Transformation Kinetics and Mechanical Properties of Copper-Alloyed and Copper-Nickel Alloyed ADI

Abstract

In this study the effect of copper and nickel in shaping the structure and properties of ADI (Austempered Ductile Iron) was investigated. The austenitization and austempering transformations were studied in order to follow the changes exhibited in transformation kinetics. The dilatometric results indicated that the addition of Cu and the addition of both Cu and Ni resulted in reducing relative expansion during austenitization, due to a larger pearlite fraction in the microstructure. In the initial stage of the austempering process, the addition of Cu, and to a greater extent, additions of both Cu and Ni led to a reduction in the transformation rate, shifting the maximum transformation rate values toward longer times. X-ray diffraction, dilatometric, metallographic and magnetic examinations allowed us to determine the phases fraction in the structure of ADI with the presence of Cu and Ni. From SEM-EDS analysis, it follows that in the copper alloyed ADI, highly dispersed particles are formed containing Mg and Cu, whose size does not exceed <1 µm. The exhibited mechanical properties were determined as a function of Cu and Ni additions and also variable austempering period of time. It was found that the addition of Cu resulted in increased tensile strength and hardness but simultaneously decreased the impact strength of ADI. The outcome of this work indicates that in order to obtain a satisfactory combination of static and dynamic mechanical properties of ADI, an optimal combination - aside from proper heat treatment - Cu and Ni should be selected.