Effects of Specimen Thickness and Notch Shape on Fracture Modes in the Drop Weight Tear Test of API X70 and X80 Linepipe
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INTRODUCTION
LINEPIPE steels used for long-distance transportation under high pressure generally require high strength and have become thicker and larger to improve transportation efficiency. They also need excellent lowtemperature toughness, since oil drilling in extremely cold regions is activated because of serious resource depletion.[1–7] In these linepipe steels, structural integrity at low temperatures is critical, and thus precise and reliable evaluation techniques of fracture resistance are needed for practical applications of linepipe steels. The drop weight tear test (DWTT) was established in the mid 1960s as a method to evaluate the fracture propagation transition temperature (FPTT) of linepipe steels.[2,8,9] Unlike Charpy V-notch (CVN) impact testing using specimens of a certain thickness (10 mm), SEOKMIN HONG, Research Assistant, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea. SANG YONG SHIN, Research Professor, is with the Department of Materials Science and Engineering, Pohang University of Science and Technology. SUNGHAK LEE, Professor, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, is jointly appointed with the Department of Materials Science and Engineering, Pohang University of Science and Technology. Contact e-mail: [email protected] NACK J. KIM, Professor, is the with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, is jointly appointed with the Department of Materials Science and Engineering, Pohang University of Science and Technology. Manuscript submitted July 2, 2010. Article published online April 23, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
specimens with the same thickness as actual linepipe steels are used in the DWTT, which can better explain changes in fracture resistance and transition temperature. Since DWTT specimens have a longer fracture path than CVN specimens, DWTT has proven to be an adequate test method of fracture propagation behavior.[2,5,8–12] As high-strength high-toughness linepipe steels were developed recently, the American Petroleum Institute (API) currently recommends using a DWTT specimen having a pressed notch (PN) for the FPTT evaluation of low-toughness linepipe steels, while a specimen having a chevron notch (CN) is recommended for high-toughness steels.[8,9] Because of the longer crack propagation path in the DWTT specimens, DWTT absorbed energy was known as a more reliable parameter to evaluate the resistance to fracture propagation than CVN absorbed energy. However, there still remain problems including uncertainties in the analysis criteria for fracture modes that occur in the broken DWTT specimens, which are varied with specimen thickness and notch shape. In the case of thick steels of over 19 mm in thickness, the transition temperature is indiscriminately lowered after the specimen thickness is reduced to 19 mm. For example, according to the API RP 5L3 specifications,[8] th
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