Improve oxidation resistance at high temperature by nanocrystalline surface layer

Abstract

An interesting change of scale sequence occurred during oxidation of nanocrystalline surface layer by means of a surface mechanical attrition treatment. The three-layer oxide structure from the surface towards the matrix is Fe 3 O 4, spinel FeCr 2 O 4 and corundum (Fe,Cr) 2 O 3, which is different from the typical two-layer scale consisted of an Fe 3 O 4 outer layer and an FeCr 2 O 4 inner layer in conventional P91 steel. The diffusivity of Cr, Fe and O is enhanced concurrently in the nanocrystalline surface layer, which causes the fast oxidation in the initial oxidation stage. The formation of (Fe,Cr) 2 O 3 inner layer would inhabit fast diffusion of alloy elements in the nanocrystalline surface layer of P91 steel in the later oxidation stage, and it causes a decrease in the parabolic oxidation rate compared with conventional specimens. This study provides a novel approach to improve the oxidation resistance of heat resistant steel without changing its Cr content.