The key is that the carbon is in the form of cast iron. Carbon in gray cast iron exists in free flake graphite, which has a great influence on the weakening of matrix. Malleable iron is better. Carbon exists in the form of flocculated graphite and in spherical graphite. In cast iron, carbon exists in the form of spheroidal graphite.
Because of the extremely low fracture sensitivity of ductile iron, it directly forms high mechanical properties.