Degradation of impact toughness due to formation of R phase in high nitrogen 25Cr-7Ni-Mo duplex stainless steels

Abstract

Influences of the formation behavior of the R phase on the hardness and the impact toughness were experimentally studied for high nitrogen duplex stainless steels containing 25 mass% of Cr, 7 mass% of Ni, 3 to 5 mass% of Mo, 0.14 to 0.27 mass% of N, and 0.02 mass% of C. The steels were solution heat treated at 1323 K for 3.6 ks, and then aged at 873 K for various times between 1.0 and 1.08×104 ks. For the aged steels, the hardness and the impact toughness were examined with a Vickers hardness test and a Charpy impact test, respectively, whereas the microstructure was observed by transmission electron microscopy as well as by optical microscopy. The experimental results indicate that thin and fine platelet precipitates of the R phase are formed along the {110} planes of body-centered cubic (b.c.c.) ferrite (α phase) at early stages of the aging. The formation of the platelet precipitates of the R phase causes the brittle fracture of the α phase and thus the significant degradation of the impact toughness.