Influences of Mo/Zr minor-alloying on the phase precipitation behavior in modified 310S austenitic stainless steels at high temperatures

High-Cr/Ni austenitic stainless steels (ASSs) have attracted more attention as fuel cladding materials of super-critical water reactors due to their excellent comprehensive properties. In order to further improve their microstructural stability at high temperatures, the present work investigated systematically the influences of Mo/Zr contents and Zr/C ratios on the phase precipitation behaviors and mechanical properties of modified 310S ASSs. The designed alloy ingots were hot-rolled, solid-solutioned at 1423 K for 0.5 h, stabilized at 1173 K for 0.5 h, and then aged at 973 K for different hours. The microstructure and precipitated phases at different heat-treatment states were characterized with OM, SEM, EPMA and TEM, respectively. All the results indicated that the excess addition of Mo and Zr and the inappropriate Zr/C ratios would promote the formation of Cr23C6, G-Ni16Si7Zr6 and (Ni, Fe)(23)Zr-6 phases, resulting in the s phase precipitation at the early stabilization stage. Furthermore, the formation mechanism of sigma phase was discussed. The effects of the precipitated phases on the mechanical properties of alloys were then studied. It was found that the Fe-22Ni-25Cr-0.046C-0.37Mo-0.35Zr (wt%) alloy with appropriate Mo content and Zr/C ratio of 1/1 exhibits the best microstructural stability and good tensile mechanical property, in which only a few s particles are precipitated from the matrix even after aging at 973 K for 408 h.