Composition Gray iron is commercially produced over a wide range of compositions. Foundries meeting the same specifications may use different compositions to take advantage of lower cost raw materials locally available and the general nature of the type of castings produced in the foundry. For these reasons, inclusion of chemical composition in purchase specifications for castings should be avoided unless essential to the application. Nitrogen is generally present in the range of 20 to 92 ppm. The concentration of some elements may exceed the limits shown above, but generally the ranges are less than shown.
Carbon is by far the most important element in gray iron.
With the exception of the carbon in the pearlite of the matrix, the carbon is present as graphite. The graphite is present in flake form and as such greatly reduces the tensile strength of the matrix. It would be impossible to produce gray iron without an appropriate amount of silicon being present. The addition of silicon reduces the solubility of carbon in iron and also decreases the carbon content of the eutectic. The eutectic of iron and carbon is about 4.3 percent.