The demand for castings is based on the nature of cast iron as engineering materials and their economic cost advantages. Cast irons offers a tremendous range of the metallic properties of strength, hardness, machinability, wear resistance, abrasion resistance, corrosion resistance and other properties. Furthermore, the foundry properties of cast irons in terms of yield, fluidity, shrinkage, casting soundness, ease of production and others make the material highly desirable for casting purposes.in general, a cast iron is an alloy of iron ,carbon (up to 4 per cent) and silicon (up to about 3.5 per cent) which ordinarily is not usefully malleable as cast.
Because of a fortunate combination of engineering properties, availability, excellent casting characteristics and favourable cost, grey irons are produced in tonnages exceeding all other castings. Gray cast iron is having a chemical composition such that after solidification, a large portion of its carbon is distributed throughout the casting as free or graphic carbon in “flake form". Gray cast iron always presents a Gray sooty surface when fractured.
Ductile Iron is also known as Spheroidal Graphite Iron, SG Cast Iron, Nodular Cast Iron, Spherulitic Graphite Cast Iron is a type of cast iron that has been treated while molten with an element such as magnesium or cerium to induce the formation of free graphite as nodules or spherulites. The ductile iron family offers the design engineer a unique combination of strength, wear resistance, fatigue resistance, and toughness, as well as excellent ductility characteristics. The common defining characteristic of this group of materials is the shape of the graphite. In ductile irons, the graphite is in the form of nodules rather than ”flakes” as it is in grey iron.