Effect of treating atmosphere in plasma post-oxidation of nitrocarburized AISI 5115 steel

In this study quenched and tempered AISI 5115 steel was plasma-nitrided and nitrocarburized at 550 °C for 5 h in atmospheres of 80% N2 balanced with various amounts of CO2 and H2 gases. The amount of CO2 varied from 0 to 10 vol%. The highest amount of ε phases was formed in the compound layer after treating in atmosphere containing 7 vol% CO2. Optimized compound layer was post-oxidized for 1 h at 450 °C under O2/H2 volume ratios of 1/1 and 3/1 as well as 100% oxygen. The treated samples were characterized using metallographic techniques, XRD, SEM, roughness measurement and potentiodynamic methods. The results showed that the growth rate of the oxide layer increased with increasing O2 in the oxidizing gas mixture. X-ray diffraction analysis of oxidized layers confirmed the formation of highest amount of magnetite at post-oxidation in an atmosphere with the O2/H2 volume ratio of 1/1. Electrochemical polarization tests proved the enhancement of corrosion resistance by plasma post-oxidation and the highest corrosion resistance obtained after oxidizing under an O2/H2 volume ratio of 1/1.