Creep behavior of an alumina-forming austenitic steel with simple alloy design

An Fe-18Ni-12Cr-based alumina-forming austenitic (AFA) steel with simple alloy design free of Nb and C was developed for potential application in advanced ultra-supercritical power plants. Creep behavior of the material was studied at the target application temperature of 700 °C. Creep properties were analyzed in terms of stress exponent (n), Monkman-Grant relationship and creep damage tolerance factor (λ). The microstructural evolution of crept samples was investigated by scanning electron microscope (SEM), electron back scatter diffraction (EBSD) and transmission electron microscope (TEM). The minimum creep rate decreased from 5.8 × 10−8 to 1.6 × 10-9 1/s with the applied stress decreasing from 140 to 80 MPa. Precipitation of B2-NiAl during the prolonged creep process contributed to improve creep resistance of the material. The related creep mechanism was found to be dislocation creep, which was supported by the value of n = 7.0 and by the crept microstructure of dislocations tangled with precipitations. Characterization results of creep fracture morphology indicate that the creep failure was resulted from microstructural degradation of precipitation coarsening.