WELDING METALLURGY SECOND EDITION

WELDINGMETALLURGYSECOND EDITION

Sindo KouProfessor and ChairDepartment of Materials Science and EngineeringUniversity of Wisconsin

A JOHN WILEY & SONS, INC., PUBLICATION

Copyright © 2003 by John Wiley & Sons, Inc. All rights reserved.

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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 0-471-43491-41. Welding. 2. Metallurgy. 3. Alloys. I. Title.TS227 .K649 2002671.5¢2–dc21

2002014327

Printed in the United States of America.

10 9 8 7 6 5 4 3 2 1

To Warren F. Savagefor his outstanding contributions to welding metallurgy

CONTENTS

Preface xiii

I INTRODUCTION 1

1 Fusion Welding Processes 3

1.1 Overview 31.2 Oxyacetylene Welding 71.3 Shielded Metal Arc Welding 111.4 Gas–Tungsten Arc Welding 131.5 Plasma Arc Welding 161.6 Gas–Metal Arc Welding 191.7 Flux-Core Arc Welding 221.8 Submerged Arc Welding 221.9 Electroslag Welding 241.10 Electron Beam Welding 271.11 Laser Beam Welding 29

References 33Further Reading 34Problems 34

2 Heat Flow in Welding 37

2.1 Heat Source 372.2 Analysis of Heat Flow in Welding 472.3 Effect of Welding Parameters 532.4 Weld Thermal Simulator 58

References 60Further Reading 62Problems 62

3 Chemical Reactions in Welding 65

3.1 Overview 653.2 Gas–Metal Reactions 683.3 Slag–Metal Reactions 82

References 92

vii

Further Reading 95Problems 95

4 Fluid Flow and Metal Evaporation in Welding 97

4.1 Fluid Flow in Arcs 974.2 Fluid Flow in Weld Pools 1034.3 Metal Evaporation 1144.4 Active Flux GTAW 116

References 117Further Reading 119Problems 120

5 Residual Stresses, Distortion, and Fatigue 122

5.1 Residual Stresses 1225.2 Distortion 1265.3 Fatigue 1315.4 Case Studies 137

References 140Further Reading 141Problems 141

II THE FUSION ZONE 143

6 Basic Solidification Concepts 145

6.1 Solute Redistribution during Solidification 1456.2 Solidification Modes and Constitutional Supercooling 1556.3 Microsegregation and Banding 1606.4 Effect of Cooling Rate 1636.5 Solidification Path 166

References 167Further Reading 168Problems 169

7 Weld Metal Solidification I: Grain Structure 170

7.1 Epitaxial Growth at Fusion Boundary 1707.2 Nonepitaxial Growth at Fusion Boundary 1727.3 Competitive Growth in Bulk Fusion Zone 1747.4 Effect of Welding Parameters on Grain Structure 1747.5 Weld Metal Nucleation Mechanisms 1787.6 Grain Structure Control 187

viii CONTENTS

References 195Further Reading 197Problems 197

8 Weld Metal Solidification II: Microstructure within Grains 199

8.1 Solidification Modes 1998.2 Dendrite and Cell Spacing 2048.3 Effect of Welding Parameters 2068.4 Refining Microstructure within Grains 209

References 213Further Reading 213Problems 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 239Further Reading 241Problems 241

10 Weld Metal Chemical Inhomogeneities 243

10.1 Microsegregation 24310.2 Banding 24910.3 Inclusions and Gas Porosity 25010.4 Inhomogeneities Near Fusion Boundary 25210.5 Macrosegregation in Bulk Weld Metal 255References 260Further Reading 261Problems 261

11 Weld Metal Solidification Cracking 263

11.1 Characteristics, Cause, and Testing 26311.2 Metallurgical Factors 26811.3 Mechanical Factors 28411.4 Reducing Solidification Cracking 28511.5 Case Study: Failure of a Large Exhaust Fan 295References 296Further Reading 299Problems 299

