The impact of intended service temperature on the optimal composition of Laves and M23C6 precipitate strengthened ferritic creep resistant steels

Abstract

Recently, the Laves phase precipitate family has been identified as a promising alternative for Precipitation Hardening (PH) in ferritic creep resistant steels owing to its significant contribution to creep strength. A computational alloy design approach to create novel ferritic creep resistant steels strengthened by optimized Laves phase is presented. This novel approach aims at a simultaneous optimization of the steel’s chemical composition involving 9 chemical elements to obtain a ferritic steel optimally strengthened by Laves phases with an ideal combination of a high volume fraction and a reduced coarsening rate at the intended service temperature. Using this approach, three alloys possessing ideal PH contributions at their intended service temperatures, i.e. 650, 700 and 750°C, respectively, are designed. This approach is validated by analyzing PH contributions in existing 15Cr ferritic steels at different service temperatures. Good agreement between reported experimental performance and model predictions is found. The newly designed alloys are predicted to have a substantially higher PH contribution at corresponding service temperature than those of the existing alloy. While Laves phases generally precipitate slowly during service and have a more significant strengthening effect at the late stage, the precipitation of the alternative M23C6 precipitates can be used to increase the initial strength of ferritic steel. Hence, our computational approach was extended to aim for a high strengthening contribution at both initial and late stages at the intended service temperature.