Relationship between the Effective Grain Size of Brittle Crack Propagation and Microstructural Size in Low-carbon Low-alloy Bainitic Steels

Abstract

Crack propagations of a lathlike upper bainite with martensite-austenite (MA) regions were examined, and a fracture unit of the crack propagation was determined by means of a scanning electron microscope equipped with an electron backscatter diffraction detector. Fracture surfaces of the cracks were parallel to {001} planes in not only bainite but also the MA regions, and the cracks were arrested at the MA regions. In tempered specimens, the crack planes were not always parallel to the {001} planes, additionally the MA regions were deformed by the cracks. Although the MA regions arrest the cracks, macroscopic crack planes were flat in the similar crystal orientated regions in the bainite. To consider the macroscopic fracture unit of the crack propagation, a crystallographic unit which is a single crystal grain and a morphological unit which corresponds to a single bain group region were measured. From the measurements, it was found that the crystallographic unit was better than the morphological unit in the crack propagation. One of the reasons was that the prior austenite grain boundaries change to low angle grain boundaries after bainitic transformation, variant selection rules constrain the bainitic ferrites to form in both the austenite grains with a similar crystal orientation.