Welding Metallurgy / Edition 2

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  • Updated to include new technological advancements in welding
  • Uses illustrations and diagrams to explain metallurgical phenomena
  • Features exercises and examples
  • An Instructor's Manual presenting detailed solutions to all the problems in the book is available from the Wiley editorial department.
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Editorial Reviews

From the Publisher
"…well presented, comprehensive, and accurate…a welcome addition to the bookshelves of metallurgists, academics, postgraduate students, as well as non-specialized engineers…" (JOM, February 26, 2004)

"The second edition, a valuable resource for practitioners, researchers and students, contains more exercises and offers a solution manual upon request..." (Materials Evaluation, February 2003)

"For many years this review has been looking, without success, for a book on the metallurgy of welding. This...second edition fulfills all those needs and expectations...all those who need a basic understanding of...welds will greatly benefit...a valuable acquisition…highly recommended." (Choice, Vol. 40, No. 7, March 2003)

From The Critics
In this new edition, Kou (science and engineering, U. of Wisconsin) incorporates extensive revisions and includes sharper photomicrographs and line drawings; integration of the phase diagram, thermal cycles, and kinetics with the microstructure to explain microstructural development and defect formation in welds; and additional exercise problems. Topics include introductory material (fusion welding processes, heat and fluid flow, chemical reactions, and residual stresses, distortion and fatigue), and the fusion, partially melted, and heat-affected zones. Illustrated with b&w images. Annotation c. Book News, Inc., Portland, OR
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Product Details

  • ISBN-13: 9780471434917
  • Publisher: Wiley
  • Publication date: 10/18/2002
  • Edition description: REV
  • Edition number: 2
  • Pages: 480
  • Sales rank: 1,089,447
  • Product dimensions: 6.36 (w) x 9.23 (h) x 1.03 (d)

Meet the Author

SINDO KOU, PhD, is Professor and Chair of the Department of Materials Science and Engineering at the University of Wisconsin. He graduated from MIT with a PhD degree in metallurgy. He is a Fellow of American Welding Society and ASM International. He is the author of Transport Phenomena and Materials Processing, also published by Wiley.

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Read an Excerpt

Welding Metallurgy

By Sindo Kou

John Wiley & Sons

ISBN: 0-471-43491-4

Chapter One

Fusion Welding Processes

Fusion welding processes will be described in this chapter, including gas welding, arc welding, and high-energy beam welding. The advantages and disadvantages of each process will be discussed.


1.1.1 Fusion Welding Processes

Fusion welding is a joining process that uses fusion of the base metal to make the weld. The three major types of fusion welding processes are as follows:

1. Gas welding:

Oxyacetylene welding (OAW)

2. Arc welding:

Shielded metal arc welding (SMAW)

Gas-tungsten arc welding (GTAW)

Plasma arc welding (PAW)

Gas-metal arc welding (GMAW)

Flux-cored arc welding (FCAW)

Submerged arc welding (SAW)

Electroslag welding (ESW)

3. High-energy beam welding:

Electron beam welding (EBW)

Laser beam welding (LBW)

Since there is no arc involved in the electroslag welding process, it is not exactly an arc welding process. For convenience of discussion, it is grouped with arc welding processes.

1.1.2 Power Density of Heat Source

Consider directing a 1.5-kW hair drier very closely to a 304 stainless steel sheet 1.6mm (1/16 in.) thick. Obviously, the power spreads out over an area of roughly 50mm (2 in.) diameter, and the sheet just heats up gradually but will not melt. With GTAW at 1.5kW, however, the arc concentrates on asmall area of about 6mm (1/4 in.) diameter and can easily produce a weld pool. This example clearly demonstrates the importance of the power density of the heat source in welding.

The heat sources for the gas, arc, and high-energy beam welding processes are a gas flame, an electric arc, and a high-energy beam, respectively. The power density increases from a gas flame to an electric arc and a high-energy beam. As shown in Figure 1.1, as the power density of the heat source increases, the heat input to the workpiece that is required for welding decreases. The portion of the workpiece material exposed to a gas flame heats up so slowly that, before any melting occurs, a large amount of heat is already conducted away into the bulk of the workpiece. Excessive heating can cause damage to the workpiece, including weakening and distortion. On the contrary, the same material exposed to a sharply focused electron or laser beam can melt or even vaporize to form a deep keyhole instantaneously, and before much heat is conducted away into the bulk of the workpiece, welding is completed.

Therefore, the advantages of increasing the power density of the heat source are deeper weld penetration, higher welding speeds, and better weld quality with less damage to the workpiece, as indicated in Figure 1.1. Figure 1.2 shows that the weld strength (of aluminum alloys) increases as the heat input per unit length of the weld per unit thickness of the workpiece decreases. Figure 1.3a shows that angular distortion is much smaller in EBW than in GTAW. Unfortunately, as shown in Figure 1.3b, the costs of laser and electron beam welding machines are very high.

