High temperature gas nitriding of Cr containing martensitic steels

High temperature gas nitriding (HTGN) was performed on 9 different Cr containing martensitic steels by exposing the materials to 1 bar of N₂ gas at 1110 °C for 7.2 ks. Thin foils were used in order to obtain a homogeneous distribution of nitrogen. The experimentally measured nitrogen contents were compared with thermodynamic predictions using the commercial software Thermo-Calc. Additionally, the microstructure of the material was evaluated after conventional cooling to room temperature, as well as after additional treatment at cryogenic temperatures, applying optical microscopy and hardness measurements. Information was compared with the results of well-established predictive equations for the austenite-to-martensite transformation start temperature, M_S. Data shows excellent agreement between predicted and measured nitrogen content in the steels, with the exception of materials characterized by the presence of large bulky carbides, where the predictions significantly overestimate the N uptake. In absence of bulky carbides in the material microstructure, the phase distribution of austenite and martensite can be predicted applying existing equations for M_S.