Separation Phenomenon Occurring during the Charpy Impact Test of API X80 Pipeline Steels
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INTRODUCTION
MANY studies on steels have focused on reducing alloying elements in order to decrease production costs and achieve environmental friendliness while simultaneously obtaining excellent mechanical properties. Intensive studies on high-strength, high-toughness pipeline steels used for transporting crude oil or natural gas in which the low-alloying method is applied in order to cope with the soaring prices of raw materials are good examples. Despite their high strength, however, pipeline steels fabricated with reduced alloying elements often have the shortcoming of low toughness, which can cause critical safety problems for structures because the toughness further deteriorates as separations occur during the Charpy impact test.[1,2] The separation refers to the phenomenon of opening the impact specimen in the vertical direction to the fracture surface after the impact test, and is known to occur more seriously in steel plates rolled with higher reduction ratios.[3] Thick steel plates, such as pipeline steel plates, can also experience problems with separations, because quite high reduction ratios are applied in the beginning stage of the rolling process.[1–3] Factors SANG YONG SHIN, Postdoctoral Research Associate, and SEOKMIN HONG, Research Assistant, are 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 with the Materials Science and Engineering Department, Pohang University of Science and Technology. Contact e-mail: [email protected] JIN-HO BAE, Principal Researcher, and KISOO KIM, Group Leader, are with the Sheet Products & Process Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Korea. Manuscript submitted December 10, 2008. Article published online August 14, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
associated with the separation phenomenon include the following: (1) inclusions such as MnS, (2) textures, (3) intergranular failure along prior austenite boundaries, (4) segregation of P and S, (5) anisotropy of microstructures, (6) band structures elongated along the rolling direction, and (7) combinations of these factors.[3] Because the factors related to separations in pipeline steels are diverse and complex, it is necessary to systematically investigate the mechanisms involved in separations. This investigation into the factors of separations can present ways to prevent or minimize the toughness reduction, thereby leading to the successful development of high-strength, high-toughness pipeline steels having excellent properties using the low-alloying method. In the present study, therefore, various API X80 pipeline steels were fabricated by varying chemical compositions and rolling conditions. The Charpy impact test was conducted on these steels, and the results were classified into different separation types. Separation mechanisms involved in each type were interpreted i
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