Retardation of 20%Cr steel oxidation with laves phase precipitation

A new method to improve oxidation resistance of ferritic stainless steel was studied. Increase in oxidation resistance of ferritic stainless steel with laves phase precipitation was succeeded in addition to the conventional method such as addition of rare-earth element. Ultra low carbon 20%Cr steels with various niobium content continuously oxidized at 1073-1173K in air. A mass gain with exposure at 1073K decreased with increasing niobium content and saturated more than 0.1% in niobium content. In steels contained niobium less than 0.1%, mass gain were also decreased with increase in initial ferritic grain diameter. In steels contained niobium more than 0.1%, laves phase was observed at the ferritic grain boundary by using transmission electron microscope. But then a mass gain with exposure at 1173K did not decreased with niobium content and laves phase was not observed. On the assumption that oxidation behaviors of steels contained niobium more than 0.1% obey the parabolic law, the parabolic rate constant obtained at 1173K is similar to that controlled by grain boundary diffusion, and parabolic rate constant obtained at 1073K is similar to that controlled by lattice diffusion. These results suggest that precipitation of laves phase retard growth of oxide film effectively with suppressing grain boundary diffusion of cation forming oxide film.