Effect of Grain Boundary on Microcracking of Austenitic stainless steel multi-pass weldments

在奥氏体金属材料的多道焊件中，经常观察到微裂纹。为了阐明奥氏体不锈钢多层焊件微裂纹产生的机理，研究了微裂纹与焊接条件的关系。研究了第一层焊缝晶界组织与取向差的关系对微裂纹的影响，发现微裂纹主要发生在第一层焊缝靠近第二层的区域，沿沿着分布。由于焊件组织为全奥氏体，裂纹严重加剧，第二层焊接速度较低，增加了第一层焊件微裂纹的倾向。从显微裂纹断口的SEM观察来看，裂纹的产生是由于晶界的液化和塑性下降。一般来说，在第一层焊件中，在具有大取向差的晶界处观察到由于后续运行而导致的显著的微裂纹倾向。分析认为，晶界处的取向差较大，S、P等杂质元素偏聚较多，是导致微裂纹产生的主要原因。而用CMA和EPMA方法观察到的晶界偏聚现象均未出现，可以认为偏聚晶界由于其宽度较窄而无法检测。

In multi-pass weldments of austenitic metallic materials, microcracking is frequently observed. In this study, to clarify mechanism of microcracking occurring in austenitic stainless steel multilayer weldments, relation between microcracking and welding conditions were investigated. And also effect of relation between misorientation and structure of grain boundary in 1st layer of weldments on micro-cracking was studied.Microcracking occur along grain boundaries in the region of 1st layer which locate vicinity of 2nd layer of the welds. As microstructure of weldment is fully austenitic, cracking is very enhanced, and lower welding speed of 2nd layer increase microcracking tendency in 1st layer of weldments.Judging from the observations of fracture surface of microcrack by SEM, cracking occur due to liquation and ductility-dip of grain boundaries.In general, pronounced microcracking tendency in 1st layer of weldments due to following run is observed at grain boundary having large misorientation. It seemthat more segregation of impurity elements, such as S and P, occur at grain boundary having large misorientation cause microcracking. Nevertheless, the segregation at grain boundary observed by CMA and EPMA methods could not be found. It may be concluded that segregational grain boundary could not detect by reason of its narrow width.