ESSC & DUPLEX 2019

In the present work high and low temperature (surface) hardening of X30Cr13 (AISI 420) martensitic stainless steel is addressed. High temperature solution nitriding (HTSN) was performed on thin specimens for obtaining a uniform distribution of nitrogen over the specimen thickness, reflecting the equilibrium nitrogen solubility. Three different nitrogen pressures were analysed, leading to three different equilibrium nitrogen contents in the steel and associated microstructures. The purpose of this work is to investigate the nitrogen absorption as a function of nitrogen pressure and the fraction of retained austenite after martensite formation (on cooling) as a function of nitrogen content. Also, low temperature surface hardening (LTSH) of the three microstructures obtained by HTSN, was investigated. LTSH was achieved by gaseous nitriding in ammonia gas in the temperature range 380 to 420°C. The process was performed in ammonia for a fixed nitriding time, with the aim of analysing the nitrogen content and the final microstructure obtained as a function of LTSH temperature. This work leads to a better understanding of the combination of high and low temperature hardening for optimizing a heat treatment chain in order to improve the surface performance of this martensitic stainless steel (fatigue and tribological properties, i.e. adhesion and galling resistance) without impairing, but preferably improving the corrosion resistance. ThermoCalc is used for calculating the appropriate HTSN parameters. Reflected light microscopy, scanning electron microscopy, with energy dispersive spectroscopy, EDS, and electron back-scattered diffraction, EBSD, and X-ray diffraction were applied for the characterization of the morphology and composition of the developing case. Micro-hardness Vickers indentation provides information on the depth dependence of the hardness.