13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 5. Creep rupture times of the base metals of the high B-9Cr steels and the dissimilar weld joints between high B-9Cr steels and Ni base alloys for A-USC boiler at 650 ˚C (a) MARBN10 steel-Alloy 617 weld joint, 650 ˚C, 100MPa, 22766h (b) MARBN12 steel-Alloy 617 weld joint, 650 ˚C, 100MPa, 22518h Figure 6. Dissimilar weld joint specimens creep ruptured at 650 ˚C and 100MPa for 22000h We have investigated the microstructures and creep damages in the crept welds. Figure 6 shows the dissimilar weld joint specimens creep ruptured at long-term test condition. It was observed that a creep crack initiated in the fusion boundary at the lower surface of U-groove and then propagated toward to the HAZ of the high B-9Cr steel. Figure 7 shows microstructures and creep damages observed in the HAZ of the dissimilar welds between MARBN12 steel and Alloy 617 ruptured at 650 ˚C and 140 MPa for 5619h. In the HAZ at 1.5 mm from the fusion boundary, small amounts of creep voids were observed as shown in Fig. 7. This area corresponds to Fig. 4(b) where the grain size of the base metal was retained and small grains formed only on the prior-austenite grain boundaries. Creep voids in the HAZ of high B-9Cr
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