Ferritic coating of graphite in flake graphite cast iron or nodular graphite cast iron. In particular for pearlitic types, silicon accumulation around the spherolites promotes formation of ferrite areas.
Ferrite and pearlite are formed during eutectoid transformation of austenite (s. in the Iron-carbon phase diagram).
In this condition, concurrent reactions occur in the cast iron between formation of ferrite and graphite, on the one hand, and pearlite formation, on the other. Ferrite formation takes place at slightly higher temperatures and lower cooling rates but may also be considerably influenced by alloying elements. The ferrite shell that is formed, for example encases the nodular graphite present, which grows due to adsorption of graphite that is diffused during austenite decomposition. The ferrite shell also grows until the the ferrite shells form contact with each other (fully ferritic structure), unless this process is stopped by pearlite formation
These types of structures also referred to as “bulls-eye ferrite” impair crack propagation from graphite and thus increase fracture toughness of the material. Figures 1 to 2 show the different formations of ferrite borders in nodular graphite cast iron.