investigation. Based on the mechanism of failure, sequence of events that led to failure can be drawn and corrective actions can be suggested for preventing re-occurrence of similar failure. Hence, it may be concluded that investigation into the cause of boiler tube failure is very much helpful in improving the availability and reliability of boilers.

References 1. G. Bernasconi, G. Piatti (eds.), Creep of Engineering Materials and Structures (Applied Science Publishers, London, 1979) 2. R. Viswanathan, J.R. Foulds, D.I. Roberts, Proceedings on boiler tube failures in fossil power plants. vol. 10 (Palo Alto, CA, EPRI, 1987) pp. 3.35–53 3. R. Viswanathan, J.R. Foulds, Service experience, structural integrity, severe accidents and erosion in nuclear and fossil plants. ASME Press Vess. Pip. 303, 187–205 (1995) 4. R. Viswanathan, in Damage Mechanisms and Life Assessment of High Temperature Components (Metals Park (OH), ASM International, 1989) 5. M.C. Coleman, J.D. Parker, D.T. Welters, Behaviour of ferritic weldments in thick section Cr–Mo–V pipe at elevated temperatures. Int. J. Press Vess. Pip. 18, 277–310 (1985) 6. G.A. Lamping, R.M. Arrowwood, Manual for investigation and correction of boiler tube failures. Report CS-3945, Electric Power Research Institute, Palo Alto, CA, April (1985) 7. F. Garofalo, Fundamentals of creep and creep rupture in metals. Mae Millan series in material science NY (1965) 8. J. Bressers (ed.), Creep and Fatigue in High Temperature Alloys (Applied Science Publishers, London, 1981)

14

1 Introduction

9. A. Saha, Boiler tube failures: some case studies, Chapter 3. in Handbook of Materials Failure Analysis with Case Studies from Chemical, Concrete, and Power Industries (2016). ISBN No. 978-0-08-100116-5 10. M.I. Reznikov, Y.M. Lipov, Steam Boiler of Thermal Power Station (MIR publication, Moscow, 1985) 11. ASME Boiler and Pressure Vessel Code, Section I (1995) 12. Manual for investigation and correction of boiler tube failure EPRI, CS-3945 (1991)

Chapter 2

Failure/Damage Mechanism of Boiler Tube

Much of work has been carried out and still going on regarding boiler tube failure mechanisms by renowned organizations and individuals. Some of the organizations who are in the field are Electric Power Research Institute (EPRI, USA), Central Electricty Generating Board (CEGB, UK), ASME Boiler and Pressure Vessels, USA, Combustion Engineering, USA, etc. Failure of boiler tubes is the prime reason of boiler outage in most of the countries. Boiler tube failures are of immense concern to utility companies and boiler manufacturers. There are many reports regarding failure modes and mechanisms. Damage mechanisms mostly contribute the maximum percentage of tube failures [1–3] are: 1. 2. 3. 4.

Short-term overheating High-temperature creep Caustic corrosion and Hydrogen Damage.

2.1 Short-Term Overheating Stress and temperature influence the useful life of tubing steels operating in a boiler. The strength of a boiler tube is dependent on the level of stress as well as temperature when the tube metal temperatures are in the creep range. Since an increase in either stress or temperature can reduce the time to rupture, attention must be given to both factors when investigation is to be carried out regarding failure caused by stress-rupture mechanism. Stress-rupture failure mechanisms are predominantly experienced in steam cooled superheater and re-heater sections where the operating temperature is in the creep range. Stress rupture can also be experienced in water cooled tubing if abnormal heat transfer conditions exists that results in an increase in the tubes operating temperature [4, 5]. If the thickness of the tube decreases by corrosion, or erosion, the hoop stress will increase and hence the likelihood of failure.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_2

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2 Failure/Damage Mechanism of Boiler Tube

The circumferential hoop stress in a tube is determined by the diameter and thickness of the tube as stated in Eq. (2.2). The “overheating failure” means a failure resulting from operation of a tube at temperature higher than expected in design selection of the tube steel for a period of time sufficient to cause a stress rupture failure. Time at temperature is an important factor, and these types of failures are called “short-term” and “long-term” overheating. Causes of short-term overheating A short-term overheating failure is one in which incident/incidents exposes the tube steel to an excessively high temperature to the point where deformation or yielding occurs. Overheating results from abnormal conditions [6] such as loss of coolant flow and excessive boiler gas temperature. Such types of abnormalities are created by: (i) (ii) (iii) (iv)

Internal blockage of the tube Loss of coolant circulation or low water level Loss of coolant due to an upstream tube failure and Overfiring or uneven firing of boiler fuel burners.

The abnormalities created by serial nos. (i), (ii) and (iii) are produced due to starvation or low coolant flow failure. A tube can be blocked by erection and repair debris, tools, steel shots, deposits from carry over or spray water or loose pieces of internal non-pressure part hardware such as nuts, bolts and other steel parts. In tubing containing water, blockage will reduce coolant flow which will in film boiling and produce local metal temperature approaching the furnace gas temperature. Loss of coolant circulation can have several causes, such as low drum-water level or failure in the same tube at a different location. In adequate coolant, turbulence or circulation in a region of high heat flux can result in a deviation from the normal nucleate type of boiling condition that is desired inside a water cooled tube. A departure from nucleate boiling condition results when steam bubbles in the hot tube surface begin to interface with the flow of water coolant to the tube metal surface. The bubbles can eventually cover the inside surface and produce a film of steam which restricts the flow of heat away from the tube. When film type of boiling exists in a water cooled tube, the metal temperature can exceed 540 °C. Film boiling can also be produced when overfiring or uneven firing of fuel burners results in the region of high heat flux. In general, short-term overheating failure involves considerable tube deformation in the form of metal elongation and reduction in wall area or cross section. Such types of failure are often characterized as knifeedge fracture surface. The elongation and deformation normally encountered with short-term overheating. Wall thinning and local bulging precede the actual fracture, because the strength of the material is very much reduced at the higher temperature. A fish-mouth appearance with thin-edge fracture surfaces and considerable swelling is typical for a ferritic steel tube that has failed before its temperature has exceeded the upper critical temperature (Ac3 ). If the tube temperature was high enough to transform the steel from ferrite to austenite, there will be no noticeable necking down

2.1 Short-Term Overheating

17

or reduction in wall thickness of the fracture edges. There will still be metal elongation and tube swelling so that measurement of the tube diameter will show an increase. A metallurgical analysis of the microstructure of the steel should be performed to confirm that the tube temperature prior to failure was high enough to transform the ferrite to austenite. Changes in tube ID and OD measurement can be the indicator of overheating. Increase of 5% or more is indicator of short-term overheating, and also significant changes in microstructure in carbon steel will occur when the steel is overheated. A normalized microstructure of carbon steel boiler tubing consists of ferrite and pearlite phases. Above the lower critical temperature (Ac1 ), the pearlite will begin to transform to austenite. At the upper critical temperature (Ac3 ), the conversion to austenite is complete. Upon the rapid cooling that occurs when the tube bursts, the austenite will transform to martensite. Figure 2.1 shows the locations where short-term overheating can occur. Figures. 2.2 and 2.3 show the typical short-term overheating failure of boiler tube. Preventive measure to avoid short-term overheating In general, quality control measures should be enforced to prevent the followings: (i) Tube blockage (ii) Low coolant flow rates

Fig. 2.1 Boiler locations where short-term overheating can occur [2]

18

2 Failure/Damage Mechanism of Boiler Tube

Fig. 2.2 Unusual short-term overheating failure. Excessive swelling but thick-edged fracture surfaces are produced when a tube’s metal temperature exceeds its upper critical temperature (Ac3 ) and the iron in the steel has been completely transformed from ferrite to austenite [2]

Fig. 2.3 Typical short-term overheating failure. Excessive swelling and thin-edged fracture surfaces are produced when the tube’s metal temperature becomes several hundred degrees above its normal operating temperature but below its upper critical temperature (Ac3 ). Most tubes fall before reaching about 600 °F (300 °C) over operating conditions [2]

(iii) Low drum water levels and (iv) Excessive firing rates. Maintenance procedures should be followed during welding of tube joints to prevent tools, cutting debris, weld spatter from entering the tube circuit. Operating instructions should be followed to avoid low water levels, excessive firing rates, improper-fuel-burner operation, excessive de-superheating sprays or low heat transfer through water walls.

2.2 High Temperature Creep Boiler tube failure can result from high-temperature creep of superheater and reheater tube steel. Metal degradation and permanent deformation will occur with time depending on the actual stress level and temperature. If temperatures and stresses

2.2 High Temperature Creep

19

exceed design selection values, the tube steel will exhibit a higher creep rate and will fail earlier than expected. High-temperature creep failures are called “long-term” or extended overheating failures. Such a failure results from a relatively continuous extended period of slight overheating, a slowly increasing level of temperature or stress, or accumulation from several periods of excessive overheating. The creep occurs along the grain boundaries of the steel and is aligned 90° from the direction of applied tensile stress. Creep deformation results in little or no reduction in wall thickness but produces measurable creep elongation or increase in diameter in ferritic steel tubes. The creep cavitation of the superheater/re-heater tube due to exposure at higher temperature and stress was shown by earlier investigators [6–9]. Figure 2.6 shows the locations of the boiler where high-temperature creep can occur. Figure 2.7 shows blister-type hightemperature creep failure, and Figure 2.8 shows fish-mouth-type high-temperature creep failure. Causes of high-temperature creep High-temperature creep develops from insufficient boiler-coolant circulation, elevated boiler gas temperature and material properties that are inadequate for actual operating conditions. These abnormal conditions are created by the following circumstances: (i) Internal restriction of tube coolant (ii) Reduction of heat transfer capacity due to internal (steam side) surface oxide scales or chemical deposits (iii) Periodic overfiring or uneven firing of fuel burners (iv) Blockage of boiler gas passage (v) Operation of a tube material at a higher temperature than allowable temperature and (vi) Increase in stress due to wall thinning. High-temperature creep usually results in a longitudinal fracture on the heated side of the tube. A small fracture forms a blister-type opening, whereas a longer fracture will exhibit a wide, fish-mouth-type appearance. The fracture surface has thick edges or thick lips because the creep damage creates link up of individual voids and black oxide-filled cracks. Secondary cracking adjacent to the main fracture is extensive and is a positive indication of creep. The thickness of the tube at the fracture edge is an indication of a very long-term creep failure. ASME boiler and pressure vessel code allows 1% creep in 100,000 hours of operating time. If temperature and stresses are higher than the design selection temperature or stress, then the tube steel will experience a higher creep rate and will fail earlier than expected. A commonly used parametric analysis method is called the Larson–Miller method, where time and temperature are related by the following equation [10] T (20 + log10 tr ) × 10−3 = P

(2.1)

20

2 Failure/Damage Mechanism of Boiler Tube

where P Larson–Miller parameter T Absolute temperature in ° R (° F + 460) t r Rupture time in hours. Plot of the rupture stress as a function of LMP will result in a single line which can be used to assess the changes in expected tube steel life when operating conditions are varied. If the operating stress, temperature and time can be determined, the LMP parameter can be used to calculate the remaining time to rupture for tube material as shown in Figs. 2.4 and 2.5 for 1 Cr 1 /2 Mo and 21 /4 Cr 1 Mo steel, respectively. The time to rupture will decrease if the applied stress and/or temperature is increased. Cyclic applications of stresses and temperature from boiler load changes will also aggravate these effects on tube life. Wall thickness from erosion, corrosion or radiation will increase the stresses. The stress level of tube can be estimated by knowing the OD, ID and wall thickness. Since the wall thickness decreases with time due to corrosion, erosion, etc., periodic measurement of the actual wall dimension is necessary to determine the corrosion rate to estimate correct and future stress level. Fig. 2.4 Variation of Larson–Miller rupture parameter with stress for wrought 11 /4 Cr−1 /2 Mo-Si steel [11]

2.2 High Temperature Creep

21

Fig. 2.5 Variation of Larson–Miller rupture parameter with stress for annealed 21 /4 Cr−1 Mo steel [11]

An estimation of the hoop stress can be obtained from the mean diameter formula σH = PDM /2W

(2.2)

where σH P DM W

Estimated hoop stress Internal stress pressure Mean tube diameter Tube wall thickness.

Figure 2.6 shows the locations of the boiler where high-temperature creep can occur. Figure 2.7 shows blister-type high-temperature creep failure. Figure 2.8 shows the fish-mouth-type high-temperature creep failure. Preventive measure to avoid high-temperature creep Prevention of high-temperature creep failure involves keeping the tube metal stress and temperature within the capabilities of tube material. Overheating and/or overstressing the tube material beyond its design limit as established by ASME or the boiler manufacturer accelerates creep deformation and results in premature tube failure.

22

2 Failure/Damage Mechanism of Boiler Tube

Fig. 2.6 Boiler locations where high-temperature creep can occur [2]

Corrective action for control of high-temperature creep failure depends on the specific cause for overheating or overstressing [12]. • Failure from overheating caused by internal flow restrictions or tube heat transfer reductions can be avoided by removal of scale, debris or deposits that have accumulated inside the tube. High-pressure fluid flushing or chemical cleaning may be necessary to restore the design coolant flow or tube heat transfer characteristics.

