Evolution of microstructure in nickel-based C-HRA-2 alloy during welding thermal simulation

Abstract The welding heat-affected zone (HAZ) of C-HRA-2 nickel-based alloy at different welding peak temperatures (T_p) (850 to 1450 °C) was obtained through welding thermal simulation. The microstructure of the different simulated HAZs was characterized by an optical microscope (OM), a scanning electron microscope (SEM), and a transmission electron microscope (TEM). The microhardness of simulated HAZs was examined using a microhardness tester. The results indicated that fine and dispersed M₂₃C₆ carbides were identified along the grain boundaries in the simulated HAZs at 1050 and 1150 °C. In the simulated HAZs above 1250 °C, eutectic microstructure of γ matrix bonded with M₂₃C₆ carbides was identified close to the grain boundaries due to constitutional liquation. The liquefaction of γ matrix and M₂₃C₆ carbides primarily accounted for the simulated HAZ liquation of C-HRA-2 alloy during continuous welding heating. The content of the liquefied phase was increased as T_p was increased. The liquefied phase was converted into the eutectic microstructure of γ matrix and M₂₃C₆ carbides in the cooling process. Besides 1050, 1150, and 1450 °C, the microstructures under other T_p were similar to the microstructure of the Base Metal (BM). The microhardness distribution of simulated HAZs tended to be higher in the middle and lower at both ends with the increase of T_p. Furthermore, the result indicated that the microhardness of the simulated HAZ was the closest to BM at 1050 °C.