EFFECT OF AUSTENITIZING CONDITIONS ON THE IMPACT PROPERTIES OF AN ALLOYED AUSTEMPERED DUCTILE IRON OF INITIALLY FERRITIC MATRIX STRUCTURE

The effect of austenitizing conditions on the microstructure and impac t properties of an austempered ductile iron (ADI) containing 1.6 % Cu and 1.6 % Ni as the main alloying elements was investigated. Impact te sts were carried out on samples of initially ferritic matrix structure and which had been first austenitized at 850, 900, 950, and 1000 degr ees C for 15 to 360 min and austempered at 360 degrees C for 180 min. Results showed that the austenitizing temperature, T-gamma and time, t (gamma) have a significant effect on the impact properties of the allo y. This has been attributed to the influence of these variables on the carbon kinetics. The impact energy is generally high after short t(ga mma), and it falls with further soaking. In samples austenitized at 85 0 and 900 degrees C, these trends correspond to the gradual disappeara nce of the pro-eutectoid ferrite and the attainment of fully developed ausferritic structures. In initially ferritic structures, the carbon diffusion distances involved during austenitization are large compared to those in pearlitic structures. This explains the relatively long s oaking periods required to attain fully ausferritic structures, which in spite of the lower impact energy values, have a better combination of mechanical properties. Microstructures of samples austenitized at 9 50 and 1000 degrees C contain no pro-eutectoid ferrite. The impact pro perties of the former structures are independent of t(gamma), while th ose solution treated at 1000 degrees C are generally low and show wide variation over the range of soaking time investigated. For fully ausf erritic structures, impact properties fall with an increase in T-gamma , This is particularly evident at 1000 degrees C. As the T-gamma incre ases, the amount of carbon dissolved in the original austenite increas es. This slows down the rate of austenite transformation and results i n coarser structures with lower mechanical properties. Optimum impact properties are obtained following austenitizing between 900 and 950 de grees C for 120 to 180 min.