Pig iron, also known as crude iron, is an intermediate product of the iron industry in the production of

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...

which is obtained by smelting

iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...

in a

blast furnace A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheric ...

. Pig iron has a high carbon content, typically 3.8–4.7%, along with silica and other constituents of dross, which makes it brittle and not useful directly as a material except for limited applications. The traditional shape of the molds used for pig iron ingots is a branching structure formed in sand, with many individual ingots at right angles to a central channel or "runner", resembling a litter of piglets being nursed by a sow. When the metal had cooled and hardened, the smaller ingots (the "pigs") were simply broken from the runner (the "sow"), hence the name "pig iron". As pig iron is intended for remelting, the uneven size of the ingots and the inclusion of small amounts of sand cause only insignificant problems considering the ease of casting and handling them.

History

Casting pig iron, Iroquois smelter, Chicago

Smelting and producing wrought iron was known in ancient Europe and the Middle East, but it was produced in

bloomeries A bloomery is a type of metallurgical furnace once used widely for smelting iron from its oxides. The bloomery was the earliest form of smelter capable of smelting iron. Bloomeries produce a porous mass of iron and slag called a ''bloom''. ...

by direct reduction. Pig iron was not produced in Europe before the Middle Ages. The Chinese were making pig iron by the later Zhou dynasty (which ended in 256 BC).Wagner, Donald. ''Iron and Steel in Ancient China''. Leiden 1996:

Brill Publishers Brill Academic Publishers (known as E. J. Brill, Koninklijke Brill, Brill ()) is a Dutch international academic publisher founded in 1683 in Leiden, Netherlands. With offices in Leiden, Boston, Paderborn and Singapore, Brill today publishes 27 ...

Furnaces such as Lapphyttan in Sweden may date back to the 12th century; and some in Mark (today part of Westphalia, Germany) to the 13th. It remains to be established whether these northern European developments derive from Chinese ones. Wagner has postulated a possible link via Persian contacts with China along the

Silk Road The Silk Road () was a network of Eurasian trade routes active from the second century BCE until the mid-15th century. Spanning over 6,400 kilometers (4,000 miles), it played a central role in facilitating economic, cultural, political, and reli ...

and Viking contacts with Persia, but there is a chronological gap between the Viking period and Lapphyttan. The phase transition of the iron into liquid in the furnace was an ''avoided'' phenomenon, as decarburizing the pig iron into steel was an extremely tedious process using medieval technology.

Uses

Traditionally, pig iron was worked into wrought iron in finery forges, later puddling furnaces, and more recently, into

.R. F. Tylecote, ''A history of metallurgy'' (2nd edition, Institute of Materials, London, 1992). In these processes, pig iron is melted and a strong current of air is directed over it while it is stirred or agitated. This causes the dissolved impurities (such as silicon) to be thoroughly oxidized. An intermediate product of puddling is known as ''refined pig iron'', ''finers metal'', or ''refined iron''. Pig iron can also be used to produce gray iron. This is achieved by remelting pig iron, often along with substantial quantities of steel and

scrap Scrap consists of Recycling, recyclable materials, usually metals, left over from product manufacturing and consumption, such as parts of vehicles, building supplies, and surplus materials. Unlike waste, scrap Waste valorization, has monetary ...

iron, removing undesirable contaminants, adding alloys, and adjusting the carbon content. Some pig iron grades are suitable for producing ductile iron. These are high purity pig irons and depending on the grade of ductile iron being produced these pig irons may be low in the elements silicon, manganese, sulfur and phosphorus. These types of pig iron are used to dilute all the elements (except carbon) in a ductile iron charge which may be harmful to the ductile iron process.

Modern uses

Until recently, pig iron was typically poured directly out of the bottom of the

through a

trough Trough may refer to: In science * Trough (geology), a long depression less steep than a trench * Trough (meteorology), an elongated region of low atmospheric pressure * Trough (physics), the lowest point on a wave * Trough level (medicine), the l ...

into a ladle car for transfer to the

steel mill A steel mill or steelworks is an industrial plant for the manufacture of steel. It may be an integrated steel works carrying out all steps of steelmaking from smelting iron ore to rolled product, but may also be a plant where steel semi-finish ...

in mostly liquid form; in this state, the pig iron was referred to as ''hot metal''. The hot metal was then poured into a steelmaking vessel to produce steel, typically an electric arc furnace, induction furnace or basic oxygen furnace, where the excess carbon is burned off and the

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...

composition controlled. Earlier processes for this included the finery forge, the puddling furnace, the

Bessemer process The Bessemer process was the first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. The key principle is removal of impurities from the iron by oxidation ...

, and the

open hearth furnace An open-hearth furnace or open hearth furnace is any of several kinds of industrial furnace in which excess carbon and other impurities are burnt out of pig iron to produce steel. Because steel is difficult to manufacture owing to its high me ...

. Modern steel mills and direct-reduction iron plants transfer the molten iron to a ladle for immediate use in the steel making furnaces or cast it into pigs on a pig-casting machine for reuse or resale. Modern pig casting machines produce stick pigs, which break into smaller 4–10 kg piglets at discharge.

References

{{Authority control Ancient Egyptian technology Ancient Roman technology Chinese inventions Ferrous alloys Iron Metalworking Smelting Steelmaking