Austempering kinetics of a ductile iron

The austempering kinetics of a ductile iron (ADI) was studied by means of diiatometry. Small samples (10 mm length, 4mm diameter) were austenitized between 850-900°C and austempered at different temperatures in the range 320-370°C. The austenite decomposition to form acicular ferrite and Carbon enriched austenite is evidenced by a net expansion which follows the typical kinetics of diffusional transformations, described by the JMA equation x(t)=1- exp(-(kt)n). The activation energy was calculated assuming fixed stages of transformation, namely 10 and 90% at different austempering temperatures. At the lowest austenitizing temperature (850°C) the transformation may be described by a single value of the activation energy (23,95 kJ/mol). On the other side, at 900°C the kinetics showed an influence of the austempering temperature: in the lowest temperature range the activation energy was in the same range of that calculated at 850°C (26,07 kJ/mol), whilst in the highest one it was considerably lower (6,2 KJ/mol). This behavior could be correlated to the coarser a us ferrite microstructure arising from high temperature austenitizing and the different carbon redistribution during isothermal treatment.