Post-oxidizing treatments of the compound layer on the AISI 4135 steel produced by plasma nitrocarburizing

Plasma nitrocarburizing and post-oxidation treatments were performed on AISI 4135 steel by a pulsed plasma-ion nitriding system. Plasma oxidation process resulted in the formation of a very thin ferritic oxide layer 1–2 μm in thickness on top of a 15- to 25-μm-thick ɛ-Fe2–3(N,C) nitrocarburized layer. The growth rate of the oxide layer increased with increasing treatment temperature and time. However, the oxide layer was easily spalled from the compound layer either for oxidation temperatures above 450 °C, or for oxidation time longer than 2 h at oxidation temperature of 400 °C. It was confirmed that the relative amount of Fe2O3, compared with Fe3O4, increased rapidly as the oxidation temperature increased. The amounts of γ′-Fe4(N,C) and θ-Fe3C phases, generated from dissociation of the ɛ-Fe2–3(N,C) phase during O2 plasma sputtering, also increased with the oxidation temperature. It was demonstrated that the corrosion characteristics of the nitrocarburized compound layer were further improved by post-oxidation treatment.