Plasma Thermochemical Processing of Austenitic Stainless Steel for Combined Wear and Corrosion Resistance

Plasma nitriding has long been used to harden the surfaces of austenitic stainless steels. However, conventional nitriding at temperatures above 500°C normally results in a significant reduction in the corrosion resistance of material, due to the formation of chromium nitride precipitates in the nitrided case and the depletion of free chromium from the substrate. Attempts have been made in the past decade to tackle this problem mainly by lowering the nitriding temperature. It has been established that when the nitriding temperature is sufficiently low, precipitation of chromium nitrides can be suppressed and a hard and corrosion resistant layer, known as S phase, can be produced on austenitic stainless steel surface, achieving combined improvement in wear, fatigue and corrosion resistance. More recently, it has been found that carbon can also be used as the major alloying species to engineer the surfaces of austenitic stainless steels. This has led to the development of a low temperature plasma carburising process for austenitic stainless steels to achieve not only improved wear and corrosion resistance, but also enhanced load bearing capacity and toughness. The present paper discusses various aspects of both the low temperature plasma nitriding (LTPN) and low temperature plasma carburising (LTPC) processes, in terms of process characteristics, structural evolution, mechanical and chemical properties. The industrial potential of this class of surface engineering technology in diverse sectors including the valving, nuclear and biomedical sectors are also discussed.