The carbon content of gray iron is higher (for 2.7%~4.0%), it can be regarded as the matrix of carbon steel and flake graphite.
According to the matrix structure of different gray iron divided into three categories: ferrite matrix Gray cast iron, pearlite one ferrite matrix Gray cast iron, pearlite matrix Gray cast iron.
Ferrite Gray cast iron is a large and coarse graphite sheet distributed on the ferrite substrate, and its strength and hardness are very poor and seldom used.
The Pearlite Gray cast iron is distributed evenly and fine graphite sheet on the base of Pearlite, its strength and hardness are relatively high, and it is often used to make important parts such as bed body and organism.
The Pearlite-ferritic Gray cast iron is formed on the base of the mixture of pearlite and ferrite, distribution of thicker graphite, this kind of cast iron strength, hardness, although lower than the former, but still can meet the general requirements of the body, its casting, shock resistance are good, and easy to smelting, is the most widely used gray cast iron. The difference of microstructure of gray iron is essentially the difference of the form of carbon in cast iron. The carbon in gray iron is composed of combined carbon (FE3C) and graphite carbon. When the combined carbon is 0.8%, it belongs to the pearlite Gray cast iron, when the combined carbon is less than 0.8%, it belongs to the pearlite-ferritic gray iron, and the ferrite gray iron when all the carbon is in graphite state.