The research project aiming to commercialize 700°C class pulverized coal power system; advanced ultra-super critical (A-USC) pressure power generation has been conducted in Japan from 2008. In A-USC boilers, Ni base or Ni-Fe base alloys are used for high temperature parts at 650–700°C and advanced high Cr ferritic steels are planning to be used at the temperatures lower than 650°C. Because the high B-9Cr steel developed in National Institute for Materials Science (NIMS) has improved creep strength in weldments, it is one of the candidate materials for A-USC boilers. In the present paper, the creep tests of the dissimilar welds between high B-9Cr steels and Ni base alloys were conducted. In the heat affected zone (HAZ) of the high B-9Cr steels, fine-grained microstructures were not formed and grain size of the base metal was retained. Free boron on the grain boundaries is considered to affect the mechanisms of the α-γ transformation during weld thermal cycle. Consequently, the creep rupture lives of the 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.

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