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1、29 The Metallurgy of Welding;Welding Design and Process Selection 焊接冶金，焊接设计与工艺制定29.1 Introduction Weld joints can be gotten by the welding processes described in chapter 27 and chapter 28.Is the joint strong enough(good),so-so or fail?,Because of the heating process,there must be some metallurgical
2、and physical changes（冶金与物理变化）in the materials.,other property,strength,ductility,and toughness of a welded joint,mechanical property,For example,the rate of heat application and the thermal properties of metals are important,in that they control the magnitude and the distribution of temperatures in
3、the joint.The microstructure and grain size of the welded joint（焊接接头的微观结构与晶粒尺寸）depend on the amount of heat applied and the consequent temperature rise,on the degree of prior cold work of the metals（冷加工程度）,and on the rate of cooling after the weld is produced.Control of such factors is essential to
4、the creation of reliable welds that have acceptable mechanical properties.,Weld quality depends on many factors,among them the geometry of the weld bead and the presence of cracks,residual stresses,inclusions,oxide films and so on.Weld qualitythe microstructure and grain size of the welded joint 焊接接
5、头的微观结构与晶粒尺寸the geometry of the weld bead 焊道的几何因素defects in joint 接头缺陷Temperature and its distribution 温度分布heatingduration at high temperaturecooling,Temperature is decisive,29.2 The welded joint Three distinct zones can be identified in a typical fusion-weld joint:base metal;母材 heat-affected zone(HA
6、Z);热影响区 weld metal.焊缝,Figure 29.1 Characteristics of a typical fusion weld zone in oxyfuel gas and arc welding.,The metallurgy and properties of the HAZ and weld metal depend strongly on the metals joined,焊接金属 the welding process,焊接方法 the filler metals used,if any,填充金属 and the process variables.焊接参数
7、,the metals joined,焊接金属 Temperature distribution for welding of different materials under same energy input and same welding speed.,the welding process,焊接方法 Energy density for fusion welding,Compare of the energy distribution of(a)arcs,and(b)the consumable arc with flame(a)(b),the process variables.
8、焊接参数 Profiles of weld bead and HAZ.different welding current and voltage for CO2 weldinggas metal arc,29.2.1 Solidification of the Weld Metal 焊缝凝固 Weld metal:Melting during welding and then cooled down(solidification).The solidification process of the weld metal is similar to that in casting and beg
9、ins with the formation of columnar(dendritic)grains.relatively long and parallel to the heat flow.or perpendicular(垂直于)to the surface of the base metal(shallow weld).,Figure 29.2 Grain structure in(a)a deep weld(b)a shallow weld.Note that the grains in the solidified weld metal are perpendicular to
10、the surface of the base metal.In a good weld,the solidification line at the center in the deep weld shown in(a)has grain migration,which develops uniform strength in the weld bead.,The solidification of welding pool under:a)low speed welding b)high speed welding columnar(dendritic)grains.,Grain stru
11、cture and size depend on the specific alloy,the specific welding process employed,(cooling velocity)and the specific filler metal.The way to improve the mechanical properties of the joint:The proper selection of filler-metal composition,control of cooling rate,or adoption of heat treatments followin
12、g welding Cooling rates may be controlled and reduced by a preheating of the general weld area prior to the welding.,The weld metal has,basically,a cast structure and,because it has cooled slowly,it has coarse grains（粗大晶粒）.,29.2.2 Heat-Affected Zone(HAZ)热影响区 It has a microstructure different from th
13、at of the base metal prior to welding,because it has been subjected to elevated temperatures for a period of time during welding;The properties and microstructure of the HAZ depend on(Thermal cycling)the rate of heat input and cooling,and the temperature to which this zone was raisedCarbon steel Mar
14、tensite 马氏体 Pearlite 珠光体 Cementite 渗碳体 ferrite 铁素体 Austenite 奥氏体 annealing,normalizing,quenching,and tempering The size of HAZ depend on temperature distribution.温度分布,Figure 29.4 Schematic illustration of various regions in a fusion weld zone(and the corresponding phase diagram)for 0.30%carbon steel
15、,The strength and hardness of the HAZ depend partly on how the original strength and hardness of the base metal was developed prior to the welding.The heat applied during welding recrystallizes(再结晶）the elongated grains of the cold-worked（冷轧拉长的晶粒）base metal.Grains that are away from the weld metal wi
16、ll recrystallize into fine equiaxed grains（等轴晶）.Grains close to the weld metal,on the other hand,have been subjected to elevated temperatures for a longer period of time，consequently they will grow.This growth will cause their region to be softer and to have less strength.For some materials under a
17、certain welding processes,harden region will appear after cooling.Quenching 淬火,29.3 Weld defects As a result of a history of thermal cycling and its attendant microstructrual changes,a welded joint may develop various discontinuities.不连续性 Welding discontinuities can also be caused by inadequate or c
