In order to achieve a rapid carburizing process, a novel method is proposed in which a solid carburizing compound of cast iron powder (Fe-2.55%Si-3.22%C) and sodium silicate solution (Na2O・SiO2) are placed in contact with steel samples (JIS-SCr420) at a temperature above the eutectic temperature of steels. First, the carbon diffusion into the steel was experimentally investigated at a temperature range between 1373K and 1673K intended for developing a reaction model of this carburizing method. It was found that the surface carbon content can be controlled from 0.5 to 1.5%, similar to conventional gas carburizing processes. At the contact face between the steel sample and cast iron powder, a solidified layer of cast iron is formed. The thickness of the solidified layer and carbon inflow are proportional to the square root of the carburizing duration. The microstructure investigation, elemental analysis and numerical calculations by DICTRA revealed that the carbon transfer mechanism from the molten cast iron through the solidified layer into the steel specimen surface is as follows : (a) constant carbon inflow by local equilibrium on liquid/solid interface of molten cast iron and solidified layer, (b) carbon diffusion in solidified layer, (c) carburization by local equilibrium on solid/solid interface of solidified layer and steel specimen, (d) carbon diffusion into steel specimen toward the core area.
With this method, the carbon potential (Cp) controlling the reaction is determined by the concentration of the solidus line of the carburizer. Moreover, the carbon concentration of the solid/liquid interface of the carburizer is kept constant and it can be predicted by diffusion-controlled reaction theory on the condition that sufficient carbon is supplied from the carburizer.