2.2 High Temperature Creep

23

Fig. 2.7 Blister-type high-temperature creep failure. Typical features of a long-term, local overheating, creep rupture failure include small fracture opening at the apex of a bulge, creep elongation only in blister area, and little or no wall reduction in non-blistered area [2]

Fig. 2.8 Fish-mouth-type high-temperature creep failure. Typical features of a long-term, general overheating, creep rupture failure include an open-mouthed longitudinal split and thick-edged fracture surface. Oxide scale on the external and internal surfaces also indicates a high-temperature creep failure [2]

• Failure from overstressing caused by wall thinning can be controlled and assessed well in advance by applying ultrasonic wall thickness measurements with residual life estimates. • Failure from creep damages caused by periods of operation at metal temperatures above the design limit can be controlled by restoring boiler design conditions or by upgrading tube material. Measurements of actual tube metal temperature can show where design limits are being exceeded. When actual temperatures cannot be reduced, the tube material should be replaced with a higher-chromium-content ferritic steel or austenitic stainless steel. Residual life estimates can be performed to determine when tube failures can be expected so that corrective action can be taken prior to their occurrence.

24

2 Failure/Damage Mechanism of Boiler Tube

2.2.1 Temperature Measurement Based on Steam Side Oxide Scale Thickness Measurement One of the crucial parameters in estimation of creep life is the operating mean metal temperature. Due to load fluctuation and steam side oxide scale growth during operation, it is unlikely that a constant metal temperature is maintained during service. It is, therefore, more convenient to established an “equivalent” or mean metal temperature in service by examination of such parameters as hardness, microstructure and thickness of the steam side oxide scale. Because the changes of these parameters are functions of time and temperature, their values may be used to estimate an equivalent thermal history for a given operating time. Extensive data indicate that in relatively pure steam, the growth of oxide scale is a function of temperature and time of exposure alone and is presumed to obey specific rate laws. Several expressions have been proposed in literature to describe oxide scale growth kinetics [13–15] as given in Table 2.1. Figure 2.9 shows the empirical relationship between oxide scale thickness and Larson–Miller parameter for 21 /4 Cr−1 Mo steel. Table 2.1 Expressions for oxide-growth kinetics in Cr–Mo steels No

Expression (a)

Steel

Temperature range, °C (°F) Units

1

Log X = −7.1438 + 2.1761 × 10−4 T (20 + Log t)1 − 3% Cr

Below FeO formation

X in mils T in °R

2

Y 2 = kt

T in K

For 1 Cr-1 /2 Mo: Log k = (−7380/T ) + 2.23 [T ≤ 585 °C (1085 °F)] 1 Cr-1 /2 Mo

585 (1085)

Log k = (−48,333/T ) + 49.28 [T > 585 °C (1085 °F)] 1 Cr-1 /2 Mo

585 (1085)

For 21 /4 Cr-1 Mo: Log k = (−7380/T ) + 1.98 [T ≤ 595 °C (1103 °F)] 21 /4 Cr-1 Mo

595 (1103)

Log k = (−48,333/T ) + 49.2 [T > 595 595 (1103) °C (1103 °F] 21 /4 Cr-1 Mo 3

Log X = −6.8398 + 2.83 × 10 −4 T (13.62 + Log t) 21 /4 Cr-1Mo

429–649 800–1200

X in mils T in °R

(a) X is scale thickness; y is metal loss (penetration); T is temperature; t is time, in hours; all logarithms to base 10. °R = °F + 460; K = °C + 273; 1 mm = 103 μm = 40 mils; y = 0.42x

2.3 Hydrogen Damage

25

Fig. 2.9 Empirical relationship between oxide scale thickness and a time temperature parameter for 21 /4 Cr−1 Mo steel in isothermal steam environments [15]

2.3 Hydrogen Damage Boiler tube failure caused by hydrogen damage resulted from liberation of atomic hydrogen during corrosion process [16, 17]. This hydrogen is capable of diffusing into the metal wall. It may pass through the wall into the furnace environment. If the atomic hydrogen reacts with carbide (Fe3 C) phase of tube wall metal to form methane (CH4 ). 4H + Fe3 C = CH4 + 3Fe, the outward diffusion ceases and methane begins to accumulate within the tube wall. The accumulation sites for the CH4 are grain boundaries adjacent to colonies of pearlite, the Fe3 C-containing constituent of the microstructure. CH4 gas pressure at the grain boundaries eventually exceeds the grain boundary strength, resulting in short, discontinuous, randomly oriented microcracks. Interlinkages of these microcracks diminish the load carrying cross section of the tube wall metal. Eventually, a thick-walled-ruptured tube metal occurs. Fig. 2.10 shows the locations of the boiler where hydrogen damage can occur (Fig. 2.11). Causes of Hydrogen Damage Hydrogen is generated under the following circumstances: (i) Operation of boiler with low PH water chemistry from the ingress of acidic salts through condenser leakage, contamination from chemical cleanings, etc. (ii) Concentration of the corrosive contaminants within the deposits on the internal tube wall, especially in crevices, shallow pits and under weld backing rings. Hydrogen damage usually produces a blow out of a rectangular section of the tube in a manner described as a “window opening”. The edges of the fracture are thick because the tube steel has been weakened by the decarburization of the iron carbide (cementite) and by the formation of the methane-filled microfissures. Hydrogendamaged tube steel will crack if tensile stress is applied because the metal fails in a

26

2 Failure/Damage Mechanism of Boiler Tube

Fig. 2.10 Boiler locations where hydrogen damage can occur [2] Fig. 2.11 Hydrogen damage, “window opening” and “thick-edge split”-type failure [2]

2.4 Caustic Corrosion/Gouging

27

brittle manner. The amount of hydrogen damage is directly related to the corrosion rate since it controls the rate of hydrogen-ion reduction and diffusion through the metal. Where the corrosion rate is very high in localized areas, the hydrogen generation and the diffusion rates can result in hydrogen concentrations in the steel that will initiate the hydrogen carbon reaction and result in significant decarburization and cracking. Corrective action to prevent hydrogen damage Boiler water chemistry monitoring and control practices are important factors in prevention of internal tube deposit and hydrogen damage corrosion attack. The deposit thickness and amount as well as its constituents are important criteria in determining when chemical cleaning of the boiler is necessary. Chemical cleaning should be immediately considered when the boiler water PH has been below 7 for more than one hour due to ingress of saline condenser cooling water or acidic chemicals from water treatment facility. Successful chemical cleaning can remove the internal deposits and stop the further generation of hydrogen. If insufficient wall thickness and material strength remain for sustaining the operating pressure, the tube metal will yield and ductile-type fracture will be experienced later in service. Ultrasonic wall thickness measurement could be carried out to locate the reduced wall thickness areas. The hydrostatic test should be carried out at 1.5 times the operating pressure and held for several hours to observe pressure drop due to leakage.

2.4 Caustic Corrosion/Gouging Caustic corrosion/gouging occurs when alkalinity of boiler water increases. Caustic corrosion is also called caustic attack. Caustic corrosion develops from deposition of feed water corrosion products in which NaOH can concentrate to high PH levels. At high PH level, the tube steel’s protective magnetic oxide coating is solubilized, and rapid corrosion occurs [10] as per the reaction given below: 4 NaOH + Fe3 O4 = 2 NaFeO2 + 2 H2 O With the destruction of protective magnetic oxide layer, concentrated NaOH reacts with the tube material and forms atomic hydrogen as per the reaction Fe + 2 NaOH = Na2 FeO2 + 2 H The atomic hydrogen so produced reacts with Fe3 C of pearlite constituent and form CH4 which ultimately causes hydrogen damage discussed earlier. The tube surface deposits accumulate at locations where flow is disrupted such as welds with backing rings, at bends, in horizontal tube weld and at high heat input locations.

28

2 Failure/Damage Mechanism of Boiler Tube

Causes of caustic corrosion Caustic corrosion occurs through (i) Selective deposition of feed water system or preboiler corrosion products at locations of high heat flux. (ii) Concentration of NaOH from boiler water chemicals or from upsets in water chemistry. Depositions occur when boiler water is converted into steam, and the dissolved solids concentrate in a residual film. The solids’ concentration in this film increases with the increase of temperature. When the deposit thickness becomes great enough to make caustic concentrations locally corrosive, caustic corrosion can proceed to failure in a very short time. Caustic corrosion results in irregular wall thinning or gouging of tube’s water-side surface, and failures are the result of the corrosion metal loss. As the thickness is reduced by corrosion, the hoop stress imposed by the water pressure is increased and a ductile tensile failure results. Considerable wall thinning can occur (up to 75% wall thickness) before failure. The corrosion process can be stopped by proper water quality and by chemical cleaning to remove wall deposits. Once corrosion is stopped, thinned tubes that can withstand the higher stress level will not continue to fail. Since water wall tubes of drum-type boilers are not operated in the high-temperature creep range, the higher stress level has no longer effect. Figure 2.12 shows the locations of the boiler where caustic corrosion can occur (Fig. 2.13). Corrective actions to prevent Caustic Corrosion In general, caustic corrosion occurs under conditions of relatively high boiler water alkalinity. Such corrosion damage may be reduced by minimizing the ingress of Fig. 2.12 Boiler locations where caustic corrosion can occur [18]

References

29

Fig. 2.13 Caustic corrosion gouging. Severe localized gouging on the heat absorption side of this SA-210 water wall tube was caused by caustic corrosion beneath a deposit. Once the caustic concentration within a deposit becomes great enough to cause corrosion, the reaction becomes self-perpetuating and causes a through-wall leak within a very short time [2]

deposit-forming materials and by performing periodic removal of water-side deposits by chemical cleaning. Rigorous monitoring and control of the boiler water chemistry are necessary to prevent high caustic level. Elimination of welds with backing rings and other irregularities of welded joint can be beneficial.

References 1. Manual for investigation and correction of boiler tube failure EPRI, CS-3945 (1991) 2. G.A. Lamping, R.M. Arrowwood, Manual for investigation and correction of boiler tube failures. Report CS-3945, Electric Power Research Institute, Palo Alto, CA, April (1985) 3. A. Saha, Boiler tube failures: some case studies, Chapter 3. in Handbook of Materials Failure Analysis with case studies from Chemical, Concrete, and Power Industries (2016). ISBN No. 978-0-08-100116-5 4. R.B. Dooley, D. Briske, (Ed.) Boiler tube failures in fossil fuel power plant project no. CS 5500, SR, EPRI (1988) 5. R. Viswanathan, R.B. Dooley, Creep life assessment technique for fossil power plant boiler pressure parts. in Proceedings of International Conference on Creep ASTM-ASME, Tokyo, (1986) 6. G. Joseph, Singer (Ed.), in Combustion in Fossil Power System, 3rd edn (Windsor, Combustion Engineering Inc., 1981) 7. A. Saha, H. Roy, A.K Shukla, Failure of a pendent reheater tube in a 110 MW thermal power plant. J. High Temp. Mater Process 33(4), 299–304 (2014) 8. A. Saha, A.K. Shukla, Failure of a secondary superheater tube in a 140 MW thermal power plant. J. Fail. Anal. Prev. 14, 10–12 (2014)

30

2 Failure/Damage Mechanism of Boiler Tube

9. A. Saha, H. Roy, A.K. Shukla, Failure of a final superheater tube in a 140 MW thermal power plant. J. Fail. Anal. Prev. 15, 184–189 (2015) 10. F. R Larson, J. Miller, A time-temperature relationship for rupturing and creep stresses Transactions of ASME (1952) 11. R. Viswanathan, Damage mechanisms and life assessment of high temperature components. (Metals Park (OH), ASM International, 1989) 12. O.I. Roberts, Magnetic oxide thickness, time-temperature model for 21 /4 Cr-1Mo operating at high pressure steam, G.A. Technologies, San Diego (1986) 13. S.R. Paterson, T.W. Ratting, Remaining life estimation of boiler pressure parts-21 /4 Cr-1 Mo superheater and reheater tubes, Project No. RP 2253–5, EPRI, Palo, Atlo, CA (1987) 14. M. Dewitte, J. Stubbe, Personal Communication to D.A. Roberts of G.A. Technologies, San Diego, Laboratories Co., Belgium (1986). 15. H.A. Grabowski, H.A. Klein, Corrosion and Hydrogen Damage in High Pressure Boilers (National association for corrosion engineers, USA, 1964) 16. Hydrogen damage Symposium, ASME IEEE National Power Conference, (Combustion, USA ,1963) 17. R. D. Port, Identification of corrosion damage in fossil fuel utility boiler. in Boiler Tube Failure in Fossil Fuel Power Plant, Conference Proceeding, EPRI (1988) 18. A.F. Armor, Boiler tube failure, the number one availability problems for utilities, Failure and inspection of fossil fired boiler tube conference and workshop, EPRI (1983)

Part II

Case Studies

All possible types of boiler tube failure mechanisms have been discussed in detail in section 4.0. Investigation into the probable causes of failure requires gathering of physical evidence or data through a systematic problem analysis. The important parameters that may require determination by calculation, observation or measurement are as follows: • Tube metal temperature • Tube metal stress • Tube metal thickness • Tube metal diameter • Tube metal material properties • Tube metal microstructure • Boiler water quality • Boiler water flow • Fuel constituents • Fuel gas temperature • Tube deposit constituents and • Steam side scale thickness Online monitoring periodic sampling laboratory testing is some of the three ways to quantify these three critical parameters.

