Morphology and Crystallography of Plate-like Lower Bainite in Fe–C Alloys

抄録

The aim of this study is to characterise the microstructure of plate-like lower bainite and the effect of carbon on this microstructure using Fe–C alloys. The carbon content in the specimens ranged from 0.6 to 1.4 in mass percentage. The specimens were heat-treated at 573 K after austenitisation, followed by water cooling. The specimens were observed using optical microscopy and scanning electron microscopy, and their crystal orientations were analysed using electron backscatter diffraction. Microstructural observations and crystallographic analysis show that plate-like lower bainite contains a packet and block structure within the prior austenite grains, similar to lath martensite and lath-like bainite. The most frequently observed crystallographic orientation relationship between the bainitic plates is the one 20° away from the twin orientation relationship, i.e., the V1/V6 relationship. The paired variants compose a microstructure unit and the bundled bainitic plates have similar long directions. The morphology of the interface is different from those of lath martensite and upper bainite. The variant pairing and paired block morphology, including interface shape, are considered distinct features of plate-like lower bainite.