The role of the austenite grain size in the martensitic transformation in low carbon steels

There is sufficient experimental evidence to propose that the formation kinetics of athermal martensite directly depends on the austenite grain structure from which the martensite forms. Yet, this dependence is frequently ignored. The present study investigates the role of the prior austenite grain size (PAGS) in the martensitic transformation in low-carbon steels. The transformation kinetics was experimentally studied for PAGS in the range from 6 to 185 μm and theoretically analysed based on the nucleation rate and the thermodynamic balance between the chemical driving force and the resistance exerted by the austenite against the progress of the transformation. It is observed that grain refinement shifts the martensite start temperature (MS) to lower values and accelerates the transformation rate at initial stages. At a later stage, when approximately 30% martensite has formed, the transformation rate decreases rapidly for small PAGS, whereas higher rates are maintained in coarse-grained microstructures. The change in martensite formation rate with the grain size depends on the nuclei density and on the austenite strength. This research enables an optimised selection of processing parameters for the design of ultra-high strength steels that require the formation of a controlled fraction of martensite. The austenite grain size influences the martensitic transformation through the nuclei density provided by the grain boundary area and through the strengthening of the austenite phase as the transformation progresses. [Display omitted] •The nuclei density and the thermodynamic balance between chemical driving force and austenite strength determine the martensite transformation rate•Grain refinement increases the austenite resistance against the austenite/martensite interface motion and suppresses strain relaxation mechanisms•Austenite grain refinement lowers the MS and increases the initial transformation rate through the repeated nucleation of parallel martensite laths•After approximately a 30% of martensite has formed, the transformation rate decreases rapidly for small prior austenite grain sizes