Effect of post-oxidizing on tribological and corrosion behaviour of plasma nitrocarburized AISI 4140 steel

AISI 4140 steel was initially plasma nitrocarburized at 570 and 700 degrees C for 4 h in a gas mixture of 60 % N-2 + 37 % H-2 + 3 % CO2. After nitrocarburizing, post-oxidation treatment was performed at 450 degrees C for 1 h using oxidation gas mixture ratios of 50 % H2 + 50 % O-2 and pure oxygen. The structural, mechanical, tribological, and corrosion properties of nitrocarburized and post-oxidized steel were analysed using an X-ray diffraction, microhardness tester, scanning electron microscopy, pin-on-disc tribotester, and electrochemical polarization. The experimental results showed that oxide layer included Fe2O3 and Fe3O4 phases while the compound layer formed after both ferritic and austenitic nitrocarburizing consisted Of Fe2-3(N,C) and Fe-4(N,C) phases. Post-oxidation treatment improved further tribological and corrosion properties in consequence of nitrocarburizing. The contribution of post-oxidation treatment made after the ferritic nitrocarburizing was much more than the contribution of post-oxidation treatment made after the austenitic nitrocarburizing. Furthermore, it was seen that using hydrogen in oxidation gas mixture had useful effects such as better tribological properties and corrosion resistance.