Cast iron (metallurgy)


May 20, 2022

Cast iron, in metallurgy, is an alloy of iron and carbon whose content is greater than 2%. Carbon, which is supersaturated in cast irons, can precipitate in the form of graphite or Fe3C cementite. They are distinguished from other iron alloys by their excellent flowability. Several classifications of cast irons exist, but the most used, based on the fracture surface of a witness, defines two categories: white cast irons, with white breakage, made up of iron and cementite and gray cast irons, with gray breakage, made up iron and graphite.


Cast irons are all alloys. They are distinguished from other alloys by their excellent flowability (this term includes the thermal inertia and the fluidity of the alloy in fusion, it is measured in a standardized way by a spiral test piece with a triangular section). Cast iron has a melting temperature ranging from 1135 to 1350°C, depending mainly on the percentage of carbon and silicon it contains. When molten, its maximum carbon content depends on its temperature. At the moment of solidification, the quantity of carbon precipitating in the form of graphite in the metallic matrix depends on the other elements present (essentially silicon) and on the cooling rates. It can be a precursor in the manufacture of steel from iron ore. It is the alloy which comes out of the blast furnace and which will be refined into steel (by decarburization). It is then called pig iron, to distinguish it from foundry iron, generally resulting from fusion in a cupola, of determined composition, and intended for the production of molded parts.


Cast iron was discovered in China during the Warring States period (4th century BC). In Europe, it was during the 19th century that cast iron took an essential place in the economy, through the generalization of the "indirect process" (production of iron in a blast furnace with the obtaining of cast iron as an intermediate product). Pig iron was produced in charcoal blast furnaces. Abraham Darby, who was originally a malt roaster (for making beer), succeeded in producing pig iron using coke (the "coak" which was later written "coke" , from English to cook, to cook). In 1709, using low-sulphur coal, he made the first casting of coke iron, at his factory in Coalbrookdale. But the product, reputed to be of lower quality than wood-based cast iron, took fifty years to establish itself and become one of the major products of industrialization. Between 1777 and 1779, Abraham Darby III built the Iron Bridge in Coalbrookdale, the first large metal bridge in history, made entirely of cast iron. The production of wood-based cast iron nevertheless persisted, partly because of the protectionism practiced by the producing countries (France, Germany), partly because of the quality attributed to this type of cast iron and the reluctance of certain forge masters.

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From a chemical point of view, cast irons are iron-carbon alloys containing a eutectic phase, called ledeburite. On the metastable iron-carbon phase diagram, it is therefore iron-carbon alloys having more than 2.11% carbon (but this diagram is no longer valid in the presence of alloying elements). The different cast irons are distinguished by their percentage of carbon. In the case of a pure alloy of iron and carbon (theoretical case because cast iron always contains silicon and manganese in non-negligible quantities), the following thresholds are noted: hypoeutectic cast iron: from 2.11 to 4.3% carbon; eutectic iron: 4.3% carbon; this cast iron has the lowest melting temperature at 1148°C; hypereutectic melting: from 4.3 to