Active screen plasma nitriding of Fe-24Mn-2Al-0.45C TWIP steel: Microstructure evolution and a synergistic selective oxidation mechanism

In this study, Fe-24Mn-2Al-0.45C twinning-induced plasticity (TWIP) steel was investigated after active screen plasma nitriding (ASPN) at 350–480 °C. Up to 450 °C, the ASPN-treated TWIP steel showed a double-layered structure, with a topmost Fe4N-type γ' layer and an underlying interstitial solid solution. The underlying nitrogen interstitial solid solution obtained on ASPN-treated TWIP steel can be identified as γN(i), being expanded austenite with small lattice expansions at low nitrogen (and carbon) absorption levels. However, the 480 °C ASPN-treated TWIP steel surface is composed of an unexpected MnO-containing ferritic topmost layer and a thick underlying γN(i) layer, which corresponds to a hardness-depth profile different to those obtained at 350–480 °C. Hence, we propose and elucidate a selective oxidation mechanism (that occurred alongside nitrogen inward diffusion) on TWIP steel during ASPN treatment, which can be attributed to (a) an elevated treatment temperature that allowed Mn migration, (b) a processing atmosphere with low oxygen partial pressure, and (c) a high-Mn substrate chemical composition with low Al/Mn (or Si/Mn) ratio.