Tempering in Cast-iron
Cast-iron comes in many types, depending on the carbon-content. However, they are usually divided into grey and white cast-iron, depending on the form that the carbides take. In grey cast iron, the carbon is mainly in the form of graphite but, in white cast-iron, the carbon is usually in the form of cementite. Grey cast-iron consists mainly of the mircrostructure called pearlite, mixed with graphite and sometimes ferrite. Grey cast-iron is usually used as-cast, with its properties being determined by its composition.
White cast-iron is composed mostly of a microstructure called ledeburite mixed with pearlite. Ledeburite is very hard, making the cast-iron very brittle. If the white cast-iron has a hypoeutectic composition, it is usually tempered to produce malleable cast-iron. Two methods of tempering are used, called "white tempering" and "black tempering." The purposes of both tempering methods is to cause the cementite to decompose from the ledeburite, increasing the ductility.
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