Bainitic microstructures are receiving much attention, since their exceptional combination of strength and ductility. From the industrial point of view, the design of heat treatments has primary importance, since it can lead to maximise the performance and/or reducing the time required to complete the transformation. Among the variable that should be considered there is the effect of prior austenite grain size (PAGB) and austenitization temperatures, which affect both the morphology of the transformation product and the kinetic of the transformation. However, the effect of PAGS on the bainitic transformation is still unclear and under debate, owing to the controversial results exposed in literature . In this work, a medium carbon (~0.4C) high silicon (3.2%) steel has been austempered at 325 °C after different austenitization temperatures (900-1100 °C), and the transformation kinetics and microstructural changes evaluated by means of dilatometry, XRD, SEM, and hardness (HV10).

Microstructural observation on the specimens showed that increasing the austenitization temperature from 900 °C to 1100°C led to an almost identical microstructure in terms of phase and composition distribution, i.e. ~75% of bainitic ferrite with a C content of ~ 0.16wt.%, and ~25% of carbon-enriched retained austenite (1.21 wt.%), with a hardness HV10 = 450. But, due to the lower strength of the subcooled austenite and its larger size, the bainitic ferrite plates obtained were thicker and longer.

Dilatometry analysis of the isothermal transformation, showed that, in the range of 900-1000 °C grain refinement leads to an acceleration of the bainitic transformation due to the increase in grain boundary nucleation sites high amount of available nucleation sites. While for higher austenitization conditions, 1100°C, the coarser austenite grain, leads to an acceleration since the compensating effect from the autocatalytic nucleations. Fitting of the curves with the models proposed in the literature confirmed the predominant effect of autocatalytic nucleation for large grains leading to an acceleration of bainitic transformation above 1000 °C. Furthermore, it was found that the extent of autocatalytic reaction and the contribution of carbon partitioning is dependent on the phase of the formed bainitic ferrite.