Pig iron, also known as crude iron, is an

intermediate good Intermediate goods, producer goods or semi-finished products are Good (economics), goods, such as partly finished goods, used as inputs in the production of other goods including final goods. A firm may make and then use intermediate goods, or mak ...

used by the

iron industry Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's ...

in the production of

steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...

. It is developed by

smelting Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a desired base metal product. It is a form of extractive metallurgy that is used to obtain many metals such as iron-making, iron, copper extraction, copper ...

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 f ...

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 supplied above atmospheric pressure. In a ...

. Pig iron has a high

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

content, typically 3.8–4.7%, along with

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

and other

dross Dross is a mass of solid impurities floating on a molten metal or dispersed in the metal, such as in wrought iron. It forms on the surface of low- melting-point metals such as tin, lead, zinc or aluminium or alloys by oxidation of the metal. Fo ...

, which makes it

brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. ...

and not useful directly as a material except for limited applications.

Etymology

The traditional shape of the molds used for pig iron

ingot An ingot is a piece of relatively pure material, usually metal, that is Casting, cast into a shape suitable for further processing. In steelmaking, it is the first step among semi-finished casting products. Ingots usually require a second procedu ...

s is a branching structure formed in

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

, 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 are insignificant issues when compared to the ease of

casting Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or ...

and handling.

History

The Chinese were already making pig iron during the later

Zhou dynasty The Zhou dynasty ( ) was a royal dynasty of China that existed for 789 years from until 256 BC, the longest span of any dynasty in Chinese history. During the Western Zhou period (771 BC), the royal house, surnamed Ji, had military ...

(which ended in 256 BC).Wagner, Donald. ''Iron and Steel in Ancient China''. Leiden 1996:

Brill Publishers Brill Academic Publishers () is a Dutch international academic publisher of books, academic journals, and Bibliographic database, databases founded in 1683, making it one of the oldest publishing houses in the Netherlands. Founded in the South ...

Furnaces such as Lapphyttan in Sweden may date back as far back as the 12th century; and some in the

County of Mark The County of Mark (, colloquially known as ) was a county and Imperial State, state of the Holy Roman Empire in the Lower Rhenish–Westphalian Circle. It lay south of Lippe (river), Lippe river on both sides of the Ruhr river along the Volme a ...

dating back to the 13th century, which is now part of

Westphalia Westphalia (; ; ) is a region of northwestern Germany and one of the three historic parts of the state of North Rhine-Westphalia. It has an area of and 7.9 million inhabitants. The territory of the region is almost identical with the h ...

, Germany. It remains to be established whether these northern European developments were derived from the Chinese ones. Wagner has postulated a possible link via Persian contacts with China along the

Silk Road The Silk Road was a network of Asian trade routes active from the second century BCE until the mid-15th century. Spanning over , it played a central role in facilitating economic, cultural, political, and religious interactions between the ...

and Viking contacts with Persia, but there is a chronological gap between the Viking period and Lapphyttan. Smelting and producing

wrought iron Wrought iron is an iron alloy with a very low carbon content (less than 0.05%) in contrast to that of cast iron (2.1% to 4.5%), or 0.25 for low carbon "mild" steel. Wrought iron is manufactured by heating and melting high carbon cast iron in an ...

were known in ancient Europe and the Middle East, but it was produced in bloomeries by

direct reduction read online, part I, pp. 240; 250-254; 257. (capacity 650,000 t/year).(fr) Adolf Ledebur (trans. Barbary de Langlade revised and annotated by F. Valton), ''Manuel théorique et pratique de la métallurgie du fer, Tome I et Tome II'', t. 2, Librairie ...

