The role of Cu, Mo and Ni on the kinetics of the bainitic reaction during the austempering of ductile irons

The kinetics of the reaction I occurring during the austempering treatment of five types of ductile cast iron containing different concentration of Cu , Ni, and Mo was studied using dilatometric analysis in the range of 280 to 420 degrees C. Assuming the rate of this reaction proportional to the line al expansion, the kinetic parameters were estimated making use of the Johns on-Mehl-Avrami model. The isothermal transformation diagrams were built for transformed fractions of 0.05, 0.5, and 0.95, it was found the presence of two "C" curves that overlap in the vicinity of 350 degrees C, The curve ab ove this temperature corresponds to the upper probainite formation, while t he curve occurring between 270 and 350 degrees C is associated to the forma tion of lower pro-bainite, TEM observations allowed to characterize these m icrostructures, and in the case of lower probainite revealed title presence of carbides, It was found that the addition of alloying elements retards t he beginning and ending time of reaction I.