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Iron is an important engineering metals. Iron exists in the solid state in two crystalline forms. At low temperatures it takes up a body-centred cubic crystal structure. At 910C, if changes to a face-centred cubic form, and then, just below its melting point, it reverts to the body-centred cubic form. The low temperature form is known as ?-iron or ferrite and that which forms about 910C is called ?-iron or austenite. Carbon, too is uncommon among the alloying elements, in that it is a very small atom that forms interstitial solid solutions with iron; in other words, it tucks itself into the spaces between iron atoms instead of replacing them in their space lattice. The maximum solubility of carbon in ?-iron by weight is 0.05%, whereas the solubility in y-iron is 2.0%. The phase diagram for the iron-rich end of the iron-carbon system is shown in Fig. 1.
Cast irons are carbon-iron alloys with the carbon contain 2% to 4.5% carbon. Although the maximum solubility of carbon in solid solutions in 2.0%, carbon is even more soluble in liquid iron.
Cast irons have comparatively low melting points and are very fluid in the molten state. For these reasons, such alloys can be cast directly into moulds to form complicated shapes which require little machining to make them suitable for use in engineering structures. However, because of the variety of structures it is possible to produce during solidification and heat treatments, a remarkably wide range of property combinations is possible.
As cast irons solidify, any carbon in excess of 2% comes out of solution as graphite and a compound of iron and carbon of composition Fe3C known as cementite. The brittle, needle-like crystals of cementite, as well as the flakes of graphite, weaken the structure in which they are held. For this reason, cast irons are usually brittle. However, with appropriate alloying agents, such as cerium or magnesium, the graphite can be made to precipitate in a spheroidal form that affects the strength and ductility to only a minor degree. If required, the formation of graphite can be suppressed in favour of cementite, making the material harder and more wear-resistant. Cast irons can be even further modified by heat-treatment to produce castings which are as strong and tough as many steels (i.e., the malleable cast irons).
There are so many variations of composition and treatment that a full description of cast irons is beyond the scope of this text, but they have been and will continue to be valuable engineering materials.