Microstructural Origins for Quench Cracking of a Boron Steel: Boron Distribution

Boron is an important trace element and intentionally added for hardenability of low-alloy steels. The forms of B presence in microstructure are believed to be crucial for success or failure of boron steel production. This research addresses atomistic distribution of boron responsible for quench cracking of a boron steel. Boron steel sheets (6 mm thick) with/without quench cracking are carefully characterized with transmission electron microscopy and atom probe tomography to reveal B distribution and to understand quench cracking mechanisms. The quench cracking is brittle intergranular fracture along prior austenite grain boundaries (PAGBs). The PAGBs are decorated with boron-containing Fe3(C, B) precipitates in the cracked steel, but segregated with boron in the uncracked steel. Without the segregation of B at PAGBs, the formation of ferrite and massive acicular-shaped Fe3C during quenching makes cracks easy to initiate and propagate in the steel with deteriorated mechanical properties. Processing parameters important for engineering the distribution of boron are discussed.