Effect of Ti Addition on Oxidation Behavior of High-Purity Ferritic Stainless Steel

The effect of Ti addition on the oxidation resistance of high-purity 19%Cr ferritic stainless steels has been investigated during isothermal heat treatment at temperatures between 1073 K and 1273 K in air. The microstructures of scale and the scale/metal interface were investigated in detail by means of SEM-EBSD and FE-TEM together with μ-EDS.The Ti addition increases the oxidation mass gains but simultaneously improves the critical temperature of the oxidation resistance. The formed scale is mainly Cr2O3 regardless of the addition of Ti but Ti addition increases the thickness of Cr2O3. Considerable reduction in the grain size of Cr2O3 by Ti addition was recognized, which was inferred to increase the oxidation mass gain due to the easy diffusion through the grain boundary. Furthermore, Ti is oxidized beneath the scale/metal interface as internal complex oxides, such as Al2TiO5 and Al2Ti7O15 because of the small amount of aluminum in the steels used. In a somewhat deeper region from the interface, θ-Al2O3 forms where Ti cannot be oxidized. The oxygen getter effect by Ti atoms is postulated to be responsible for improving the oxidation resistance.