Gray Iron Castings

Gray Iron (or grey iron) is the most common type of ferrous cast material in the world. It is named after the gray color of the fracture surfaces created by it, which is caused by the presence of graphite. Gray Iron is commonly used in static castings where the rigidity or stiffness of the part is more important than the tensile strength or elongation, such as engine blocks, various housings, valve bodies, mold castings, and so on.

Common grades of gray iron include Class 30, Class 40, and Class 50.

Gray Iron Chemical Composition

A normal chemical composition for a graphitic gray iron microstructure for gray iron castings is 2.5 to 4.0% carbon and 1 to 3% silicon by weight. Graphite may occupy 6 to 10% of the volume for the material. Silicon is important as opposed to white cast iron, because silicon is a graphite stabilizing element in cast iron, which means it allows the alloy to produce graphite instead of iron carbides; at 3% silicon almost no carbon is held in chemical combination with the iron. Another factor affecting graphitization is the solidification rate; the slower the rate, the greater the time for the carbon to diffuse and accumulate into graphite. A moderate cooling rate forms a more pearlitic matrix, while a fast cooling rate forms a more ferritic matrix. To achieve a fully ferritic matrix the alloy must be annealed. Rapid cooling partly or completely suppresses graphitization and leads to the formation of cementite, which is called white iron.

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. The presence of graphite flakes makes the Gray Iron easily machinable as they tend to crack easily across the graphite flakes. This material also has very good damping capacity and hence it is mostly used as the base for machine tool mountings.

Gray Iron Castings are commonly used for housings of many types including cylinder heads and casings, engine blocks, and other pressure containing applications where strength and cost must be balanced.