Crystallographic Microstructure Analyses below Cleavage Triggers in Bainitic Low Carbon Steels

Microstructures at the cleavage triggers were investigated in Bainitic steels. Bainitic steels with different Austenite grain size and second phase were prepared, and the fracture absorbed energies in cleavage fracture were examined under static loading with Charpy type bend specimens. Cleavage trigger point was identified on each fracture surface by fractography, and was exposed into an etching process. The correspondence of cleavage facets around a trigger with Bainite microstructural unit was investigated. Bainite packet boundaries were frequently observed on a continuous cleavage facet. Fracture surfaces were sectioned, and the cross sections which included a trigger were polished. These cross sections were exposed onto EBSD observations, and the correspondence of cleavage facets with Bainite microstructure was crystallographically investigated. Optically observed microstructure boundary did not always act as a resistance against a cleavage cracking, and EBSD analyses suggested that the crystalline orientation units such as a Bain group controlled a cleavage facet unit.