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Results
Initiation of Cleavage Fracture In a Weld Simuled Low Carbon Low Alloy Arctic Steel
Brandt, Kristin (Dept. Material Sci. Eng., Norwegian University of Science and Technology) | Solberg, Jan Ketil (Dept. Material Sci. Eng., Norwegian University of Science and Technology) | Akselsen, Odd Magne (SINTEF Materials and Chemistry, SINTEF) | Østby, Erling (SINTEF Materials and Chemistry, SINTEF)
ABSTRACT The risk of brittle fracture is an important aspect when oil and gas exploration move to Arctic regions. Therefore, the present work addresses examination of fracture surfaces in scanning electron microscope to identify brittle fracture mechanisms. The study is based on Charpy and CTOD bend testing at -60°C of weld thermal simulated specimens corresponding to both coarse grained HAZ (CGHAZ) and intercritically reheated coarse grained HAZ (ICCGHAZ). Brittle fracture was obtained by both test methods, and SEM investigation of the fracture surfaces revealed that slag particles like sulphides and oxides had initiated cleavage in the CGHAZ specimens, and that martensite-austenite (MA) particles probably were responsible for cleavage in the ICCGHAZ specimens. In both types of weld simulated structures, the crack initiation points were located close to the pre-crack tip. INTRODUCTION The increased focus on oil and gas exploration in the high north will inevitably give renewed attention to the low temperature embrittlement of structural steels. The ductile to brittle fracture transition temperature that occurs in such steels will consequently be very critical due to low service temperature. There are low alloy steels available with sufficiently low transition temperature, but they tend to loose their excellent properties after welding, primarily the toughness. If steels are to be made with the substantial improvements in weldability, further developments in steel processing technology, combined with optimization in alloying practice, must be made. In parallel, a better understanding of brittle fracture mechanisms of different weld microstructures must be sought. Such information has been collected from scanning electron microscopy (SEM) examination of fracture surfaces of Charpy V or CTOD (crack tip opening displacement) tests taken from either real weldments or weld thermal simulation. However, this may not be straight forward because of difficulties in finding the first point of fracture initiation (first microcrack to form).
- Europe (0.29)
- North America > United States (0.29)
Determination of Ccrystallographic Facet Orientations On Fracture Surfaces of an Arctic Steel By Using EBSD
Mohseni, Peyman (Norwegian University of Science and Technology (NTNU)) | Solberg, Jan Ketil (Norwegian University of Science and Technology (NTNU)) | Karlsen, Morten (Norwegian University of Science and Technology (NTNU), and Statoil ASA) | Akselsen, Odd M. (Norwegian University of Science and Technology (NTNU), and SINTEF) | Østby, Erling (SINTEF)
ABSTRACT Electron backscatter diffraction (EBSD) has been increasingly used to identify the crystallographic planes and orientation of cleavage facets with respect to the rolling direction in fracture surfaces. The crystallographic indices of cleavage planes can be determined either directly from the fracture surface or indirectly from metallographic sections perpendicular to the plane of the fracture surface. The commonly observed crystallographic mode (i.e. the micromechanism) of brittle fracture in ferritic steels is cleavage, which involves the separation of atomic bonds along low-index {100} crystallographic planes. The main purpose of this work was to identify the crystallographic orientations of cleavage facets in fracture surfaces with respect to the rolling direction of a well known steel at low temperatures. The material used for the work was a steel (API X80 grade) that has been developed for applications at low temperatures. The crystallographic indices of the cleavage crack planes were identified to be {100}, {110}, {211} and {310} at all temperatures. INTRODUCTION High strength and toughness pipeline steels have been used at high operation pressures to improve the transport efficiency of oil and gas. The major motivation behind the development of such steels has been to obtain the best combination of strength and toughness (Bose-Filho, Carvalho, Strangwood, 2007). Weldability, high fracture toughness and fatigue resistance at low temperatures are additional requirements for transmission of oil and gas through pipelines (Das, Sivaprasad, Dasa, Chatterjee and Tarafder, 2006). The balance of high strength and high toughness can be deteriorated by welding thermal cycles, producing local poor toughness in the welded joints (Lambert-Perlade, Gourgues, Besson, Sturel, and Pineau, 2004, Davis and King, 1996). The heat affected zone (HAZ) is in many cases considered to be the most critical part of a weld. The region of lowest toughness after single pass welding is the coarse grained heat affected zone (CGHAZ) (Qiu, Mori, Enoki and Kishi (2000) and Akselsen, Solberg and Grong, 1988).
- Energy > Oil & Gas (1.00)
- Materials > Metals & Mining > Steel (0.48)