In order to estimate fatigue reliability of super-rapid induction quenched spheroidal graphite cast iron, FCD500, FCD700, the influence of complex surface microstructure on the fatigue strength was examined through fractography. The main results obtained are as follows : (1) Fatigue strength of quenched FCD500 was superior to that of quenched FCD700 in the long-life region (N> 5.0×10
5), in spite of tensile strength of FCD500 was lower than that of FCD700. (2) Fatigue property of super-rapid induction quenched spheroidal graphite cast iron could be estimated using a concept of "S-N globe". (3) Initial stress intensity factor at crack initiation under the surface of quenched FCD500 was comparable to that of quenched FCD700, therefore the improvement in fatigue strength of FCD500 depended on the crack propagation behavior through the complex surface microstructure, not the crack initiation behavior. (4) The prevention of crack propagation through the complex surface microstructure gave and appearance of high level fatigue reliability of FCD500.
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