Effect of cobalt on the microstructure of tempered martensitic 9Cr steel for ultra-supercritical power plants

The effect of Co on the microstructure of advanced 9Cr–3W–0/5Co–0.2V–0.05Nb–0.08C–0.05N steel, designed for application at highly efficient ultra-supercritical power plants, has been systematically studied, using specimens containing 0%, 1%, 3% and 5% Co. Calculation by Thermo-Calc suggested and observation of microstructure after heat treatment confirmed the presence of about 6% and 0.4% of δ-ferrite in Co-free and 1% Co steel, respectively. Coarse primary Nb-rich MX particles preferentially form inside δ-ferrite, as according to Thermo-Calc Nb solubility in the body-centered cubic phase is approximately double of that in the face-centered cubic phase. Detailed examination of precipitates was conducted on samples after heat treatment. The total amount of precipitation increases with increasing Co content, which is especially significant between 1% and 3% Co steels. Precipitate density around prior austenitic grain boundaries is highest for 3% Co steel, and lowest for 1% Co steel. The mean diameter of precipitates does not differ significantly.