核用SA508-4N钢粗晶区再热裂纹敏感性评价

采用等温恒速拉伸方法评价了核用SA508-4N钢焊接粗晶区(CGHAZ)的再热裂纹敏感性。使用激光共聚焦、扫描电镜和透射电镜观测和分析粗晶区和断口处的组织、裂纹以及断口形貌，结果表明：SA508-4N钢的母材为回火马氏体，较高含量的碳和铬影响碳化物的大小和分布状态，粗晶区马氏体的形成不利于抑制再热裂纹的产生。碳化物的析出使晶内和晶界的强度不同，当晶内强度大于晶界强度时形成沿晶脆性断裂；当晶内和晶界强度的差距较小时断裂形式包括穿晶和沿晶断裂。SA508-4N钢粗晶区对再热裂纹不敏感，成分A粗晶区的抗再热裂纹能力比成分B的高。在实际生产中，优选的工艺参数为：成分A、焊接t_8/5为25 s、焊后热处理温度580℃。

Reheat Cracking Susceptibility in Coarse Grained Heat-affected Zone of SA508-4 Steel for Nuclear Pressure Vessel

The reheat cracking susceptibility of the coarse-grained heat affected zone (CGHAZ) of SA508-4N steel for nuclear pressure vessel was evaluated by stress-rupture tests at high temperature. The microstructure of the CGHAZ and base metal (BM), as well as the crack- and fracture- morphology were characterized by means of Laser confocal microscope, scanning electron microscopy and transmission electron microscopy. The results show that the microstructure of base metal is tempered martensite, while carbon and chromium content can affect the size and distribution of carbides. The formation of martensite in CGHAZ is not conducive to the suppression of reheat cracking. The precipitation of carbides causes the difference in the strength for grain and grain boundary. When the strength of grain is greater than that of grain boundary, intergranular brittle fracture could emerge. However, when the strength difference between the grain and grain boundary is small, both of the transgranular- and intergranular-fractures could occur. The CGHAZ of SA508-4N steel is not sensitive to reheat cracking, and the resistance to reheat cracking in the CGHAZ of steel A is better than that of Steel B. It follows that the optimum parameters in the actual welding for steel A are as follows: the welding t_8/5 is 25 s, and the post weld heat treatment temperature is 580^oC.