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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 2 integration of the phase diagram, thermal cycles, and kinetics with the microstructure to explain microstructural development and defect ation in welds; and 3 additional rcise problems. Specifi c 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 effi ciency has been updated and the melting effi ciency 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 fl ow 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 solidifi cation path added. Chapter 7 now includes nonepitaxial growth at the fusion boundary and ation of non- dendritic equiaxed grains.In Chapter 8 solidifi cation modes are explained with more illustrations. Chapter 9 has been expanded signifi cantly to add ferrite ation mechanisms, new ferrite prediction s, the effect of cooling rate, and factors affecting the austenite–ferrite transation. 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 solidifi cation 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 solidifi cation,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 标准分享网 w w w .b z f x w .c o m 免费下载 w w w . b z f x w . c o m reheat cracking and Chapter 18 the grain boundary chromium depletion in a sensitized 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 xivPREFACE w w w . b z f x w . c o m PART I Introduction Welding Metallurgy, Second Edition. Sindo Kou Copyright2003 John Wiley b capital equipment cost. Reprinted from Mendez and Eagar 2. w w w . b z f x w . c o m TABLE 1.1Overview of Welding Processesa MaterialThicknessbSMAWSAWGMAWFCAWGTAWPAWESWOFWEBWLBW CarbonS✕✕✕✕✕✕✕ steelsI✕✕✕✕✕✕✕✕ M✕✕✕✕✕✕✕ T✕✕✕✕✕✕✕ Low-alloyS✕✕✕✕✕✕✕ steelsI✕✕✕✕✕✕✕ M✕✕✕✕✕✕ T✕✕✕✕✕✕ StainlessS✕✕✕✕✕✕✕✕ steelsI✕✕✕✕✕✕✕✕ M✕✕✕✕✕✕✕ T✕✕✕✕✕✕ Cast ironI✕✕ M✕✕✕✕✕ T✕✕✕✕✕ NickelS✕✕✕✕✕✕✕ and alloysI✕✕✕✕✕✕✕ M✕✕✕✕✕✕ T✕✕✕✕ AluminumS✕✕✕✕✕✕ and alloysI✕✕✕✕ M✕✕✕ T✕✕ a Process code SMAW, shielded metal arc welding; SAW, submerged arc welding; GMAW, gas–metal arc welding; FCAW, fl ux-cored arc welding; GTAW, gas–tungsten arc welding; PAW, plasma arc welding; ESW, electroslag welding; OFW, oxyfuel gas welding; EBW, electron beam welding; LBW, laser beam welding. b Abbreviations S, sheet, up to 3mm 1/8in.; I, intermediate, 3–6mm 1/8–1/4in.; M, medium, 6–19mm 1/4–3/4in.; T, thick, 19mm 3/4in. and up; X, recommended. Source Welding Handbook 3. 6 标准分享网 w w w .b z f x w .c o m 免费下载 w w w . b z f x w . c o m aluminum alloys 3. For one example, GMAW can be used for all the materi- als 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 fl ux, such as SMAW, SAW, FCAW, and ESW, is not applicable to aluminum alloys. 1.1.4Types 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. 1.2OXYACETYLENE WELDING 1.2.1The Process Gas welding is a welding process that melts and joins metals by heating them with a fl ame caused by the reaction between a fuel gas and oxygen. Oxy- acetylene welding OAW, shown in Figure 1.7, is the most commonly used gas welding process because of its high fl ame temperature.A fl ux may be used to deoxidize and cleanse the weld metal. The fl ux melts, solidifi es, and s a slag skin on the resultant weld metal. Figure 1.8 shows three different types of fl ames in oxyacetylene welding neutral, reducing, and oxidizing 4, which are described next. 1.2.2Three Types of Flames A. Neutral FlameThis refers to the case where oxygen O2 and acetylene C2H2 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 fl ame OXYACETYLENE WELDING7 a butt joint c T-joint b lap joint d edge joint e corner joint Figure 1.4Five basic types of weld joint designs. w w w . b z f x w . c o m Figure 1.8a.The inner cone is the area where the primary combustion takes place through the chemical reaction between O2and C2H2,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 H2,react with O2 from the surrounding air and CO2and H2O.This is the secondary com- bustion, which accounts for about one-third of the total heat generated. The area where this secondary combustion takes place is called the outer enve- lope. It is also called the protection envelope since CO and H2 hereconsume the O2entering from the surrounding air, thereby protecting the weld metal from oxidation. For most metals, a neutral fl ame is used. B. Reducing Flame When excess acetylene is used, the resulting fl ame is called a reducing fl ame.The combustion of acetylene is incomplete.As a result, a greenish acetylene feather between the inert cone and the outer envelope characterizes a reducing fl ame Figure 1.8b.This fl ame 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 fl ame in this case because excess oxygen can oxidize carbon and CO gas porosity in the weld metal. 