Development of post-oxidation treatments to improve wear and corrosion resistance

Gas nitrocarburizing and post-oxidation treatments were performed on 42CrMo4 steel and GS600 cast iron, to improve wear and corrosion resistance. In both materials the compounds layer after nitrocarburizing, mainly composed by ε-Fe2-3(N,C) and γ′- Fe4N phase. First the oxidizing step, a subsequent impregnation with two different lubricating oils was performed in order to further enhance corrosion resistance. Wear resistance was measured using a ball-on-disk tribometer with Al 2O3 and 100Cr6 counterpart and the corrosion resistance of the samples was studied using salt spray test in accordance with ASTM B117 and electrochemical cell. Experimental results show that the nitrocarburizing treatment improves significantly the wear resistance of the steel but not the wear resistance of the studied cast irons. The application of nitrocarburizing alone does not significantly improve corrosion resistance and it may even promote localized corrosion. The subsequent postoxidation 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 by one order of magnitude.