Effect of plasma oxynitriding temperature on wear and corrosion resistance of the AISI 4140 steel

This work presents the effect of oxynitriding process at different temperature on the corrosion resistance and wear behavior of the quenching-and-tempering-treated AISI 4140 steel. The AISI 4140 was plasma nitrided at 500°C. Subsequently, the plasma oxynitriding was performed on the nitrided AISI 4140 at different temperatures under H₂O atmosphere. Microstructure and phases of the plasma-oxynitrided samples are investigated, indicating that phase formation of the oxide layer is strongly dependent on processing temperature during plasma oxynitriding: Formation of Fe₃O₄ is preferred over Fe₂O₃ at lower processing temperature. Also, it is believed that ε-Fe_2–3N phase formed by nitriding process plays an important role to promote the formation of Fe₃O₄ phase during plasma oxynitriding. In order to investigate the mechanical, wear, and corrosion properties of the plasma-oxynitrided samples, Vickers hardness, friction coefficient, and potentiodynamic curves are evaluated, respectively. Compared to a plasma-nitrided sample, the Vickers hardness of the plasma-oxynitrided sample at optimal processing temperature shows a slight decrease of the hardness, but, improved wear and corrosion resistances were observed. It is suggested that wear and corrosion resistance of the oxynitrided sample is strongly dependent on the volume fraction of Fe₃O₄ phase in the oxide layer.