Effects of microstructure on inverse fracture occurring during drop-weight tear testing of high-toughness X70 pipeline s
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I. INTRODUCTION
RECENTLY, rapid technological advancements of the thermomechanical control process (TMCP), composed of controlled low-temperature hot rolling and accelerated cooling, have considerably improved the toughness of pipeline steels.[1–5] In these high-toughness pipeline steels, however, an abnormal fracture appearance frequently occurring during the drop-weight tear test (DWTT) has caused many difficulties in evaluating DWTT properties.[6–18] The abnormal fracture appearance can roughly be divided into two types. First, the test is invalid, in accordance with the API RP 5L3 specification,[15] as shear fracture initiates at the notch of the DWTT specimen. In the second case, the socalled “inverse fracture” of a cleavage-fracture mode occurs in the hammer-impacted region of the DWTT specimen, after cleavage fracture initiated at the notch propagates in a shear-fracture mode. Thanks to intensive studies, the former problem was partially solved by inserting a static precrack, a fatigue precrack, an electron-beam weld, a chevron notch, etc., into the DWTT specimen, instead of the conventional standard pressed notch.[7,8] However, the latter problem still waits for clear solutions or alternatives to prevent or reduce its occurrence. Empirically speaking, the size of this inverse fracture tends to become more serious, as the DWTT speciBYOUNGCHUL HWANG, Postdoctoral Research Associate, and YANG GON KIM, Research Assistant, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. SUNGHAK LEE and NACK J. KIM, Professors, Center for Advanced Aerospace Materials, Pohang University of Science and Technology, are also with the Materials Science and Engineering Department, Pohang University of Science and Technology. Contact e-mail: shlee@ postech.ac.kr JANG YONG YOO, Principal Researcher, is with the Plate Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Korea. Manuscript submitted April 1, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
men thickness increases and the test temperature decreases.[6–9] It is also known that the inverse fracture is associated with the high fracture-initiation energy of the recently developed high-toughness pipeline steels.[11,12] Nevertheless, in the case of the chevron-notch (CN) DWTT specimen, in which the fracture-initiation energy is reduced by inserting the sharp notch, the inverse fracture also occurs in the hammerimpacted region, leaving the problem unsolved.[12,13] Few works have been done on how inverse fracture occurs in the recently developed high-toughness pipeline steels and on how microstructures and other properties, together with DWTT type and test conditions, affect the inverse fracture. Iwasaki et al.[6] investigated the plastic deformation and fracture behavior of DWTT specimens and reported that a high compressive prestrain was introduced inside the DWTT specimen as both the three-point bending and the hammer impact were working in the hammer-impacted region. Recently, Hwang et
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