A large family of alloys, generally containing more than 2% carbon and between 1% and 3% silicon. Unlike steels, they are not malleable when solid, and most have low ductility and very poor resistance to impact loading. However, cast irons are very useful
when intricate or inexpensive castings are required, and they provide a high damping capacity (the ability of a material to absorb vibration) which can be important for precision machinery. Cast irons have a low melting temperature, are very fluid when molten, and shrink very little during solidification.
Unlike steels, cast irons contain free graphite grains, and it is the shape and distribution of the free graphite grains which have the strongest effect on the properties of the cast iron. Also important is the matrix in which they occur. The microstructure of the matrices depends on the alloys present in the metal, and the rate at which it solidifies and cools. If this sequence is very rapid, the dissolved carbon does not have enough time to nucleate as graphite during solidification; while the matrix transforms to harder microstructure. Subsequent heat treatments are also important to temper the very hard structures.
Silicon is added to cast iron primarily to control the solubility of carbon, and therefore the characteristics of the graphite. Additionally, silicon serves as a deoxidizer, promotes fluidity, and decreases shrinkage. Sulfur might be present in the alloy but is not added intentionally, since it causes hot cracks and can produce porosity if present in high concentrations. Phosphorous is also undesirable, because it produces a hard, brittle compound; its low melting temperature contributes to hot-cracking problems. However, phosphorous increases the fluidity of these irons, which is a desirable characteristic when casting very thin sections. Manganese is added to tie up the sulphur as a high-melting compound in order to reduce the problem of hot cracking. Manganese is also used to control the microstructure of welds, improving the strength and ductility as well as machinability.
The four basic types of cast iron are gray, white, ductile, and malleable. The gray cast irons contain flake graphite, which imparts a gray surface in fractures, and are the most common of the cast irons. The gray irons are readily machinable.
The white cast irons exhibit crystalline, whitish fractures because the carbon remains in solution during solidification, producing massive carbides in a pearlitic matrix. They are very brittle and hard, but very wear-resistant.
The ductile irons are also known as nodular irons. They contain alloys which cause the graphite to nucleate as spheres. These nodules are encased in a layer of ferrite and are in a pearlitic matrix, making them very ductile. Some nodules exhibit elongations of up to 18%.
Malleable cast irons are produced by heat treating specially alloyed white cast irons. Heat treating results in the development of graphite nodules (temper carbon) in a ferrite matrix. Malleable cast irons are used when good strength, toughness, and casting and machining properties are required.