Cast irons are differentiated by the shape of the graphite particles. Grey cast iron is characterized by randomly oriented graphite flakes, while the graphite in ductile iron exists as individual spheres. The graphite particles in CGI are randomly oriented and elongated as in grey iron, but they are shorter, thicker and have rounded edges.
The difference between the three types of graphite is more striking when viewed in the three-dimensional deep-etch scanning electron micrographs.
In comparison to either grey iron or ductile iron, the entangled compacted graphite 'clusters' interlock themselves into the iron matrix to provide a strong adhesion. This graphite shape suppresses crack initiation and propagation and is the source of both the increased mechanical properties relative to grey iron and the improved thermal conductivity relative to ductile iron.