Fracture-Toughness Analysis in Transition-Temperature Region of Three American Petroleum Institute X70 and X80 Pipeline
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TRODUCTION
PIPELINE steels used to transport crude oil or natural gas over a long distance essentially require high strength and toughness and have become thicker and larger, to improve transportation efficiency under highpressure conditions.[1–3] Because natural resources are seriously depleted by industrialization, active oil drilling, even in extremely cold areas, has been needed recently. Because structural integrity at low temperatures is important in pipeline steels used in extremely cold areas, precise and reliable evaluation techniques of the ductilebrittle transition temperature and fracture resistance are required for the practical applications of pipeline steels. Because high strength is generally obtained at the expense of reduced ductility and toughness, it is SANG YONG SHIN, Research Assistant, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang, 790-784, Korea. SUNGHAK LEE, Professor, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, is jointly appointed to the Materials Science and Engineering Department, Pohang University of Science and Technology. Contact e-mail: [email protected] KUK JE WOO, Principal Engineer, is with the Plate Quality Control Team, Quality Control Department, POSCO, Pohang, 790-785, Korea. BYOUNGCHUL HWANG, Senior Researcher, is with the EcoMaterials Research Center, Korea Institute of Machinery & Materials, Changwon, 641-831, Korea. SANGHO KIM, Senior Researcher, is with the Plate Research Group, Technical Research Laboratories, POSCO, Pohang, 790-785, Korea. Manuscript submitted June 29, 2007. Article published online February 4, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
necessary to scrutinize the structural integrity related to low-temperature toughness, in order to safely manage pipeline steels. To evaluate the fracture properties of pipeline steels, various laboratory-scale test methods that can correspond to the full-scale fracture behavior have been studied.[4–7] Among them, the Charpy V-notch impact test and drop-weight tear test are most widely used.[1–5,8] However, these tests do not use fracture mechanics as a basis for evaluating fracture toughness; indeed, because they largely depend on specimen size and geometry, their data may have large deviations in the transition-temperature region. Thus, in order to quantitatively evaluate fracture toughness, there is an essential need for advanced standard test methods that are based on probabilistic and statistical analysis. The fracture toughness of pipeline steels can be evaluated by measuring the plane-strain fracture toughness (KIc) in the lower shelf region and the resistance to ductile crack propagation (JIc) in the upper shelf or upper transition region, in accordance with the recently unified American Society for Testing and Materials (ASTM) E1820-06 standard test method;[9] this method, however, has some limitations in measuring the fracture toughness in the transition-temperature region. Because the plastic zone size at
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