Chapter 3

Case 1A: Short-Term Overheating

Power plant authority had experienced a failure of a secondary superheater tube by the way of wide open burst over a localized area and referred the problem to probe into the metallurgical cause/causes of such failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) (ii) (iii) (iv) (v)

Date of commissioning/capacity: September, 1966/140 MW Load at the time of failure: 61 MW Location of failure: Secondary superheater (inlet tube) front side Feed water inlet temperature: 150 °C Secondary superheater steam at the time of failure: Designed pressure 136.5 Kg/cm2 Actual pressure 101 Kg/cm2 Designed temperature 540 °C Actual temperature 402 °C

(vi) Material specification: BS 3059-Part 2-S2-ST-622.50 Cat 2–1968 (vii) Dimension of original tube: 11 /4 inch OD X 0.372 inch thickness

3.1 Scope of Work (i) (ii) (iii) (iv)

Visual inspection Dimensional measurement Chemical analysis of the tube material Evaluation of mechanical properties (a) Hardness measurement (b) Tensile strength at room temperature

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_3

33

34

3 Case 1A: Short-Term Overheating

Fig. 3.1 General view of the burst and typical cracking of scale

(v) Metallographic examination to analyze microstructural features of the tube material (vi) Analysis of findings and identification of probable causes of failure.

3.2 Examination Details 3.2.1 Visual Inspection (i) The tube sample was visually inspected, and following physical evidences were noted. (ii) The tube failed by longitudinal cracking, and the failed zone was accompanied by noticeable swelling of the tube circumstances. The general view of failed tube sample is shown (Fig. 3.1). (iii) There was a thick and tightly adherent scale on the outside surface over the length of the failed tube and which had cracked along the tube axis in a typical manner (Fig. 3.1) (iv) A thick and tightly adherent scale on internal surface over the length of the failed tube was also observed, and the scale had cracked in an identical manner (Fig. 3.2).

3.2.2 Dimensional Measurement Outside diameter (OD) of the failed tube was measured at selected locations and is shown in following Table 3.1.

3.2 Examination Details

35

Fig. 3.2 General view of internal surface of ruptured tube and longitudinal cracking of scale

Table 3.1 Outside diameter of the failed tube sample Locations

OD along the line of rupture with scale (mm)

OD with scale transverse to the line of rupture (mm)

1

46.15

47.40

2

47.50

47.00

3

52.10

50.50

4

56.50

59.80

5

61.00

59.50

6

56.00

57.00

7

58.50

59.50

8

59.50

58.80

Fig. 3.3 Dimensional measurement of the ruptured tube

3.2.3 Chemical Analysis The materials of the failed tube as well as the fresh tube have been chemically analyzed, and analytical results in percentage weight of the major constituent elements are incorporated in Table 3.2. For the sake of comparison, the chemical

36 Table 3.2 Chemical composition

3 Case 1A: Short-Term Overheating

Constituent elements Failed in % Wt tube

Fresh tube

BS: 3059-Pt. II-S2-St 622/50

C

0.15

0.13

0.08–0.15

Si

0.32

0.28

0.50 max

Mn

0.37

0.44

0.40–0.70

Cr

2.40

2.10

2.00–2.50

Mo

0.97

1.05

0.90–1.20

composition of the recommended material under specification BS: 3059-Pt. II-S2-ST 622/50 [1] is also inserted in this table.

3.2.4 Mechanical Properties (Tensile Test) Standard tensile test pieces were prepared from the ruptured tube as well fresh tube. The test pieces were subjected to tensile test to evaluate yield strength, tensile strength and percentage elongation of the tube material, and test results are tabulated in Table 3.3. The tensile properties of the recommended tube material are also inserted in this table for reference.

3.2.5 Metallographic Examination Samples from different locations of the failed tube as well as the fresh tube have been sectioned and subsequently prepared for metallographic examination. The significant structural features associated with the investigation are illustrated through the following micrographs. Micrograph in Fig. 3.4 reveals the presence of double-layer appearance of the tightly adherent porous scale about 1.10 mm thick on the external surface of the ruptured tube. Oxide-filled crack propagating transverse to the tube axis through the scale is also reflected from the figure. Similarly, double-layer appearance of tightly adherent and relatively less porous scale of about 1.00 mm thick on the internal surface of the rupture tube was observed and is illustrated through Figs. 3.5 and 3.6. Oxide-filled cracks propagating transverse to the tube axis through the scale are also evident in these figures. Oxide-filled cracks propagating transverse to the tube axis through the tube wall beyond the internal scale thickness have been detected in the ruptured zone and are represented by Fig. 3.7. Microstructural features through cross section in the vicinity of rupture revealed a mixture of ferrite (light) and bainite (dark) in association with isolated creep cracks

Ultimate tensile strength (MPa) 404 387 384 574 561 572

Mean ± Standard deviation

270 ± 6

428 ± 10

Yield strength (MPa)

264 271 276

435 417 432

Identification of sample

Ruptured tube

Fresh tube

Table 3.3 Tensile properties of the tube samples

569 ± 7

392 ± 11

Mean ± Standard deviation

20 25 24

22 29 24

% Elongation

23 ± 3

25 ± 4

Mean ± Standard deviation

3.2 Examination Details 37

38 Fig. 3.4 Unetched

Fig. 3.5 Unetched

Fig. 3.6 Unetched

3 Case 1A: Short-Term Overheating

3.2 Examination Details

39

Fig. 3.7 Unetched

as well as interlinkage of creep cracks. These microstructural evidences are reported in Figs. 3.8 and 3.9. Microstructural features at adjacent area along the line of rupture are depicted in Fig. 3.10, indicating mixture structure of ferrite (light) and bainite (dark). On the other hand, microstructural features at the area diametrically opposite to rupture exhibit ferrite grain (light) and intense coalescence of fine alloy carbides along the grain boundaries (Fig. 3.11). Microstructural characteristic away from the rupture consists of ferrite (light) grains and agglomeration of fine alloy carbides at the grain boundaries in association with isolated creep voids. These evidences can be seen in Fig. 3.12 illustrating a sharp change in microstructure of the burst tube in service condition from the normal structure of the fresh tube metal consisting of ferrite (light) and pearlite colonies (dark) in Fig. 3.13. Fig. 3.8 Etched in Nital

40 Fig. 3.9 Etched in Nital

Fig. 3.10 Etched in Nital

Fig. 3.11 Etched in Nital

3 Case 1A: Short-Term Overheating

3.2 Examination Details

41

Fig. 3.12 Etched in Nital

Fig. 3.13 Etched in Nital

3.2.6 Estimation of Peak Temperature An attempt has also been made to estimate the temperature at the time of failure based on oxide scale thickness at the internal surface at the location of rupture using the data developed by Rehn et al. correlating the scale thickness with the Larson–Miller (L-M) parameter [2]. Log X = 0.00022 P−7.25. where X = Oxide scale thickness in mils, P = L−M parameter, P = T (20 + Log t).

42

3 Case 1A: Short-Term Overheating

where T = Absolute temperature in ° R. t = Exposure time (hours) in service condition. In this case, X = 40 mils. t = 2, 08,780 hours (Calculating from the data of commissioning including shut-down periods). Hence, log 40 + 7.25 = 40, 237. 0.00022 40, 237 ◦ ◦ = 1590 ◦ R = 1130 F = 610 C. T = 20 + 5.3197 P=

Hence, estimated exposure temperature of the tube based on steam side oxide scale thickness was in the order of 610 °C which is much higher than the design working temperature (520 °C).

3.2.7 Discussion Physical observations on the ruptured tube sample revealed that the tube had failed by a typical narrow open burst accompanied by noticeable plastic deformation. Internal and external adherent scale had cracked in a typical manner in view of its non-ductile nature. These evidences generally reflect that the tube had failed by stress rupture. Thick and tightly adherent scale on fire-side surface of the failed tube was due to oxidation by gas metal reaction. High-temperature or other corrosive species in the flue gas generally lead to an oxidation, metal wastage and eventual failure. Steam side adherent scale had resulted from steam-iron reaction which generally takes place in the temperature range from 450 to 700 °C. The oxides usually grow both inward and outward from the surface to the tube to form a double-layer structure. The oxidation can occur either by inward diffusion of oxygen of the steam or by outward diffusion of iron. Chemical composition of both failed and fresh tubes generally conform to recommend specification BS: 3059, Pt-II-S2, steel 622/50 (Standard specification for steel boilers and superheater tubes). Microscopic examination revealed that the microstructural features in the vicinity of rupture were composed of bainite (dark) and ferrite (light). From the relative amounts of ferrite (light) and bainite (dark), the tube metal temperature at the time of failure, presumably was above its A1 (730 °C) temperature. From this two-phase austenite and ferrite temperature region, the tube metal was cooled rapidly by onrushing steam, transforming the austenite to bainite. Microstructural features at the areas diametrically opposite to rupture as well as away from the rupture exhibited intense agglomeration of fine alloy carbides at the ferrite grain boundaries in association with creep voids. These evidences prove

References

43

beyond doubt that the tube metal had been exposed to a temperature above its safe working limit but below A1 temperature for a prolonged period. Penetration of oxide through the tube wall beyond internal scale at the zone of rupture forming intergranular fissures is also a positive evidence of stress-rupture failure. From the results of the tensile test, it is apparent that the tensile strength of the ruptured tube is below the recommended value, while the tensile strength, yield strength and percentage elongation of the fresh tube are within the limiting respective value as per BS: 3059, Pt-II- S2, steel 622/50. The appreciable drop in tensile strength of the ruptured tube is attributed to structural changes of the material due to long exposure at elevated temperature below A1.

3.2.8 Conclusion for Case Study 1A Reviewing the technical data on the tube failure and assimilating them with physical evidences as well as laboratory experimental results, it is concluded that the tube had failed due to stress rupture. The tube metal had been exposed to a temperature above the safe operating limit for long duration. As a result, the tube metal had lost its strength, suffered plastic deformation and failed to cope with the working stress resulting in final rupture. The adherent scale, particularly on internal surface, had acted as an insulating blanket in view of its much lower thermal conductivity than that of tube metal. The progressive build-up of scale had greatly affected heat transfer and raised the tube metal temperature above A1 for a short duration before bursting of the tube. The overheating in this case may be due to poor circulation of boiler over the localized area. From this investigation study, it is apparent that the healthiness of the ruptured tube over the supplied length had been significantly deteriorated. The tubes should be degraded and replaced. The healthiness of the remaining portion of the subject tube over its length should be thoroughly inspected to ensure its service worthiness.

References 1. BS-3059 Part 2–78, Steel boiler and superheater tubes, Carbon alloy and Austenitic stainless steel tubes with specified elevated temperature properties 2. F.R. Larson, J. Miller, A time-temperature relationship for rupture and creep stresses, Transactions ASME, July (1952)

Chapter 4

Case IB: Short-Term Overheating

Power plant authority had experienced a failure of a water wall tube by the way of wide open burst rupture over a localized area and referred the problem to probe into the metallurgical cause/causes of such failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) (ii) (iii) (iv)

Working temperature: 398 °C Working pressure: 214 kg/cm2 Working temperature : 398 °C Working pressure : 214 kg/cm2

4.1 Scope of Work (i) (ii) (iii) (iv)

Visual inspection. Dimensional measurement (OD and wall thickness measurement). Chemical analysis of the tube material. Metallographic examination and hardness measurement.

4.2 Examination Details 4.2.1 Visual Inspection (i) The failure region consists of wide open burst rupture (Fig. 4.1). (ii) Noticeable bulging/swelling in and around the failure zone of the tube circumference. (iii) Deformation of the tube along the failure side (Fig. 4.2). (iv) Noticeable wall thinning in and around the failed zone. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_4

45

46

4 Case IB: Short-Term Overheating

Fig. 4.1 Failed water wall tube

Fig. 4.2 Cross section of the tube showing deformation

4.2.2 Dimensional Measurement Wall thickness measurements using ultrasonic thickness meter (Type: DM-3, Krautkramer, Germany) and outside diameter measurements using vernier caliper were carried out on supplied tube. The results are self-explanatory and shown in Fig. 4.3.

4.2 Examination Details

47

Fig. 4.3 Outer diameter and wall thickness measurement of water wall tube sample (mm)

Table 4.1 Observed chemical composition (Wt%)

Sample No.

C

Si

Mn

Failed water wall tube

0.112

0.367

0.720

4.2.3 Chemical Analysis Chemical analysis of the tube sample to estimate the weight percentage of the constituent elements was carried out using Spectrograph (Model: Q4 TASMAN, Bruker, Germany). The chemical compositions are detailed in Table 4.1.

