Cast irons are Fe-C-Si alloys with 2-4%C, 1-3%Si and variable amounts of other elements (Mn, P, S, etc.). Similarly to steels, alloying elements can be added to improve their properties. Alloy cast irons are typically divided into three categories: abrasion-resistant, heat resistant and corrosion resistant.

Cast irons are cheaper than steels and manufacturing and machining are relatively simple. As their name implies, they are produced by casting. Forming operations are rare. 

Depending on the distribution of C, cast irons are classified into four main categories:

-White irons: C is present mainly as Fe₃C, which is very brittle. The name derives from the white appearance of the fracture surface. Their microstructures can be interpreted from the metastable Fe-Fe₃C phase diagram. High cooling rates and low equivalent carbon promote the formation of white irons, also known as chilled irons. Ledeburite (a eutectic aggregate of austenite and Fe₃C) is the most characteristic feature of these irons. Note that austenite transforms to pearlite upon cooling.

-Gray cast irons: lamellar of flake graphite is visible in these irons, which can be explained by following the equilibrium Fe-C phase diagram. Silicon, a graphitizer element, is in the range between 1.5 to 3% as opposed to <1%Si in white irons. In addition to Si, graphite is also favoured by slow cooling. Gray irons are the most common ones for engineering applications due to their combination of properties (fludity, machinability, wear resistance, damping capacity) and low cost. 

-Malleable irons: they are obtained from white irons by applying an annealing heat treatment known as malleabilization. This process decomposes cementite into irregular graphite nodules. Note that the size of the specimens is limited due to high cooling rates required for the production of the white iron.

-Ductile irons: free carbon is present as spheroids. This can be achieved by the addition of small amounts of Mg to the molten iron prior casting. Their chemical composition is similar to that of gray irons, although a stricter control of S and P is required. They are amenable for heat treatment, offering a wide range of mechanical properties in combination with reasonable ductility.

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