Ductile iron is an engineering material for various applications [1-6]. One of the most important advantages it offers is the possibility of shaping its mechanical properties by heat treatment. Therefore numerous research centres carry out investigations on the kinetics of phase transformations and, consequently, on the improvement of cast iron properties. Most of the research is, however, focussed on the analysis of phase transformations taking place on cooling. Only few studies are devoted to the analysis of effects that take place during heating and austenitising [7-9]. The mechanical properties of cast iron, i.e. the tensile strength, elongation and hardness, largely depend on the cast iron structure in initial condition, i.e. upon casting solidification, and on changes that take place in this structure after the successive operations of heat treatment.