In the stable eutectoid transformation of austenite in ductile cast iron, ferrite forms around the graphite spheroid. As carbon diffuses through the ferrite ring, the graphite spheroid must enlarge and the ferrite ring must deform plastically to accomodate this growth. A model has been proposed to clarify this mechanism. The alloying effects of molybdenum, nickel and copper were studied in a series of heats cast into a range of casting section sizes. Regression analysis was used to illustrate the effects of alloying and cooling rate on the microstructure of ductile cast iron.