Development and investigation of austempered ductile iron (ADI) for thick-walled gear components

Austempered ductile irons (ADI) are attractive for thick-walled and highly stressed gear components due to the possibility to design complex structures requiring a combination of high strength and moderate ductility. In this study, ADI was developed and optimized for industrial planetary carriers with shaft diameters ranging from 120 to 460 mm. The material was characterized by correlating fatigue properties to microstructural parameters using samples prepared from cast and heat treated carriers. The optimized castings exhibit a tension-compression fatigue strength enhanced by 25% compared with castings of the same size of nominally the same material grade. A scaling behavior was developed in order to estimate fatigue strength on the basis of the maximum defect size in the tested volume. The maximum defect can be either a graphite nodule or a micropore. By optimizing the microstructure of a thick-walled casting according to the typical loading conditions, a structurally graded component with a uniform degree of utilization can be obtained. This allows efficient castings and heat treatment of thick-walled ADI parts.