iron carbon digram
Steels which are cooled relatively slowly from high temperature develop a microstructure of ferrite (a) and pearlite (the eutectoid mixture of ferrite and iron carbide, Fe3C). The pearlite forms at about 723°C as a product of the last of the austenite (g).
Most steels have carbon contents of up to 0.4% carbon, which gives an approximate balance of 50% ferrite and 50% pearlite in the alloy. This can be demonstrated by using the lever-rule with the phase diagram.
The proportion of ferrite and pearlite in the microstructure of the screw is close to this 50% balance, which indicates that the steel is close to the 0.4%C content. This gives a microstructure of reasonable strength and toughness
A cast iron with 3% carbon is in the liquid state. When cooled under quick conditions solidification begins at 1300 degrees where dendrites of austenite containing 1% carbon and liquid solution first form. As the temperature drops to 1140 degrees austenite containing 1.75 carbon will be in equilibrium with liquid solution of composition 4.35 % carbon. The eutectic reaction occurs and the liquid solidifies as a mixture of austenite and cementite. On further cooling a eutectoid reaction takes place at 723 degrees. Below this temperature the final structure would consist of pearlite and cementite
