Gaseous surface hardening of martensitic stainless steels

The present work addresses heat and surface treatments of martensitic stainless steel EN 1.4028. Different combinations of heat treatments and surface treatments were performed: conventional austenitisation, cryogenic treatment and in particular high temperature solution nitriding (HTSN) and low temperature surface hardening (LTSH). Controlled low temperature gaseous treatment was monitored with thermogravimetry. Reflected-light microscopy, X-ray diffraction and hardness-depth profiling were applied for the characterisation of the morphology and composition of the developing case. It was found that cubic lath martensite in conventionally austenitised EN 1.4028 dissolves nitrogen and develops expanded martensite (ferrite) during LTSH. HTSN leads to a microstructure of tetragonal plate martensite and retained austenite. The content of retained austenite can be reduced by a cryo-treatment and develops metastable expanded austenite during LTSH. Consistently, the case depth obtained after LTSH was shallowest after a prior cryo-treatment. Hardness values range up to 18 GPa after LTSH.