Improvement of wear and corrosion resistance of ferrous alloys by post-nitrocarburizing treatments

Gas nitrocarburizing and post-oxidation treatments were performed on 42CrMo4 and 20MnCr5 steels, G30 and GS600 cast irons, to improve wear and corrosion resistance. In both materials the thickness of the compounds layer, mainly composed by epsilon- and gamma’- phase, resulted about 15-20 micron and the measured diffusion layer was about 150 micron thick. A subsequent oxidizing step, followed by impregnation with a two different lubricating oils was performed in order to further enhance corrosion resistance. Wear resistance against alumina was measured using a ball-on-disk tribometer and the corrosion characteristics of the samples were studied using salt spray test in accordance with ASTM B117. Experimental results show that the nitrocarburizing treatment improves significantly the wear resistance of the steels but not the wear resistance of the studied cast irons. The steels present almost the same tribological behaviour, ascribable to the formation of similar compounds, while cast irons present different wear resistance due to their chemical composition and the graphite morphology. The application of nitrocarburizing alone does not significantly improve corrosion resistance and it may even promote localized corrosion. The subsequent post-oxidation step leads to a slight decrease of the corrosion rate, because it partially seals the porous nitrocarburized layer. The final oil impregnation step resulted much more effective in further decreasing the corrosion rate and this final treatment in some cases improves also the wear resistance.