Ductile Iron is a type of cast iron that has a nodular or spheroidal graphite microstructure. The addition of a small amount of magnesium to molten iron causes the graphite to form in small nodules rather than flakes, providing the enhanced ductility that gives the alloy its name. Ductile iron castings, duplicating many grades of steel, possess high strength, ductility, corrosion resistance, and resistance to both mechanical and thermal shock.

Annealed ductile cast iron can be bent, twisted, or deformed without fracturing. It is considerably less expensive than cast steel and only moderately more expensive than grey iron. Its uses include: valves, gears, housings, pipe, and crankshafts. The most commonly specified ductile iron is AS 1831/BS EN 1563/ASTM-A536.