Low temperature glow-discharge nitriding of a low nickel austenitic stainless steel

In the last years an increasing interest has been taken in AISI 200 series austenitic stainless steels for use in industrial applications. In these steels nickel is partly replaced by other austenite stabilising elements, like manganese and nitrogen. However, their chromium content is usually lower than that of the largely used Fe–Cr–Ni based AISI 300 series stainless steels, so that their corrosion resistance may be lower. For AISI 300 series steels low temperature nitriding treatments have been successfully employed to increase their corrosion resistance, due to the formation of modified surface layers consisting of the so called S phase. In this research, a low nickel AISI 202 austenitic stainless steel was subjected to glow-discharge nitriding treatments in the range 653–773 K, for 5 h, at 10 hPa, and the microstructural, mechanical and corrosion resistance characteristics of the nitrided samples were investigated and compared to those of the untreated steel. The treatments produce modified surface layers having a double layer structure. When the treatments are carried out at temperatures up to 673 K, the outer modified layer consists mainly of the S phase and a nitrogen induced h.c.p. martensite; in the inner modified layer a solid solution of nitrogen in γ-Fe is present. As the treatment temperature increases, also chromium and iron nitrides are observed in the outer modified layer, and their amount increases as the treatment temperature is higher. High surface hardness values (from 940 to 1750 HK0.025 depending on treatment temperature) have been observed on the treated samples. Corrosion resistance tests, performed in 5% NaCl aerated solution with the potentiodynamic method, show that, with the used treatment parameters, glow-discharge nitriding at temperatures up to 703 K allows to increase significantly the corrosion resistance in respect of the untreated alloy, reducing the anodic current values up to about 5 orders of magnitude.