Phase transformation and fatigue properties of austempered ductile irons

A series of studies were carried out regarding ausferrite transformation, the growth of ferrite from austenite, during isothermal holding. The mechanical properties associated with microstructures of alloyed and unalloyed austempered ductile irons (ADIs) were also investigated. Heat released from ausferrite transformation has been quantitatively measured in a previous study. The effect of alloying elements on the heat evolution rate and on the morphology of ausferrite has also been discussed. Mechanical property testing was carried out on specimens of ductile iron austempered at 593 K and 633 K for 2 hrs. Tensile properties and high-cycle fatigue life tests were also made. In addition, phase transformation of ADIs during ultrasonic vibration treatment was also investigated. SEM micrographs of deformed samples indicate and confirm the occurrence of stress-induced transformation of ADIs. The alloyed ADIs isothermally-held at 633 K develop an optimum tensile strength, elongation and superior impact toughness and good high-cycle fatigue strength. These properties are attributed mostly to a large amount of dispersed blocky austenite. This retained austenite may become work hardened during deformation.