Precipitation Behavior of Laves Phase in the Vicinity of Oxide Film of Ferritic Stainless Steel: Selective Oxidation-Induced Precipitation

Abstract

Anti-corrosion behavior of Fe–Cr alloys at elevated temperature is greatly influenced by the protective nature of oxide scale formed on them. However, precipitates formed underneath the scale can affect the diffusion of reactive elements, growth mechanism and failure mechanism of oxide film. Here, we elucidate the precipitation mechanism of Laves phase in the vicinity of oxide film during oxidation at 950–1050 °C in air. Two types of automotive exhaust manifold-used ferritic stainless steels were designed by replacing Mo with different contents of W and adding small content of Ce. By systematic investigations of the precipitates in the vicinity of steel/oxide interface, we conclude that Laves phase precipitated near the oxide film is related to the selective oxidation of chromium, which is dramatically different from conventional precipitation mechanism. Nb was localized at the interface due to rejection from the growing Cr2O3 scale, which promoted the formation of Laves phase. The experimental steels were characterized by excellent oxidation resistance and Ti–rich oxide formed in the internal oxidation zone that hindered the further diffusion of oxygen.