Fracture Toughness of Different Region Materials from a Dissimilar Metal Welded Joint in Steam Turbine Rotor

This study systematically evaluated the fracture toughness of a CrMoV/NiCrMoV dissimilar metal welded joint (DMWJ) with buttering layer technology in a steam turbine rotor. The fracture resistance curves and parameters of base metals (BM-1 and BM-2), weld metal (WM), buttering layer (BL), and heat-affected zones (HAZ-1 and HAZ-2) in the welded joint were all obtained. The characteristic microstructures, carbides, and fracture surfaces were observed by optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results revealed a different fracture toughness of each region in the DMWJ. The BM-1 showed a brittle fracture mode, mainly related to the directional needle-shaped carbide M3C. However, HAZ-1, BL, WM, HAZ-2, and BM-2 illustrated ductile fracture mode. The tempered microstructure and dispersed carbides increased the toughness of each material. Except for BM-1, the ductile fracture toughnesses of BL and WM were low in DMWJ due to coarse spherical carbide M7C3. The fracture toughness in the middle of HAZs was higher than that of the corresponding BMs owing to the fine tempered martensite and bainite. The fracture toughness along DMWJ appeared uneven. In sum, these findings look promising for the accurate integrity evaluation of DMWJs.