Impact of austenite grain boundaries and ferrite nucleation on bainite formation in steels

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.