13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Creep Strength of Dissimilar Welds for Advanced USC Boiler Materials Masaaki Tabuchi1,*, Hiromichi Hongo1, Fujio Abe1 1 National Institute for materials Science, Tsukuba Science City 305-0047, Japan * Corresponding author: TABUCHI.Masaaki@nims.go.jp Abstract 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 welds, it is one of the candidate materials for A-USC boilers. In the present paper, the creep tests of the dissimilar welds between Ni base alloy and high B-9Cr steel for A-USC boilers were conducted at 650˚C. 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 Type-IV failure was not occurred in the present dissimilar welds and their creep rupture lives were 5-10 times longer than those of the conventional 9Cr steel welds at 650˚C. Keywords A-USC boiler, High B-9Cr steel, Dissimilar weld, Creep 1. Introduction Research project aiming to commercialize 700 ˚C class pulverized coal power system; A-USC power generation, with 46-48 % power generation efficiency, and to reduce CO2 emissions more than 10 % has been conducted from 2008 in Japan. In the A-USC boiler, Ni base or Ni-Fe base alloys such as Alloy 617, Alloy 263 and HR6W are planning to be 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. In the dissimilar weld between Ni base alloy and high Cr steel, Type-IV failure and thermal-fatigue (creep-fatigue) are the major concerns. Because the 9Cr-3W-3Co-VNb steel containing higher boron and lower nitrogen (high B-9Cr steel) developed in NIMS has improved creep strength in the HAZ and weld joint [1, 2], it is one of candidate materials for A-USC boilers [3]. In the present paper, creep tests of the dissimilar welds between high B-9Cr steels and Ni base alloys for A-USC boilers (Alloy 617 and Alloy 263) were conducted at 650 ˚C. Microstructures and creep damages in the welds were investigated. 2. Experimental procedure Two kinds of the high B-9Cr steels; MARBN10 steel (9Cr-3W-3Co-VNb-0.01B-0.003N) and MARBN12 steel (9Cr-2.6W-3Co-VNb-0.01B-0.007N) were prepared. The chemical compositions are shown in Table 1. In the present steels, about 0.01% boron was added aiming to strengthen the grain boundaries. Nitrogen content was lower than 0.01 % to avoid the formation of boron nitride (BN) and to maximize the grain boundary strengthening effect of boron. MARBN12 steel contains higher nitrogen than MARBN10 steel. These steels were normalized at 1100 ˚C for 1 h and tempered at 800 ˚C for 1 h. Four kinds of dissimilar welds between high B-9Cr steels (MARBN10 and MARBN12) and Ni base alloys for A-USC boilers (Alloy 617 and Alloy 263) were prepared by gas tungsten arc (GTA)
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