Creep Behavior at 1273 K (1000 A°C) in Nb-Bearing Austenitic Heat-Resistant Cast Steels Developed for Exhaust Component Applications

A series of Nb-bearing austenitic heat-resistant cast steels with variations of N/C ratios were investigated, and the morphological change of Nb(C,N) from faceted blocks, mixed flake-blocks to Chinese-script was observed as N/C ratios decreased. The creep behavior of these alloys was studied at 1273 K (1000 A degrees C), and the longest creep life and lowest creep rate occurred in model alloys with script Nb(C,N). Residual delta-ferrites and (Cr,Fe)(23)C-6 were adverse to creep properties. This work indicates that the control of N/C ratio is required for the as-cast microstructural strengthening.