4.2.4 Metallographic Examination Metallographic specimens from the supplied tube sample were prepared as per the standard ASTM E-3, 2003 and ASTM E 407–2003. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out in Vickers hardness tester under 20 Kgf load. The microstructural characteristics and hardness values are detailed in Table 4.2.

4.2.5 Discussion (i) Visual observation of the failed water wall tube revealed that the failure is wide open burst associated with considerable bulging/swelling in and around the failure zone. (ii) Dimensional measurement revealed reduction of wall thickness and increase in diameter in and around the failure zone. Considerable deformation along the failure side is also noticed.

48

4 Case IB: Short-Term Overheating

Table 4.2 Details of microstructural characteristics and mean hardness values Sample No.

Location

Microstructural characteristics

Hardness range in Hv

Water wall tube

(A) Adjacent to failure zone

Ferrite and low carbon martensite (Fig. 4.4) Crack originated from outside diameter side of the tube sample (Fig. 4.5)

231

(B) Diametrically opposite to failure zone

Ferrite and pearlite (Fig. 4.6)

153

(C) Away from failure zone Ferrite and pearlite (Fig. 4.7)

148

Fig. 4.4 Etched in Nital

(iii) The chemical composition of the tube sample supplied confirms to the desired specification (i.e., SA 210 Gr.C). (iv) Microstructural examination at the adjacent to failure location of the tube material of the tube revealed ferrite and low carbon martensite along with crack originated from outside diameter side of the tube. This indicates that the temperature at the time of failure was above the upper critical temperature, whereas the microstructures at the opposite to failure and away from failure locations revealed normal ferrite and pearlite. A comparison of the microstructures presented above indicates that the overheating was limited to a localized section of the tube.

4.2 Examination Details

49

Fig. 4.5 Etched in Nital

Fig. 4.6 Etched in Nital

4.2.6 Conclusion for Case Study IB Based on technical information provided and the findings of laboratory study, it is concluded that the tube had failed due to localized short-term overheating. Overheating results from abnormal conditions such as loss of coolant flow and excessive boiler gas temperature.

50 Fig. 4.7 Etched in Nital

4 Case IB: Short-Term Overheating

Chapter 5

Case IIA: High-Temperature Creep

Power plant authority had experienced a failure of a platen superheater tube by the way of wide open burst over a localized area and referred the problem to probe into the metallurgical cause/causes of such failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) (ii) (iii) (iv) (v)

Material specification: BS 3059/ 622–50 S2 Dimension of original tube: 38 mm OD X 5.9 mm thickness Working temperature: 480 °C Working pressure: 158.2 kg/cm2 Running hours: 63,177

5.1 Scope of Work (i) (ii) (iii) (iv)

Visual inspection Dimensional measurement Chemical analysis of the tube material Metallographic examination to analyze microstructural features of the tube material (v) Hardness measurement (vi) Analysis of findings and identification of probable causes of failure.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_5

51

52

5 Case IIA: High-Temperature Creep

Fig. 5.1 General view of burst of platen superheater tube

5.2 Examination Details 5.2.1 Visual Inspection The tube sample was visually inspected, and following physical evidences were noted. (i) The failure is open burst rupture. The general view of failed tube sample is shown (Fig. 5.1). (ii) Appreciable bulging/swelling at and adjacent to failure area. (iii) Reduction in wall thickness at the vicinity of rupture. (iv) Presence of thick black scale on the inner surface of the tube. (v) No significant evidence of corrosion on the outer surface of the tube. (vi) Presence of adherent black scale on the outer surface of the tube.

5.2.2 Dimensional Measurement Wall thickness measurement using thickness meter (type: DM-3, Krautkramer, Germany) and outside diameter measurements using vernier caliper were carried out on the tube sample. The locations of wall thickness and outside diameter measurement are shown in Fig. 5.2, and the measured values are shown in Table 5.1.

Fig. 5.2 Locations of wall thickness and outside diameter measurement of the tube sample

5.2 Examination Details

53

Table 5.1 Measured values of wall thickness and outside diameter of the failed tube sample Tube identification Location (Fig. 5.2) Measured wall thickness (mm)

Measured outside diameter (mm)

On flue gas side On diametrically ‘XX’ opposite to flue gas side Platen superheater A tube B

‘YY’

5.2

6.1

39.62 41.1

2.7

–

54.4

48.6

C

2.3

–

–

–

D

3.9

5.9

40.9

40.5

E

5.5

6.3

39.5

40.0

F

5.6

5.8

–

–

Table 5.2 Chemical composition Tube identification Platen superheater tube

Weight percentage C

Cr

Mo

Ni

Si

Mn

0.09

2.10

0.90

–

–

–

5.2.3 Chemical Analysis The material of the failed tube has been chemically analyzed, and analytical results in percentage weight of the major constituent elements are incorporated in Table 5.2.

5.2.4 Metallographic Examination and Hardness Measurement Metallographic specimens from the failed tube sample were selected, sectioned, polished and etched as and when required as per the standard ASTM E-3, 1995 and ASTM E- 407, 1993. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out. The microstructural characteristics and hardness values are incorporated in Table 5.3.

54

5 Case IIA: High-Temperature Creep

Table 5.3 Microstructural characteristics and mean hardness values Sample

Location of examination

Microstructural characteristics

Platen Adjacent to failure Elongated grains of ferrite and coalescence superheater tube of alloy carbide at ferrite grain boundaries (Fig. 5.3)

Mean hardness value, HV 136

Diametrically Dispersed alloy carbide in ferrite matrix and 128 opposite to failure coalescence of alloy carbide at ferrite grain boundaries along with isolated creep cavities (Fig. 5.4) Away from failure Ferrite, alloy carbide mostly at grain 127 boundaries along with isolated creep cavities at the grain boundaries (Fig. 5.5)

Fig. 5.3 Etched in Nital

5.2.5 Scale Thickness Measurement and Estimation of Tube Metal Temperature Inside diameter scale thickness measurement was carried out on three specimens from failed platen superheater tube. Tube metal temperature of the tube was estimated based on maximum measured oxide scale thickness on ID surface for 21 /4 Cr. 1 Mo steel. The observed maximum scale thickness and estimated tube metal temperatures are given in Table 5.4.

5.2 Examination Details

55

Fig. 5.4 Etched in Nital

Fig. 5.5 Etched in Nital

Table 5.4 Observed ID scale thickness and mean tube metal temperature of the failed platen superheater tube Location of scale thickness measurement

Maximum measured ID scale thickness (mm)

Estimated mean tube metal temperature (°C)

Figure No

(a) Adjacent to rupture of the failed tube

0.60

603

Fig. 5.6

(b) Diametrically opposite to rupture

0.50

594

Fig. 5.7

(c) Away from rupture

0.50

594

Fig. 5.8

56

5 Case IIA: High-Temperature Creep

Fig. 5.6 Unetched

Fig. 5.7 Unetched

5.2.6 Discussion Visual examination revealed that the failure was open burst-type rupture. Thick adherent scale (black in colour) was noticed on the inner surface of the tube sample. Dimensional measurement showed appreciable bulging/ swelling and reduction in wall thickness at and adjacent to location of failure. The chemical composition of the tube material conforms to BS-3059/622–50-S2. Estimated average metal temperature of the tube sample based on maximum ID scale thickness was 603 °C which is much higher than the expected metal temperature (520 °C) of the tube under design conditions.

5.2 Examination Details

57

Fig. 5.8 Unetched

Microstructure revealed material degradation in the way of dispersed alloy carbide in ferrite matrix and coalescence of alloy carbides at the ferrite grain boundaries along with isolated creep cavities. Elongated grains reveal that the material experienced high-temperature deformation at the location of rupture.

5.2.7 Conclusion for Case Study IIA Based on technical information provided and the findings of laboratory study, it is concluded that the tube had failed due to severe creep damage caused by high metal temperature during service. The probable causes of high metal temperature may be in sufficient flow of steam due to partial blockage, presence of thick oxide scale on ID surface, high flue gas temperature, etc.

Chapter 6

Case Study IIB: High-Temperature Creep

Power plant authority had experienced a failure of a bend of secondary superheater pendent tube by the way of through-thickness longitudinal fracture and referred the problem to probe into the metallurgical cause/causes of such failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) (ii) (iii) (iv) (v)

Material specification: TP347 H Dimension of original tube: 53.97 mm ODX 6.01 mm thickness Working temperature: 540 °C Working pressure: 135 kg/cm2 Running hours: 3,00,000

6.1 Scope of Work (i) Visual inspection. (ii) Dimensional measurement (Outside diameter and wall thickness measurement). (iii) Chemical analysis of the tubes material. (iv) Metallographic examination and hardness measurement.

6.2 Examination Details 6.2.1 Visual Inspection (i) The failure region consists of through-thickness longitudinal fracture of length bout 52 mm on the tube sample (Fig. 6.1). © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_6

59

60

6 Case Study IIB: High-Temperature Creep

Fig. 6.1 Secondary superheater pendent tube showing through-thickness longitudinal fracture

(ii) Slight bulging/swelling in and around the failure zone of the tube circumference. (iii) Absence of loose scale on the inner surface of the failed tube. (iv) No noticeable wall thinning was noted at the tube sample.

6.2.2 Dimensional Measurement Wall thickness measurements using ultrasonic thickness meter (Type: DM-3, Krautkramer, Germany) and outside diameter measurements using vernier caliper were carried out on tube. The results are self-explanatory and shown in Fig. 6.2.

6.2.3 Chemical Analysis Chemical analysis of the tube sample to estimate the weight percentage of the constituent elements was carried out using Spectrograph (Model: Q4 TASMAN, Bruker, Germany). The chemical compositions are detailed in Table 6.1.

6.2.4 Metallographic Examination Metallographic specimens from the supplied tube sample were prepared as per the standard ASTM E-3, 2003 and ASTM E 407–2003. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out in Vickers

6.2 Examination Details

61

Fig. 6.2 Outer diameter and wall thickness measurement in mm. of secondary superheater pendent tube

Table 6.1 Observed chemical composition (Wt %) Sample identification

C

Si

Mn

Cr

Mo

Ni

Bend of secondary superheater pendent tube

0.055

0.402

1.656

17.460

–

10.160

Table 6.2 Details of microstructural characteristics and mean hardness values Sample identification

Location

Microstructural characteristics

Hardness in H v

Bend of secondary superheater pendent tube

(A) Adjacent to failure zone

(i) OD oxide-filled intergranular nature of cracks in the vicinity of crack tip (Figs. 6.3 and 6.4) (ii) Presence of slip lines, twinning along with carbide precipitation and micro cracks along the austenite grain boundaries (Figs. 6.5 and 6.6)

185–192

(iii) Diametrically opposite to failure zone

Slip bands along with twin boundaries in austenitic stainless steel (Fig. 6.7)

203–205

(iv) Away from failure zone

Slip bands along with twin boundaries in austenitic stainless steel (Fig. 6.8)

194–207

hardness tester under 20 Kgf load. The microstructural characteristics and hardness values are detailed in Table 6.2.

62

6 Case Study IIB: High-Temperature Creep

Fig. 6.3 Unetched

Fig. 6.4 Etched in ferric chloride

6.2.5 Discussion (i) Visual observation of the failed secondary superheater pendent tube revealed through-thickness longitudinal fracture of length about 52 mm on the tube sample. (ii) The chemical composition of the tube sample confirms to the desired specification (i.e., TP347 H). (iii) Measurement of tube wall thickness revealed no reduction in wall thickness at failed area of the secondary superheater pendent tube but slight localized swelling of the tube, as revealed from the increase in diameter was noticed.

6.2 Examination Details

63

Fig. 6.5 Etched in ferric chloride

Fig. 6.6 Etched in ferric chloride

(iv) Microstructure of the tube sample of secondary superheater pendent tube revealed a multitude of deep oxide-filled cracks on the OD and carbide precipitation along with microcracks along the austenite grain boundaries. The presence of oxide-filled cracks and appreciable amount of carbide precipitation along the austenite grain boundaries suggests that the tube metal temperature was well above the design temperature prior to failure of the tube.

64

6 Case Study IIB: High-Temperature Creep

Fig. 6.7 Etched in ferric chloride

Fig. 6.8 Etched in ferric chloride

Microstructure of the tube sample also revealed presence of slip bands and twin boundaries. Presence of slip lines and twinning reflects that the material experienced high-temperature deformation at the location of failure.

6.2 Examination Details

65

6.2.6 Conclusion for Case Study IIB Secondary superheater pendent tube of had failed due to high-temperature creep or stress rupture. Void formation by grain boundary sliding may be observed in the vicinity of the crack tip. The conclusion is that this tube alone was partially blocked, for reasons unknown, and had been operating at higher than design temperatures for several months or a few years.

