Analysis of Fracture Toughness in the Transition Temperature Region of API X70 Pipeline Steels Rolled in Two-Phase Regio
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INTRODUCTION
RECENTLY produced line pipe steels contribute greatly to energy conservation, cost reduction, and improved transportation efficiency, because they allow long-distance transportation of a large amount of crude oil or natural gas under high pressure.[1,2,3] They have become stronger, tougher, thicker, and larger because the gas composition has become richer and many drilling activities have been undertaken in severe conditions such as extremely cold or deep regions. As high strength is generally achieved at the expense of reduced toughness and ductility, it is imperative to scrutinize the structural integrity related to low-temperature toughness in order to safely manage line pipe steels at extremely cold or deep regions. To evaluate fracture properties of line pipe steels, various laboratory-scale testing methods, which correspond closely to the fullscale fracture behavior, have been studied.[4,5,6] Among them, the Charpy V-notch impact test and drop-weight tear test (DWTT) are most widely used.[1–5,7] However, these tests are not based on fracture mechanics to evaluate fracture toughness, and their data may have large deviations in the transition temperature region because they largely depend on specimen size and SANG YONG SHIN, Research Assistant, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. GUILIANG GONG, Engineer, is with Cold Rolling Mill Technology–Quality Control Department, Wu Han Iron and Steel Processing Corporation, Ltd., Hu Bei Province 430083, China. SANGHO KIM, Principal Researcher, is with the Plate Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, 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. Contact e-mail: [email protected] Manuscript submitted February 8, 2006. Article published online May 8, 2007. 1012—VOLUME 38A, MAY 2007
geometry. Furthermore, contrary to the values obtained from fracture toughness tests in accordance with ASTM standards, those measured from the Charpy impact test and DWTT are not transferable to structural components. In order to quantitatively evaluate fracture toughness and its distribution, advanced standard testing methods based on the probabilistic and statistical analysis of fracture mechanics are needed. Recently, the American Society for Testing Materials (ASTM) suggested the ASTM E1921-05 standard test method,[8] in which the variation of fracture toughness in the transition temperature region is considered as a property of ferritic steels. According to this test method, variations in fracture toughness as a function of temperature can be explained using a master curve characterized by a reference temperature. In the present study, fracture toughness in the transition temperature region of three kinds of API X70 line pipe steels rolled in the two-phase (a + c) region was analyzed in accordance with the ASTM E1921-05 standard
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