Mod.9Cr-1Mo steel has been used for boiler components in ultra-supercritical (USC) thermal power plants. The creep strength of welded joint of this steel decreases due to Type-IV creep cracking formed in heat affected zone (HAZ) at high temperatures. The present paper aims to clarify the damage process and microstructural degradation of the welded joint. Long-term creep tests for base metal, simulated HAZ and welded joints were conducted at 550, 600 and 650°C. Furthermore, creep tests of thick welded joint specimens were interrupted at several time steps for 600°C, 90MPa, and the damage distribution was measured. It is found that creep voids initiate at early stage of creep life (t/tr = 0.2), and coalesce into the crack at the later stage of life (t/tr = 0.9). Creep damages concentrate mostly at a quarter depths of the plate thickness in the fine-grained HAZ. The experimental creep damage distribution was compared with the computed one by using the damage mechanics analysis.

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