Ductile cast irons (DCI) are characterized by their excellent mechanical properties, good castability, and low costs. This combination predestines DCI for an application as large structural components, such as wind turbines. The microstructure of ductile cast iron (DCI) is defined by spherical graphite nodules embedded in a ductile matrix, ranging from ferritic to pearlitic. In the case of a ferritic matrix, additional solid solution strengthening with silicon can significantly enhance the mechanical properties of the matrix. Thus, solution strengthened ferritic ductile irons (SSF-DI) offer improved mechanical properties compared to traditional cast iron grades. The macroscopic mechanical properties of SSF-DI are significantly impacted by the complex residual stress state that is produced by the mismatch in thermal contraction between ferrite and graphite as well as the phase transition that takes place during cooling to ambient temperature [1, 2].

Due to the local stress concentration around graphite nodules acting as a notch, they are considered as a primary fatigue crack initiation site. Therefore, it is important to consider the large local residual stresses that are induced during the cooling process and that surround the graphite nodules. These residual stresses may increase the potential for crack initiation, and consequently the risk of fatigue failure of a component. Thus, it is crucial to take such process-related residual stresses into account when assessing the macroscopic mechanical properties of SSF-DI, particularly under cyclic loading.

In order to predict the residual stress field in the SSF-DI microstructure, this work presents a finite element model that evaluates the coupled thermal, mechanical, and metallurgical mechanisms during cooling from casting temperature. Therefore, the implemented model depicts the fully solidified SSF-DI's cooling behavior and considers local phase transitions. Based on the locally varying strains due to thermal contraction and phase transition, residual stresses are computed. The implemented model is used to conduct sensitivity analysis concerning variations of the microstructure (e.g. graphite nodule size) and the cooling process (e.g. cooling rate). Validation is performed on two scales using micromechanical nanoindentation experiments and X-ray diffraction measurements.

References
[1] T. Andriollo; Y. Zhang; S. Faester; J. Thorborg; J. Hattel Journal of the Mechanics and Physics of Solids, 2019, 125, 714-735.
[2] Y. B. Zhang; T. Andriollo; S. Faester; W. Liu; J. Hattel; R. I. Barabash Acta Materialia, 2016, 121, 173-180.