Influence of microstructure on high-cycle fatigue behavior of austempered ductile cast iron

An investigation was carried out to examine the influence of austempering heat treatments and the resultant microstructure of austempered ductile cast iron, on the fatigue crack growth rate, fatigue threshold, and high-cycle fatigue strength of the material. Two different approaches were used to study the fatigue behavior of this relatively new material, that is, a traditional S-N curve approach for determination of fatigue strength and a fracture mechanics-based approach for determination of the fatigue threshold. Compact tension and cylindrical specimens prepared from alloyed nodular ductile cast iron were given three different austempering heat treatments to produce three different microstructures. The fatigue threshold and high-cycle fatigue behavior of these specimens were studied in room temperature ambient atmosphere. The results of the present investigation demonstrate that the fatigue threshold of the material increases with increase in volume fraction of carbon-saturated austenite. The fatigue strength of the material, on the other hand, was found to increase with decrease in austenitic grain size. The crack growth process in the material was a combination of ductile striations and microvoid coalescence, and crack propagation by connecting the graphite nodules along its path.