Oxide scale formation and morphology on austenitic stainless steel at medium annealing temperatures

Abstract

This paper considers the effect that surface finish has on early oxide growth. Austenitic stainless steel of grades 304 and 316L, with ground (240 grit) and polished (1 µm) surface finishes were subjected to temperatures of 750°C and 800°C for times of up to 24 h. It was found that the oxide morphology was heavily dependent on surface finish and substrate microstructure. The ground surfaces were found to initially produce a homogenous oxide scale, while the polished samples formed a scale made up of many oxide islands. The formation of the oxide islands was due to the fast diffusion of chromium through grain boundaries and prior ferrite regions. These fast diffusion paths provided localised oxidation resistance at the surface which caused breaks in the oxide scale. With the ground surface finish, these fast diffusion paths become negated as the grinding of the surfaces creates an even distribution of fast diffusion paths. Mechanisms of early oxide growth in relation to the substrate microstructure and surface finish have been summarised in a schematic diagram.