Development and characterization of lower, upper, and inverse bainite in an experimental 0.8wt.% C steel

Abstract

In this study, we have successfully developed lower, upper, and inverse bainite morphologies in a 0.8wt.% C steel through a meticulously designed heat treatment process. The steel underwent a precise sequence of thermal treatments, commencing with austenitization at 900 °C for 30 minutes, followed by austempering at temperatures of 400 °C, 480 °C, and 500 °C, each for a duration of 60 minutes. The deliberate adjustment of carbon content to a higher level played a pivotal role in facilitating the intriguing inverse bainitic transformation. Our findings, corroborated by optical microscope, scanning electron microscope and energy dispersive x-ray diffraction analysis, unequivocally validate the successful formation of these uniquemicrostructures. Further insights emerge from the micro-Vickers hardness testing, which reveals the order of hardness among the samples. The upper bainite structure outperforms all, showcasing the highest hardness. Following closely is the enigmatic inverse bainite, trailed by lower bainite. The traditional ferrite-pearlite structure, which has long been a benchmark, lagsbehind in terms of hardness. This research not only advances our understanding of the bainitic transformations in high-carbon steels but also highlights the potential for tailored material properties for various engineering applications.