오스테나이트계 스테인레스강에 대한 질소 고용화 처리 및 그 효과

Abstract

As a case hardening process for stainless steels, nitriding is more preferred and widely used than carburizing which deterioates corrosion resistance severely. In order to add the nitrogen into the stainless steels, passive film on the surface must be removed effectively before nitriding. Conventional gas nitriding process is performed in the temperature range of 500 to 600℃ with NH₃ gas, which often leads to sensitization of stainless steels. In this study, we tried to activate passive film of austenitic stainless steels by heating at low pressure.(900℃, 5×10^(-2) Torr.) Nitriding was performed at the solution treatment temperature of 1100℃ with nitrogen molecules instead of NH₃gas. An attainable nitrogen content in a case depends on the nitrogen gas pressure at constant nitriding temperature. A case depth is proportional to the square not of solution time, which suggests that inward diffusion of nitrogen follows the Fick`s 2nd law. Surface nitrogen atoms are dissolved as interstitial solutes, or precipitated in the form of MN, M₂N nitrides, which increase the case hardeness. Dissolved nitrogen in the case enhances the cavitation resistance of austenitic stainless steels dramatically.