Low temperature glow-discharge nitriding of stainless steels

In the last years nitriding treatments of stainless steels have been the subject of intensive research, especially regarding austenitic grades. In order to inhibit the formation of chromium nitrides, nitriding is performed at temperatures lower than about 450 °C, allowing the formation of metastable phases, as the so called S phase in austenitic stainless steels or the expanded martensite in martensitic grades. The aim of the present research was to study the effects of low temperature glow-discharge nitriding on the microstructural, mechanical and corrosion resistance characteristics of AISI 202 and AISI 316L austenitic stainless steels and of AISI 431 martensitic stainless steel. The obtained results show that all the nitriding treatments allow to increase the surface microhardness in respect of the untreated steels. The corrosion resistance of the nitrided samples, tested in 5% NaCl aerated solution with the potentiodynamic method, depends on the treatment conditions. When low amounts of nitrides are formed in the modified surface layers, the corrosion resistance of both AISI 202 and AISI 316L austenitic stainless steels is significantly improved. With the proper treatment conditions, low temperature nitriding allows to decrease corrosion damage also on AISI 431 martensitic stainless steel.