It is an iron carbon alloy where most of the carbon is present as meta‐stable iron carbide called cementite. The upper limit of carbon content is 2%. Phase diagram helps us guess the structure of alloys and their properties.

Depending on its composition we may have 3 types of structures. (i) % carbon < 0.02 (ii) 0.02 < % carbon < 0.8 (iii) 0.8 < % Carbon < 2.0.

Fig explains the solidification behaviour of steel having greater than 0.8% carbon but less than 2.0% carbon with the help of a set of schematic diagrams.

Such steels are known as hyper‐eutectoid steel. The sketch on the left shows a part of the equilibrium diagram (Fe‐Fe3C) with the location of the alloy as a vertical dotted line. It has around 1.0% carbon.