Chapter 7

Case Study III: High-Temperature Failure

Power plant authority had experienced a failure of a low-temperature superheater (LTSH) hanger tube by the way of wide-open ductile-type burst (fish mouth type), and the burst was on the centerline of the tube at the highest heat input zone. The problem was referred to probe into the metallurgical cause/causes of failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) (ii) (iii) (iv) (v)

Material specification: SA-210 Gr. C Dimension of original tube: 51 mm ODX 11 mm thick thickness Working temperature: 394 °C Working pressure: 207.3 kg/cm2 Running hours: 5500

7.1 Scope of Work (i) (ii) (iii) (iv) (v)

Visual inspection. Dimensional measurement (outside diameter and wall thickness measurement). Chemical analysis of the tubes material. Metallographic examination and hardness measurement. Preparation and submission of test report based on technical data generated through laboratory tests/examinations.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_7

67

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7 Case Study III: High-Temperature Failure

7.2 Examination Details 7.2.1 Visual Inspection (i) A wide-open ductile-type burst (fish mouth type) (Fig. 7.1). The burst was on the centerline of the tube at the highest heat input zone. (ii) Thick and tightly adherent scale on the outside surface over the length of the failed tube which had cracked along the tube axis. (iii) Thick and tightly adherent scale on the internal surface of the tube. (iv) Noticeable bulging/swelling at failure location (Fig. 7.1). (v) Wall thinning was noted at the failed side of the tube (Fig. 7.2).

Fig. 7.1 LTSH hanger tube showing wide open fish mouth rupture

Fig. 7.2 Wall thickness and outside diameter measurement in mm. of LTSH hanger tube

7.2 Examination Details

69

Table 7.1 Observed chemical composition (Wt%) Sample identification

C

Si

Mn

Cr

Mo

Ni

LTSH hanger tube

0.275

0.359

0.853

0.136

0.015

0.009

7.2.2 Dimensional Measurement Wall thickness measurements using ultrasonic thickness meter (Type: DM-3, Krautkramer, Germany) and outside diameter measurements using vernier caliper were carried out on supplied tube. The results are self-explanatory and shown in Fig. 7.2.

7.2.3 Chemical Analysis Chemical analysis of the tube samples to estimate the weight percentage of the constituent elements was carried out using atomic emission spectrometer. The chemical compositions are detailed in Table 7.1.

7.2.4 Metallographic Examination Metallographic specimens from the supplied tube samples were prepared as per the standard ASTM E-3, 2011 and ASTM E 407–2007. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out in Vickers hardness tester under 20 Kgf load. The microstructural characteristics and hardness values are detailed in Table 7.2.

7.2.5 Discussion (i) Visual observation of the failed LTSH hanger tube revealed wide-open ductiletype rupture (fish mouth type). The rupture was on the centerline of the tube at the highest heat input zone. Thick and tightly adherent scale on the outside surface over the length of the failed tube which had cracked along the tube axis was also observed. (ii) The chemical composition of the tube sample confirms to the desired specification (i.e., SA 210 Gr.C).

70

7 Case Study III: High-Temperature Failure

Table 7.2 Details of microstructural characteristics and mean hardness values Sample identification

Location

LTSH hanger tube

Adjacent to failure zone Elongated grains of ferrite and complete spheroidized carbides along with some subgrains within the grains (Figs. 7.3 and 7.4) Diametrically opposite to failure zone

Microstructural characteristics

Avg. hardness in H v 137

Ferrite and dispersed of 137 spheroidized carbides (Fig. 7.5)

Away from failure zone Ferrite and dispersed of 154 spheroidized carbides (Fig. 7.6)

Fig. 7.3 Etched in Nital

(iii) Measurement of tube wall thickness revealed localized reduction in wall thickness at failed and bulged area of the LTSH hanger tube. In addition, considerable localized swelling of LTSH Hanger tube as revealed from the increase in diameter was noticed. (iv) Microstructure at the adjacent to failure location of the tube sample revealed material degradation in the way of elongated grains of ferrite and complete spheroidized carbides indicating prolonged exposure to relatively high metal temperature. It is also noted that elongated grains and some subgrains within the grains indicate rapid high-temperature deformation. The microstructures at the opposite to failure and away from failure locations also revealed complete

7.2 Examination Details

71

Fig. 7.4 Etched in Nital

Fig. 7.5 Etched in Nital

spheroidized carbides. Hardness values at the adjacent to failure and diametrically opposite to failure location of the tube sample are reduced due to spheroidization of the steel structure.

7.2.6 Conclusion for Case Study III Based on technical information provided and the findings of laboratory study, it is concluded that the tube had failed due to prolonged exposure to high metal temperature. Evidence of complete spheroidization of the steel structure indicates prolonged

72

7 Case Study III: High-Temperature Failure

Fig. 7.6 Etched in Nital

exposure to high metal temperature. The tube metal temperature rose rapidly until the strength of the tube was inadequate to retain the steam pressure, and the tube ruptured. As the tube metal rose, the tube began to bulge when the yield point was reached, the wall thickness decreased, and the stress increased until rupture occurred as a wide-open ductile burst. Exposure to high-temperature results from abnormal conditions such as loss of coolant flow and excessive boiler gas temperature.

Chapter 8

Case Study IV: Erosion Failure

Power plant authority had experienced failure of two wall re-heater tubes by the way of thin-lip rupture/pin hole puncture on the flue gas side of the tubes and referred the problem to probe into the metallurgical cause/causes of failure through systematic diagnostic approach. Technical data relating to the problem are as follows: 1. 2. 3. 4. 5. 6.

Location of the failure: Wall re-heater Material: 12Cr1MoVG Working temperature: 400 °C Working pressure: 41 kg/cm2 Dimension: 50 mm ODX 4.0 mm thickness Running hours: 65,250 h (approx.).

8.1 Scope of Work (i) (ii) (iii) (iv)

Visual examination. Dimensional measurement (OD and wall thickness measurement). Chemical analysis of the tubes material. Metallographic examination and hardness measurement.

8.2 Examination Details 8.2.1 Visual Inspection

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_8

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8 Case Study IV: Erosion Failure

Table 8.1 Details of visual examination Sample No.

Tube identification

Observations

1

Wall re-heater (i) Thin-lip rupture on the flue gas side of the tube (Figs. 8.1 and 8.3) tube (Tube A) (ii) The tube was exposed to the high-velocity steam erupting from the point of failure (iii) Presence of steam wash mark close to rupture region of the tube (Figs. 8.1 and 8.3) (iv) No significant evidence of corrosion and scaling on the outer and inner surface of the tube (v) Noticeable reduction in wall thickness at the vicinity of rupture and entire steam wash portion of the tube (Figs. 8.1, 8.5 and 8.8)

2

Wall re-heater (i) Pin hole puncture on the flue gas side of the tube (Fig. 8.6) tube (Tube B) (ii) The tube was exposed to the high-velocity steam erupting from the point of failure (iii) Presence of steam wash mark at the flue gas side of the tube (Figs. 8.2 and 8.7) (iv) No significant evidence of corrosion and scaling on the outer and inner surface of the tube (v)Noticeable reduction in wall thickness at the steam wash portion of the tube (Figs. 8.2, 8.5, 8.7 and 8.9)

Fig. 8.1 As received wall re-heater tube sample (Tube A)

8.2.2 Dimensional Measurement Wall thickness measurements using ultrasonic thickness meter (Type: DM-3, Krautkramer, Germany) and outside diameter measurements using Vernier caliper were carried out on supplied tubes. The results are self-explanatory and shown in Figs. 8.8 and 8.9.

8.2 Examination Details

75

Fig. 8.2 As received wall re-heater tube sample (Tube B)

Fig. 8.3 Failed wall re-heater tube (Tube A)

8.2.3 Chemical Analysis Chemical analysis of the tube samples to estimate the weight percentage of the constituent elements was carried out using atomic emission spectrometer. The chemical compositions are detailed in Table 8.2.

76 Fig. 8.4 Failed wall re-heater tube (Tube A)

Fig. 8.5 Cross section of the wall re-heater tubes showing wall thinning along with deformation

Fig. 8.6 Wall re-heater tube (Tube B) showing pin hole puncture 6

8 Case Study IV: Erosion Failure

8.2 Examination Details

77

Fig. 8.7 Failed wall re-heater tube (Tube B)

Fig. 8.8 Outer diameter and wall thickness measurement in mm. of wall re-heater tube (Tube A)

8.2.4 Metallographic Examination Metallographic specimens from the supplied tube samples were prepared as per the standard ASTM E-3, 2003 and ASTM E 407-2003. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out in Vickers hardness tester under 20 Kgf load. The microstructural characteristics and hardness values are detailed in Table 8.3.

78

8 Case Study IV: Erosion Failure

Fig. 8.9 Outer diameter and wall thickness measurement in mm. of wall re-heater tube (Tube B) Table 8.2 Observed chemical composition (Wt%) Sample No.

C

Si

Mn

S

P

Cr

Mo

V

Wall re-heater tube (Tube A)

0.10

0.23

0.58

0.016

0.017

1.02

0.27

0.18

Wall re-heater tube (Tube B)

0.11

0.22

0.55

0.018

0.019

1.05

0.27

0.19

Table 8.3 Details of microstructural characteristics and mean hardness values Sample No.

Location

Wall (A) Adjacent to failure zone re-heater tube (Tube A)

Microstructural characteristics

Hardness values in Hv

(i) There is no grain flow at and 143 adjacent to fracture tip (Fig. 8.10) (ii) Ferrite, pearlite and spheroidized carbide (Fig. 8.11)

(B) Diametrically opposite to failure zone

Ferrite, pearlite and sheroidized carbide (Fig. 8.12)

142

(C) Away from failure zone

Ferrite and pearlite (Fig. 8.13)

149

(i) Oxide scale growth inside the tube of 49.03 µm (Fig. 8.14) (ii) Ferrite and partially spheroidized pearlite (Fig. 8.15)

162

(B) Diametrically Opposite to failure zone

Ferrite and partially spheroidized pearlite (Fig. 8.16)

170

(C) Away from failure zone

Ferrite and partially spheroidized pearlite (Fig. 8.17)

165

(A) Adjacent to failure zone Wall re-heater tube (Tube B)

8.2 Examination Details

79

Fig. 8.10 Etched in Nital

Fig. 8.11 Etched in Nital

8.2.5 Discussion (i) Visual observation reveals the presence of seam wash marks on both the tubes. It shows the effects of steam-washed tubes caused by adjacent tube ruptures. The severity of the erosion is observed in both the tubes. The tubes were exposed to the high-velocity steam erupting from the point of failure. (ii) Dimensional measurement of the tubes reveals considerably lower outer diameter in XX' direction of the tubes. Ultrasonic wall thickness measurement indicates significantly lower wall thickness to varying degree along its length. In addition, cross section of the tubes shows flattened outer surface of the tubes.

80

8 Case Study IV: Erosion Failure

Fig. 8.12 Etched in Nital

Fig. 8.13 Etched in Nital

All these observations together suggest that the tubes suffered loss of material from outer surface due to erosion caused by high-velocity steam erupting from the point of failure. (iii) The chemical composition of the tube material confirms to the specification. (iv) Microstructural examination of the tubes material of the tubes revealed ferrite and spheroidized pearlite.

8.2 Examination Details

81

Fig. 8.14 Etched in Nital

Fig. 8.15 Etched in Nital

8.2.6 Conclusion for Case Study IV Tubes in the vicinity of a rupture are often exposed to the high-velocity steam erupting from the point of failure. The high-velocity steam eroded the tubes surface, and the tubes had been thinned down to the point at which the tubes metal could not cope with the working stress in the tubes resulting in eventual failure.

82 Fig. 8.16 Etched in Nital

Fig. 8.17 Etched in Nital

8 Case Study IV: Erosion Failure

Chapter 9

Case Study VA: Dissimilar Metal Weld

Power plant authority had experienced a failure of a secondary superheater element being attachment welded of tubes by the way of puncture over a localized area and referred the problem to probe into the metallurgical cause/causes of such failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) (ii) (iii) (iv) (v)

Material specification: TP347 H Dimension of original tube: 53.97 mm ODX 7.62 mm thickness Working temperature: 540 °C Working pressure: 135 kg/cm2 Running hours: 300,000.

9.1 Scope of Work (i) (ii) (iii) (iv)

Visual inspection. Dimensional measurement (outside diameter and wall thickness measurement). Chemical analysis of the tubes material. Metallographic examination and hardness measurement.

9.2 Examination Details 9.2.1 Visual Inspection (i) The failure consists of puncture at the attachment weld (Fig. 9.1). (ii) Severe dent mark on the tube surface adjacent to the attachment weld joint. (iii) No sign of bulging/swelling adjacent to failure area. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_9

83

84

9 Case Study VA: Dissimilar Metal Weld

Fig. 9.1 Secondary superheater element showing puncture at the attachment weld

(iv) Presence of scale on the inner side of the tubes. (v) No sign of reduction in wall thickness of the tube sample.

9.2.2 Dimensional Measurement Wall thickness measurements using ultrasonic thickness meter (Type: DM-3, Krautkramer, Germany) and outside diameter measurements using Vernier caliper were carried out on supplied tubes. The results are self-explanatory and shown in Fig. 9.2.

9.2.3 Chemical Analysis Chemical analysis of the tube samples to estimate the weight percentage of the constituent elements was carried out using spectrograph (Model: Q4 TASMAN, Bruker, Germany). The chemical compositions are detailed in Table 9.1.

