Through-thickness fracture of a Ti-V-N plate steel
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I.
INTRODUCTION
T H E through-thickness fracture properties of plate steels are important because of the large through-thickness tensile stresses which can develop in welded joints (e.g., welded tubular offshore drilling platforms) tl] and the effects of rolling-plane delamination fractures on the integrity of large-diameter line pipe. t2) Previous work on delamination fractures and through-thickness tensile testing indicates that the critical cleavage fracture stress is generally lower for the through-thickness direction than the longitudinal direction for controlled-rolled microalloyed steel plate; t3] i.e., trc/(Z) < ~rci (L).* It is im*In this paper, L = rolling direction and Z = normal to plate surface. Longitudinal Charpy specimens are oriented with length in the L direction and notch along the Z direction. Longitudinal and through-thickness tensile specimens are oriented with length in the L and Z directions, respectively. Thus, tr~ (L) and try, (Z) are the cleavage fracture stresses with respect to planes normal to L and Z, respectively. Through-thickness properties pertain to tests where the maximum tensile stress is in the Z direction, e.g., through-thickness tensile tests.
portant to know how this anisotropy of fracture resistance is affected by rolling schedule and to identify the critical microstructural features. It is well established that the fracture of low-carbon steels is a heterogeneous process initiated at a particular microstructural site. Ductile fracture is primarily controlled by the distribution of nonmetallic inclusions. For controlled-rolled plate, through-thickness ductility is influenced by planar arrays of inclusions parallel to the rolling plane t2,4] and by certain C-rich transformation products in low-residual ("clean") steels. ]~1 Cleavage fracture also requires a brittle initiating phase, t6) which suggests there should be a strong dependence of lowtemperature fracture properties on microstructure. This is well illustrated by numerous studies of rolling-plane
B. DOGAN, formerly Visiting Research Fellow, Metals Technology Laboratories, CANMET, Ottawa, ON, Canada, is Research Scientist, GKSS Forschungszentrum Geesthacht GmbH, Max-Planck-Strasse, D-2054 Geesthacht, Federal Republic of Germany. J.D. BOYD, formerly Research Scientist, Metals Technology Laboratories, CANMET, is Professor, Department of Metallurgical Engineering, Queen's University, Kingston, ON K7L 3N6, Canada. Manuscript submitted April 10, 1989. METALLURGICAL TRANSACTIONS A
delaminations in controlled-rolled plate and the various cleavage-nucleating mechanisms proposed. 14,7-121 The objective of the present study was to elucidate the effects of various microstructural features on throughthickness fracture properties. A distinctive range of microstructures was produced by subjecting a single TiV-N steel composition to different thermomechanical processing schedules. Through-thickness fracture properties were characterized directly by means of through-thickness tensile tests and indirectly by studying delamination
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