Nickel and Nitrogen Alloying Effects on the Strength and Toughness of Austenitic Stainless Steels at 4 K

Abstract

The tensile strength and fracture toughness at 4 K were studied as a function of Ni (6–15 wt.%) and N (0.90–0.28 wt.%) contents for eight austenitic stainless steels. Results indicate that Ni increases the tensile yield strength and decreases the fracture toughness, K_Ic(J), and Ni has little effect on tensile yield strength but increases the fracture toughness. The temperature dependence of the yield strength is given by σ_y = σ_o e^-AT, where σ_o is the yield strength at O K, and A is the slope of In σ_y vs. T. The parameter A is proportional to the stacking fault energy. Lower Ni alloys exhibited brittle facets on fracture surfaces. The quality index, a new parameter = σ_y • K_Ic(J), relates to the capacity of the alloy to achieve greater strength or toughness, but not at the expense of the other parameter. Nickel alloying increases the quality factor; nitrogen has little effect.