Effect of Nb on Thermal-Shock Resistance of Austenitic Heat Resistant Cast Steel

Abstract

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 M₂₃C₆ at grain boundary in as cast steel. Decomposition of primary M₂₃C₆ 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.

Fig. 12 Partitioning of alloying elements analyzed by STEM-EDS for micro-crack in Nb steel occurred when thermal shocked at 1223 K for 300 cycles.