Comparative study of the corrosion behaviour of plasma nitrocarburised AISI 4140 steel before and after post-oxidation

In this study, the corrosion behaviour of plasma nitrocarburised AISI 4140 steel before and after post-oxidation was investigated. Plasma nitrocarburising was carried out at 530 °C, 570 °C and 630 °C for 5 h in an atmosphere consisting of 80 vol.% N₂, 17 vol.% H₂ and 3 vol.% CO₂. After nitrocarburising, plasma post-oxidation was performed at 450 °C for 1 h in a gas mixture of 50 vol.% O₂ and 50 vol.% H₂. The microstructure of the treated samples was characterized using X-ray diffraction, scanning electron microscopy and surface roughness techniques. Dynamic polarization test was also used to evaluate the corrosion resistance of the samples. The results indicated that the compound layer was composed of ε-Fe_2–3(N, C) and γ′-Fe₄(N, C) phases. The amount of ε-Fe_2–3(N, C) phase increased as the treatment temperature rose from 530 °C to 570 °C and decreased at 630 °C. The X-ray diffraction patterns of post-oxidized samples confirmed the formation of the highest amount of magnetite after post-oxidation of samples that had been nitrocarburised at 570 °C as compared with the samples that had been treated at 530 °C and 630 °C. Nitrocarburising at 570 °C followed by post-oxidation also provided the highest corrosion resistance among all treatment conditions.