Study on Type IV Cracking Behavior of T92/Super304H Dissimilar Steel Welded Joints in Long-Term Service

The microstructure and residual mechanical properties of several groups of T92/Super304H dissimilar steel welded joints (hereinafter referred to as welded joints) in service for 70,000 similar to 85,000 h were analyzed. The results show that the early service history of the welded joint results in the polygonization of the martensite lath and the coarsening of the precipitated phase on the side of T92 steel. In the further creep process, the cavities nucleate along the precipitated phase interface and the triple junction grain boundary. Under the same load, the creep life of the joint decreases rapidly with the increase in service time and, finally, type IV cracking occurs. Type IV cracking needs to meet two conditions: 1. a large degree of precipitated phase coarsening and cavity nucleation in the fine grain heat-affected zone (FGHAZ) and 2. a much lower loading stress than the yield strength.