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Malleable iron is cast as white iron, the structure being a metastable carbide in a pearlitic matrix. Through an annealing heat treatment, the brittleness structure as first cast is transformed into the ductility form. Carbon agglomerates into small roughly spherical aggregates of graphite, leaving a matrix of ferrite or pearlite according to the exact heat treatment used.
Three basic types of malleable iron are recognized within the casting industry: blackheart, whiteheart, and pearlitic.Robert R. Brown (1999). 075064284X, Butterworth-Heinemann. 075064284X
Malleable iron also exhibits better fracture toughness properties in low temperature environments than other nodular irons, due to its lower silicon content. The ductile to brittle transition temperature is lower than many other ductile iron alloys.
In order to properly form the spherical-shaped nodules of graphite (called temper graphite nodules or temper carbon nodules) in the annealing process, care must be taken to ensure that the iron casting will solidify with an entirely white iron cross section. Thicker sections of a casting will cool slowly, allowing some primary graphite to form. This graphite forms random flake-like structures and will not transform to carbide during heat treatment. When stress is applied to such a casting in application, the fracture strength will be lower than expected for white iron. Such iron is said to have a 'mottled' appearance. Some countermeasures can be applied to enhance the formation of the all-white structure, but malleable iron foundries often avoid producing heavy sections.
After the casting and heat treatment processes, malleable iron can be shaped through cold working, such as stamping for straightening, bending or coining operations. This is possible due to malleable iron's desirable property of being less strain rate sensitive than other materials.
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