Cast iron usually refers to grey iron, but also identifies a large group of ferrousalloys, which solidify with a eutectic. The colour of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due to its carbide impurities which allow cracks to pass straight through. Grey cast iron is named after its grey fractured surface, which occurs because the graphitic flakes deflect a passing crack and initiate countless new cracks as the material breaks.
Iron (Fe) accounts for more than 95% by weight (wt%) of the alloy material, while the main alloying elements are carbon (C) and silicon (Si). The amount of carbon in cast irons is 2.1 to 4 wt%. Cast irons contain an appreciable amount of silicon, normally 1 to 3 wt%, and consequently, these alloys should be considered ternary Fe-C-Si alloys. The principle of cast iron solidification, however, is understood from the binary iron-carbon phase diagram, where the eutectic point is at 1,154 °C (2,109 °F) and 4.3 wt% carbon. Since cast iron approximates this composition, its melting point of 1,150 to 1,200 °C (2,102 to 2,192 °F) is about 300 °C (572 °F) lower than the melting point of pure iron.
Cast iron tends to be brittle, except for malleable cast irons. With its low melting point, good fluidity, castability, excellent machinability, resistance to deformation, and wear resistance, cast irons have become an engineering material with a wide range of applications, and are used in pipes, machines, and automotive industry parts, such as cylinder heads (declining usage), cylinder blocks, and gearbox cases (declining usage). It is resistant to destruction and weakening by oxidisation (rust).
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