The relation between microstructure and creep property of austenitic heat-resistant cast steels with and without Nb addition under the condition of repeated vacuum carburizing and quenching was investigated. Cr-carbide scale is formed on the sample surface by a carburizing reaction, resulting in the depletion of Cr in the matrix adjacent to the scale. A carburized layer consisting of various fine carbides is observed below the Cr-depleted layer, and the carburized layer depth is suppressed by Nb addition. When process of vacuum carburizing and quenching is repeated, formation of voids caused by heating and rapid cooling is more remarkable in primary Cr carbides than in primary Nb carbides. As the carburized layer depth increases, creep rupture time of both cast steels shifts to the shorter time side ; however, Nb addition is effective for extending creep rupture time at 1303K.

  In die cast products, further increasing the J factor of the molten metal as it passes through the gate into the die cavity as an atomized flow has yielded differing experimental results, where porosity decreases in the thick wall part in the die casting pistons but increases in the rim part in the die casting wheel, which is the last parts to be filled. To clarify the mechanisms underlying the variation in porosity at positions far from the gate when the molten metal is atomized at the gate and forms a multiphase flow, we developed a multiphase flow analysis system using large-scale non-steady-state turbulence calculations. The developed system performs model calculations by a high-order precision finite volume method that couples large eddy simulation with the volume of fluid method. The developed system clarified by visualization the relationship between the process of ejection to overflow after void formation in the thick wall part and changes in vortex structure as the volume fraction of aluminum changes. Furthermore, the amount of air expelled from the overflow part during the filling process was quantified and a process was clarified in which air was initially expelled followed by a multiphase flow of aluminum and air. The developed system was able to clarify the differences in the relationship between the J factor and porosities due to differences in shape between the piston and wheel.