The Role of Prior Austenite Grain Size in the Kinetics and Morphology of Carbide-Free Bainite.

: This study examines how prior austenite grain size (PAGS) influences the kinetics and microstructural evolution of carbide-free bainite in a medium-carbon, high-silicon steel isothermally transformed at 325 °C. Using controlled austenitization, fine (∼28 μm), intermediate (∼42 μm), and coarse (∼77 μm) grains were generated. While all conditions yielded a comparable final phase balance of ∼75 vol.% bainitic ferrite and ∼25 vol.% retained austenite, their kinetic pathways diverged significantly. Fine grains achieved rapid kinetics via abundant grain-boundary nucleation, whereas coarse grains relied on extended autocatalytic sheaf growth within large, unobstructed volumes. The intermediate grain size represented a distinct transitional regime, lacking a dominant nucleation mechanism and resulting in the slowest overall transformation. Furthermore, kinetic modelling and microstructural analysis revealed that bainitic plate dimensions are heavily governed by transformation kinetics and dynamic "hard impingement" events, rather than austenite yield strength alone. High initial nucleation rates forced early impingement and finer plates, whereas a suppressed nucleation rate reduced impingement frequency, allowing plates to grow thicker and longer, geometrically promoting a higher fraction of blocky retained austenite. Ultimately, PAGS dictates bainitic microstructure through a complex interplay of nucleation pathway selection, spatial constraints, and the dynamics of plate thickening.