Roles of pre-formed martensite in below-Ms bainite formation, microstructure, strain partitioning and impact absorption energies of low-carbon bainitic steel

Abstract

The roles of pre-formed martensite (PM) in below-Ms bainite formation, microstructure, crystallography, strain partitioning and mechanical properties of a low-carbon bainitic steel were investigated using electron-backscattered diffraction, transmission electron microscopy, micro digital image correlation technique and mechanical tests. It is demonstrated that the pre-formation of martensite eliminates the incubation time for bainite transformation at various austempering temperatures below Ms, indicative of its acceleration effect at the early stage of transformation. This effect is mainly attributed to the surfaces or tips of the PM acting as the nuclei of subsequently-formed bainite, with initial bainite tending to form around the PM. However, the finishing time for below-Ms bainite transformation, especially at even lower temperatures, is retarded, owing to the dividing effect of PM on parent austenite grains, the decreasing effect of lowered isothermal temperature on the diffusion rate of carbon atoms and the strengthening effect of lowered isothermal temperature on supercooled austenite. PM and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block. The pre-formation of martensite largely refines the bainitic blocks/laths and retained austenite. The specimens with PM show relatively uniform strain partitioning among various phases, contrasting with the specimens without PM, for which strains are highly concentrated in the bainite region nearby fresh martensite/austenite (M/A) blocks or between adjacent M/A blocks. The impact absorption energies of the specimens with PM, when austempered at 30–60 °C below Ms, are more than twice higher than those of the specimens without PM, at no expense of tensile properties.