Beneficial Effect of Sulfur State on High-Temperature Steam Oxidation Resistance in High Chromium Ferritic Steels

Abstract

The presence of sulfur (S) at an impurity level in high chromium (Cr) ferritic steels improves remarkably high-temperature steam oxidation resistance. However, it still remains unknown which S state in the steels gives such a beneficial effect. There are two possible S states in the steels; one is the soluble S state in a solid solution of the steel, and the other is the precipitated S state which occurs through the sulfide formation or the S segregation in grain boundaries. Either state appears depending on the S content and the heat-treatment temperature. In this study in order to elucidate the effective S state, high-temperature steam oxidation resistance was investigated with high Cr ferritic steels, by changing the S states in them by proper heat treatments. As the result, it was found that the precipitated S state operated more effectively to the improvement of steam oxidation resistance, as compared to the soluble S state.