The rate of bainite formation depends on several factors such as austenite grain size, decomposition of austenite into other phases and austenite composition. Although studies have been carried out to understand various factors affecting bainite formation, open lines of investigation still remain. In this work, the effect of ferrite formation prior to bainite formation as well as the effect of parent austenite grain boundary composition on the bainite formation kinetics is investigated. With this aim, bainite formation treatments directly after complete austenitization and in combination with an intermediate heat treatment step prior to bainite formation were applied to a low-carbon silicon-containing steel. The intermediate heat treatment step leads to ferrite formation at and/or elemental segregation to austenite grain boundaries which were characterized using scanning electron microscopy, electron backscatter diffraction analysis and atom probe tomography. The results indicate that the kinetics of bainite formation can be accelerated with the help of an intermediate heat treatment step prior to bainite formation. The acceleration of bainite formation is mainly due to increase in the density of bainite nucleation sites.

Bainite formation in steels typically starts at austenite grain boundaries and continues through nucleation of bainite at newly formed bainitic ferrite/austenite interfaces. Recent experimental evidence has pointed out that austenite to bainite transformation can also proceed in the presence of martensite. Studies suggest that the presence of athermal martensite formed prior to bainite formation can accelerate the kinetics of bainite formation with the martensite/austenite interfaces acting as potential nucleation sites. In this work, a kinetic model based on the displacive mechanism of bainite formation is adapted to isolate the impact of martensite/austenite interfaces on the overall rate of bainite formation when bainite formation occurs in the presence of previously formed martensite. This adapted kinetic model is validated using dilatometer studies published in the literature on a silicon-containing low-carbon steel in which bainite formation experiments are performed both below and above the M_s temperature. The results suggest that the formalism of the existing kinetic theory can describe the effects of martensite/austenite interfaces on the bainite formation.

Low temperature bainite formation in high-carbon steels leads to highly refined microstructures. However, the rate of bainite formation becomes impractically low as the bainite formation temperature decreases. Thus, it is important to create strategies to accelerate the low-temperature bainite formation kinetics in high-carbon steels. In this work, it is observed that decorating the parent austenite grain boundaries with grain-boundary cementite prior to bainite formation leads to an appreciable acceleration of subsequent bainite formation kinetics due to decrease in activation energy for bainite nucleation at cemenite/austenite interfaces.

Over the years, a quantitative theory to explain bainite formation kinetics has been proposed based on the nucleation kinetics of bainitic sub-units. Although the theory shows acceptable correlation with experimental results, it is observed that the kinetic models show a certain degree of discrepancy with actual kinetics. It is identified that these mainly arise due to the inadequate estimation of autocatalytic nucleation, especially as a function of progress of bainite formation. With the help of this observation, the kinetic model is modified and a better insight into the process of autocatalytic nucleation, essential in bainite formation, is obtained.