The influence of rolling practice on notch toughness and texture development in high-strength linepipe
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I. INTRODUCTION
IN modern linepipe technology, both high strength and toughness are of primary interest. In the first case, appreciable savings in construction costs can be achieved by employing stronger materials and reducing the pipe wall thickness. Examples can be found in the literature where new X100 (690 MPa)[1,2,3] and X80 (550 MPa)[4] grades have been designed to replace the X70 (480 MPa) highstrength low-alloy pipeline steels. In addition, achieving high toughness and crack-propagation resistance are crucial to the development of any high-strength linepipe. Therefore, in practice, appropriate combinations of strength and toughness are desired. One way to improve the final properties is by changing the chemical composition of the steel.[1] Thermomechanical processing (TMP) is another means of influencing the final microstructures and crystallographic textures. To control these properties, the roles of the alloying additions and processing parameters (soaking temperature, finish rolling temperature, cooling rate, and cooling interrupt temperature) have been studied extensively during the last two decades.[1–17] This has led to a better understanding of microstructural evolution and of the relation between texture and anisotropy of the mechanical properties. In this way, mechanical-property anisotropies, in the range between ambient G.J. BACZYNSKI, formerly Research Associate with McGill University, Montreal, PQ, Canada H3A 2B2, is now Research Scientist with Alcan Int. Limited, Kingston, Ontario, Canada, K7L 5L9. J.J. JONAS, Professor, is with the Department of Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. L.E. COLLINS, Research Director, is with Research and Development, IPSCO Inc., Regina, Saskatchewan, Canada SK S4P 3C7. Manuscript submitted November 20, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature and 2196 8C, have been successfully simulated by means of computer modeling.[18,19,20] No such predictions are available in the literature for the anisotropy in notch toughness. Instead, ductile-to-brittle transition temperature curves are measured, and the onset of brittleness is deduced from these experiments. The lower the transition temperature, the greater the fracture toughness of the material.[21] Toughness is most commonly measured by the amount of energy absorbed by a Charpy V-notch specimen during impact testing. Low energy levels are associated with brittle behavior at low temperatures due to cleavage-type fractures. Transgranular cleavage is typical in this case. At higher temperatures, high-energy ductile fracture occurs by microvoid coalescence.[22,23] Allen et al. showed first[24] that the brittle fracture of single crystals of high-purity iron of various orientations involves separations on {001} cleavage planes. More recently, this characteristic was confirmed on polycrystalline samples tested at low temperatures.[18,19] Cheng et al.[8] and Sun and Boyd[9] also indicated the importance of the {001} textures formed parallel to the rolling plane. Th
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