Experimental investigation of the influence of the austenite grain size on the mechanism and kinetics of the bainite transformation in steels

This study describes selected results of an extensive characterisation of the influence of the reduction of the austenite grain size on the mechanism of bainite transformation in several high carbon low-alloy steels. For quite large grain sizes, it is generally admitted that the transformation starts with the nucleation and growth of a first platelet at the austenite grain boundary. It further proceeds by the autocatalytic nucleation and growth of new platelets at the tip of the previous ones, bringing about the typical sheaf morphology. This process then stops when the sheaf reaches the grain boundary. Previous studies have shown that the increase of the grain boundary area resulting from smaller grains brings about an increase of the nucleation rate and thus faster kinetics for the bainite transformation. However, no systematic work was carried out for very small grain sizes, particularly when the austenite grain becomes of the same order as the length of the individual platelets of bainitic ferrite. This study shows how the conditions of nucleation and growth of the bainitic ferrite platelets as well as the carbon redistribution are influenced when the austenite grain size is ranging from I to 50 mum