Influence of Excessive Nitrogen on Sensitization and Hardness of a Supermartensitic Stainless Steel

Supermartensitic Stainless Steel (SMSS) might be considered an advance in ordinary martensitic steel grade development, achieved by lowering carbon content and increasing key alloy elements such as nickel and molybdenum. Nitrogen may be added to improve mechanical resistance through a solid solution and, in specific cases, secondary hardening in the tempering treatment. Therefore, this work aims to investigate how the heat treatments might affect both the hardness and the degree of sensitization (DOS) of a SMSS type 13Cr-5Ni-2Mo with N micro-addition (0.18%). In this regard, double loop electrochemical potentiokinetic reactivation (DL-EPR) tests were used to obtain the DOS, after which the surface of specimens was observed in the light optic microscope (LOM) and scanning electron microscope (SEM). Thermo-Calc® analyses indicate several possible deleterious phases, and the X-ray diffraction (XRD) measurements confirm low austenite content after 1-hour tempering. The results showed that this N-alloyed SMSS has higher DOS and hardness than the other 13Cr-5Ni-2Mo without nitrogen tempered in the 550-600°C range. Finally, it was possible to correlate the DOS and hardness.