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

Abstract

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 dilatometrie analysis in the range of 280 to 420°C. Assuming the rate of this reaction proportional to the lineal expansion, the kinetic parameters were estimated making use of the Johnson-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°C. The curve above this temperature corresponds to the upper pro-bainite formation, while the curve occurring between 270 and 350°C is associated to the formation of lower pro-bainite. TEM observations allowed to characterize these microstructures, and in the case of lower pro-bainite revealed the presence of carbides. It was found that the addition of alloying elements retards the beginning and ending time of reaction I.

Keywords:

• Austempering
• ductile irons
• kinetics
• isothermal transformation diagrams