Effects of Microstructure on Tensile, Charpy Impact, and Crack Tip Opening Displacement Properties of Two API X80 Pipeli
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RECENTLY, pipeline steels used to transport crude oil or natural gas over a long distance have become thicker and larger, and require high strength and toughness simultaneously to improve transportation efficiency under a high-pressure condition.[1–3] Oil-drilling operation in very cold areas, e.g., Alaska and Siberia, has been actively conducted because of worldwide resource depletion, and thus pipeline steels having excellent low-temperature toughness are highly demanded. There are several methods to form a pipe shape from a plate shape, especially, UOE (U-ing, Oing, and Expansion) and spiral piping methods are generally used.[1,4,5] The mechanical properties of the UOE pipes are affected by the mechanical properties of the perpendicular direction to the rolling direction of plates, while the mechanical properties of the spiral pipes are affected by the mechanical properties of a certain angle bending to the rolling direction of plates (Figures 1(a) and (b)). Therefore, the investigation into the correlation between microstructures and mechanical SANG YONG SHIN, Postdoctoral Researcher, is with the Department of Microstructure Physics and Alloy Design, MaxPlanck-Institut fu¨r Eisenforschung GmbH, Du¨sseldorf 40237, Germany. Contact e-mail: [email protected] Manuscript submitted September 7, 2012. Article published online January 24, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
properties in the direction of plates is needed essentially for guarantees of mechanical properties of steel pipes. Fine bainitic microstructure has been generally used in API (American Petroleum Institute) X80 pipeline steels to satisfy high strength and toughness. In order to generate fine grains, it is necessary to carry out hot rolling process with high pressure.[1,5] Thus, elongated grains along the rolling direction is formed by the hot rolling process, because of which the direction of the plates having elongated grains along the rolling direction will significantly change the mechanical properties of steel pipes formed by different piping methods. Mintz et al. reported that tensile and Charpy impact properties of materials having elongated grains by rolling process were most affected by the direction of elongated grains.[6] Kim et al. reported that the least tough Charpy orientation also happens to coincide with the circumferential direction which experiences the largest hoop stress in spiral pipes.[7] In addition, bainitic microstructural steels consist of various hard phases such as martensite, cementite, retained austenite, and martensite–austenite constituents (MA) to improve strength, in which the size and the volume fractions of hard phases have a huge impact on the mechanical properties. Therefore, systematic studies on the effects of elongated grains along the rolling direction, and size and volume fractions of hard phases on strength and toughness of API pipeline steels are also needed. VOLUME 44A, JUNE 2013—2613
Fig. 1—Schematic diagrams of (a) the UOE and (b) spiral piping methods.[1]
Tensile and Charpy impact tests a
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