1.1.3 Welding Processes and Materials

Table 1.1 summarizes the fusion welding processes recommended for carbon steels, low-alloy steels, stainless steels, cast irons, nickel-base alloys, and aluminum alloys. For one example, GMAW can be used for all the materials of almost all thickness ranges while GTAW is mostly for thinner workpieces. For another example, any arc welding process that requires the use of a flux, such as SMAW, SAW, FCAW, and ESW, is not applicable to aluminum alloys.

1.1.4 Types of Joints and Welding Positions

Figure 1.4 shows the basic weld joint designs in fusion welding: the butt, lap, T-, edge, and corner joints. Figure 1.5 shows the transverse cross section of some typical weld joint variations. The surface of the weld is called the face, the two junctions between the face and the workpiece surface are called the toes, and the portion of the weld beyond the workpiece surface is called the reinforcement. Figure 1.6 shows four welding positions.


1.2.1 The Process

Gas welding is a welding process that melts and joins metals by heating them with a flame caused by the reaction between a fuel gas and oxygen. Oxyacetylene welding (OAW), shown in Figure 1.7, is the most commonly used gas welding process because of its high flame temperature. A flux may be used to deoxidize and cleanse the weld metal. The flux melts, solidifies, and forms a slag skin on the resultant weld metal. Figure 1.8 shows three different types of flames in oxyacetylene welding: neutral, reducing, and oxidizing, which are described next.

1.2.2 Three Types of Flames

A. Neutral Flame This refers to the case where oxygen ([O.sub.2]) and acetylene ([C.sub.2][H.sub.2]) are mixed in equal amounts and burned at the tip of the welding torch. A short inner cone and a longer outer envelope characterize a neutral flame (Figure 1.8a). The inner cone is the area where the primary combustion takes place through the chemical reaction between [O.sub.2] and [C.sub.2][H.sub.2], as shown in Figure 1.9. The heat of this reaction accounts for about two-thirds of the total heat generated. The products of the primary combustion, CO and [H.sub.2], react with [O.sub.2] from the surrounding air and form C[O.sub.2] and [H.sub.2]O. This is the secondary combustion, which accounts for about one-third of the total heat generated. The area where this secondary combustion takes place is called the outer envelope. It is also called the protection envelope since CO and [H.sub.2] here consume the [O.sub.2] entering from the surrounding air, thereby protecting the weld metal from oxidation. For most metals, a neutral flame is used.

B. Reducing Flame When excess acetylene is used, the resulting flame is called a reducing flame. The combustion of acetylene is incomplete. As a result, a greenish acetylene feather between the inert cone and the outer envelope characterizes a reducing flame (Figure 1.8b). This flame is reducing in nature and is desirable for welding aluminum alloys because aluminum oxidizes easily. It is also good for welding high-carbon steels (also called carburizing flame in this case) because excess oxygen can oxidize carbon and form CO gas porosity in the weld metal.

C. Oxidizing Flame When excess oxygen is used, the flame becomes oxidizing because of the presence of unconsumed oxygen. A short white inner cone characterizes an oxidizing flame (Figure 1.8c). This flame is preferred when welding brass because copper oxide covers the weld pool and thus prevents zinc from evaporating from the weld pool.

1.2.3 Advantages and Disadvantages

The main advantage of the oxyacetylene welding process is that the equipment is simple, portable, and inexpensive. Therefore, it is convenient for maintenance and repair applications. However, due to its limited power density, the welding speed is very low and the total heat input per unit length of the weld is rather high, resulting in large heat-affected zones and severe distortion. The oxyacetylene welding process is not recommended for welding reactive metals such as titanium and zirconium because of its limited protection power.


1.3.1 The Process

Shielded metal arc welding (SMAW) is a process that melts and joins metals by heating them with an arc established between a sticklike covered electrode and the metals, as shown in Figure 1.10. It is often called stick welding. The electrode holder is connected through a welding cable to one terminal of the power source and the workpiece is connected through a second cable to the other terminal of the power source (Figure 1.10a).

The core of the covered electrode, the core wire, conducts the electric current to the arc and provides filler metal for the joint. For electrical contact, the top 1.5 cm of the core wire is bare and held by the electrode holder. The electrode holder is essentially a metal clamp with an electrically insulated outside shell for the welder to hold safely.

The heat of the arc causes both the core wire and the flux covering at the electrode tip to melt off as droplets (Figure 1.10b). The molten metal collects in the weld pool and solidifies into the weld metal. The lighter molten flux, on the other hand, floats on the pool surface and solidifies into a slag layer at the top of the weld metal.

1.3.2 Functions of Electrode Covering

The covering of the electrode contains various chemicals and even metal powder in order to perform one or more of the functions described below.

A. Protection It provides a gaseous shield to protect the molten metal from air. For a cellulose-type electrode, the covering contains cellulose, [([C.sub.6][H.sub.10][O.sub.5]).sub.x]. A large volume of gas mixture of [H.sub.2], CO, [H.sub.2]O, and C[O.sub.2] is produced when cellulose in the electrode covering is heated and decomposes. For a limestone-(CaC[O.sub.3-]) type electrode, on the other hand, C[O.sub.2] gas and CaO slag form when the limestone decomposes. The limestone-type electrode is a low-hydrogen-type electrode because it produces a gaseous shield low in hydrogen. It is often used for welding metals that are susceptible to hydrogen cracking, such as high-strength steels.

