Microstructure evolution and mechanical properties of 16Cr–25Ni superaustenitic stainless steel weld metal during 600 °C aging

The effect of high temperature aging on the microstructure evolution and mechanical properties of 16Cr–25Ni superaustenitic stainless steel weld metal was investigated at 600 °C for up to 5000 h. Interdendritic micro-size M6C carbide, nano-size M6C and M23C6 carbides were identified in the 200 h aged weld metal, which appeared in clusters and distributed dispersedly. With the aging time increased to 1000 h, the interdendritic region was filled with newly formed precipitates, which were identified as nano-size M12C and M23C6 carbides. Compared with M6C carbide, M12C carbide was preferred to precipitate for lower Gibbs free energy, and the pre-existed M6C carbide had transformed to M12C carbide through the release of C atoms to the matrix. As the aging time further increased to 5000 h, the M12C carbide kept stable with the M23C6 carbide. Due to the precipitation and coarsen of M12C and M23C6 carbides that consumed massive solute atoms from matrix, which weakened the precipitation strengthening effect and suppressed the dynamic strain aging, the tensile properties kept superior stability during aging. The evolution of impact toughness was also closely related to the interdendritic precipitates. As the interdendritic region was occupied by precipitates since the aging time increased from 200 h to 1000 h, the impact toughness rapidly decreased from 76 J to 39 J, and then slightly decreased to 33 J with the aging time increased to 5000 h

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

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