Effect of B on Coarsening Behavior of M23c6 Carbides in 9.5cr-1.5mocovnbnb Heat-Resistant Steel During Aging at 620 ℃

9.5Cr-1.5MoCoVNbNB heat-resistant steel is designed to serve at the temperature of 620 ℃, and has been acknowledged as the most promising martensitic heat-resistant steel for turbine rotors in ultra-supercritical power plants. Due to the addition of 100 ppm B, the evolution of M23C6 carbides in the steel possesses some specialties during thermal aging. B added in steel can prevent the coarsening of M23C6 carbide particles during long-term aging at 620 ℃. However, there is a lack of direct observation and quantitative analysis of B segregated at boundaries and M23C6 carbide in 9.5Cr-1.5MoCoVNbNB heat-resistant steel. The results of SIMS reveal that B is segregated at boundaries after tempering, and the results of APT reveal that B atoms are evenly in M23C6 carbide particles during aging at 620 ℃. Si is segregated at the interface and solutes in the matrix. P is observed to be segregated at the interface between M23C6 carbide particles and the martensitic matrix. The concentration of B decreases, and Mo increases slightly in M23C6 carbide particles with the increase of aging. It is indicated that the coarsening of M23C6 carbides is a process of alloy element redistribution.