Effect of Preheating in Air on Gas Nitriding of Austenitic Stainless Steels

Three types of austenitic stainless steels, SUS304, SUS316 and SUS310, were preheated in air in the temperature range of 293 to 1273 K after diamond polishing or #120 emery paper polishing, followed by nitriding in NH3 gas at 843 K for 20 h. The influence of atmospheric preheating on nitriding behavior was examined in connection with the composition of the surface film. In the case of diamond polishing, atmospheric preheating in the temperature range of 673 to 1023 K for 1 h in SUS304 steel and in the temperature range of 293 to 1023 K for 1 h in SUS316 and SUS310 steels result in the formation of a uniformly nitrided layer. The depth of nitrided layer reached a maximum at the preheating temperature of about 1000 K in SUS304 steel, in the preheating temperature range of 800 to 1000 K in SUS316 steel and at the preheating temperature of about 800 K in SUS310 steel, respectively. The nitriding reaction was promoted with increasing preheating time in SUS304 and SUS316 steels, but it was independent of preheating time in SUS310 steel. The nitriding reaction after emery paper polishing is inferior to that after diamond polishing. It has been confirmed that gas nitriding of austenitic stainless steels is possible without any chemical treatment, by applying atmospheric preheating which forms the surface film consisting mainly of iron oxides at the surface and containing a low concentration of Cr2O3 and a high concentration of FeO.