Effect of La on oxidation properties and mechanism of high-speed steel rolls

The formation mechanism of oxide film on the surface of high-speed steel for rolls was investigated using various characterization methods, and the effect of La modification on the oxidation properties of high-speed steel for rolls was studied. The results showed that differential oxidation occurred in the high-speed steel matrix and eutectic carbide regions, where the MC eutectic region was the first to be oxidized while the M 6 C region was the last to be oxidized. The matrix oxide film has a three-layer structure: the inner layer of Cr 2 O 3 , the middle layer of Fe–Cr spinel oxide, and the outer layer of Fe 2 O 3 /Fe 3 O 4 . In contrast, the MC eutectic region has an additional layer of FeVO 4 in the outermost layer. In addition, the addition of La does not change the type of oxide film formed by isothermal oxidation of high-speed steel. However, the La element improved the size and distribution of MC-type carbides, and these improvements in microstructure make the oxide film more evenly dispersed and ultimately improve the oxidation resistance of HSS used in rolls, where 0.25 wt % of La reduced the weight gain of HSS by 32% when oxidized at 700 °C.