This work aimed at studying the kinetic aspects of the incomplete bainite transformation of ductile cast iron containing 2.36 and 2.68% silicon (in wt %). Samples were initially austenitized at 900°C during 90 minutes and then austempered at 320 and 370°C in times ranging from 1.5 to 60 minutes. Microstructural characterization was performed by light optical microscopy (LOM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Vickers microhardness tests. A LOM-based method for transformed fractions quantification has been developed. Due to its high silicon content, both alloys presented the bainite transformation stasis phenomenon. Samples austempered at 320°C displayed more refined microstructures and higher transformed volume fractions at the reaction stasis beginning. Low resolution is the main drawback for using light optical microscopy in the phase quantification. Transformed volume fractions determined by XRD showed smaller values than those obtained by LOM, mainly for the lower austempering temperature. Kinetic transformation curves have been determined for all studied conditions. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model was used to study the incomplete bainite transformation kinetics. Avrami exponents varied from 0.15 to 0.67.
Austempered ductile iron; phase transformations; microstructure; retained austenite; bainitic ferrite
