The tensile properties of ferritic spheroidal graphite cast irons are strongly influenced by ferrite grains, and the Hall-Petch rule holds for 0.2% proof stress and ferrite grains. The ferrite grain size tends to increase with decreasing cooling rate during the eutectoid transformation, however, this trendency significantly decreases when the modulus exceeds 3 cm. Impact properties show no difference in absorbed energy at modulus 1.25 to 5cm in the ductile region. On the other hand, in the transition region, the influence of ferrite grains is stronger than that of graphite grains, and the absorbed energy of modulus 1.25 cm increases markedly due to the increase in crack initiation energy. However, the impact transition curves for modulus 3 and 5 cm with ferrite grains larger than about 60 μm show similar impact transition curves, and there is no difference in the impact transition temperature.
