ESSC & DUPLEX 2019

The case developing during low temperature surface hardening of austenitic stainless steel by nitriding, carburizing or nitrocarburizing consists of a supersaturated interstitial solid solution of nitrogen and/or carbon in austenite. The favorable wear, fatigue and anti-corrosion properties of this so-called expanded austenite depend on the profiles of interstitial concentration and associated composition-induced residual stress over the case. The prediction of composition and stress profiles for a certain steel grade from the process parameters gas composition, temperature and treatment time would enable targeted surface engineering of stainless steels. Furthermore, such a numerical model would enable the design of new stainless steel grades that are tailored for optimal performance during low temperature surface hardening, equivalent to the well-known nitriding steels. Preferably, such a numerical model departs from the state of knowledge of microstructure evolution in order to arrive at accurate predictions. For this purpose the availability of fundamental parameters as determined experimentally on homogeneous powders and foils under well-defined conditions is essential. In the present contribution an overview of the current state of understanding of the evolution of the microstructure, composition and residual stress during low temperature surface hardening of stainless steels is presented. The presentation showcases the joint achievements of many co-workers in the last 20 years. The overview concerns theoretical, experimental and modelling aspects of expanded austenite, both as a homogeneous phase and as a case on stainless steel.