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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 effect of Nb on microstructure and thermal-shock resistance was investigated for as cast and annealed JIS SCH21 steels. Primary NbC carbide were crystallized together with M 23 C 6 at grain boundary in as cast steel. Decomposition of primary M 23 C 6 was detected, while NbC was also hardly decomposed when annealed at elevated temperature. It was found that chromium carbides had low thermal stability but primary NbC carbides played a role in preventing propagation of micro-cracks and restraining shape deformation under condition both heating and quenching. As a result, thermal-shock resistance was improved in the Nb-added steel.

The effect of Nb on microstructure and thermal shock resistance was investigated for as cast and annealed JIS SCH21 steels. Primary NbC carbide were crystallized together with M 23 C 6 at grain boundary in as cast steel. Decomposition of primary M 23 C 6 was detected, while NbC was also hardly decomposed when annealed at elevated temperature. It was found that chromium carbides had low thermal stability but primary NbC carbides played a role in preventing propagation of micro cracks and restraining shape deformation under condition both heating and quenching. As a result, thermal shock resistance was improved in the Nb added steel.