9.2.4 Metallographic Examination Metallographic specimens from the supplied tube samples were prepared as per the standard ASTM E-3, 2003 and ASTM E 407-2003. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out

9.2 Examination Details

85

Fig. 9.2 Outer diameter and wall thickness measurement in mm. of secondary superheater element

Table 9.1 Observed chemical composition (Wt %) Sample identification

C

Si

Mn

Cr

Mo

Ni

Secondary superheater element

0.055

0.459

1.473

17.550

–

10.620

in Vickers hardness tester under 20 Kgf load. The microstructural characteristics and hardness values are detailed in Table 9.2.

9.2.5 Discussion (i) Visual observation of the failed secondary superheater element revealed puncture along with severe dent mark adjacent to the attachment weld. Oxide scale on the inner side of the element was also noticed. (ii) The chemical composition of the tubes material confirms to the desired specification (i.e., TP 347 H). (iii) Dimensional measurement did not show any increase in outside diameter and reduction in wall thickness of the element tubes. (iv) Microstructural examination of the element adjacent to failure location revealed intergranular cracking at the austenite grain boundaries and presence of brittle

86

9 Case Study VA: Dissimilar Metal Weld

Table 9.2 Details of microstructural characteristics and mean hardness values Sample identification

Location

Microstructural characteristics

Hardness in Hv

Secondary superheater element

(A) Adjacent (i) A thin layer of metal carbide along the weld to failure zone interface and intergranular cracking at the austenite grain boundaries (Figs. 9.3 and 9.4) (ii) Oxide wedge on OD surface at the interface between base metal and weld deposit (Fig. 9.5) (iii) Dark band in the stainless steel indicative of carbon diffusion (Fig. 9.6)

193–199

(B) Diametrically opposite to failure zone

Equiaxed grains of austenite and precipitation of carbides at the austenite grain boundaries (Fig. 9.7)

188–191

(C) Away from failure zone

Precipitation of carbides surrounding the 164–170 austenite grains, giving the appearance of a string of pearls (Fig. 9.8)

Fig. 9.3 Unetched

metal carbide layer along the weld interface. Microstructure at the opposite to failure and away from failure location revealed precipitation of carbides surrounding the austenite grains.

9.2.6 Conclusion for Case Study VA Based on technical information provided and the findings of laboratory study, it is concluded that secondary superheater element welded between tubes had failed due to migration of carbon and formation of brittle carbide layer along the weld interface at elevated temperature.

9.2 Examination Details

87

Fig. 9.4 Etched in ferric chloride

Fig. 9.5 Etched in ferric chloride

9.2.7 Suggestion Since welded joints of the elements between austenitic grade stainless steel tubes become susceptible to brittle failure after long service at elevated temperature due to metallurgical reasons, all the similar welded joints, which are in service for a long time, may be checked by dye penetrant/magnetic particle method to ensure their suitability for further service.

88 Fig. 9.6 Unetched

Fig. 9.7 Etched in ferric chloride

9 Case Study VA: Dissimilar Metal Weld

9.2 Examination Details Fig. 9.8 Etched in ferric chloride

89

Chapter 10

Case Study VB: Dissimilar Metal Weld

Power plant authority had experienced a failure of a secondary superheater tube welded between low alloyed ferritic steel and austenitic stainless steel by the way of circumferential continuous crack in heat-affected zone, very close to fusion boundary in stainless steel side. Dissimilar metal joints between low alloy ferritic and austenitic stainless steels are used widely in steam generators of power station. In fossil fuel-fired plants, the primary boiler and heat exchanger tubes made of chromium–molybdenum steels, especially 2.25Cr–1Mo steel, are sometimes welded to austenitic stainless steel tubes used in the high-temperature sections such as superheaters and re-heaters. In this case, failure has occurred in the heat-affected zone (HAZ) of ferritic steel through the propagation of circumferential cracks very close to the fusion boundary (Fig. 10.1). Due to differential thermal expansion between the ferritic and austenitic tubes, failures occur in the ferritic steel heat-affected zone (HAZ). The following facts have been established regarding dissimilar metal weld failures: (a) use of Inconel filler metals results in more weld live than use with other filler metals; (b) failures are brittle, with little evidence of wall thinning, necking, or other deformation; (c) the fracture front occurs at a location one to two grains away from the fusion line in the heat-affected zone of ferritic steel tube and (d) the fracture front is intergranular with austenitic and ferritic filler metals but follows a continuous interface of carbides in the case of Inconel filler metals [1–3]. The problem was referred to probe into the metallurgical cause/causes of failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) Material specification: SA 213 T11 (2.25Cr–1Mo ferritic steel) and SA213 Gr. TP347H (austenitic stainless steel) (ii) Working temperature: 540 °C (iii) Working pressure: 137 kg/cm2 (iv) Welding process: Gas tungsten arc welding (GTAW) (v) Running hours: 223,050 (vi) Capacity of the boiler: 140 MW (vii) Location of failure: Secondary superheater outlet side. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_10

91

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10 Case Study VB: Dissimilar Metal Weld

Fig. 10.1 Failed superheater tubes A, B and C denote the areas from specimens which have been taken for microstructural analysis

10.1 Scope of Work (i) (ii) (iii) (iv)

Visual inspection. Dimensional measurement. Chemical analysis of the tube material. Metallographic examination to analyze microstructural features of the tube material. (v) Analysis of findings and identification of probable causes of failure.

10.2 Examination Details Visual examination of the failed secondary superheater tube was done on the entire welded joint of the tube. It was carried out by the unaided eye and also with a portable magnifying lens. The outside diameter of the failed tube was measured at failure location and also in the unfailed zone. The base metals on both sides of the welded tube were analyzed using spectrograph (Model: Q4TASMAN, Bruker, Germany), and the results are presented in Tables 10.1 and 10.2. In order to study the metallographic examination, three small specimens (marked as A, B and C) from promising areas of the tube were removed and mounted for suitable edge retention. The various locations are shown in Fig. 10.1. Table 10.1 Chemical composition of low alloy ferritic steel side (wt%) Sample identification

C

Cr

Mo

Low alloy ferritic steel side of secondary superheater tube

0.13

2.07

0.88

Table 10.2 Chemical composition of stainless steel side (wt%) Sample identification

C

Cr

Mo

Stainless steel side of secondary superheater tube

0.05

17.40

11.30

10.4 Conclusion for Case Study VB

93

All the mounted specimens were polished with successive grades of emery papers up to 1000 grit size followed by cloth polishing with 1-lm diamond paste. Two specimens (marked as A and B) were etched by ferric chloride, and one specimen (C) was etched by Nital. The microstructures of all the materials were examined using a metallurgical optical microscope (WIDEFIELD METALLOGRAPH, REICHERTJung, MeF3, Austria). Hardness was also measured using Vickers hardness equipment under 20 kg load. Three sets of indentations were taken on specimens A, B and C, and the average values are being reported.

10.3 Discussion A circumferential continuous crack in the HAZ of the weld joint of the tube in the stainless steel side was found during visual examination (Fig. 10.1). The crack was very close to the fusion boundary. Dimensional measurement revealed no reduction in wall thickness or swelling of the tube. The chemical composition of the dissimilar welded tube conformed to the material specifications BS: 3059/622/50 and TP 347H. Metallographic analysis adjacent to failure location (stainless steel side) revealed the microstructure as given in Fig. 10.2a. The microstructure contains intergranular cracking at the austenite grain boundaries and a thin layer of metal carbide along the weld interface. The microstructure away from the failure location (stainless steel side) of the welded tube is shown in Fig. 10.2b. It reveals equiaxed austenite grains. Considerable amount of carbide precipitation has been seen at the austenite grain boundaries. The microstructure away from failure location (low alloy steel side) as shown in Fig. 10.2c exhibits ferrite and alloy carbides at the grain boundaries. Similar observation has been found by earlier investigator [4, 5]. Average hardness values of specimens A, B, and C are found to be 180, 184, and 148 HV, respectively, which are as per recommended values. The microstructural degradation suggests that the welded joint between low alloy (2.25Cr–1Mo) and stainless steels failed due to migration of carbon and formation of brittle carbide layer along the weld interface due to long exposure at elevated temperature.

10.4 Conclusion for Case Study VB 1. Welded joint between low alloy (2.25Cr–1Mo) and stainless steels failed due to migration of carbon and formation of brittle carbide layer along the weld interface during long exposure at elevated temperature.

94

10 Case Study VB: Dissimilar Metal Weld

Fig. 10.2 Microstructure of a failure location adjacent to stainless steel side showing intergranular nature of cracks, b base metal of stainless steel, c base metal of low alloy (2.25Cr–1Mo) steel

2. As welded joints between stainless steel and 2.25Cr–1Mo steel become prone to brittle failure after long service at elevated temperature, all the welded joints, which are in service for a long time, may be checked by dye-penetrant/ radiographic method to ensure their suitability for further service.

References 1. A. Saha, A.K. Shukla, Failure of a secondary superheater tube in a 140 MW thermal power plant. J. Fail. Anal. Prev. 14, 10–12 (2014) 2. Dissimilar Welds in Fossil-Fired Boilers, Report CS 3623 (Electric Power Research Institute, Palo Alto, CA, 1985) 3. Dissimilar Weld Failure Analysis and Development Program, Report CS 4252, vol. 1–9 (Electric Power Research Institute, Palo Alto, CA, 1985) 4. D. French, Metallurgical Failure in Fossil Fired Boilers (Wiley, New York, 1982) 5. S. Ghosh, V. Kain, A. Roy, H. Sivaprasad, S. Tarafdar, K.K. Ray, Deterioration in fracture toughness of 304LN austenitic stainless steel due to sensitization. Metall. Mater. Trans. A 40A, 2938–2949 (2009)

Chapter 11

Case Study VIA: Hydrogen Damage

Power plant authority had experienced a failure of a furnace wall tube by the way of wide-open burst over a localized area and referred the problem to probe into the metallurgical cause/causes of such failure through systematic diagnostic approach.

11.1 Scope of Work (i) Visual inspection. (ii) Chemical analysis of the tube material. (iii) Metallographic examination to analyze microstructural features of the tube material. (vii) Hardness measurement. (iv) Analysis of findings and identification of probable causes of failure.

11.2 Examination Details 11.2.1 Visual Inspection (i) The failure is thick lip wide-open burst (Fig. 11.1). (ii) The maximum width of the failure is about 32 mm, and the length of failure is about 200 mm. (iii) Noticeable bulging/swelling around the failure zone was observed. (iv) No appreciable wall thickness reduction around the failure zone. (v) No adherent deposits on inner side of the tube.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_11

95

96

11 Case Study VIA: Hydrogen Damage

Fig. 11.1 Failed furnace wall tube

Fig. 11.2 Locations of wall thickness and outside diameter measurement of the tube sample

11.2.2 Dimensional Measurement Wall thickness measurement using thickness meter (type: DM-3, Krautkramer, Germany) and outside diameter measurements using Vernier caliper were carried out on the tube sample. The results are self-explanatory and shown in Fig. 11.2.

11.2.3 Chemical Analysis Drilled chips from tube sample were carefully collected and chemically analyzed to estimate the weight percentage of the constituent elements. The chemical compositions are shown in Table 11.1.

11.2 Examination Details Table 11.1 Chemical composition

97

Tube identification

Weight percentage C

Si

Mn

Furnace wall tube

0.15

0.12

0.47

11.2.4 Metallographic Examination and Hardness Measurement Metallographic specimens from the failed tube sample were selected, sectioned, polished and chemically etched as and when required as per the standard ASTM E-3, 1995 and ASTM E- 407, 1993. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out. The microstructural characteristics and hardness values are incorporated in Table 11.2. Table 11.2 Microstructural characteristics and mean hardness values Tube identification

Microstructural features

Mean hardness, Hv

Adjacent to failure zone

(i) Discontinuous cracking (Fig. 11.3) (ii) Discontinuous grain boundary cracking initiated from ID surface and presence of decarburization zone (Fig. 11.4)

121.5

Slightly away from the failure zone

Ferrite, complete breakdown of pearlite in association with intergranular grain boundary cracking and decarburized zone (Fig. 11.5)

–

Diametrically opposite to failure

Ferrite and partial breakdown of pearlite (Fig. 11.6)

126

Away from the failed zone

Ferrite and pearlite (Fig. 11.7)

124.5

Fig. 11.3 Unetched

98

11 Case Study VIA: Hydrogen Damage

Fig. 11.4 Etched in Nital

Fig. 11.5 Etched in Nital

Fig. 11.6 Etched in Nital

11.2.5 Discussion “Thick lip” wide-open burst associated with considerable bulging/swelling in and around the failure zone was found during visual examination (Fig. 11.1). Dimensional measurement revealed no reduction in wall thickness in and around the failure zone. The percentage increase in diameter is found to be 18% around the failure zone, which is evidence by considerable bulging/swelling. The chemical composition of the

11.2 Examination Details

99

Fig. 11.7 Etched in Nital

failed tube conformed to the material specifications. Metallographic analysis adjacent to failure location revealed the microstructure as given in Figs. 11.3 and 11.4. The microstructure contains ferrite, complete breakdown of pearlite in association with discontinuous intergranular cracking and decarburized zone. The microstructure diametrically opposite to failure and away from the failure location of the tube are shown in Figs. 11.6 and 11.7, respectively. It exhibits normal ferrite and pearlite. The microstructure at the failure location as depicted was probably due to hydrogen embrittlement. The carbon in the material reacted with the hydrogen to form methane (CH4 ). Methane was a large molecule, which exerted pressure and caused discontinuous cracking along the grain boundaries. Removal of the carbon from the sample caused the decarburization zone. The hardness measurement revealed no abnormal variation of hardness adjacent to the failure zone.

