The name ‘ductile iron’ is a type of cast iron that is different from other cast irons with the respect to the form or shape of the graphite in the iron. It is to be noted that in ductile iron, the graphite particles feature a spherical shape due to the addition and retention of a comparatively small amount of magnesium content in the iron. In the case of an ordinary gray cast iron, the graphite is available as flakes. Because of the shape of the graphite iron, it is also known as ‘spheroidal graphite’ cast iron or ‘nodular graphite cast iron’ especially outside of the US and Canada. The spherical shape of the graphite accounts for its ductility and toughness, compared to the ordinary flake graphite cast iron.

Technically speaking, the relatively flat, interlocking flakes of graphite for network disturb the continuity of the iron matrix and forms notches. As a matter of combination, these effects of flake graphite are primarily accountable for the comparative weakness and certain lack of toughness in the case of ordinary cast irons. The spherical graphite in ductile iron is distributed in such a way that it helps maintain the continuity of the spherical graphite in iron matrix. In addition, how the graphite particles are shaped will limit the notch effects.

Basically if you take the ductile-iron pipe, it features graphite distributed in a uniform manner in the form of spheroidal particles all throughout a ferritic matric such that the desired strength with a maximum level of ductility, in addition to an appreciable effect on resistance to corrosion, is achieved.  Malleable cast iron is different from ordinary grey cast iron in the distribution and form of graphite particles. The benefit of ductile iron is offset partly, by the relatively less amount of graphite in malleable iron.