Development of Plasma Nitrocarburizing and Post-Oxidation Technology for the Replacement of Cr6+ Plating for Application to Vehicle Engine Shafts

Abstract

Plasma nitrocarburizing and post-oxidation treatments were performed to improve the wear and corrosion resistance of S45C steel. Plasma nitrocarburizing was conducted for 3 h at 570°C in a nitrogen, hydrogen and methane atmosphere to produce the e-Fe2−3(N,C) phase. It was found that the compound layer produced by plasma nitrocarburising was predominantly composed of the ∈-phase with traces of the γ′-Fe4(N,C) phase. The thickness of the compound layer was approximately 12 μm and the diffusion layer was approximately 300 μm in thickness. Plasma post oxidation was performed on nitrocarburized samples with various oxygen/hydrogen ratios at a constant temperature of 500°C for 1 h. The very thin magnetite (Fe3O4) layer 1 μm to 2 μm in thickness on top of the compound layer was obtained by plasma post oxidation. It was also confirmed that further improvement of the corrosion characteristics of the nitrocarburized compound layer was possible with an application of the superficial magnetite layer. Finally, throttle valve shafts of S45C steel were treated under optimum plasma processing conditions. Accelerated life time test results using a throttle body assembled with a shaft treated by plasma nitrocarburising and post oxidation showed that plasma nitrocarburizing and plasma post-oxidation processes could be a viable technology in the very near future in place of Cr6 plating.