27th IFHTSE CONGRESS & EUROPEAN CONFERENCE ON HEAT TREATMENT 2022

High temperature solution nitriding (HTSN) of stainless steel was originally introduced in the early 1990s and is in many aspects the “stainless” analogue to carburizing of steels. Instead of introducing carbon to the surface, nitrogen is used. The process is normally carried out at temperatures above, say, 1050°C, using an atmosphere of molecular nitrogen at a fixed pressure in order to control how much nitrogen is introduced. Austenitic, ferritic, duplex and martensitic stainless steels can all be high temperature solution nitrided but the strategies to obtain optimal properties can be quite different. For martensitic (and ferritic) stainless steel the concept is to obtain a nitrogen containing martensitic case 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 treatment of martensitic stainless steel - including the different challenges associated with HTSN. New developments in the optimization of the HTSN process and the resulting microstructures will be presented. For HTSN treatment of martensitic stainless steels, special heat treatment processes are required in order to exploit the full potential of HTSN. It will be shown that microstructures and properties equivalent to the expensive nitrogen containing “Cronidur30” type martensitic stainless steel can be achieved in conventional martensitic stainless steel grades via HTSN and specialized heat treatment. Such materials are highly effective in combatting failures relating to “white etching cracks” (WEC) – a widely known challenge in many bearing applications.