“Ductile cast iron, previously known as nodular iron or spheroidal-graphite (SG) cast iron (the international term is ductile iron), is cast iron in which the graphite is present as tiny spheres (nodules). In ductile iron, eutectic graphite separates from the molten iron during solidification in a manner similar to that in which eutectic graphite separates in grey cast iron.” “However, because of additives introduced in the molten iron before casting, the graphite grows as spheres, rather than as flakes of any of the forms characteristic of grey iron. Cast iron containing spheroidal graphite is much stronger and has higher elongation than grey iron or malleable iron. It may be considered as a natural composite in which the spheroidal graphite imparts unique properties to ductile iron.” “The relatively high strength and toughness of ductile iron give it an advantage over gray iron or malleable iron in many structural applications. Also, because ductile iron does not require heat treatment to produce graphite nodules (as does malleable iron to produce temper-carbon nodules), it can compete with malleable iron even though it requires a treatment and inoculation process.” “The mould yield is normally higher than with malleable iron. Ductile iron can be produced to x-ray standards because porosity stays in the thermal center.
Malleable iron cannot tolerate porosity because voids migrate to the surface of hot spots such as fillets and appears as cracks. Typically, the composition of unalloyed ductile iron differs from that of grey iron or malleable iron.” “The raw materials used for ductile iron must be of higher purity. All cast irons can be melted in cupolas, electric arc furnaces, or induction furnaces. Ductile iron, as a liquid, has high fluidity, excellent castability, but high surface tension. The sands and moulding equipment used for ductile iron must provide rigid moulds of high density and good heat transfer. The formation of graphite during solidification causes an attendant increase in volume, which can counteract the loss in volume due to the liquid-to-solid phase change in the metallic constituent.” “Ductile iron castings typically require only minimal use of risers. Grey irons often do not require risers to ensure shrinkage-free castings. On the other hand, steels and malleable iron generally require heavy risering. Thus, the mould yield of ductile iron castings (the ratio of the weight of usable castings to the weight of metal poured) is much higher than that of either steel castings or malleable iron castings, but not as high as that of grey iron.” “There are some cases of ductile iron castings being made without risers.
Often designers must compensate for the shrinkage of cast iron during both solidification and subsequent cooling to room temperature by making patterns with dimensions larger than those desired in the finished castings. Typically, ductile iron requires less compensation than any other cast ferrous metal. The allowances in patternmaker rules (shrink rules) are usually: Shrinkage allowance can vary somewhat from the percentages given above, and often different percentages must be used for different directions in one casting because of the influence of the solidification pattern on the amount of contraction that takes place in different directions. Shrinkage is volumetric, and the ratio of dimensions to volume influences each dimension. As ductile iron approaches a condition of shrinkage porosity, the graphite nodules tend to become aligned and can result in lower fatigue strength.”
“During recent years, producers and users of ductile iron castings have observed that many potential users of ductile iron castings are not aware of the wide range of properties offered by the family of ductile iron alloys.” “Since their commercial introduction ductile iron castings have proven to be a cost effective alternative to malleable iron castings, steel castings, forgings, and fabrications. This is for a multitude of reasons and ductile iron castings are found in every field of engineering and in every geographic area of the world.”
“Ductile cast irons are used extensively in automotive applications such as clutches, gears, carriers, shafts, bearings, cams and hubs. Castings are made in a wide range of sizes with sections that can be either very thin or very thick.” “Since its introduction, the growth of ductile iron applications has exceeded all expectations. Whether in an automobile component, a water pipe, or a plow, ductile iron has made major inroads to the casting market in every industrially developed country.
There can be little doubt that the major motivating factor for this was “more strength for less expense” compared to just about every other cast alloy. The lesser expense comes not only from the readily available raw materials and the efficiencies of the foundry operation, but also from reduced cleaning and machining costs of ductile iron castings. The application of ductile iron is a notable engineering achievement of our age.”