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Grey Iron

A typical chemical composition to obtain a graphitic microstructure is 2.5 to 4.0% carbon and 1 to 3% silicon. Silicon is important to making grey iron as opposed to white cast iron, because silicon is a graphite stabilizing element in cast iron, which means it helps the alloy produce graphite instead of iron carbides. Another factor affecting graphitization is the solidification rate; the slower the rate, the greater the tendency for graphite to form. A moderate cooling rate forms a more pearlitic matrix, while a slow cooling rate forms a more ferritic matrix. To achieve a fully ferritic matrix the alloy must be annealed.[1][4] Rapid cooling partly or completely suppresses graphitization and leads to formation of cementite, which is called white iron.[5]

The graphite takes on the shape of a three dimensional flake. In two dimensions, as a polished surface will appear under a microscope, the graphite flakes appear as fine lines. The graphite has no appreciable strength, so they can be treated as voids. The tips of the flakes act as preexisting notches; therefore, it is brittle.[5][6]. The presence of graphite flakes makes the Grey Iron easily machinable as they tend to crack easily across the graphite flakes.Grey iron also has very good damping capacity and hence it is mostly used as the base for machine tool mountings.

Ductile iron is not a single material but is part of a group of materials which can be produced to have a wide range of properties through control of the microstructure. The common defining characteristic of this group of materials is the morphological structure of the graphite. In ductile irons, the graphite is in the form of spherical nodules rather than flakes (as in grey iron), thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its name.[5] The formation of nodules is achieved by addition of nodulizing elements, most commonly Magnesium (note Magnesium boils at 1100C and Iron melts at 1500C) and, less often now, Cerium (usually in the form of Misch metal), into the melt.[6] Tellurium has also been used. Yttrium, often a component of Misch metal, has also been studied as a possible nodulizer.