曹彪-焊接-joining processes and equipment3.ppt
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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 metallur
2、gical 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 temper
3、atures in 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 i
4、s essential to 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 quality the microstructure and grain size of
5、 the welded joint 焊接接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 heating duration at high temperature cooling,Temperature is decisive,29.2 The welded joint Three distinct zones can be identified in a typical fusion-weld joint: base me
6、tal; 母材 heat-affected zone (HAZ); 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,
7、 填充金属 and the process variables. 焊接参数,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
8、 arc with flame (a) (b),the process variables. 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
9、the weld metal is similar to that in casting and begins 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 tha
10、t the grains in the solidified weld metal are perpendicular to 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 sp
11、eed welding b) high speed welding columnar (dendritic) grains.,Grain structure 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-met
12、al composition, control of cooling rate, or adoption of heat treatments following 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
13、(粗大晶粒).,29.2.2 Heat-Affected Zone (HAZ) 热影响区 It has a microstructure different from that 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 o
14、f heat input and cooling, and the temperature to which this zone was raised Carbon steel Martensite 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
15、 various regions in a fusion weld zone (and the corresponding phase diagram) for 0.30% carbon steel,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)t
16、he elongated grains of the cold-worked (冷轧拉长的晶粒)base metal. Grains that are away from the weld metal will 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
17、grow. This growth will cause their region to be softer and to have less strength. For some materials under a 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 wel
18、ded joint may develop various discontinuities. 不连续性 Welding discontinuities can also be caused by inadequate or careless 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 melt
19、ing 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 contaminants. 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 m
20、ay be random or the porosity may be concentrated in a certain region. The classification of porosity related to gas?,Porosity 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 o
21、r an increase in the rate of heat input; c. proper cleaning, and the prevention of contaminants from entering the weld zones; d. reduced 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 materia
22、ls that are trapped in the weld zone. Welding conditions are important; with proper techniques, the molten slag will float to the surface 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 b
23、eads,Figure 29.6 Low-quality weld beads, the result of incomplete fusion,Incomplete penetration occurs when the depth of the welded joint is insufficient.,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)resul
24、ts 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 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 locati
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