Ductile iron is also referred to as nodular iron or spheroidal graphite iron, and is a material patented in 1948. It is not a single material but one part of a group. By controlling the microstructure of this group of materials, it can be produced with a range of properties. The shape of the graphite in ductile iron is nodular as opposed to the flakes that are in grey iron.
These rounded nodules prevent cracks within the metal matrix, making the iron more ductile, hence the name, while the sharp graphite flakes in grey iron create stress concentrated points. To solidify the iron into this structure a very small, but specific, amount of Mg is added to the molten iron. The reaction it creates with the sulphur and oxygen in it, modifies the way the graphite is formed. Ductile iron, like malleable iron, exhibits a linear stress strain relation, a considerable range of yield strengths and, of course, ductility. Castings are made in a wide range of sizes with sections that can be either very thin or very thick.
Some of the main differences between Ductile Iron and Grey Iron are:
- Ductile iron has a greater resistance to impacts. It can resist a minimum of 7 pounds of impact, versus 2 pounds of impact for grey iron
- Ductile iron has a lower thermal conductivity than grey iron
- Grey iron dampens vibrations more effectively than ductile iron