Long-term creep strength and fracture of Gr.91 steel welds

Mod. 9Cr-1Mo steel (ASME Gr.91 steel) is widely used as a main structural material for boiler components in ultra-supercritical (USC) thermal power plants at about 600 degrees C. Decrease of the creep strength of welded joints due to Type-IV failure is a critical issue, however creep rupture data longer than 10,000 h are scarcely obtained. The present paper conducted long-term creep tests of welded joints from 30,000 to about 70,000 h at 600 and 650 degrees C. For the specimens fractured after about 35,000 h at 600 and 650 degrees C, creep damage was mainly observed in the fine-grained heat-affected zone (HAZ) (Type-IV), however damage was also observed in the weld metal. The welded joints crept at 600 degrees C and 50 MPa for about 50,000 and 70,000 h fractured in the weld metal. It was found that the hardness of 9Cr weld metal decreased after about 30,000 h creep at 600 degrees C due to the recovery of microstructures. Therefore it is considered to be necessary to monitor creep damage not only in the HAZ but also in the weld metal.