Microstructure and modeling of creep resistance of modified 9Cr-1Mo steel

Ferritic steels are widely used in steam turbine components such as boilers, pressure vessels, heat exchangers, and other high-temperature components. One of the critical properties of such materials in the application is their long-term creep rupture strength. 9Cr-1Mo-V-Nb is considered as the benchmark in the 9Cr steel class. Although the 9Cr-1Mo-V-Nb steel is designed to have a tempered martensitic structure with precipitation of M23C6 and MX to increase its creep-resistance, its microstructure will change due to the evolution of precipitates at high-temperature under stress. Notably, intermetallic particles, i.e., Laves phase [(FeCr)2Mo], which are not present in the original state of the material, will form during long-term creep, which can shorten the creep life. In this study, the creep behavior of 9Cr-1Mo-V-Nb steel aged for 5,000 hours at 600 o C is evaluated. Microstructure examination is also conducted with regards to the Laves phase formation and its microstructural locations. The creep behavior is analyzed using a mechanism-based true-stress creep model with the consideration of dislocation climb, dislocation glide, and grain boundary sliding.