Micro-cracks of the NiCrFe-7 weld metal with large thickness

Micro-cracks in a NiCrFe-7 weld metal with large thickness were characterized and analyzed by metallographic microscopy, scanning microscopy, and electron backscatter diffraction. Two kinds of welding methods, shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW), were applied in our study. The results showed that the crack was a kind of reheating crack along the grain boundaries in the heat affected zone (HAZ). These cracks were induced by the stress relaxation in the heat affected zone during welding process, which resulted in local stress concentration around the large-size MC carbides and oxides. The cracks nucleated and propagated within these particles or along the interfaces of the particles/matrix. Compared with the GTAW process, there were a large number of oxides in the weld metal of the SMAW process. Therefore, the tendency of reheating cracks in the SMAW was much higher than that of the GTAW, which resulted in the bending properties of the former weld failed to meet the requirements of the national standard. The GTAW process with better effect of protective gas was recommended for the Ni-base weld metal with large thickness.