There are four commercial forms of iron. These forms differ from each other mainly in their carbon contents.
1. PIG IRON
It is the impure form of iron and contains about 3 to 4 % carbon.. ft also contains other impurities such as Si, P,Mn and sulphur. It is brittle and usually used to get other forms of iron.
2. CAST IRON
When pig iron is melted, run into moulds and cooled, it changes to cast iron. It contains about 2 to 4% carbon. There are two forms of cast iron:
a) WHITE CAST IRON
Rapid cooling of molten pig iron produces white cast iron. The white appearance is due to the presence of iron carbide.
b) GREY CAST IRON
When molten pig iron is cooled slowly, the carbon separates as graphite giving iron a grey colour. This is called grey cast iron.
It is the purest form of iron and contains only 0.12 to 0.25% carbons. It is obtained by melting pig iron with an excess of iron oxide which oxidizes impurities. Wrought iron is soft, malleable and ductile.
Iron which contains 0.25 to 1.5% carbon and hardened by quenching is called steel. Other metals such as Mn, Ni, Cr, Mo, W and V are added to iron in small amounts to produce different types of alloy steels. Steel posses the useful properties of both cast iron and wrought iron. Its hardness and elasticity can be varied by annealing and quenching.