On the Microstructure and Fracture Surface of Bainite in JIS SK5 Steel

The microstructure and fracture surface of bainite in JIS SK5 steel have been investigated using transmission electron microscopy (TEM) and electron back-scatter diffraction (EBSD) in this study. Specimens were austenitized at 880 for 30 min and then austempered at 300 and 400 for 1hr, as a result, lower bainite and upper bainite were formed, respectively. The lower bainite phase consists of plate-like bainitic ferrites and embedded cementite platelets, the cementite precipitated unidirectionally at an angle of 55 to 60 deg deviated from the long axes of the ferrites; in addition, the upper bainite phase consists of a parallel array of ferrite laths and discrete cementite layers sandwiched between them. Both the bainite structures have the same ferrite/cementite orientation relationship, which is close to that of the Bagaryatskii relation. The impact fractographs of lower and upper bainite structures exhibit brittle failure with cleavage facets, of which the size is correlated with the width of the bainite sheaves. Moreover, the crystallographic orientation of cleavage facets has been determined directly by EBSD. The results showed that the cleavage facet planes of lower bainite structure are close to the {001} plane of ferrite, and they were close to the {001}, {112} and {123} planes of ferrite for the upper bainite structure.