Cast iron is an iron material characterized by a high carbon content of at least 2,06% and an increased silicon content. Due to their chemical composition, cast iron materials differ significantly from steel, especially in terms of processing options: While steel can be further processed by plastic forming, such as forging, this is not possible with cast iron due to its brittleness. These properties result from the high carbon concentration, which influences the structure of cast iron and distinguishes it from malleable steel.
A key aspect of cast iron is the formation of graphite. In gray cast iron, graphite precipitation occurs in various forms that significantly determine the metallographic structure. Typical graphite forms are the lamellar form (lamellar graphite) and the spherical form (globular graphite). The specific formation and size of the graphite directly influences the mechanical and physical properties of the material. A fine, even graphite distribution can, for example, increase the strength and toughness of the material. These shapes and sizes are classified according to the DIN EN ISO 945-1 standard, which enables a uniform metallographic analysis and evaluation of the graphite formation.
Unlike gray cast iron, white cast iron does not contain free graphite. Instead, the carbon remains bound in the form of cementite (Fe₃C), resulting in a hard, brittle structure. The term "white cast iron" is derived from the white appearance of the fracture surface, which is caused by the lack of graphite. Technically, white cast iron is often used in areas where high wear resistance is required, such as chilled casting for camshafts.
One of the most common forms of corrosion affecting cast iron is spongiosis. This corrosion process selectively dissolves the metallic structural components such as ferrite and pearlite, leaving the graphite untouched. The result is a porous structure reminiscent of an iron sponge, while the graphite holds the component together in the form of a weak, porous framework. This corrosion mechanism can significantly impair the mechanical integrity of a component, as the load-bearing structural components are broken down, leaving only an unstable framework of graphite.
