Microstructural changes in creep exposed P91 steel weld joint

Cast plates made of 9Cr – 1MoV creep resistant steel were joined together using the GTAW and SMAW methods. Creep testing was carried out at a temperature range from 525°C to 625°C and a stress range from 50 to 240MPa. The microstructure of crept specimens was compared with weld joint at conditions after PWHT and the base material before welding. Substructures were investigated in the base material, the heat affected zones and the weld metal. Different kinetics of precipitation reactions in individual zones caused changes in creep strength and unexpected transition from Type IV cracking to Type I cracking at long creep exposure at 625°C. After creep tests at temperatures above 550°C for about 30,000 hours, coarse particles of Laves phase were observed. In addition, small plates of Z-phase were detected after creep tests at 625°C. Precipitation of Laves phase occurred in the weld metal predominately and caused depletion of solid solution about molybdenum. If the lowest stress were applied at 625°C, the growth of the Laves phase was the dominant process, which caused a significant softening and rupture in the weld metal. This effect prevailed over accumulation of cavitation failure in the heat affected zones that caused fracture of the other specimens.