CALPHAD-based alloy design of cast austenitic heat-resistant steels with enhanced strength at 1000 °C

Abstract

Cast austenitic heat-resistant steels are attractive candidates for ultra-high temperature applications, owing to their relatively high strengths and low costs. In the present study, a new alloy was developed with improved mechanical properties at 1000 °C based on two commercial cast austenitic heat-resistant steels. The alloy composition was designed by predicting the quantities of different precipitates in these steels under the equilibrium and non-equilibrium conditions through the CALPHAD method. The new steel exhibited much better creep resistance than the commercial ones at 1000 °C, despite of the similar tensile strengths. It was strongly attributed to the increased Nb content that accelerated the precipitation of the “Chinese-script” Nb(C,N) and strengthened the interdendritic regions. The W addition was completely removed to prevent the formation of the residual δ-ferrite and Cr23C6 that accelerated the propagation of cracks and degraded the mechanical properties. A small addition of N was also demanded to reduce the quantity of the residual δ-ferrite and brittle intermetallic phases, but its content should be strictly limited to avoid the precipitation of Cr2N. Based on this alloy design, the reliability of the CALPHAD method is well experimentally confirmed by comparing the phases in the microstructure of the commercial and new steels.