Studies on the effect of the prior austenite grain size on the phase transformation kinetics of bainite are contradictory. Literature explains these contradictions by the presence of two different types of nucleation sites, the prior austenite grain boundaries and the tips of previously formed bainitic ferrite sub-units. The difference in their activation energies for nucleation, ΔQ, is known to determine whether the phase transformation kinetics of bainite are accelerated by prior austenite grain refinement or coarsening. However, the factors that influence ΔQ are not entirely understood, which is the reason why the contradictory results regarding the effect of the prior austenite grain size on the phase transformation kinetics of bainite observed in the different studies cannot be explained yet.

This master thesis investigates the effect of the prior austenite grain size on the phase transformation kinetics of bainite at different transformation temperatures in a low-carbon high-silicon steel. The experiments were divided into two groups, with one group consisting of specimens with finer prior austenite grains and one with coarser prior austenite grains. Specimens were transformed to bainite at three different isothermal transformation temperatures, 440 °C, 410 °C and 380 °C. Bainite formation was investigated by in-situ synchrotron XRD experiments performed at DESY to study the evolution of the phase fractions, lattice parameters and microstrains of bainitic ferrite and austenite. Furthermore, microstructure investigations on the specimens transformed at DESY and additional interrupted quenching experiments were conducted to understand the effect of the prior austenite grain size and the isothermal transformation temperature on the microstructure and the sheaf morphology formed in the early stages of the phase transformation. Finally, simulations were performed to determine the effect of the prior austenite grain size and the transformation temperature on ΔQ.

The experiments showed that, by decreasing the transformation temperature, the phase transformation kinetics of the group with fine prior austenite grains were decelerated, whereas the phase transformation kinetics of the group with coarse prior austenite grains were accelerated. The simulations exhibited an increase in ΔQ as the isothermal transformation temperature was decreased, indicating that sheaf growth by successive nucleation events at the tips of previously formed sub-units becomes increasingly prevalent. While the specimens with coarse prior austenite grains provide more potential nucleation sites at the tips of previously formed sub-units, the specimens with fine prior austenite grains provide more nucleation sites for grain boundary nucleation, which explains the reverse effect of the transformation temperature on the phase transformation kinetics of the two experiment groups. Microstructure observations have shown that the effect of the transformation temperature on processes, such as carbon partitioning and the transition from upper to lower bainite, could play an important role in explaining the observed effect of the prior austenite grain size on the phase transformation kinetics of bainite.