Pig iron, also known as crude iron, is an intermediate good 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 iron ore in a blast furnace. 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, which makes it brittle 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 (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 dating back to the 13th century, which is now part of Westphalia, 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 were known in ancient Europe and the Middle East, but it was produced in bloomeries by direct reduction. Small prills of pig iron dispersed in slag 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 wrought iron in finery forges, 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. 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 iron, removing undesirable contaminants, adding alloys, and adjusting the carbon content. Ductile iron 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 blast furnace through a channel into a ladle car for transfer to the steel mill 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

, 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 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 finery forge, the

, the Bessemer process, and the open hearth furnace. 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