Oxidation Behavior and Subsurface Phase Transformation of Novel High Mn Cryogenic Steel during Heat Treatment

Abstract

The oxidation behavior and subsurface phase transformation of novel high Mn cryogenic steel Fe–(0.35–0.45)C–(0.2–0.25)Si–(25–25.8)Mn–(3–5)Cr are investigated in the temperature range of 600–1200 °C under air atmosphere. The results show that the weight gain curves of oxidation with time are in good agreement with linear equations during 90 min holding below 800 °C, implying that the interface reaction is the rate‐controlling process. Above 900 °C, the oxidation reaction kinetics is governed by the parabolic rate law due to the formation of thick oxide scale. The inner oxide scale mainly consisting of Cr oxide at 1000 °C is thicker than that at 900 °C, but the thickness of the whole oxide scale is thinner than that at 900 °C. After 90 min holding, the oxidation weight gain at 900 °C is more than that at 1000 °C, which is dominated by the combined effect of thermodynamic and kinetic factors. The Mn‐depleted zone occurs in the steel matrix epidermis, leading to the subsurface phase transformation. The γ‐Fe transforms into α‐Fe in the steel matrix epidermis, which will result in deterioration of steel properties. Protective atmosphere should be adopted for heat treatment process of the novel high Mn steel.