Effect of Titanium 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 and 1273K in air. The microstructures of the scale and scale/metal interface were investigated in detail using scanning electron microscopy-electron backscatter diffraction, field emission-transmission electron microscopy, together with micro-energy dispersive X-ray spectroscopy. Ti addition increased the oxidation mass gains and simultaneously improved the oxidation resistance limit temperature. The formed scale consisted mainly of Cr2O3 regardless of the addition of Ti, but the addition of Ti increased the thickness of the Cr2O3 layer. Moreover, the addition of Ti considerably reduced the grain size of Cr2O3, and this was inferred to increase the oxidation mass gain owing to the easy diffusion of metal ion through grain boundaries. Furthermore, Ti was oxidized in the region underneath the scale/metal interface and formed internal complex oxides, such as Al2TiO5 and Al2Ti7O15 owing to the presence of small amounts of Al in the used steels. theta-Al2O3 also formed in the somewhat deeper region from the interface, where Ti could not be oxidized. The oxygen-getter effect of Ti atoms was postulated to be responsible for improving the oxidation resistance of the alloys.