CONTENTS ix

III THE PARTIALLY MELTED ZONE 301

12 Formation of the Partially Melted Zone 303

12.1 Evidence of Liquation 30312.2 Liquation Mechanisms 30412.3 Directional Solidification of Liquated Material 31412.4 Grain Boundary Segregation 31412.5 Grain Boundary Solidification Modes 31612.6 Partially Melted Zone in Cast Irons 318References 318Problems 319

13 Difficulties Associated with the Partially Melted Zone 321

13.1 Liquation Cracking 32113.2 Loss of Strength and Ductility 32813.3 Hydrogen Cracking 32813.4 Remedies 330References 336Problems 338

IV THE HEAT-AFFECTED ZONE 341

14 Work-Hardened Materials 343

14.1 Background 34314.2 Recrystallization and Grain Growth in Welding 34714.3 Effect of Welding Parameters and Process 349References 351Further Reading 352Problems 352

15 Precipitation-Hardening Materials I: Aluminum Alloys 353

15.1 Background 35315.2 Al–Cu–Mg and Al–Mg–Si Alloys 35915.3 Al–Zn–Mg Alloys 36715.4 Friction Stir Welding of Aluminum Alloys 370References 371Further Reading 372Problems 372

16 Precipitation-Hardening Materials II: Nickel-Base Alloys 375

16.1 Background 375

x CONTENTS

16.2 Reversion of Precipitate and Loss of Strength 37916.3 Postweld Heat Treatment Cracking 384References 390Further Reading 392Problems 392

17 Transformation-Hardening Materials: Carbon and Alloy Steels 393

17.1 Phase Diagram and CCT Diagrams 39317.2 Carbon Steels 39617.3 Low-Alloy Steels 40417.4 Hydrogen Cracking 41017.5 Reheat Cracking 41817.6 Lamellar Tearing 42217.7 Case Studies 425References 427Further Reading 429Problems 430

18 Corrosion-Resistant Materials: Stainless Steels 431

18.1 Classification of Stainless Steels 43118.2 Austenitic Stainless Steels 43318.3 Ferritic Stainless Steels 44618.4 Martensitic Stainless Steels 44918.5 Case Study: Failure of a Pipe 451References 452Further Reading 453Problems 454

Index 455

CONTENTS xi

PREFACE

Since the publication of the first edition of this book in 1987, there has beenmuch new progress made in welding metallurgy. The purpose for the secondedition is to update and improve the first edition. Examples of improvementsinclude (1) much sharper photomicrographs and line drawings; (2) integrationof the phase diagram, thermal cycles, and kinetics with the microstructure toexplain 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 andelectrochemical 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 andmicrosegregation expanded and the solidification path added. Chapter 7 nowincludes nonepitaxial growth at the fusion boundary and formation of non-dendritic equiaxed grains. In Chapter 8 solidification modes are explained withmore illustrations. Chapter 9 has been expanded significantly to add ferriteformation mechanisms, new ferrite prediction methods, the effect of coolingrate, and factors affecting the austenite–ferrite transformation. Chapter 10now includes the effect of both solid-state diffusion and dendrite tip under-cooling on microsegregation. Chapter 11 has been revised extensively toinclude the effect of eutectic reactions, liquid distribution, and ductility of the solidifying metal on solidification cracking and the calculation of fractionof liquid in multicomponent alloys.

Chapter 12 has been rewritten completely to include six different liquationmechanisms in the partially melted zone (PMZ), the direction and modes ofgrain boundary (GB) solidification, and the resultant GB segregation. Chapter13 has been revised extensively to include the mechanism of PMZ crackingand 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 ofInconel 718. Chapter 17 now includes the effect of multiple-pass welding on

xiii

reheat cracking and Chapter 18 the grain boundary chromium depletion in asensitized austenitic stainless steel.

The author thanks the National Science Foundation and NASA for supporting his welding research, from which this book draws frequently.He also thanks the American Welding Society and ASM International for per-missions to use numerous copyrighted materials. Finally, he thanks C. Huang,G. Cao, C. Limmaneevichitr, H. D. Lu, K.W. Keehn, and T.Tantanawat for pro-viding technical material, requesting permissions, and proofreading.

Sindo Kou

Madison, Wisconsin

xiv PREFACE