抄録
This study investigates the relationship between the morphology and the fracture behavior of upper bainite in JIS SK5 steel. The cleavage crack path was found to lie on {001}α, {112}α or {123}α using electron backscatter diffraction (EBSD) from the fracture surface. Additionally, most of the bainite sheaf boundaries were found to be high-angle boundaries. If the cleavage planes are presumed to lie uniquely on {001}α, then most of the deviations of the angles between two K-S variants in a given austenite grain are high-angle deviations. According to TEM diffraction analysis, the orientation relationship of cementite/bainitic ferrite satisfies the Bagaryatskii relation, and the habit plane of cementite precipitated in the bainite sheaf locates on (0\\bar11)α||(100)θ. Cementite does not significantly affect the propagation of a cleavage crack. Hence, cleavage cracking deflects at grain boundaries or bainite sheaf boundaries, but only reinitiates at the cementite/bainitic ferrite interface.
