Effect of plasma nitriding temperature on the corrosion behavior of AISI 4140 steel before and after oxidation

In this study, the corrosion behavior of plasma nitrided AISI 4140 steel before and after oxidation was investigated. Plasma nitriding was carried out at 530 °C, 570 °C and 630 °C for 5 h in an atmosphere of 80 vol.% N2 and 20 vol.% H2. After nitriding, plasma post-oxidation was performed at 450 °C for 1 h in a gas mixture of 50 vol.% O2 and 50 vol.% H2. The treated specimens were characterized by using XRD, SEM and surface roughness techniques. Dynamic polarization test was also used to evaluate the corrosion resistance of the samples. The results confirmed formation of ε and γ′ phases in compound layer in all treatment temperatures. The amount of ε phase increased as the treatment temperature rose from 530 °C to 570 °C and decreased at 630 °C. The XRD patterns of post-oxidized samples confirmed formation of the highest amount of magnetite after oxidation of samples that had been nitrided at 570 °C as compared with the samples that had been treated at 530 °C and 630 °C. Nitriding at 570 °C plus post-oxidation also provided the highest corrosion resistance among all treatment conditions.