11.2.6 Conclusion for Case Study VIA Hydrogen damage in the plain carbon steel normally occurs in high heat flux region in the area of flow disruption such as welded joints with backing rings or protrusions, bends or deposits. Hydrogen is caused by operation with low PH water chemistry from ingress of acidic salts through condenser leakage, contamination from chemical cleaning or malfunctioning of chemical control components and concentration of corrosive contaminants within deposits on the inner tube wall. Deposits are formed from feed water system corrosion from condenser in-leakage constituent. Corrective action involves the restoration of boiler water chemistry to the proper values and the consideration of boiler chemical cleaning. Chemical cleaning are performed to remove the internal deposits and to further generation of hydrogen on the tube surface.

Chapter 12

Case Study VIB: Hydrogen Damage

Power plant authority had experienced a failure of a water wall corner panel tube by the way of longitudinal cracking (fracture) on the OD surface of the tube over a localized area and referred the problem to probe into the metallurgical cause/causes of such failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) (ii) (iii) (iv) (v)

Material specification: SA 210 Gr.C Dimension of original tube: 51 mm ODX 5.6 mm thickness Design temperature : 396 °C Design pressure: 214 kg/cm2 Running hours: 14,928.

12.1 Scope of Work (i) Visual inspection. (ii) Chemical analysis of the tube material. (iii) Metallographic examination to analyze microstructural features of the tube material. (iv) Hardness measurement. (v) Analysis of findings and identification of probable causes of failure.

12.2 Examination Details 12.2.1 Visual Inspection (i) Longitudinal cracking (fracture) on the OD surface of the tube (Fig. 12.1). (ii) Presence of gouging mark and deposits on ID surface of the tube (Fig. 12.2). © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_12

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12 Case Study VIB: Hydrogen Damage

Fig. 12.1 As received failed water wall tube

Fig. 12.2 Bi-furcated water wall tube showing gouging marks

(iii) No sign of bulging/swelling of the tube. (iv) Presence of gray scale on OD surface. (v) Appreciable wall thinning in and around thegouged zone (Fig. 12.3).

12.2.2 Dimensional Measurement Wall thickness measurements using ultrasonic thickness meter (Type: DM-3, Krautkramer, Germany) and outside diameter measurements using vernier caliper were carried out on supplied tubes. The results are self-explanatory and shown in Fig. 12.3.

12.2.3 Chemical Analysis Chemical analysis of the failed tube sample to estimate the weight percentage of the constituent elements was carried out using spectrograph (Model: Q4 TASMAN, Bruker, Germany). The chemical compositions are detailed in Table 12.1.

12.2 Examination Details

103

Fig. 12.3 Outer diameter and wall thickness values (in mm) of failed water wall corner panel tube

Table 12.1 Observed chemical composition (Wt %) Sample identification

C

Si

Mn

Cr

Mo

S

P

Failed water wall corner panel tube

0.27

0.23

0.88

0.044

0.017

0.007

0.015

12.2.4 Metallographic Examination Metallographic specimens from the supplied tube samples were prepared as per the standard ASTM E-3, 2003 and ASTM E 407-2003. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out in Vickers hardness tester under 20 Kgf load. The microstructural characteristics and hardness values are detailed in Table 12.2.

12.2.5 Discussion (i) Visual observation of the tube revealed shallow gouging mark on inner surface and presence of deposit in gouged area. (ii) The chemical composition of the tube sample confirms to the desired specification (i.e., SA 210 Gr.C).

104

12 Case Study VIB: Hydrogen Damage

Table 12.2 Details of microstructural characteristics and mean hardness values Sample Location Microstructural characteristics No. Failed water wall corner panel tube

Hardness range in Hv

(A) Adjacent to gouging zone

(i) Presence of numerous discontinuous micro-fissures 114–116 traversing from ID toward OD side at corrosion damaged location (unetched) (Figs. 12.4 and 12.5) (ii) Ferrite, complete breakdown of pearlite in association with intergranular grain boundary cracking initiated from ID surface (etched) (Figs. 12.6 and 12.7)

(B) Away from gouging zone

Banded structure of ferrite and pearlite (Fig. 12.8)

Fig. 12.4 Unetched

Fig. 12.5 Etched in Nital

152–154

12.2 Examination Details Fig. 12.6 Etched in Nital

Fig. 12.7 Etched in Nital

Fig. 12.8 Etched in Nital

105

106

12 Case Study VIB: Hydrogen Damage

(iii) Localized gouging on the inner surface resulted in reduction in wall thickness at the gouged locations. (iv) Microstructural examination revealed ferrite, complete breakdown of pearlite in association with numerous discontinuous micro-fissures along the grain boundaries.

12.2.6 Conclusion for Case Study VIB Presence of localized corrosion and grain boundary micro-fissuring of the tube material on the ID surface suggests that the tube material suffered from hydrogen damage. Failure of the water wall tube may be attributed to localized reduction in wall thickness due to corrosion. Hydrogen damage in the plain carbon steel normally occurs in high heat flux region in the area of flow disruption such as welded joints with backing rings or protrusions, bends or deposits. Hydrogen damage is caused by operation with low PH water chemistry from ingress of acidic salts through condenser leakage, contamination from chemical cleaning or malfunctioning of chemical control components and concentration of the corrosive contaminants within deposits on the inner tube wall. Deposits are formed from feed water system corrosion from condenser in-leakage constituent. Corrective action involves the restoration of boiler water chemistry to the proper values and the consideration of boiler chemical cleaning. Chemical cleaning is performed to remove the internal deposits and to further generation of hydrogen on the tube surface.

Chapter 13

Case Study VIIA: Fireside Corrosion

Power plant authority had experienced a failure of a front water wall tube by the way of through-thickness narrow longitudinal fracture on the tube and referred the problem to probe into the metallurgical cause/causes of such failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) (ii) (iii) (iv) (v)

Material specification: Carbon steel Dimension of original tube: 51 mm ODX 5.6 mm thickness Design temperature: 450 °C Design pressure: 190 kg/cm2 Running hours: 49,000.

13.1 Scope of Work (i) (ii) (iii) (iv) (v)

Visual inspection. Dimensional measurement (outside diameter and wall thickness measurement). Chemical analysis of the tubes material. Metallographic examination and hardness measurement. Preparation and submission of test report based on technical data generated through laboratory tests/examinations.

13.2 Examination Details 13.2.1 Visual Inspection (i) The failure region consists of through-thickness narrow longitudinal fracture on the tube sample (Fig. 13.1). © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_13

107

108

13 Case Study VIIA: Fireside Corrosion

Fig. 13.1 Front water wall tube showing narrow longitudinal fracture along with outside deposits

(ii) Thick fireside corrosion deposits on the outer and inner surface of the tube sample (Figs. 13.1 and 13.3). (iii) Slight bulging/swelling at failure location (Figs. 13.1 and 13.2). (iv) Wall thinning was noted at the failed side of the tube (Fig. 13.4). Fig. 13.2 Front water wall tube showing swelling/ bulging

Fig. 13.3 Front water wall tube showing internal deposits

13.2 Examination Details

109

Fig. 13.4 Outer diameter and wall thickness measurement in mm of front water wall tube

13.2.2 Dimensional Measurement Wall thickness measurements using ultrasonic thickness meter (Type: DM-3, Krautkramer, Germany) and outside diameter measurements using vernier caliper were carried out on supplied tubes. The results are self-explanatory and shown in Fig. 13.4.

13.2.3 Chemical Analysis Chemical analysis of the tube samples to estimate the weight percentage of the constituent elements was carried out using spectrograph (Model: Q4 TASMAN, Bruker, Germany). The chemical compositions are detailed in Table 13.1.

110

13 Case Study VIIA: Fireside Corrosion

Table 13.1 Observed chemical composition (Wt %) Sample identification

C

Mn

Si

S

P

Cr

Ni

Cu

Front water wall tube

0.261

0.75

0.254

0.0094

0.019

0.131

0.110

0.234

Table 13.2 Details of microstructural characteristics and mean hardness values Sample identification

Location

Microstructural characteristics

Avg. hardness in Hv

Front water wall tube

(A) Adjacent to failure zone

(i) Penetration of oxide-filled cracks from 123 OD side toward tube metal wall (Figs. 13.5, 13.6 and 13.7) (ii) Furnace wall corrosion deposits of thickness 221.93 µm (max.) of the tube metal (Figs. 13.6 and 13.7)

(B) Diametrically opposite to failure zone

Penetration of oxide-filled cracks from OD side toward tube metal wall along with ferrite and pearlite (Fig. 13.8)

156

(C) Away from failure zone

Penetration of oxide-filled cracks from OD side toward tube metal wall along with ferrite and pearlite (Fig. 13.9)

152

13.2.4 Metallographic Examination Metallographic specimens from the supplied tube samples were prepared as per the standard ASTM E-3, 2003 and ASTM E 407-2003. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out in Vickers hardness tester under 20 Kgf load. The microstructural characteristics and hardness values are detailed in Table 13.2.

13.2.5 Discussion (i) Visual observation of the failed front water wall tube revealed throughthickness narrow longitudinal fracture along with penetration of corrosion products on the fireside. Thick deposit on the fireside of the tube was also noticed. (ii) The chemical composition of the tube sample confirms to the desired specification. (iii) Dimensional measurement revealed reduction in wall thicknesses and slight swelling of the tube sample.

13.2 Examination Details

111

Fig. 13.5 Etched in Nital

Fig. 13.6 Unetched

(iv) Microstructure of the tube sample of front water wall tube revealed corrosion products along with penetration of oxide-filled cracks from fireside toward the tube metal surface. The microstructure at the opposite to failure location and away from failure location also revealed penetration of oxide-filled crack from fireside toward the tube metal surface along with ferrite and pearlite.

112

13 Case Study VIIA: Fireside Corrosion

Fig. 13.7 Unetched

Fig. 13.8 Etched in Nital

13.2.6 Conclusion for Case Study VIB Penetration of the corrosion products on the fireside of front water wall tubes suggests that the tubes had suffered from severe fireside corrosion attack. In the most severe case, the wall punctured and a steam leak resulted.

13.2 Examination Details Fig. 13.9 Etched in Nital

113

Chapter 14

Case Study VIIB: Fireside Corrosion

Power plant authority had experienced a failure of a pendent re-heater tube by the way of open rupture along with deep material dislodged at the bend portion of tube and referred the problem to probe into the metallurgical cause/causes of such failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) (ii) (iii) (iv) (v)

Material specification: BS 3059 PT2-1990-622-490-S2-CAT2 Dimension of original tube: 47.6 mm ODX 3.6 mm thickness Design temperature: 540 °C Design pressure: 25.10 kg/cm2 Running hours: 235,298.

14.1 Scope of Work (vi) Visual inspection. (vii) Dimensional measurement (outside diameter and wall thickness measurement). (viii) Chemical analysis of the tubes material. (ix) Metallographic examination and hardness measurement. (x) Preparation and submission of test report based on technical data generated through laboratory tests/examinations.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_14

115

116

14 Case Study VIIB: Fireside Corrosion

Fig. 14.1 As received tube sample

Fig. 14.2 Pendent re-heater tube showing deep corrosion marks and deposits on the outer surface of the tube sample

14.2 Examination Details 14.2.1 Visual Inspection (i) The failure is open rupture along with deep material dislodged at the bend portion of tube sample (Figs. 14.1 and 14.2). (ii) Thick scale on the external surface of the tube (Fig. 14.2). (iii) No sign of bulging/swelling adjacent to failure area. (iv) Appreciable corrosion marks and deposits on the outer surface of the tube sample (Figs. 14.1 and 14.2). (v) Reduction of wall thickness due to removal of oxide scale at the vicinity of failure (Fig. 14.3).

14.2.2 Dimensional Measurement Wall thickness measurements using ultrasonic thickness meter (Type: DM-3, Krautkramer, Germany) and outside diameter measurements using vernier caliper were carried out on supplied tube. The results are self-explanatory and shown in Fig. 14.3.

