Influence of the austenite matrix microstructure on the phase transformations during austempering of ductile iron

This article reviews the influence of the austenite matrix microstructure and its inhomogeneity on the phase transformations during austempering of ductile iron. Significant differences in the austenitic matrix before austempering result from different solidification conditions and thermal cycles. Higher cooling rates during solidification result in a finer distribution of the microsegregated regions. Reheating above the eutectoid temperature range before austempermig causes a marked grain refinement of the austenite. The austempering transformation of ductile iron was studied for austenitic matrices of different characteristics, ranging from very coarse grained, obtained through hot shake out, to very fine grained, obtained through recrystallization. The marked difference in the availability of nucleation sites for ferrite needles causes noticeable changes in the austempering microstructure. The relative absence of grain boundaries in the as cast austenite causes the nucleation of ferrite to take place preferentially at the graphite/matrix interface. The actual grain size of the austenite after solidification was directly observed by using a novel technique.