Abstract
Three forged low-density high manganese steels Mn28Al10, Mn28Al8 and Mn20Al10 were used as experimental materials in this study. The forged microstructure and external oxidation characteristics at 1323K and 1373K for 5–25h in air were investigated by microstructural observation and X-ray diffraction (XRD) technique. The phase compositions and abundance in the forged and oxidized samples were quantitatively obtained by Rietveld method on the basis of XRD pattern analysis. The results showed that an austenitic microstructure formed in steels Mn28Al10 and Mn28Al8, and 18.02wt% ferrite could be found in Mn20Al10. The relative amount of ∼5.28wt% κ-carbide (Fe3AlC0.5) in Mn28Al10 was far greater than that in Mn28Al8 and Mn20Al10. The oxidation kinetics of forged steels oxidized at 1323K for 5–25h had two-stage parabolic rate laws; and the oxidation rate of the first stage was lower than that of the second stage. When they were oxidized at 1373K for 5–25h, the oxidation kinetics followed only a parabolic law and the oxidation rates were respectively greater than those at 1323K for 5–25h. When they were oxidized at 1323K for 25h, detached external scales contained Fe2MnO4 and α-Fe2O3 oxides. α-Al2O3 and (Fe, Mn)2O3 oxides could only be indexed in steels Mn28Al8 and Mn28Al10, respectively. When they were oxidized at 1373K for 25h, Fe2MnO4, Fe3O4, α-Fe2O3 and α-Al2O3 oxides could all be indexed in the external detached scales. The main phase of detached external scales was Fe2MnO4; and the relative amount of α-Al2O3 in steel Mn28Al8 was higher than that in steels Mn28Al10 and Mn20Al. The external oxidation layers of these three forged steels oxidized at 1323K and 1373K for 25h were essentially followed the sequence of α-Al2O3, Fe2MnO4, Fe3O4, FeMnO3, and Fe2O3 from the substrate to the outside surface. The forged Mn28Al10 steel with austenitic microstructure and a certain amount of κ-carbide (∼5.28wt% in the present work) possessed a better combination of strength, ductility, specific strength, and oxidation rate when compared to that of the forged Mn28Al8 and Mn20Al10 steels.
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