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.

  Molten metal properties and mechanical properties of cast iron are considered to change if the molten cast iron is held in the furnace for a long time due to sintering work or line trouble.

  In this study, the chill depth, number of graphite counts, shrinkage cavities, fluidity, and tensile strength of spheroidal graphite cast iron were investigated when the molten cast iron (added with Zr inoculant in advance) was held at 1773K for a long time. Compared with the sample without Zr inoculant added, the decrease in the number of graphite counts, was small even when the cast iron was held for a long time. In addition, the chill depth decreased stably to the same level as the samples in which the shrinkage amount could be maintained. The mechanical properties did not change much from the samples not added with the Zr inoculant, indicating that the impact of Zr inoculation is small. However, a large difference was seen in the graphite area ratio and the number of graphite counts in the test piece microstructure.

  In the development of spheroidal graphite cast iron, there is a need to analyze the crystal structure distribution in spheroidal graphite particles, as well as observe the shape and morphological distribution. In this study, the c-axis distribution in spheroidal graphite particles was investigated at the nanoscale by an electron diffraction mapping using a transmission electron microscope. In the cross section of the spheroidal graphite particles, uniform c-axis distribution towards the center was observed. Our method paves the way to elucidate the mechanism of spheroidal graphite particle formation.