Creep Strength of Dissimilar Welded Joints Using High B-9Cr Steel for Advanced USC Boiler
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ODUCTION
THE commercialization of a 973 K (700 °C) class pulverized coal power system, advanced ultra-supercritical (A-USC) power generation, is the target of an ongoing research project initiated in Japan in 2008.[1] By raising the current USC steam conditions of 873 K (600 °C) and 25 MPa to A-USC conditions of 973 K (700 °C) and 35 MPa, the net thermal efficiency is improved from 42 to 46-48 pct, and CO2 emissions can be reduced by more than 10 pct. In the A-USC boiler, Ni or Ni-Fe base alloys such as Alloy 617, Alloy 263, Alloy 740, Alloy 141, HR6W, and HR35 would be used for the high-temperature parts at 923 K to 973 K (650 °C to 700 °C). At temperatures lower than 923 K (650 °C), advanced high-Cr ferritic steels would be used in order to reduce the cost of materials. High boron 9Cr (high B-9Cr) steel, low carbon 9Cr (LC-9Cr) steel, and SAVE12AD steel are candidate high-Cr steels.[1] In the A-USC research project, creep rupture, fatigue, oxidation, corrosion, welding, and bending tests are being conducted on the above alloys and steels. In the dissimilar welds between Ni base alloys and high-Cr steels, Type IV failure in the heat-affected zone (HAZ)[2–4] is a concern. We have been studying the 9Cr-3W-3CoMASAAKI TABUCHI, Group Leader, is with the Creep Resistant Materials Group, Materials Reliability Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan. Contact e-mail: [email protected] HIROMICHI HONGO, Principal Engineer, is with the Materials Information Station, National Institute for Materials Science, Tsukuba, Japan. FUJIO ABE, NIMS Invited Researcher, is with the Materials Reliability Unit, National Institute for Materials Science. Manuscript submitted December 26, 2013. Article published online July 25, 2014 5068—VOLUME 45A, OCTOBER 2014
VNb steel containing about 0.01 pct B with low N (high B-9Cr steel) aiming to improve the grain boundary strength, and consequently, the long-term creep strength.[5] Since the high B-9Cr steel developed at the National Institute for Materials Science (NIMS) has improved creep strength not only in the base metal but also in the welded joint,[6–8] it is one of the candidate materials for the Japanese A-USC boiler.[1] In the present study, creep tests on the dissimilar welded joints between high B-9Cr steels and Ni base alloys for A-USC boilers were conducted at 923 K (650 °C). Microstructures and creep damage in the dissimilar welded joints were investigated. Creep damage distribution in the dissimilar welded joint was calculated using damage mechanics equation and compared with the experimental result.
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MATERIALS AND EXPERIMENTAL PROCEDURE
A. Material Two kinds of high B-9Cr steel, MARBN10 (9Cr3W-3Co-VNb-0.01B-0.003N) and MARBN12 (9Cr2.6W-3Co-VNb-0.01B-0.007N), were prepared. The chemical compositions are shown in Table I. In the present steels, 0.01 pct boron was added to maximize the grain boundary strengthening effect of boron. From the solubility product for boron nitride (BN),[9] nitrogen content was lower than 0.01 pct to a
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