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6章.ppt

6.2 Weldability of low carbon steel There is evidence from Auger analysis of crack surfaces that, in the brittle intergranular mode, sulphur separates out to the crack tip（裂纹尖端）. Two alternative models have been proposed to describe this behaviour. In the first model, it is assumed that solute atoms（溶质原子） are driven to the crack-tip vicinity by elastic interaction（弹性相互作用） with the crack-tip stress field（应力场）. When sufficient concentration is reached, local embrittlement occurs and the crack jumps forwards by brittle fracture into a fresh region of unsegregated grain boundary, so that growth occurs in a stepwise manner (Hippsley, 1985). This model is similar to that for hydrogen-induced cracking, except that sulphur atoms occupy substitutional, not interstitial（间隙）, locations and the detailed mechanism of migration is not the same. Reheat Cracking 6.2 Weldability of low carbon steel The second proposal assumed that intergranular sulphides, which are originally precipitated by quenching from high austenitizing temperatures（奥氏体化温度）, dissolve when they are exposed on the crack surface. Elemental sulphur so formed then diffuses across the surface to the crack tip, enabling brittle fracture to progress at a steady rate. For both models, the rate of crack growth is governed by the rate of diffusion of sulphur.Reheat cracks may also form or extend in service if the welded component is operating at elevated temperature and if joints are exposed to tensile stress, due to either inadequate stress relieving or service loads. Reheat Cracking 6.2 Weldability of low carbon steel Reheat Cracking Reheat cracking tests may be divided into three types: self-restraint tests, high-temperature tensile tests and stress relaxation tests. One technique is to make up butt welds（对接焊缝） with about two-thirds of the weld completed. The samples are cut into strips, and the strips welded to an austenitic stainless-steel bar. This assembly is then heated and held for 2 h at the postweld heat treat