Oxidation Behavior of High-speed Steels in Dry and Wet Atmospheres

Abstract

The oxidation behavior of high-speed steel (HSS), which is used as the work rolls in hot strip mills, was examined under both wet and dry oxidation conditions. In a dry atmosphere, carbides as well as the martensite matrix were oxidized, while only the matrix was oxidized in the wet atmosphere. After dry oxidation, the M₂C-, M₆C- and M₇C₃-type carbides maintained their original shapes, while the MC-type carbides were oxidized into parallelepiped (orthorhombic) crystals. The parallelepiped oxides were easily removed from the sample surface due to their low adhesion strength. Double-layered oxides were formed after oxidizing the matrix, in dry as well as wet atmospheres. The outer layer showed a dense structure after dry oxidation, while a columnar and porous layer was formed in the wet atmosphere. In the early stages of oxidation, the high-speed steels oxidized following the parabolic rate law in both the dry and wet atmospheres. The parabolic weight change transformed to a linear one, when the oxide thickness was >1.3 μm by dry oxidation. The transition to a linear weight change was not observed in the wet oxidation. It is believed that, in a dry atmosphere, cracks occurred due to stress accumulation in the oxide layer, while the porous oxide layer hindered crack formation during wet oxidation.