Characterization on the gradient microstructure near the fusion interface of dissimilar metal between high Cr heat-resistant steel and Ni-based Alloy 617

The gradient microstructure occurring adjacent to fusion interface of dissimilar metal between high Cr steel and nickel-based Alloy 617 was characterized in detail, and its formation mechanism was discussed in this paper. The island-like and peninsula-like structures present various characters due to difference in elements mixing during welding process. It is found that an unmixed zone (UMZ), a partial mixed zone (PMZ) with lamellar structure and a transition zone (TZ) together make up island-like gradient microstructures. In UMZ, the tempered martensite with much finer equiaxed grains was the main microstructure. In PMZ, nickel-deficient ferrite as the dendritic core, which is separated by nickel-rich fresh lath martensite in interdendritic region, lead to the lamellar structure. The formation of fresh martensite showed phase transition happened under post weld heat treatment (PWHT), which is also verified by the higher nanohardness. In TZ, the occurrence of carbide-free ferrite indicates the carbon diffusion was inhibited by the reaustenitization of TZ. It is also found that the formation of gradient microstructure has close relationship with the angle (0) between tangents of fusion line and the interface of multi-layer, it is easier to form gradient microstructure as theta > 0 due to complex fluid flow of steel. So, suitable design of multi-layer in weld could help in controlling the formation of this kind of gradient microstructure.