Microstructure-Fatigue Property Relationships for Cast Irons

Cast irons are widely used for combustion engine/exhaust system applications, not only because they are less expensive but also because they offer some attractive properties such as good thermal conductivity, relatively high specific yield strength, and good oxidation resistance. Cast irons can be made with a wide variety of microstructures containing either flake-like graphite (FG), nodular graphite (NG) or vermicular graphite (VG), or mixing of the above, which control their mechanical and fatigue properties. In this paper, a microstructure-fatigue property relationship model is developed, combining the Tanaka-Mura-Wu’s fatigue crack nucleation model with Eshelby’s solution for materials containing ellipsoidal inclusions. This applies to cast irons considering its microstructural graphite characters (shape, size, elastic modulus and Poisson’s ratio). This model is used to analyse ductile cast iron (DCI) with nodular graphite (NG) microstructure, grey cast iron (GCI) with flake-like graphite (FG) microstructure, and compacted graphite iron (CGI) with vermicular graphite (VG) microstructure. Excellent agreement is found between the model prediction and the experimental data or the Coffin-Manson-Basquin correlations at room temperature. Further development will be to incorporate creep and oxidation models with the current model of mechanical fatigue (as driven by plasticity) to formulate a cost-effective approach to address thermomechanical fatigue of cast irons on a microstructural basis. This way, it can be used as a microstructural fatigue design tool in combination with the process control to optimize properties of cast irons for engineering applications.