Mechanical characterization of heavy section ductile cast iron components

This article is about the mechanical characterization performed on different types of ductile iron with solidification times up to 20 hours, in order to identify the metallurgical and process parameters affecting fatigue resistance. For this purpose, an experimental casting apparatus, based on controlled solidification within an electric furnace, was designed and developed to reproduce on small-scale samples the typical microstructures of heavy section castings, especially concerning the graphite morphology and the porosity. Preliminary, to validate this new experimental approach, the microstructures and the mechanical properties of small-scale samples were compared with those measured on samples machined from real large castings. Test campaigns were carried out on ferritic, high-silicon ferritic and pearlitic cast irons, by performing tensile tests, Brinell hardness and rotating bending fatigue. Optical and scanning electron microscope observations of the fracture surfaces were also performed, obtaining a complete characterization of the different types of cast iron for castings up to 50 t, without the need of using real heavy parts. The results showed that the shrinkage porosity and the spiky graphite have a deleterious effect on the cracks nucleation, significantly reducing the fatigue resistance of the castings, while the chunky graphite has a minor effect, acting as a preferential path in the propagation step. Ferritic cast iron with high silicon content was the most performing for the production of thick parts undergoing fatigue, thanks to its high ductility and uniform properties.