ZENG Jiaqi, WANG Baoliang, LI Zewei, XU Hong, SONG Jianhua, ZHOU Haiyan
Through the penetrant inspection of one fixed bed reactor, it was found that there were many tiny cracks in the heat-affected zone of dissimilar steel welding head circumferential weld. The length of cracks was 1-1.5 mm with no bifurcation, and the crack direction was perpendicular to the weld. The surface cracks were analyzed by means of spectrum detection, hardness detection, alternating current electromagnetic field detection and metallographic detection. The results showed that the welding process was unreasonable, because the multi-layer and single-pass welding method generally required a large linear energy, which made the austenitic stainless-steel stay at the sensitization temperature (450-850 ℃) for an excessively long time, resulting in the precipitation of chromium carbide from the grain boundary. As a result, the chromium-poor areas were formed and the material was sensitized. Therefore, the grains were coarser, and the bonding force between grains was further weakened. In addition, the welding of dissimilar steels also caused large welding residual stress in the later cooling process of austenitic stainless steel. Moreover, due to the particularity of austenitic stainless steel, the post-weld heat treatment was generally not carried out, which made this part of residual stress preserved. Under the comprehensive action of the above reasons, some micro-cracks were produced. In view of the above problems, some improvement suggestions were put forward from the aspects of design, manufacture and use.
