Type IV cracking and creep damage evolution of P92 steel weldment after long-team service

P92 steels are widely used in Ultra-Supercritical (USC) coal-fired power plants in China. Cracking accidents of P92 welded joints occur frequently. The microstructure evolution and type IV cracking of P92 steel weld joints in an USC power plant at 605 °C for 70,000 h were investigated. The results show that type IV cracks appear in the fine-grained heat affected zone (FGHAZ), and creep cavities are distributed along the grain boundaries with a tendency to coalesce. The highest creep cavity and microstructure damage in the FGHAZ resulted from long-term creep with respect to lower dislocation density, average hardness, average kernel average misorientation (KAM) value, low angle grain boundary (LAGB) ratio, higher recrystallization ratio and equivalent diameter of cavity. The solution strengthening and the precipitation strengthening are reduced due to the coarsening of Laves phase and the precipitation of Z phase in the FGHAZ. These factors collectively promote the generation and propagation of type IV crack.