13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- 100µm 20µm steel were observed on the prior-austenite grain boundaries with large grain size. It is considered that the void coalescence and crack formation, which causes Type-IV failure, is suppressed and consequently the life of weld is improved in these HAZ microstructures of high B-9Cr steel compared with the fine-grained HAZ microstructures of conventional high Cr steels. Figure 7. Microstructure and creep damage in the HAZ of the MARBN12 steel-Alloy 617 weld joint creep ruptured at 650 ˚C and 140 MPa for 5619 h 4. Conclusions In the present paper, creep tests of the dissimilar welds between high B-9Cr steels and Ni base alloys, which are planning to be used in the 700 ˚C class pulverized coal power system (A-USC), were conducted. The results can be summarized as follows: (1) For the high B-9Cr steels, fine-grained microstructures as shown in the conventional high Cr steels did not form in the HAZ. Free soluble boron is considered to decrease the grain boundary energy and change the mechanisms of the α-γ transformation during the weld thermal cycle. (2) The creep rupture lives of the present dissimilar welds between high B-9Cr steels and Ni base alloys were 5-10 times longer than those of the conventional 9Cr steel welds at 650˚C. (3) The failure locations of the dissimilar weld joints were base metal or fusion boundary. Small amounts of creep voids were observed in the HAZ with large grain size; however Type-IV failure did not occur for about 20000 h at 650˚C.
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