INFLUENCE OF MICROSTRUCTURE ON HIGH-CYCLE FATIGUE BEHAVIOR OF AUSTEMPERED DUCTILE CAST-IRON

An investigation was carried out to examine the influence of austemper ing heat treatments and the resultant microstructure of austempered du ctile cast iron, on the fatigue crack growth rate, fatigue threshold, and high-cycle fatigue strength of the material. Two different approac hes were used to study the fatigue behavior of this relatively new mat erial, that is, a traditional S-N curve approach for determination of fatigue strength and a fracture mechanics-based approach for determina tion of the fatigue threshold. Compact tension and cylindrical specime ns prepared from alloyed nodular ductile cast iron were given three di fferent austempering heat treatments to produce three different micros tructures. The fatigue threshold and high-cycle fatigue behavior of th ese specimens were studied in room temperature ambient atmosphere. The results of the present investigation demonstrate that the fatigue thr eshold of the material increases with increase in volume fraction of c arbon-saturated austenite. The fatigue strength of the material, on th e other hand, was found to increase with decrease in austenitic grain size. The crack growth process in the material was a combination of du ctile striations and microvoid coalescence, and crack propagation by c onnecting the graphite nodules along its path.