Role of Ni content on microstructural and mechanical responses of Nb-stabilized metastable austenitic stainless steel weld metals

Abstract

Three Nb-stabilized metastable austenitic stainless steel (15Cr–9Ni–Nb steel) weld metals with Ni contents of 8.4 wt%, 9.2 wt%, and 10.2 wt% were designed and prepared by gas tungsten arc welding. The effects of Ni content on the solidification behavior, microstructure, and the mechanism of deformation-induced martensitic transformation in the tensile process were investigated via scanning electron microscopy, transmission electron microscopy, scanning transmission electron microscopy, and high-resolution transmission electron microscopy, and their effects on mechanical properties were also discussed. As the Ni content increased, the solidification mode of weld metals changed from ferrite-austenite mode to austenite-ferrite mode, which made the segregation of Nb and Si at the interdendritic region more obvious and increased the total volume and the average size of primary NbC significantly. The large amount of coarse primary NbC in the AF weld metal damaged the impact toughness and reduction of area by inducing intergranular cracks and intergranular fractures. Moreover, both the three weld metals had poor mechanical stability and the deformation-induced martensitic transformation (DIMT) (γ→α′) occurred during the tensile process, which contributed to increased tensile strength. As the Ni content increased, the mechanical stability of austenite increased, and the strengthing effect of DIMT decreased, while the uniform elongation of weld metals increased.