Microstructure characteristics and properties of fusion boundary in 9%Ni steel joint filled with Ni-based alloy

A dilution layer (DL) with homogeneous composition was found between filler metal and base metal in 9%Ni steel joint filled with nickel-based alloy. Despite the presence of high austenitic stabilizing elements, the region adjacent to weld interface was martensitic. The stress occurred at weld interface during solidification process, was the main reason for the martensite transformation in DL. In cryogenic environment, the stress increased owing to the difference of thermal expansion coefficient between base metal and filler metal. The martensite transformation continued further in DL. The martensite in DL was lenticular-like shaped, contained more M-A constituents and less high angle grain boundaries (HAGBs). The results opined that the enrichment of martensite increased the degree of deformation of surrounding area. With the increment of hardness and residual strain in DL, the low temperature toughness decreased. Fatigue crack growth rate (FCGR) test showed that cracks propagation accelerated in DL. The formation mechanism of DL and its influences on cryogenic properties is helpful to improve performance of 9% Ni steel weldment.