Structure and corrosion behaviour of stainless steels after plasma and gas nitriding

Abstract

The microstructures and corrosion properties of ferritic and austenitic stainless steels have been investigated after plasma and gas nitriding at 200-450° C. Little difference was observed between the surface layers produced by the two nitriding techniques: that on ferritic X6Cr17 consisted of ferrite plus epsilon-carbide, that on the austenitic X5CrNi18-10 was the expanded austenite S phase. Pitting resistance of X5CrNi18-10 was improved by nitriding at up to 420° C, but deteriorated for higher nitriding temperatures. The presence of epsilon-carbides in the layer on X6Cr17 slightly reduced pitting resistance for nitriding temperatures up to 440° C. Annealing the nitrided specimens degraded corrosion resistance by redistributing the nitrogen in the surface layer and promoting chromium nitride precipitation.