Ductile iron, also known as ductile cast iron, nodular cast iron, spheroidal graphite iron, spheroidal graphite cast iron, and SG iron, is a type of graphite-rich cast iron discovered in 1943 by Keith Millis. While most varieties of cast iron are weak in tension and brittle, ductile iron has much more impact and fatigue resistance, due to its nodular graphite inclusions. Ductile iron is not a single material but part of a group of materials that can be produced with a wide range of properties through control of their microstructure. The common defining characteristic of this group of materials is the shape of the graphite. In ductile irons, graphite is in the form of nodules rather than flakes as in grey iron.
Ductile iron castings are solid metal objects that result from molten ductile iron being poured into voids in a mold. The ductile iron subsequently cools and solidifies in the shape of the void it occupies. Ductile iron normally contains over 3 percent carbon and can be bent, twisted, or deformed without fracturing. Its mechanical properties are similar to steel, and far exceed those of standard cast irons. Ductile iron is specifically useful in many automotive components, where strength must surpass that of aluminum but steel is not necessarily required. Other major industrial applications include off-highway diesel trucks, class 8 trucks, agricultural tractors, and oil well pumps. In the wind power industry, ductile iron is used for hubs and structural parts like machine frames.
ASTM A536 refers to the standard specification for ductile iron castings. Ductile iron is also called spheroidal graphite iron or nodular iron. In contrast to gray iron, where the graphite occurs as a network of flakes, the graphite in ductile iron occurs as spheroids. This confers a number of advantages on ductile iron. Ductile iron, depending on grade, offers different properties. For example, A536 Grade 60-40-18 possesses maximum ductility, excellent machinability, and low-temperature toughness. A536 Grade 120-90-02, on the other hand, possesses very high strength and wear resistance.
In summary, ductile iron is a type of cast iron known for its impact and fatigue resistance, elongation, and wear resistance due to the spherical graphite structures in the metal. It is not a single material but part of a group of materials that can be produced with a wide range of properties through control of their microstructure. Ductile iron castings are solid metal objects that result from molten ductile iron being poured into voids in a mold. Ductile iron is specifically useful in many automotive components, where strength must surpass that of aluminum but steel is not necessarily required, and in other major industrial applications such as off-highway diesel trucks, class 8 trucks, agricultural tractors, and oil well pumps.
