Mechanism of Carbon Infiltration in Vacuum-Carburizing Process

Abstract

In order to estimate carbon distribution in vacuum carburized steels, the mechanism of the introduction of carbon into steel during the process was investigated. Steels bearing various amounts of Si and Cr were machined to specimens with 0.1 mm thickness, which was considered to be the surface area of carbrized parts. They were vacuum carburized at the temperature range of 1123 K to 1323 K for sufficient long time to obtain uniform microstructures and then quenched. Using Fe-Graphite diagram of each steel grade, it was clarified that the measured carbon content of each specimen corresponded to the carbon content at which graphite begins to form. And the area fraction of carbide in structures was also almost the same value calculated with the diagram. From these results, we concluded that graphite is generated from carburizing gas onto the surface of steel, and carbon is introduced into the steel under local equilibrium between the steel surface and the graphite at a treating temperature.