焊接科学与工程专业英语 教学课件 ppt 作者 吴志生 柏艳红 黎穗琼 主编第5章.ppt

* * * 5.3 The Microstructure and Properties of Heat-affected Zone热影响区的微观组织和性能 　　 Figure 5-3-3 Time spent in various ranges below the peak temperature: form Tn (max) to Tn. 5.3 The Microstructure and Properties of Heat-affected Zone热影响区的微观组织和性能 　　 If the metal that is subject to the weld thermal cycle has previously been hardened by cold work, then grain growth may be preceded by recrystallisation（再结晶） which, in turn, results in softening（软化）. Copper and aluminum are typical of metals that can acquire a useful increase in mechanical properties by cold reduction and fusion welding will usually destroy the effect of cold word in the HAZ. Similarly, alloys that are hardened by precipitation will usually be softened by fusion welding. 5.3 The Microstructure and Properties of Heat-affected Zone热影响区的微观组织和性能 　　 To prevent the grain growth in the HAZ, there are two ways in which the residence time can be reduced: firstly, by using a process that generates two-dimensional（二维的） rather than three-dimensional heat flow (residence times for two-dimensional heat flow are about half those for three-dimensional flow), and secondly by decreasing the heat input rate q/v. 5.3 The Microstructure and Properties of Heat-affected Zone热影响区的微观组织和性能 　　 Effects of the Cooling Rate（冷却速度） on Microstructure The cooling rate associated with the weld nugget depends on the total heat in the molten weld metal and the mass and temperature of the welded part. For any process the volume of molten metal will depend upon the heat input and the efficiency of utilizing this heat in producing molten metal. Most of the electric welding processes with suitable techniques utilize 40-70% of the input to produce molten metal. However, oxy-fuel (gas) welding processes may have an efficiency as low as 10 or 20%. 5.3 The Microstructure and Properties of Heat-affected Zone热影响区的微观组织和性能 　　 For a given welding method, the cooling rates depend mainly on three variables: the rate of heat input, the b