8FUSION WELDING PROCESSES Toe Toe Reinforcement T-joint; fillet weld Butt joint; square weld Toe Reinforcement Butt joint; single-V-groove weld Root Toe Lap joint; fillet weld Toe Toe d c a b Toe T-joint; single bevel weld Toe e Figure 1.5Typical weld joint variations. 标准分享网 w w w .b z f x w .c o m 免费下载 w w w . b z f x w . c o m OXYACETYLENE WELDING9 a flatb horizontal c verticald overhead Figure 1.6Four welding positions. Oxygen/acetylene mixture Filler rod Protection envelope Metal droplet Base metalWeld pool Weld metal Slag Primary combustion RegulatorFlow meter Acetylene Welding direction Gas torch Workpiece C2H2 O2 Valve Oxygen a b Figure 1.7Oxyacetylene welding a overall process; b welding area enlarged. w w w . b z f x w . c o m C. Oxidizing Flame When excess oxygen is used, the fl ame becomes oxi- dizing because of the presence of unconsumed oxygen. A short white inner cone characterizes an oxidizing fl ame Figure 1.8c. This fl ame is preferred when welding brass because copper oxide covers the weld pool and thus pre- vents zinc from evaporating from the weld pool. 1.2.3Advantages and Disadvantages The main advantage of the oxyacetylene welding process is that the equip- ment is simple,portable,and inexpensive.Therefore,it is convenient for main- tenance and repair applications.However,due to its limited power density,the 10FUSION WELDING PROCESSES inner cone inner cone acetylene feather Reducing Flame inner cone Oxidizing Flame Neutral Flame a b c Figure 1.8 Three types of fl ames in oxyacetylene welding. Modifi ed from Welding Journal 4. Courtesy of American Welding Society. C2H2 O2Gas Torch 2500 oC 1000 oC 2800 - 3500 oC inner cone outer envelope 2C2H2 2O2 from cylinder Secondary combustion in outer envelope 1/3 total heat 4CO 2H2 4CO 2O2 from air4CO2 2H2 O2 from air2H2O Primary combustion in inner cone 2/3 total heat Flame Figure 1.9Chemical reactions and temperature distribution in a neutral oxyacetylene fl ame. 标准分享网 w w w .b z f x w .c o m 免费下载 w w w . b z f x w . c o m welding speed is very low and the total heat 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. 1.3SHIELDED METAL ARC WELDING 1.3.1The 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 fi ller metal for the joint. For electrical contact, the top 1.5cm 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 fl ux covering at the electrode tip to melt off as droplets Figure 1.10b.The molten metal collects in the weld pool and solidifi es into the weld metal.The lighter molten fl ux, on the other hand, fl oats on the pool surface and solidifi es into a slag layer at the top of the weld metal. 1.3.2Functions of Electrode Covering The covering of the electrode contains various chemicals and even metal powder in order to per one or more of the functions described below. A. ProtectionIt provides a gaseous shield to protect the molten metal from air. For a cellulose-type electrode, the covering contains cellulose, C6H10O5x. A large volume of gas mixture of H2, CO, H2O, and CO2is produced when cellulose in the electrode covering is heated and decomposes.For a limestone- CaCO3 type electrode,on the other hand,CO2gas and CaO slag 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 fl uxing agents to deoxidize and cleanse the weld metal.The solid slag ed also protects the already solid- ifi ed but still hot weld metal from oxidation. SHIELDED METAL ARC WELDING11 w w w . b z f x w . c o m C. Arc StabilizationIt 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 AdditionIt provides alloying elements and/or metal powder to the weld pool. The er helps control the composition of the weld metal while the latter helps increase the deposition rate. 1.3.3Advantages and Disadvantages The welding equipment is relatively simple,portable,and inexpensive as com- pared to other arc welding processes. For this reason, SMAW is often used for maintenance,repair,and fi eld 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 35cm requires electrode changing, and this further reduces the overall production rate. 12FUSION WELDING PROCESSES Gaseous shield Core wire Flux covering Slag Metal droplet Flux droplet Base metal Weld pool Weld metal Arc a b Power Source Cable 1 Electrode holder Stick electrode Welding direction Workpiece Cable 2 Figure 1.10Shielded metal arc weldinga overall process;b welding area enlarged. 标准分享网 w w w .b z f x w .c o m 免费下载 w w w . b z f x w . c o m 1.4GAS–TUNGSTEN ARC WELDING 1.4.1The 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 elec- trode 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. Pro- tection 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 GAS–TUNGSTEN ARC WELDING13 Shielding gas nozzle Weld metal Metal droplet Shielding gas Base metalWeld pool Arc Filler rod Welding direction Filler rod Torch Cable 1 Workpiece Shielding gas cylinder Flow meter Reg
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