A problem of structure instability of cast iron in the limits of one kind, prepared by several meltings, is considered. Nonequilibrium casting conditions of metal, containing the undissolved graphite inclusions, and another hereditary signs of the initial solid phases are the reason for such instability. An effect of cast iron microstructure on viscosity of metal in liquid state is studied depending on a temperature. Morphology of graphite and its dispersity in Sch 30 cast iron are investigated via metallographic analysis.