18、areless application of established welding technologies or by substandard operator training.（不按焊接工艺执行或缺乏焊工培训）,29.3.1 Porosity 气孔 Porosity in welds is caused by gases released during melting of the weld area but tapped during solidification,析出性气孔 by chemical reactions during welding,or 反应性气孔 by conta
19、minants.污染物 侵入性气孔,Most welded joints contain some porosity,which is generally in the shape of spheres or of elongated pockets.（球形或长型）The distribution of porosity in the weld zone may be random or the porosity may be concentrated in a certain region.The classification of porosity related to gas?,Poro
20、sity in welds can be reduced by the following practices:a.proper selection of electrodes and filler metals;b.improved welding techniques,such as preheating of the weld area or an increase in the rate of heat input;c.proper cleaning,and the prevention of contaminants from entering the weld zones;d.re
21、duced welding speeds,to allow time for gas to escape.气体逸出,29.3.2 Slag Inclusions 夹渣 Slag inclusions are compounds such as oxides,fluxes,and electrode-coating materials that are trapped in the weld zone.Welding conditions are important;with proper techniques,the molten slag will float to the surface
22、of the molten weld metal and will not become entrapped.,29.3.3 Incomplete Fusion and Penetration(未熔合和未焊透）Incomplete fusion(lack of fusion)produces poor weld beads,Figure 29.6 Low-quality weld beads,the result of incomplete fusion,Incomplete penetration occurs when the depth of the welded joint is in
23、sufficient.,29.3.4 Bad Weld Profile 焊缝外形Underfilling(未填满）results when the joint is not filled with the proper amount of weld metal.Undercutting（咬边）results from the melting away of the base metal and the consequent generation of a groove in the shape of a sharp recess or notch.Overlap（焊瘤）is a surface
24、 discontinuity usually caused by poor welding practice and by the selection of improper materials.,29.3.5 Cracks 裂纹 Cracks may occur in various locations and directions in the weld area.The typical types of cracks are longitudinal,transverse,crater,underbead,and toe cracks.纵向裂纹、横向裂纹、火口裂纹、焊道下裂纹和焊趾裂纹,
25、Crack in a Weld Bead,Figure 29.9 Crack in a weld bead,due to the fact that the two components were not allowed to contract after the weld was completed.,These cracks generally result from a combination of the following factors:产生裂纹的原因Temperature gradients（温度梯度）that cause thermal stresses in the weld
26、 zone;Variations in the composition（成分不均匀）of the weld zone that cause different rates of contraction;Embrittlement of grain boundaries（晶界脆化）,caused by the segregation（隔离）of such elements as sulfur（硫）to the grain boundaries as the solid-liquid boundary moves when the weld metal begins to solidify;Hyd
27、rogen embrittlement（氢脆）;Inability of the weld metal to contract during cooling.冷却过程中焊缝不能收缩,Cracks are classified as hot or cold.热裂纹与冷裂纹 Hot cracks occur while the joint is still at elevated temperatures.Cold cracks develop after the weld metal has solidified.The basic crack-prevention measures are t
28、he following:Change the joint design,to minimize stresses from shrinkage during cooling;Change the parameters,the procedures,and the sequence of the welding process;Preheat the components to be welded;Avoid rapid cooling of the welded components.,29.3.6 Lamellar Tears（层状撕裂）because of the alignment o
29、f nonmetallic impurities and inclusions,stresses in the thickness direction（厚度方向存在非金属夹杂层）29.3.7 Surface Damage（表面伤害）spatter,arc strikes（飞溅，电弧冲击）,29.3.8 Residual Stresses 残余应力 Because of localized heating and cooling during welding,expansion and contraction of the weld area causes residual stresses i
30、n the workpiece.Residual stresses can cause the following defects:distortion,warping,and buckling of the welded parts;焊件扭曲 stress-corrosion cracking;应力腐蚀开裂 further distortion;reduced fatigue life.降低疲劳寿命,Figure 29.10 Distortion of parts after welding:(a)butt joints;(b)fillet welds.Distortion is cause
31、d by differential thermal expansion and contraction of different parts of the welded assembly.,Distortion after Welding,Residual Stresses developed during welding,Figure 29.11 Residual stresses developed during welding of a butt joint.,The type and distribution of residual stresses in welds,The reas
32、on for the producing of residual stresses:When two plates are being welded,a long narrow zone is subjected to elevated temperatures,while the plates as a whole are essentially at ambient temperature.After the weld is completed,and as time elapses,the heat from the weld zone dissipates laterally into
33、 the plates,while the weld area cools.The plates thus begin to expand longitudinally while the welded length begins to contract.These two opposing effects cause residual stresses that are distributed as shown in fig.29.11b.Note that the magnitude of the compressive residual stresses in the plates di
