High and Low Temperature Surface Hardening of Martensitic Stainless Steels

In the present work high and low temperature surface hardening of X30Cr13 (AISI 420) martensitic stainless steel is addressed. In particular, high temperature solution nitriding (HTSN) of thin samples is investigated. The use of thin samples makes it possible to obtain fully homogenous materials. Three different nitrogen partial pressures were analysed, leading to three different equilibrium nitrogen contents and resulting microstructures. This makes it possible to investigate the nitrogen absorption as a function of partial pressure and the fraction of retained austenite as a function of nitrogen content. Moreover, low temperature surface hardening (LTSH) by means of gaseous nitriding in NH3 was carried out on the three HTSN microstructures at different temperatures. Thus, insight is obtained in the combining high and low temperature surface hardening for optimizing the performance of the steel with respect to fatigue and wear, without impairing, but preferably improving, the corrosion performance. Additionally, the experiments offer information on the effect of the heat treatment cycle (thermal impact). ThermoCalc was applied adjust the appropriate HTSN parameters. Reflected light microscopy and X-ray diffraction are applied for the characterization of the morphology and phase constitution of the developing case. Micro-Vickers indentation provides information on the depth dependence of the hardness.