WELDING METALLURGY SECOND EDITION WELDING METALLURGY SECOND EDITION Sindo Kou Professor and Chair Department of Materials Science and Engineering University of Wisconsin A JOHN WILEY & SONS,INC.,PUBLICATION Copyright © 2003 by John Wiley & Sons,Inc.All rights reserved. Published by John Wiley & Sons,Inc.,Hoboken,New Jersey. Published simultaneously in Canada. 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Wiley also publishes its books in a variety of electronic formats.Some content that appears in print,however,may not be available in electronic format. Library of Congress Cataloging-in-Publication Data Kou,Sindo. Welding metallurgy / Sindo Kou.–2nd ed. p.cm. “A Wiley-Interscience publication.” Includes bibliographical references and index. ISBN 0-471-43491-4 1. Welding. 2. Metallurgy. 3. Alloys. I. Title. TS227 .K649 2002 671.5¢2–dc21 2002014327 Printed in the United States of America. 10 9 8 7 6 5 4 3 2 1 To Warren F.Savage for his outstanding contributions to welding metallurgy CONTENTS Preface xiii I INTRODUCTION 1 1 Fusion Welding Processes 3 1.1 Overview 3 1.2 Oxyacetylene Welding 7 1.3 Shielded Metal Arc Welding 11 1.4 Gas–Tungsten Arc Welding 13 1.5 Plasma Arc Welding 16 1.6 Gas–Metal Arc Welding 19 1.7 Flux-Core Arc Welding 22 1.8 Submerged Arc Welding 22 1.9 Electroslag Welding 24 1.10 Electron Beam Welding 27 1.11 Laser Beam Welding 29 References 33 Further Reading 34 Problems 34 2 Heat Flow in Welding 37 2.1 Heat Source 37 2.2 Analysis of Heat Flow in Welding 47 2.3 Effect of Welding Parameters 53 2.4 Weld Thermal Simulator 58 References 60 Further Reading 62 Problems 62 3 Chemical Reactions in Welding 65 3.1 Overview 65 3.2 Gas–Metal Reactions 68 3.3 Slag–Metal Reactions 82 References 92 vii viii CONTENTS Further Reading 95 Problems 95 4 Fluid Flow and Metal Evaporation in Welding 97 4.1 Fluid Flow in Arcs 97 4.2 Fluid Flow in Weld Pools 103 4.3 Metal Evaporation 114 4.4 Active Flux GTAW 116 References 117 Further Reading 119 Problems 120 5 Residual Stresses,Distortion,and Fatigue 122 5.1 Residual Stresses 122 5.2 Distortion 126 5.3 Fatigue 131 5.4 Case Studies 137 References 140 Further Reading 141 Problems 141 II THE FUSION ZONE 143 6 Basic Solidification Concepts 145 6.1 Solute Redistribution during Solidification 145 6.2 Solidification Modes and Constitutional Supercooling 155 6.3 Microsegregation and Banding 160 6.4 Effect of Cooling Rate 163 6.5 Solidification Path 166 References 167 Further Reading 168 Problems 169 7 Weld Metal Solidification I:Grain Structure 170 7.1 Epitaxial Growth at Fusion Boundary 170 7.2 Nonepitaxial Growth at Fusion Boundary 172 7.3 Competitive Growth in Bulk Fusion Zone 174 7.4 Effect of Welding Parameters on Grain Structure 174 7.5 Weld Metal Nucleation Mechanisms 178 7.6 Grain Structure Control 187 CONTENTS ix References 195 Further Reading 197 Problems 197 8 Weld Metal Solidification II:Microstructure within Grains 199 8.1 Solidification Modes 199 8.2 Dendrite and Cell Spacing 204 8.3 Effect of Welding Parameters 206 8.4 Refining Microstructure within Grains 209 References 213 Further Reading 213 Problems 214 9 Post-Solidification Phase Transformations 216 9.1 Ferrite-to-Austenite Transformation in Austenitic Stainless Steel Welds 216 9.2 Austenite-to-Ferrite Transformation in Low-Carbon, Low-Alloy Steel Welds 232 References 239 Further Reading 241 Problems 241 10 Weld Metal Chemical Inhomogeneities 243 10.1 Microsegregation 243 10.2 Banding 249 10.3 Inclusions and Gas Porosity 250 10.4 Inhomogeneities Near Fusion Boundary 252 10.5 Macrosegregation in Bulk Weld Metal 255 References 260 Further Reading 261 Problems 261 11 Weld Metal Solidification Cracking 263 11.1 Characteristics,Cause,and Testing 263 11.2 Metallurgical Factors 268 11.3 Mechanical Factors 284 11.4 Reducing Solidification Cracking 285 11.5 Case Study:Failure of a Large Exhaust Fan 295 References 296 Further Reading 299 Problems 299 x CONTENTS III THE PARTIALLY MELTED ZONE 301 12 Formation of the Partially Melted Zone 303 12.1 Evidence of Liquation 303 12.2 Liquation Mechanisms 304 12.3 Directional Solidification of Liquated Material 314 12.4 Grain Boundary Segregation 314 12.5 Grain Boundary