High Temperature Oxidation Behavior of Chromium Ferritic Stainless Steels

Abstract

The relation between the effects of C, N, Cr and Si contents on the oxidation resistance and the metallurgical structure of 1117%Cr steels has been studied at temperatures lower than 1 000°C. C and N have very harmful effects, and Cr and Si have very beneficial effects. It has been found, however, that their effects depend on the austenitic phase precipitates at the oxidation temperature, that is, the appearance of the austenitic phase has harmful effect against the oxidation resistance. It is suggested that decreasing C and N contents or adding stabilized elements such as Zr, Ti and Nb is effective against the oxidation resistance. Among the stabilized elements, Zr is beneficial, but Ti and Nb are not very effective. Difference of their behaviors depends on the stability of their carbides and nitrides at 1 000°C. Because Zr compound does not decompose at 1 000°C, the austenitic phase does not precipitate, whereas Ti or Nb compound decomposes at 1 000°C and the austenitic phase precipitates in Ti- or Nb-containing steel. The steel containing high C and the steel containing Nb or Ti have austenitic phases mainly around the grain boundaries at 1 000°C. It has been found that the unusual oxidation phenomenon occurs from the precipitation sites of austenitic phase, of which Cr content is less than that of ferritic phase that is determined by means of optical microscope, EPMA, IMMA and so on.