Identification of Precipitate Phases in an 11%Cr Ferritic/martensitic Steel after Short-term Creep

To further increase steam temperature in ultra-supercritical power plants, 9-12%Cr ferritic/martensitic (FM) steels with target operating temperatures up to 650 degrees C and higher are being developed. An 11% Cr F/M steel was prepared by reference to the nominal chemical composition of SAVE12 steel with an expected maximum use temperature of 650 degrees C. Precipitate phases in the 11%Cr FM steel after a short-term creep at 600 degrees C/180 MPa for 1 100 h were investigated by transmission electron microscopy with energy dispersive X-ray spectrometers. Precipitate phases in the crept steel were identified to be Fe-W-Cr-rich M6C carbide, Cr-rich M23C6 and M7C3 carbides, Fe-rich Laves phase, Nb-Ta-rich/Ta-Nb-rich/V-rich MX phases, as well as Fe-Cr-rich sigma phase. The identified M6C carbides with a f.c.c. crystal structure have a metallic element composition of about 41Fe, 31W, 20Cr, and 4Ta (in at.%) and an irregular block-like morphology with sizes ranging from 70 to 310 nm in diameter. The M6C and Cr-rich M7C3 carbides, as well as Fe-W-rich Laves phase are considered to be formed during the creep. The present investigation reveals that the M6C carbide is a dominant phase in the steel after the creep. The formation of the M6C carbide in the crept steel is also discussed.