. Small

prill A prill is a small aggregate or globule of a material, most often a dry sphere, formed from a melted liquid through spray crystallization. Prilled is a term used in mining and manufacturing to refer to a product that has been pelletized. ANFO e ...

s of pig iron dispersed in

slag The general term slag may be a by-product or co-product of smelting (pyrometallurgical) ores and recycled metals depending on the type of material being produced. Slag is mainly a mixture of metal oxides and silicon dioxide. Broadly, it can be c ...

are produced in all iron furnaces, but the operator of a bloomery had to avoid conditions causing the

phase transition In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...

of the iron into liquid in the furnace, as the prill globules or any resulting pig iron are not malleable so can't be hammered in a single piece. Alternatively, decarburizing the pig iron into steel was an extremely tedious process using medieval technology, so in Europe before the

Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the 5th to the late 15th centuries, similarly to the post-classical period of global history. It began with the fall of the Western Roman Empire and ...

the prills were discarded with the slag.

Uses

Traditionally, pig iron was worked into

finery forge A finery forge is a forge used to produce wrought iron from pig iron by decarburization in a process called "fining" which involved liquifying cast iron in a fining hearth and decarburization, removing carbon from the molten cast iron through Redo ...

s, later

puddling furnace Puddling is the process of converting pig iron to bar (wrought) iron in a coal fired reverberatory furnace. It was developed in England during the 1780s. The molten pig iron was stirred in a reverberatory furnace, in an Redox, oxidizing enviro ...

s, 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 Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

. 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 recyclable materials, usually metals, left over from product manufacturing and consumption, such as parts of vehicles, building supplies, and surplus materials. Unlike waste, scrap can have monetary value, especially recover ...

iron, removing undesirable contaminants, adding alloys, and adjusting the carbon content.

Ductile iron Ductile iron, also known as ductile cast iron, nodular cast iron, spheroidal graphite iron, spheroidal graphite cast iron and SG iron, is a type of graphite-rich cast iron discovered in 1943 by Keith Millis. While most varieties of cast iron are ...

can also be produced using certain high purity grades of pig iron; depending on the grade of ductile iron being produced, the pig irons chosen may be low in the elements silicon, manganese, sulfur and phosphorus. High purity pig iron is used to dilute any elements in a ductile iron charge which may be harmful to the ductile iron process (except carbon).

Modern uses

Pig iron was historically poured directly out of the bottom of the

through a channel 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-fini ...

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 Steelmaking is the process of producing steel from iron ore and/or scrap. Steel has been made for millennia, and was commercialized on a massive scale in the 1850s and 1860s, using the Bessemer process, Bessemer and open hearth furnace, Siemens-M ...

vessel to produce

, typically an

electric arc furnace An electric arc furnace (EAF) is a Industrial furnace, furnace that heats material by means of an electric arc. Industrial arc furnaces range in size from small units of approximately one-tonne capacity (used in foundry, foundries for producin ...

induction furnace An induction furnace is an electrical furnace in which the heat is applied by induction heating of metal. Induction furnace capacities range from less than one kilogram to one hundred tons, and are used to melt iron and steel, copper, aluminu ...

basic oxygen furnace Basic oxygen steelmaking (BOS, BOP, BOF, or OSM), also known as Linz-Donawitz steelmaking or the oxygen converter process,Brock and Elzinga, p. 50. is a method of primary steelmaking in which carbon-rich molten pig iron is made into steel. Blowi ...

, where the excess carbon is burned off and the

alloy An alloy is a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...

composition controlled. Earlier processes for this included the

, 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 steelmaking, removal of impurities and undesired eleme ...

, and the

open hearth furnace An open-hearth furnace or open hearth furnace is any of several kinds of industrial Industrial furnace, furnace in which excess carbon and other impurities are burnt out of pig iron to Steelmaking, produce steel. Because steel is difficult to ma ...

. 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 piglets at discharge.

References

External Links

Making the sand moulds for pig iron
at Smithsonian History Institute {{Authority control Ancient Egyptian technology Ancient Roman technology Chinese inventions Ferrous alloys Iron Metalworking Smelting Steelmaking