Effect of heating temperature and atmosphere on the element distribution and microstructure in a high-Mn high-Al austenitic low-density steel

The element distribution and microstructure near surface of a high-Mn high-Al austenitic low-density steel were investigated after isothermal holding at temperatures ranging from 900°C to 1200°C in different atmospheres, including air, N2 and N2 + CO2 mixed atmospheres. The results show that no ferrite formed near the surface of the experimental steel during isothermal holding at 900°C and 1000°C in air, while ferrite formed near the surface when the isothermal temperature reached 1100°C and 1200°C. The fraction of ferrite was larger at 1200°C because more C and Mn diffused to the surface and exuded from steel, which then reacted with N and O to form oxidation products. The thickness of compound scale increased due to the larger diffusion rate at a higher temperature. In addition, after isothermal holding at 1100°C in N2, Al content near the surface reduced slightly, while the contents of C and Mn did not change. Therefore, no ferrite formed near the surface. However, the contents of C and Al near the surface reduced after holding at 1100°C in N2 + CO2 mixed atmosphere, resulting in a small amount of ferrite. The thickness of compound scale was found to be the thickest in N2, followed by N2 + CO2 mixed atmosphere, and the thinnest in air. Overall, the element loss and ferrite fraction were the largest after holding in air at the same temperature. The differences in element loss and ferrite fraction were small in N2 and N2 + CO2 mixed atmospheres, but the compound scale formed in N2 was significantly thicker. Based on these results, the N2 + CO2 mixed atmosphere is the most ideal heating atmosphere for industrial production of high-Mn high-Al austenitic low-density steel.