真空浸炭中の炭素，セメンタイト分布に対する組成の影響

概要

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To develop the model to predict microstructures of case hardening steels during vacuum carburizing, the effects of chemical compositions such as Cr and Si on carburizing behaviors were investigated by applying three steels based on Fe–1.1Cr–0.8Mn–0.25Si–0.2C(SCr420H). Distributions of Carbon(C) near the carburizing surfaces and grain boundary cementite (θ) were analyzed by GD-OES and image analysis of microstructures respectively. It was revealed that the C distributions and the volume fractions of θ ware affected by the chemical compositions i.e. they increased with the Cr amount and decreased with Si amount. The analyzed results by XRD and FE-EPMA showed the C content in the retained austenite (γ) near the surface was around 1.4–1.6 mass% which was higher than the equilibrium condition.By the comparison of experimental and calculation results which was obtained from the commercial multi-elemental diffusion solver, DICTRA, it was confirmed that the C in the γ was supersaturated during vacuum carburizing and the surface condition was estimated as equal to the C activity of the meta-equilibrium boarder of γ/(γ+graphite). Additionally the result of θ distribution suggests volume fraction of θ can not be predicted by equilibrium calculations but a kinetic model for θ growth is required.