14.2 Examination Details

117

Fig. 14.3 Outer diameter and wall thickness measurement in mm of pendent re-heater tube

14.2.3 Chemical Analysis Chemical analysis of the tube samples to estimate the weight percentage of the constituent elements was carried out using spectrograph (Model: Q4 TASMAN, Bruker, Germany). The chemical compositions are detailed in Table 14.1. Table 14.1 Observed chemical composition (Wt %) Sample identification

C

Si

Mn

S

P

Cr

Mo

Ni

Pendent re-heater tube

0.086

0.266

0.560

0.018

0.012

2.23

1.03

0.130

118

14 Case Study VIIB: Fireside Corrosion

14.2.4 Metallographic Examination Metallographic specimens from the supplied tube samples were prepared as per the standard ASTM E-3, 2003 and ASTM E 407-2003. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out in Vickers hardness tester under 20 Kgf load. The microstructural characteristics and hardness values are detailed in Table 14.2. Table 14.2 Details of microstructural characteristics and mean hardness values Sample identification

Location

Pendent re-heater (A) Adjacent to tube failure zone

Microstructural characteristics

Average hardness in Hv

(i) Typical OD side corrosion (Fig. 14.4) (ii) Ferrite and complete spheroidization of carbides along with dispersion of carbides along the grain boundaries and grain body (Fig. 14.5)

130

(B) Diametrically (i) Penetration of oxide-filled cracks from 134 opposite to failure OD side toward tube metal wall Max. oxide zone scale = 392.05 µm (Fig. 14.6) (ii) Ferrite and complete spheroidization of carbides along with dispersion of carbides along the grain boundaries and grain body (Fig. 14.7) (C) Away from failure zone

Fig. 14.4 Unetched

(i) Grain-boundary attack on the OD side and spheroidized carbides (Fig. 14.8) (ii) Ferrite and complete spheroidization of carbides along the grain boundaries and grain body (Fig. 14.9)

132

14.2 Examination Details Fig. 14.5 Etched in Nital

Fig. 14.6 Unetched

Fig. 14.7 Etched in Nital

119

120

14 Case Study VIIB: Fireside Corrosion

Fig. 14.8 Etched in Nital

Fig. 14.9 Etched in Nital

14.2.5 Discussion (i) Visual observation of the failed pendent re-heater tube revealed open rupture along with deep material dislodged at the bend portion of tube sample. Thick deposit along with corrosion marks on the OD side/fireside of the tube was also noticed. (ii) Dimensional measurement revealed reduction in wall thickness adjacent to failure location. (iii) The chemical composition of the tube sample confirms to the desired specification. (iv) Microstructural examination at the adjacent to failure location of the tube material of the tube revealed typical fireside/OD side corrosion along with complete spheroidization of the carbides along the grain boundaries and grain body. The microstructures at the opposite to failure and away from failure location revealed penetration of oxide-filled cracks from fireside toward the tube metal surface along with complete spheroidization of the carbides along the grain boundaries. Complete spheroidization of carbides indicates exposure to elevated temperature for prolonged period of time.

14.2 Examination Details

121

14.2.6 Conclusion for Case Study VIIB Penetration of the corrosion products on the fireside/OD side of pendent re-heater tube suggests that the tube had suffered from severe fireside corrosion attack. In the most severe case, the wall punctured and a steam leak resulted.

Chapter 15

Case Study VIII: Failure Due to Manufacturing Defect

Power plant authority had experienced a failure of a water wall bend tube by the way of through-thickness longitudinal cracking at the extrados portion of the tube and referred the problem to probe into the metallurgical cause/causes of failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) (ii) (iii) (iv) (v)

Material specification: SA-210 Gr. C Dimension of original tube: 51 mm ODX 5.6 mm thickness Design temperature: 396 °C Design pressure: 214 kg/cm2 Running hours: 5100.

15.1 Scope of Work (i) (ii) (iii) (iv)

Visual inspection. Dimensional measurement (OD and wall thickness measurement). Chemical analysis of the tube material. Metallographic examination to analyze microstructural features of the tube material. (v) Hardness measurement. (vi) Analysis of findings and identification of probable causes of failure.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_15

123

124

15 Case Study VIII: Failure Due to Manufacturing Defect

15.2 Examination Details 15.2.1 Visual Inspection (i) The failure region consists of through-thickness longitudinal cracking at the extrados portion of the tube sample (Fig. 15.1). (ii) No noticeable bulging/swelling in and around the failure zone of the tube circumference. (iii) Absence of loose scale on the inner surface of the failed tube. (iv) Wall thinning was noted at the extrados portion of the tube sample (Fig. 15.2).

Fig. 15.1 Water wall bend tube showing through-thickness longitudinal cracking

Fig. 15.2 Wall thickness and outside diameter measurement in mm of water wall bend tube

15.2 Examination Details

125

Table 15.1 Observed chemical composition (wt%) Sample identification

C

Si

Mn

Cr

Mo

Ni

Water wall bend tube

0.308

0.285

0.826

0.108

0.039

0.065

15.2.2 Dimensional Measurement Wall thickness measurements using ultrasonic thickness meter (Type: DM-3, Krautkramer, Germany) and outside diameter measurements using vernier caliper were carried out on supplied tubes. The results are self-explanatory and shown in Fig. 15.2.

15.2.3 Chemical Analysis Chemical analysis of the tube samples to estimate the weight percentage of the constituent elements was carried out using atomic emission spectrometer. The chemical compositions are detailed in Table 15.1.

15.2.4 Metallographic Examination Metallographic specimens from the supplied tube samples were prepared as per the standard ASTM E-3, 2011 and ASTM E 407-2007. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out in Vickers hardness tester under 20 Kgf load. The microstructural characteristics and hardness values are detailed in Table 15.2. Table 15.2 Details of microstructural characteristics and mean hardness values Sample identification

Location

Microstructural characteristics

Hardness in Hv

Banded structure of ferrite and pearlite (Fig. 15.3)

176

Diametrically opposite to Banded structure of ferrite and failure zone lamellar pearlite (Fig. 15.4)

184

Away from failure zone

184

Water wall bend Adjacent to failure zone tube

Banded structure of ferrite and lamellar pearlite (Fig. 15.5)

126

15 Case Study VIII: Failure Due to Manufacturing Defect

Fig. 15.3 Etched in Nital

Fig. 15.4 Etched in Nital

15.2.5 Discussion (i) Visual observation of the failed water wall bend tube revealed throughthickness longitudinal cracking on the extrados portion of the tube sample. (ii) The chemical composition of the tube sample confirms to the desired specification (i.e., SA 210 Gr.C). (iii) Measurement of tube wall thickness revealed reduction in wall thickness at extrados portion of the water wall bend tube. No swelling/bulging of the tube was noticed. (iv) Microstructure of the tube sample of water wall bend tube revealed banded structure of ferrite and pearlite. Segregation of alloying elements during the

15.2 Examination Details

127

Fig. 15.5 Etched in Nital

initial stages of steel making changes the carbon diffusion rate to give regions of varying carbon content in the austenite. Subsequent ingot rolling and metal processing elongate these regions, and the final structure has bands of pearlite and ferrite.

15.2.6 Conclusion for Case Study VIII The failure at the extrados or outermost portion of the tube may be due to bending of the tube at manufacturing stage. Bending produces changes in tube wall thickness. During the bending process, the outer wall was put into tension; hence, wall thinning would occur. Since the inner wall was in compression, the original wall thickness increased. Because the wall thickness on the extrados, or outermost, side of the bend was at the smaller wall thickness, this resulted to a larger hoop stress, leading to failure of the tube.

Chapter 16

Case Study IX: Corrosion Fatigue

Power plant authority had experienced a failure of a economizer tube by the way of typical crack at two locations opposite to each other of the tube and referred the problem to probe into the metallurgical cause/causes of failure through systematic diagnostic approach. Technical data relating to the problem are as follows: (i) Material specification: BS3059/45S2 (ii) Dimension of original tube: 38.1 mm ODX 3.6 mm thickness (iii) Working temperature: 374 °C.

16.1 Scope of Work (i) (ii) (iii) (iv) (v) (vi)

Visual inspection. Dimensional measurement (OD and wall thickness measurement). Chemical analysis of the tubes material. Metallographic examination and hardness measurement. Scanning electron microscopic with elemental analysis. Preparation of report.

16.2 Examination Details 16.2.1 Visual Inspection (i) The failure is typical crack at two locations opposite to each other. The hair line crack is associated with corrosive marks (Fig. 16.1). (ii) Slight reduction in wall thickness is noted and no swelling/bulging sign on the tube (Fig. 16.1). © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 A. Saha, Boiler Tube Failure Mechanisms, Springer Tracts in Mechanical Engineering, https://doi.org/10.1007/978-981-99-3130-9_16

129

130 Fig. 16.1 Economizer tube showing failure zone (crack)

16 Case Study IX: Corrosion Fatigue

Crack

16.2.2 Dimensional Measurement Wall thickness measurements using ultrasonic thickness meter (Type: DM-3, Krautkramer, Germany) and outside diameter measurements using vernier caliper were carried out on supplied tubes. The results are self-explanatory and shown in Fig. 16.2.

Fig. 16.2 Outer diameter and wall thickness measurement in mm. of failed economizer tube

16.2 Examination Details

131

Table 16.1 Observed chemical composition (wt%) Sample No.

C

Si

Mn

S

P

Fe

Economizer tube

0.154

0.158

1.03

0.018

0.018

Balance

Specification of BS3059

0.16

0.10–0.35

0.40–0.80

0.035

0.035

Balance

Table 16.2 Details of microstructural characteristics and mean hardness values Sample No./location

Microstructural characteristics

Hardness values in Hv

Economizer tube (adjacent to crack)

Ferrite and banded pearlite (Figs. 16.3, 16.4 and 16.5)

156–162

16.2.3 Chemical Analysis Chemical analysis of the tube samples to estimate the weight percentage of the constituent elements was carried out using atomic emission spectrometer (modelMetaVision 1008i3 ). The chemical compositions are detailed in Table 16.1.

16.2.4 Metallographic Examination Metallographic specimens from the supplied tube samples were prepared as per the standard ASTM E-3, 2003 and ASTM E 407-2003. All the specimens were examined under optical microscope for evaluating microstructural characteristics. The hardness measurements of the metallographic specimens were also carried out in Vickers hardness tester under 20 kgf load. The microstructural characteristics and hardness values are detailed in Table 16.2.

16.2.5 SEM/EDX Analysis The SEM/EDX is carried out on the crack surface. The result indicates corrosion deposits inside the crack which reveal some corrosive constituents like sulfur and chlorine.

16.2.6 Discussion (i) Visual examination reveals crack from inner surface along the transverse and cross section of the tube. The outer diameter shows normal value, while slight reduction in wall thickness is noticed.

132

16 Case Study IX: Corrosion Fatigue

Fig. 16.3 Showing corrosion deposits inside the crack

Fig. 16.4 Shows the presence of corrosion deposits inside the crack

(ii) Visual inspection shows typical hair line crack with the presence of corrosion deposits inside the crack. (iii) The microstructure analysis adjacent to the crack shows normal microstructure of ferrite and banded pearlite. The hardness survey also shows normal value. The microstructure also shows the presence of corrosion deposits inside the crack. (iv) SEM/EDX of the corrosion deposits inside the crack reveals the presence of corrosive constituents like chlorine and sulfur. Simultaneous presence of chloride and sulfur element aggravates the corrosion phenomena due to cyclic stress and causes the failure of the tube. The source of sulfur and chlorine at the interface of the crack, however, could not be ascertained. The cracking of the tube

16.2 Examination Details

133

Fig. 16.5 Etched in Nital, shows corrosion at the inner surface

Fig. 16.6 SEM and corresponding EDX on the crack of failed economizer tube

Table 16.3 Details of elemental analysis on crack

Elements

wt%

O

10.40

S

0.28

Cl

1.07

Fe

88.25

Total

100.00

is possibly due to corrosion fatigue initiated from internal surface and rupture until crack propagation reaches the external surface causing leakage/hair line cracks.

134

16 Case Study IX: Corrosion Fatigue

16.2.7 Conclusion for Case Study IX The failure of the economizer tube is possibly due to corrosion fatigue initiated by corrosive constituents like sulfur (S) and chlorine (Cl).

16.3 Conclusion In spite of the best efforts of design engineers and material scientists, engineering components fail in service. In some cases, failure may lead to serious consequences like huge financial loss, environmental contamination and even loss of life. The failure of industrial boilers has been a prominent feature in fossil fuel-fired power plants. The contribution of several factors appears to be responsible for failures, culminating in the partial or complete shutdown of the plant. There are several mechanisms of boiler tube failure, including short-term overheating, high-temperature creep, hightemperature failure, caustic corrosion, hydrogen damage, dissimilar weld failure, manufacturing defects, corrosion fatigue, etc. These mechanisms can lead to various types of tube failures, such as fish mouth type rupture, small fracture opening at the apex of bulge, thick-edged fracture, thin-edged fracture, window type opening, thick edge split type fracture, cracks and pin hole type leaks, which can cause catastrophic incidents. In the event of a failure, it is therefore essential to investigate the root cause of failure in terms of design and quality of material and fabrication procedure. A failure analyst must have an open mind and be ready to examine and evaluate the views of other involved in operation of power plant. There are innumerable case studies which have been reported, and case studies reported indicated that actual mechanism of boiler tube failure can be identified through systematic metallurgical investigation. Based on the mechanism of failure, sequence of events that led to failure can be drawn, and corrective actions can be suggested for preventing re-occurrence of similar failure. Hence, it may be concluded that investigation into the cause of boiler tube failure is very much helpful in improving the availability and reliability of boilers.