Fatigue Strength of Austempered Ductile Cast Iron under Thermal Environment

Abstract

Austempered ductile cast iron (ADI) is attracting much attention as one of new materials. Its mechanisms of crack initiation and propagation vary with the relation between the properties of graphite and matrix structures. In this study, we focused on the following two topics : First, we considered what effect the thermal environment has on high-cycle fatigue of ADI. As an example of thermal environment, when vehicle gears come into contact with each other for a long time, due to a material change in the contacting face as well as the thermal environment, fatigue strength will decrease. There is no literature discussing fatigue strength at the Ms transformation temperature below it. Therefore, high-cycle fatigue strength under as near the Ms transformation temperature environment was compared with that under room temperature environment and discussed. Plots of fish-eye fractures on the S-N curve show a wide scatter of life. Therefore, attention, was paid to fish-eye fractures and defects considered to be the starting point of crack development. It was clear that they show stress dependence from observations of their SEM micrographs. Thus, a modified S-N curve in the fish-eye fracture region could be obtained from an arrangement using the defects and fish-eye areas.