Development of high strength 15Cr ferritic creep resistant steel with addition of tungsten and cobalt

Effects of alloying elements of W and Co on a creep strength of full annealed 15Cr ferritic steel whose chemical composition is Fe-0.1C-15Cr-1Mo-3W-0.2V-0.05Nb-0.07N-0.003B has been investigated. Increases in creep strength with increase in W content and addition of Co have been obviously observed. About 10 times and 30 times longer time to rupture than that of the base steel have been obtained at 923K-100MPa by increase in W content from 3 mass% to 6 mass% and addition of 3 mass% of Co, respectively. Moreover, significant improvement of creep strength has been observed by the combination of increase in W content and addition of Co. Time to rupture of the steel strengthened by 6 mass% of W and 3 mass% of Co is 100 times longer than that of the base steel at 923K-100MPa. Creep rupture strength of the steel containing 6 mass% of W and 3 mass% of Co is almost the same as that of ASME T92 steel. Since Cr content of conventional ferritic creep resistant steels is less than 12 mass%, oxidation resistance of the 15Cr ferritic steel investigated in this study is better than those of conventional ferritic creep resistant steels. It has been concluded that a full annealed 15Cr ferritic creep resistant steel should be one of the candidate materials for new Ultra Supercritical (USC) power plant in the 21st century.