34、minishes to zero at a point far away from the weld area.Because no external forces are acting on the welded plates,the tensile and compressive forces represented by these residual stresses must balance each other.,29.3.9 Stress Relieving of WeldsPreheating.Preheating reduces distortion by reducing t
35、he cooling rate and the level of thermal stresses.This technique also reduces shrinkage and possible cracking of the joint.Peening,hammering or surfacing rolling of the weld bead area These processes induce compressive residual stresses,which reduce or eliminate tensile residual stresses in the weld
36、.Heat treated after welding Heat treated by various other techniques in order to modify other properties also help to relieve stresses These post-welding techniques include the annealing,normalizing,quenching,and tempering of steels,et al.,29.4 Weldability(可焊性)The weldability of a metal is usually d
37、efined as its capacity to be welded into a specific structure that has certain properties and characteristics and will satisfactorily meet service requirements.,Weldability is decided by materials,design and manufacture,Weldability involves a large number of variables，so generalizations are difficul
38、t.,the material characteristics of base metal and the filler metalthe alloying elements the impurities the inclusions the grain structure the processing history the effects of melting and solidification and the consequent micro-structural changes mechanical and physical properties strength toughness
39、 ductility notch sensitivity elastic modulus specific heat melting point thermal expansion surface tension characteristics of the molten metal and corrosion resistance,Surface preparationWelding processWelding conditionsshielding gases、fluxes，the moisture content of the coatings on electrodes，weldin
40、g speed welding position，cooling rate，preheatingpost welding techniques as stress relieving and heat treating,Weldability of various metal and alloys,Plain-carbon steels excellent for low-carbon steels,fair for medium-,poor for high-Low-alloy steelsHigh-alloy steels good under well-controlled condit
41、ionsStainless steelsAluminum alloys high rate of heat inputCopper alloysMagnesium alloys protective shielding gas and fluxNickel alloysTitanium alloys proper use of shielding gasesTantalum（钽）TungstenMolybdenumNiobium（铌）See TABLE 29.1 in the book,Overview of Commercial Joining Processes,Overview of C
42、ommercial Joining Processes(cont.),Overview of Commercial Joining Processes(cont.),29.5 Testing welded joints（焊接检验）The quality of a welded joint is established by testing.29.5.1 Destructive techniques Mechanical property test Chemical analysis Microstructure examination,Figure 29.12 Two types of spe
43、cimens for tension-shear testing of welded joints.,Figure 29.13(a)Wrap-around bend test method.(b)Three-point bending of welded specimens-see also Fig.2.11.,Testing of Spot Welds,Figure 29.14(a)Tension-shear test for spot welds.(b)Cross-tension test.(c)Twist test.(d)Peel test;see also Fig.30.8.,29.5
44、.2 Nondestructive techniques visual method radiographic method magnetic-particle method liquid-penetrant method ultrasonic testing method.,29.6 Weld design and process selectionConsideration on weld joint and process selection:Material characteristics;the configuration of the components or structure
45、 to be welded,and their thickness and size；the methods used to manufacture the components；the service requirements,such as the type of loading and the stresses generated；the location，accessibility，and ease of welding；the effects of distortion and discoloration；the appearance；the costs involved in th
46、e edge preparation，the welding，and the post-processing of the weld，including those from machining and finishing operations.,Welding Design Guidelines Always make a right joint design,Figure 29.15 Design guidelines for welding.Source:J.G.Bralla(ed.),Handbook of Product Design for Manufacturing.Copyri
47、ght 1986,McGraw-Hill Publishing Company.Used with permission.,General design guidelines may be summarized as follows:Product design should minimize the number of welds,because welding can be costly unless automated.Weld location should be selected so as to avoid excessive stresses or stress concentr
48、ations in the welded structure,and secondarily for appearance.Components should fit properly before welding.Some designs can avoid or minimize the need for edge preparation.Weld-bead size should be kept to a minimum,to conserve weld metal.Weld location should be selected so as not to interfere with
49、further processing of the part or with its intended use and appearance.,Standard Identification and Symbols for Welds,Figure 29.16,Weld Design Selection,Figure 29.17,Summary The metallurgy of the welded joint is an important aspect of all welding processes,because it determines the strength and toug
50、hness of the joint.The welded joint consists of solidified metal and a heat-affected zone;each has a wide variation in microstructure and properties,depending on the metals joined and on the filler metals.Because of severe thermal gradients in the weld zone,distortion,residual stresses,and cracking
