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).