Abstract: 11Cr-0.4Mo-2W-CuVNb steel (ASME Gr.122) is used for boiler components in ultra-supercritical (USC) thermal power plants. The creep strength of high-Cr steel welds decreases due to the formation of Type IV creep damage in the heat-affected zone (HAZ) during long-term use at high temperatures. In the present study, the processes and mechanisms of Type IV creep damage were elucidated by investigating the long-term creep strength, microstructure and damage behavior of Gr.122 steel welds. Creep tests using thick welded joints were interrupted at several time steps, and the initiation, evolution and distribution of creep damage were measured. It was found that the formation and growth of creep damage was suppressed during the steady-state stage; creep voids formed at 0.5 of life, and coalesced to form a crack after 0.9 of life. Type IV creep damage was mostly observed in the area about 30\% below the surface of the plate. Differences in creep damage behavior between Gr.122 and Gr.91 steel welds were examined. Experimental creep damage distribution was compared with computed versions using the finite element method and damage mechanics.

Key words: Gr.122 steel, Welded joint, Creep void, Type IV damage, Finite element method, Damage mechanics