ESSC & DUPLEX 2019

High temperature solution nitriding (HTSN) of stainless steel was originally developed by Prof. Berns in the early 1990s and is in many aspects the “stainless” analogue to carburizing of steels. The process entails dissolution of nitrogen in stainless steel at temperatures above, say, 1050°C, from an atmosphere consisting of molecular nitrogen at a fixed partial pressure. Austenitic, ferritic, duplex and martensitic stainless steels can all be high temperature solution nitrided. In general there are two different strategies or reasons for performing HTSN: i) to obtain a nitrogen-enriched austenitic case on austenitic, duplex and high Cr ferritic stainless steels for improved corrosion performance, or ii) to obtain a nitrogen containing martensitic case on martensitic and low Cr ferritic stainless steels for improved wear, fatigue and corrosion performance. Hitherto, the HTSN process has been somewhat niche in industry owing, in part, to the high temperatures involved and in particular to the inherent challenges associated with the process and the resulting microstructures. The present contribution will provide an overview of the state of the art of HTSN and associated heat treatment processes for different classes of stainless steels - including the different challenges associated with HTSN. In addition, new developments in the optimization of the HTSN process and the resulting microstructures will be presented. In the examples shown, emphasis will be given to treatment of martensitic stainless steels, where special heat treatment processes are required in order to exploit the full potential of HTSN, and to austenitic stainless steels where drastic improvements in wear and corrosion resistance can be achieved – in particular when combined with low temperature surface hardening processes. It is anticipated that HTSN will gain more industrial interest in the future. Particularly, the rapid development in additive manufacturing of stainless steel products could benefit from improved materials properties and performance as provided by HTSN.