Solutions to improve surface hardness of stainless steels without loss of corrosion resistance

Abstract

Ion nitriding is an extensively industrialised process enabling steel surfaces to be hardened by nitrogen diffusion, with a resulting increase in wear, seizure and fatigue resistance. Unfortunately, its direct application to stainless steels, while enhancing their mechanical properties, also causes a marked degradation in their corrosion resistance. By adapting the plasma process however, it is possible to reconcile improvement of tribological properties with maintenance of corrosion resistance. This may be obtained by working with base nitrogen and/or base carbon plasmas. The material properties obtained with a nitrogen plasma are discussed, using, as an example, the application of this process to the control rod clusters of pressurised water nuclear reactors (PWR). This nitriding process is then compared with low temperature nitriding and/or carburising thermionically enhanced low pressure plasma processes which have been recently developed. Following this type of treatment it is possible to increase the wear rate resistance of austenitic stainless steels by 60 to 700 times, while fully conserving the corrosion resistance of the untreated alloy.