Effect of Chemical Compositions of Case Hardening Steels for Distribution of Carbon and Cementite during Vacuum Carburizing

抄録

In order to develop the model to predict the microstructures of case hardening steels during vacuum carburizing, the effects of chemical compositions such as Cr and Si on carburizing behaviors were investigated using three steels based on Fe–1.1Cr–0.8Mn–0.25Si–0.2C (in mass%). The distributions of carbon near the carburized surface and grain boundary cementite (θ) were analyzed by Glow Discharge Optical Emission Spectroscopy (GD-OES) and image analysis of microstructures respectively. It was revealed that the distributions of carbon and the volume fractions of θ were affected by the chemical compositions i.e. they increased with increasing amount of Cr and decreased with increasing amount of Si. The results by X-ray Diffraction (XRD) and field emission electron probe microanalyzer (FE-EPMA) showed the carbon 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 comparing the experimental and calculation results obtained using the commercial multi-elemental diffusion solver, DICTRA, it was confirmed that the carbon in the γ was supersaturated during vacuum carburizing and the surface condition was estimated to be equal to the carbon activity of the meta-equilibrium border of γ/(γ+graphite). Additionally the result of θ distribution suggests that the volume fraction of θ cannot be predicted by equilibrium calculations; rather, a kinetic model for θ growth is required.