Solidification Modes 316 12.6 Partially Melted Zone in Cast Irons 318 References 318 Problems 319 13 Difficulties Associated with the Partially Melted Zone 321 13.1 Liquation Cracking 321 13.2 Loss of Strength and Ductility 328 13.3 Hydrogen Cracking 328 13.4 Remedies 330 References 336 Problems 338 IV THE HEAT-AFFECTED ZONE 341 14 Work-Hardened Materials 343 14.1 Background 343 14.2 Recrystallization and Grain Growth in Welding 347 14.3 Effect of Welding Parameters and Process 349 References 351 Further Reading 352 Problems 352 15 Precipitation-Hardening Materials I:Aluminum Alloys 353 15.1 Background 353 15.2 Al–Cu–Mg and Al–Mg–Si Alloys 359 15.3 Al–Zn–Mg Alloys 367 15.4 Friction Stir Welding of Aluminum Alloys 370 References 371 Further Reading 372 Problems 372 16 Precipitation-Hardening Materials II:Nickel-Base Alloys 375 16.1 Background 375 CONTENTS xi 16.2 Reversion of Precipitate and Loss of Strength 379 16.3 Postweld Heat Treatment Cracking 384 References 390 Further Reading 392 Problems 392 17 Transformation-Hardening Materials:Carbon and Alloy Steels 393 17.1 Phase Diagram and CCT Diagrams 393 17.2 Carbon Steels 396 17.3 Low-Alloy Steels 404 17.4 Hydrogen Cracking 410 17.5 Reheat Cracking 418 17.6 Lamellar Tearing 422 17.7 Case Studies 425 References 427 Further Reading 429 Problems 430 18 Corrosion-Resistant Materials:Stainless Steels 431 18.1 Classification of Stainless Steels 431 18.2 Austenitic Stainless Steels 433 18.3 Ferritic Stainless Steels 446 18.4 Martensitic Stainless Steels 449 18.5 Case Study:Failure of a Pipe 451 References 452 Further Reading 453 Problems 454 Index 455 PREFACE Since the publication of the first edition of this book in 1987,there has been much new progress made in welding metallurgy.The purpose for the second edition is to update and improve the first edition.Examples of improvements include (1) much sharper photomicrographs and line drawings;(2) integration of the phase diagram,thermal cycles,and kinetics with the microstructure to explain microstructural development and defect formation in welds;and (3) additional exercise problems.Specific revisions are as follows. In Chapter 1 the illustrations for all welding processes have been re- drawn to show both the overall process and the welding area. In Chapter 2 the heat source efficiency has been updated and the melting efficiency added. Chapter 3 has been revised extensively, with the dissolution of atomic nitrogen, oxygen, and hydrogen in the molten metal considered and electrochemical reactions added.Chapter 4 has also been revised extensively, with the arc added, and with flow visualization, arc plasma dragging, and turbulence included in weld pool convection. Shot peening is added to Chapter 5. Chapter 6 has been revised extensively, with solute redistribution and microsegregation expanded and the solidification path added.Chapter 7 now includes nonepitaxial growth at the fusion boundary and formation of non- dendritic equiaxed grains.In Chapter 8 solidification modes are explained with more illustrations. Chapter 9 has been expanded significantly to add ferrite formation mechanisms,new ferrite prediction methods,the effect of cooling rate, and factors affecting the austenite–ferrite transformation. Chapter 10 now includes the effect of both solid-state diffusion and dendrite tip under- cooling on microsegregation. Chapter 11 has been revised extensively to include the effect of eutectic reactions, liquid distribution, and ductility of the solidifying metal on solidification cracking and the calculation of fraction of liquid in multicomponent alloys. Chapter 12 has been rewritten completely to include six different liquation mechanisms in the partially melted zone (PMZ),the direction and modes of grain boundary (GB) solidification,and the resultant GB segregation.Chapter 13 has been revised extensively to include the mechanism of PMZ cracking and the effect of the weld-metal composition on cracking. Chapter 15 now includes the heat-affected zone (HAZ) in aluminum– lithium–copper welds and friction stir welds and Chapter 16 the HAZ of Inconel 718.Chapter 17 now includes the effect of multiple-pass welding on xiii