High pressure nitriding of austenitic stainless steels via hot isostatic pressing

In austenitic stainless steels, nitrogen (N) is a known alloying element that has been demonstrated to improve the mechanical strength and chemical resistance of the material when dissolved interstitially in the matrix. Consequently, efforts are being made to enhance the level of dissolved N. The solubility of N in an alloy is significantly influenced by the nitrogen partial pressure (p N2 ), as well as by the temperature and chemical composition. By increasing p N2 , the total N content introduced into the alloy can be increased. This paper investigates the feasibility of a formerly unexplored approach of a high-pressure nitriding heat treatment in a hot isostatic press (HIP) with integrated gas quenching to dissolve elevated amounts of N in the austenitic stainless steel X2CrNiMo17-12-2 (DIN EN 1.4404, AISI 316 L). The heat treatment is performed in an N 2 atmosphere, resulting in a high p N2 during the process. At the end of the treatment, rapid gas quenching is performed to preserve the microstructure developed during the HIP process as the material cools to room temperature. The high pressure of the thermochemical HIP heat treatment results in the thermodynamic stabilization of the austenite, thereby enhancing the solubility of N in the alloy. The analysis of the heat-treated specimen indicates that this, in combination with the elevated p N2 , results in an increase in hardness due to substantial amounts of interstitially dissolved N of up to 0.7 wt% within the austenite, in addition to precipitates in the surface, where the solubility of N in the matrix was exceeded.