Plasma post-oxidation of nitrocarburized SUM 24L steel

Plasma nitrocarburizing and plasma oxidizing treatments were performed to improve the wear and corrosion resistance of SUM 24L steel. Plasma nitrocarburizing was conducted for 3 h at 570 °C in the nitrogen, hydrogen and methane atmosphere to produce the ɛ-Fe2–3(N, C) phase. It was found that the compound layer produced by plasma nitrocarburising was predominantly composed of ɛ-phase, with a small proportion of γ′-Fe4(N, C) phase. The thickness of the compound layer was about 10–15 μm and the diffusion layer was about 150 μm in thickness, respectively. Plasma post-oxidation was performed on the nitrocarburized samples with various oxygen/hydrogen ratio at constant temperature 500 °C for 1 h. The very thin magnetite (Fe3O4) layer 1–2 μm in thickness on top of the compound layer was obtained by plasma post-oxidation. It was observed that the thickness of the compound layer decreased with decreasing oxygen/hydrogen ratio. It was also confirmed that the corrosion characteristics of the nitrocarburized compound layer was further improved by the application of the superficial magnetite layer.