Ductile iron is cast as white iron, the structure being a metastable carbide in a pearlitic matrix. Through an annealing heat treatment the brittle as cast structure is transformed. Carbon agglomerates into small roughly speherical aggregates of graphite leaving a matric of ferrite or pearlite according to the exact heat treat used. Three basic types of malleable iron are recognized within the casting industry, Blackheart malleable iron, White-heart malleable iron and Pearlitic malleable iron.

It is often used for small castings requiring good tensile strength and the ability to flex without breaking (ductility). Electrical fittings, hand tools, pipe fittings, washers, brackets, fence fittings, power line hardware, farm equipment, mining hardware, and machine parts.