Effects of Notch Shape and Specimen Thickness on Drop-Weight Tear Test Properties of API X70 and X80 Line-Pipe Steels
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LINE-PIPE steels used to transport crude oil or natural gas are required to have high strength adequate for long-distance transportation under high pressure, and have become thicker and larger to improve transportation efficiency.[1–6] As oil drilling activities in extremely cold regions such as Siberia and Alaska are actively conducted as a result of serious resource depletion, there are increasing demands for line-pipe steels having excellent low-temperature toughness. Particularly in these steels, structural integrity at low temperatures is critical, and thus precise and reliable evaluation techniques of transition temperature and fracture resistance are needed for practical applications of line-pipe steels. A drop-weight tear test (DWTT) was developed by the Battelle Institute of the United States in the SANG YONG SHIN, Research Assistant, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. BYOUNGCHUL HWANG, Senior Researcher, is with the Eco-Materials Research Center, Korea Institute of Machinery & Materials, Changwon 641-010, Korea. SUNGHAK LEE, Professor, is with the Center for Advanced Aerospace Materials and the Materials Science and Engineering Department, Pohang University of Science and Technology, Pohang 790-784, Korea. Contact e-mail: [email protected] KI BONG KANG, Group Leader, is with the Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Korea. Manuscript submitted April 5, 2006. Article published online March 29, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
mid-1960s as a method to evaluate the fracture propagation transition temperature (FPTT) of line-pipe steels.[1,2,3] Unlike the Charpy V-notch (CVN) impact test using specimens of a certain thickness, e.g., 10 mm, DWTT using specimens with the same thickness as actual line-pipe steels can better explain changes in transition temperature. Because DWTT specimens have a longer fracture path than CVN specimens, DWTT was proved to be an adequate representation of fracture propagation behavior of line-pipe steels.[3,7,8] As various DWTT techniques have been studied and developed with increased toughness of line-pipe steels, the American Petroleum Institute (API) currently recommends the use of pressed notch (PN) DWTT for FPTT evaluation of low-toughness line-pipe steels, while chevron notch DWTT is recommended for hightoughness ones.[3] Because of the longer crack propagation path in DWTT specimens, DWTT absorbed energy was known to be a more reliable parameter to evaluate the propagation resistance to dynamic ductile fracture (DDF) of line-pipe steels than CVN absorbed energy.[9,10,11] However, there still remain problems including uncertainties in the selection criteria of the notch shape of DWTT specimens. In the case of thick steels of over 19 mm in thickness, the test temperature is indiscriminately lowered after the specimen thickness is reduced to 19 mm, which also leads to uncertainties for reliable evaluation of absorbed energy or transiti
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