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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.