Fig. 1 shows micro structure of mild steel (0.2-0.3% C). White constituent in this figure is very pure iron or having very low free carbon in iron in form of ferrite and dark patches contain carbon in iron is chemically combined form known as carbide (Cementite). Cementite is very hard and brittle. Now if the dark patches of the above figure are further observed, a substance built up of alternate layer of light and dark patches is reflected in Fig. 2. These layers are alternatively of ferrite and cementite. This substance is called as pearlite and is made up of 87% ferrite and 13% cementite. But with increase of carbon content in steel portion of pearlite increases up to 0.8% C. The structure of steel at 0.8% C is entirely of pearlite. However if carbon content in steel is further increased as free constituent up to 1.5% C, such steel will be called as high carbon steel. The micro structure of high carbon steel is depicted in Fig. 3.