High-strength and high-toughness austenitic stainless steels based on type 316LN at 4.2 K

High-strength and high-toughness prototype steel forgings, N2, N3, and N4, were developed based on type 316LN as a cryogenic structural material. By extrapolating the proportional relationship between carbon plus nitrogen content (C + N) and 0.2 % proof stress (sigma 0.2) in the range of approximately 0.10-0.25 mass% C + N content, the C + N content was successfully designed to achieve the target sigma 0.2. To suppress the degradation of the plane-strain fracture toughness (KIC(J)) while increasing the nitrogen solid-solution content, the alloying level was designed by considering the stability of the austenite phase and stacking fault energy. The steel forgings showed a good balance of strength and toughness at 4.2 K, i.e., N2 (sigma 0.2 = 1142 MPa, KIC(J) = 228 MPa root m), N3 (sigma 0.2 = 1226 MPa, KIC(J) = 219 MPa root m) and N4 (sigma 0.2 = 1296 MPa, KIC(J) = 215 MPa root m). The sigma 0.2 and KIC(J) of ITER-grade 316LN (316LNL, 316LNM, and 316LNH) thick plates with low interstitial (C + N) content of 0.13-0.17 mass% were also evaluated at 4.2 K. Then, the effects of nitrogen content on strength and fracture toughness were successfully clarified in the full range of interstitials concentrations. An alloy design with higher Ni, Mn, and Mo contents in the chemical composition range of 316LN would be desirable for improving the fracture toughness by increasing sigma 0.2 due to the higher nitrogen content.