Crystallographic and Fractographic Analysis of Upper Bainite

Abstract

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.