Nodular cast iron materials 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 order to max out the potential of these materials, an in-depth characterization of their fatigue properties is necessary. This allows for reduced safety margins.
In this study, austempered ductile iron (ADI) EN-GJS-900-8 and conventional nodular graphite iron EN-GJS-700-2 were developed and optimized for industrial planetary carriers with shaft diameters ranging from 120 mm to 460 mm. Both materials have been cast into the same patterns of differently sized planetary carriers. Fatigue specimens were prepared from the cast components and tested in order to determine the fatigue limit. Microstructural characterization and fractography including optical and scanning electron microscopy were carried out.
As a result, the correlation between mechanical and microstructural properties reveals the importance of fine and nodular graphite accompanied by the absence of micro porosities or other casting defects. A scaling behavior was developed in order to estimate fatigue strength based on the maximum defect size in the tested volume. Moreover, a minor influence of matrix structure embedding the graphite - pearlitic or ausferritic - was determined.
According to the results obtained, the fatigue strength of conventional high strength cast iron may be adequate by improving graphite quality and eliminating casting defects. Thus, the accurate design process of a cast component is essential to avoid an increase of alloying elements and the expansive heat treatment in salt bath necessary for ADI. By understanding the correlations between mechanical properties of nodular cast iron and its microstructure, the efficiency of cast components can be enhanced, while costs can be reduced.