The processing window for austempering ductile iron

A PC-controlled vacuum heat treating system was used to austemper an alloyed ductile iron with 0.77%Cu and 0.5%Ni at 400 degrees C after austenitizing at 850 degrees C. During the progress of isothermal transformation, the resistivity of austempered ductile iron vs, time was recorded and then analyzed to determine the processing window. X-ray diffraction, heat tinting technique, and microstructural examination were conducted to examine the suitability of this processing window in terms of microstructural changes. The data of mechanical properties of the ductile iron from the same casting batch and same austempering were extracted from a concurrent work to discuss the suitability of this processing window. Both the evaluation of microstructures and mechanical properties confirmed the suitability of this processing window. A slow rate transformation during the time interval of processing window was observed which was associated with the continuous decomposition of blocky austenite and resulted in a slight variation of the mechanical properties.