B. Deoxidation It provides deoxidizers and fluxing agents to deoxidize and cleanse the weld metal. The solid slag formed also protects the already solidified but still hot weld metal from oxidation.

C. Arc Stabilization It provides arc stabilizers to help maintain a stable arc. The arc is an ionic gas (a plasma) that conducts the electric current. Arc stabilizers are compounds that decompose readily into ions in the arc, such as potassium oxalate and lithium carbonate. They increase the electrical conductivity of the arc and help the arc conduct the electric current more smoothly.

D. Metal Addition It provides alloying elements and/or metal powder to the weld pool. The former helps control the composition of the weld metal while the latter helps increase the deposition rate.

1.3.3 Advantages and Disadvantages

The welding equipment is relatively simple, portable, and inexpensive as compared to other arc welding processes. For this reason, SMAW is often used for maintenance, repair, and field construction. However, the gas shield in SMAW is not clean enough for reactive metals such as aluminum and titanium. The deposition rate is limited by the fact that the electrode covering tends to over-heat and fall off when excessively high welding currents are used. The limited length of the electrode (about 35 cm) requires electrode changing, and this further reduces the overall production rate.


1.4.1 The Process

Gas-tungsten arc welding (GTAW) is a process that melts and joins metals by heating them with an arc established between a nonconsumable tungsten electrode and the metals, as shown in Figure 1.11. The torch holding the tungsten electrode is connected to a shielding gas cylinder as well as one terminal of the power source, as shown in Figure 1.11a. The tungsten electrode is usually in contact with a water-cooled copper tube, called the contact tube, as shown in Figure 1.11b, which is connected to the welding cable (cable 1) from the terminal. This allows both the welding current from the power source to enter the electrode and the electrode to be cooled to prevent overheating. The workpiece is connected to the other terminal of the power source through a different cable (cable 2). The shielding gas goes through the torch body and is directed by a nozzle toward the weld pool to protect it from the air. Protection from the air is much better in GTAW than in SMAW because an inert gas such as argon or helium is usually used as the shielding gas and because the shielding gas is directed toward the weld pool. For this reason, GTAW is also called tungsten-inert gas (TIG) welding. However, in special occasions a noninert gas (Chapter 3) can be added in a small quantity to the shielding gas. Therefore, GTAW seems a more appropriate name for this welding process. When a filler rod is needed, for instance, for joining thicker materials, it can be fed either manually or automatically into the arc.

1.4.2 Polarity

Figure 1.12 shows three different polarities in GTAW, which are described next.

A. Direct-Current Electrode Negative (DCEN) This, also called the straight polarity, is the most common polarity in GTAW. The electrode is connected to the negative terminal of the power supply. As shown in Figure 1.12a, electrons are emitted from the tungsten electrode and accelerated while traveling through the arc. A significant amount of energy, called the work function, is required for an electron to be emitted from the electrode. When the electron enters the workpiece, an amount of energy equivalent to the work function is released. This is why in GTAW with DCEN more power (about two-thirds) is located at the work end of the arc and less (about one-third) at the electrode end. Consequently, a relatively narrow and deep weld is produced.

B. Direct-Current Electrode Positive (DCEP) This is also called the reverse polarity. The electrode is connected to the positive terminal of the power source. As shown in Figure 1.12b, the heating effect of electrons is now at the tungsten electrode rather than at the workpiece. Consequently, a shallow weld is produced. Furthermore, a large-diameter, water-cooled electrodes must be used in order to prevent the electrode tip from melting. The positive ions of the shielding gas bombard the workpiece, as shown in Figure 1.13, knocking off oxide films and producing a clean weld surface. Therefore, DCEP can be used for welding thin sheets of strong oxide-forming materials such as aluminum and magnesium, where deep penetration is not required.

C. Alternating Current (AC) Reasonably good penetration and oxide cleaning action can both be obtained, as illustrated in Figure 1.12c.This is often used for welding aluminum alloys.

1.4.3 Electrodes

Tungsten electrodes with 2% cerium or thorium have better electron emissivity, current-carrying capacity, and resistance to contamination than pure tungsten electrodes. As a result, arc starting is easier and the arc is more stable. The electron emissivity refers to the ability of the electrode tip to emit electrons. A lower electron emissivity implies a higher electrode tip temperature required to emit electrons and hence a greater risk of melting the tip.

1.4.4 Shielding Gases

Both argon and helium can be used. Table 1.2


Excerpted from Welding Metallurgy by Sindo Kou Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents

Preface xiii


1 Fusion Welding Processes 3

2 Heat Flow in Welding 37

3 Chemical Reactions in Welding 65

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


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

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


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


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

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

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