Deformation Induced Microstructure Evolution and Phase Transformation in an Austempered Ductile Iron (ADI)

The effects of strain-induced austenite transformation on the microstructure evolution and phase transformation characteristics have been investigated in an austempered ductile iron during various processing routes (heat and thermomechanical treatments). The thermomechanical treatment is performed using a Gleeble 3800 thermomechanical simulator coupled with dilatometry. The deformation-induced microstructure evolution and phase transformation during the austempering process have been characterized using optical microscopy, scanning electron microscopy, and X-ray diffraction techniques. The variations in retained austenite percentage (%) and its carbon content for different austempering times are analyzed to study the strain-induced transformation of austenite. It has been observed that the thermomechanical treatment significantly influences the phase transformation kinetics during the austempering process. The thermomechanical treatment produced a martensite-free ausferritic microstructure with a high-volume fraction of carbon enriched retained austenite for all austempering times compared to the conventional heat treatment.