Abstract
Decarburization is generally avoided as it is reckoned to be a process detrimental to material surface properties. Based on the idea of duplex surface engineering, i.e., nitriding the case-hardened or through-hardened bearing steels for enhanced surface performance, this work deliberately applied decarburization prior to plasma nitriding to cancel the softening effect of decarburizing with nitriding and at the same time to significantly promote the nitriding kinetics. To manifest the applicability of this innovative duplex process, low-carbon M50NiL and high-carbon M50 bearing steels were adopted in this work. The influence of decarburization on microstructures and growth kinetics of the nitrided layer over the decarburized layer is investigated. The metallographic analysis of the nitrided layer thickness indicates that high carbon content can hinder the growth of the nitrided layer, but if a short decarburization is applied prior to nitriding, the thickness of the nitrided layer can be significantly promoted. The analysis of nitriding kinetics shows that decarburization reduces the activation energy for nitrogen diffusion and enhances nitrogen diffusivity. Moreover, the effect of decarburization in air can promote surface microstructure refinement via spinodal decomposition during plasma nitriding.
Highlights
Published: 20 January 2021M50 steel and its variant M50NiL steels are used as aero-engines bearings for hightemperature applications [1]
Ooi et al reported that duplex hardening treatment can increase surface compression stress and improve rolling contact fatigue resistance of the M50 components [3]
Summary,decarburization, decarburization, which whichisisgenerally generallyconsidered considereddetrimental detrimentalto tosurface surface properties, is innovatively applied before plasma nitriding to promote the nitriding kinetics properties, is innovatively applied before plasma nitriding to promote the nitriding kinetfor andand through-hardened
Summary
Published: 20 January 2021M50 steel and its variant M50NiL steels are used as aero-engines bearings for hightemperature applications [1]. To maintain structural integrity in a more severe service environment, excellent combined surface properties, such as hardness, wear resistance and rolling contact fatigue resistance, are required for bearings. On the other hand, nitrided layers can exhibit hardness greater than 1000 HV, but the typical case thickness is only 1/10 of the carburized case, and with a sharp decreasing hardness profile, which is not conductive to load-bearing capacities and fatigue resistance. For M50 steel with ~0.8 wt.% carbon, duplex hardening achieved by applying a thin nitride layer over the secondary-hardened matrix [2], has been applied to further improve the surface properties. Ooi et al reported that duplex hardening treatment can increase surface compression stress and improve rolling contact fatigue resistance of the M50 components [3]. Rhoads et al improved the surface hardness of Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations
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