Ductile cast irons are ferrous alloys in which precipitation of graphite in the form of spherical nodules is embedded in a metal matrix to obtain ductility on the material. Despite the importance of the shape of the nodules, the models proposed to predict the solidification of ductile irons assume a perfect spherical shape during the growing process up to the final solidification of the material, which is proved not to be the case in all castings depending on the processing conditions. The influence of the process parameters on the geometry of the nodules in ductile irons was experimentally evaluated and a model to predict the evolution of nodules during solidification was proposed. The proposed model for growth predicts changes in the nodule count as well as in the nodularity based on different laws for carbon diffusion according to the solid fraction, helping to understand the trends found experimentally. (C) The Minerals, Metals & Materials Society and ASM International 2013