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 1303 K.

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The casting defects are reduced by increasing the molten metal velocity near the defects. When the linear runner, where flowout volume through the lower runner gate was greater than the volume through the upper one, was used, the molten metal unequally filled up the cavity and the molten metal velocity suddenly decreased at a final filling portion.Therefore, the casting defects frequently occurred at the final filling portion. On the other hand, when the stair shaped runner was used, the molten metal equally filled up the cavity without a turbulent flow, and the molten metal velocity did not decrease at the final filling portion. Therefore, the molten metal filled up the cavity with inconsiderable casting defects in a short time.