Effect of austenite grain refinement on morphology of product of bainitic reaction in austempered ductile iron

The effect of austenite grain refinement on the morphology of the product of the bainitic reaction in an austempered ductile iron has been studied. The grain refinement was achieved by the rapid austenitisation of martensitic microstructure. The morphological aspects of the product of austenite decomposition during the austempering at 350°C were then compared with a diffusional model that relates morphology of precipitation to the type of nucleation site. It was observed that the grain refinement increases the proportion of grain boundary allotriomorphic ferrite. The austenite grain refinement achieved by rapid austenitisation suggests that the mechanism of austenite formation during the rapid heating of martensitic microstructure is thermally activated. The close agreement between the morphological results and the model used to explain the morphological changes suggests the existence of a diffusional component in the bainitic reaction of the austempered ductile iron investigated in this work.

MST/1737