Pig iron

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Pig iron of a type used to make ductile iron, stored in a bin Pig iron.jpg
Pig iron of a type used to make ductile iron, stored in a bin

Pig iron is an intermediate product of the iron industry, also known as crude iron, which is obtained by smelting iron ore in a blast furnace. Pig iron has a very high carbon content, typically 3.8–4.7%, [1] along with silica and other constituents of dross, which makes it very brittle and not useful directly as a material except for limited applications. [2]

Contents

The traditional shape of the molds used for pig iron ingots was a branching structure formed in sand, with many individual ingots at right angles [3] to a central channel or "runner", resembling a litter of piglets being suckled 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". [4] As pig iron is intended for remelting, the uneven size of the ingots and the inclusion of small amounts of sand caused only insignificant problems considering the ease of casting and handling them.

History

Casting pig iron, Iroquois smelter, Chicago, between 1890 and 1901 Casting pig iron, Iroquois smelter, Chicago.jpg
Casting pig iron, Iroquois smelter, Chicago, between 1890 and 1901

Smelting and producing wrought iron was known in ancient Europe and the Middle East, but iron was produced in bloomeries by direct reduction. Pig iron was not produced in Europe before the Middle Ages. The Chinese were also making pig iron by the later Zhou dynasty (which ended in 256 BC). [5] 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. [6] It remains to be established whether these northern European developments derive from Chinese ones. Wagner [7] has postulated a possible link via Persian contacts with China along the Silk Road and Viking contacts with Persia, [5] 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 steel. [8] 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. [9]

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. 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 blast furnace through a trough 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 steel, typically an electric arc furnace, induction furnace or basic oxygen furnace, where the excess carbon is burned off and the alloy composition controlled. Earlier processes for this included the finery forge, the puddling furnace, 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 4–10 kg piglets at discharge.

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Wrought iron Iron alloy with a very low carbon content

Wrought iron is an iron alloy with a very low carbon content in contrast to cast iron. It is a semi-fused mass of iron with fibrous slag inclusions, which gives it a "grain" resembling wood that is visible when it is etched or bent to the point of failure. Wrought iron is tough, malleable, ductile, corrosion-resistant and easily welded.

Cast iron iron or a ferrous alloy which has been liquefied then poured into a mould to solidify

Cast iron is a group of iron-carbon alloys with a carbon content greater than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its colour when fractured: white cast iron has carbide impurities which allow cracks to pass straight through, grey cast iron has graphite flakes which deflect a passing crack and initiate countless new cracks as the material breaks, and ductile cast iron has spherical graphite "nodules" which stop the crack from further progressing.

Steelmaking process for producing steel from iron ore and scrap

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Blast furnace type of metallurgical furnace used for smelting to produce industrial metals

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 pressure.

Steel mill Plant for steelmaking

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 describe plants where steel semi-finished casting products are made, from molten pig iron or from scrap.

Open hearth furnace historic steel producing technology

Open hearth furnaces are one of a number of kinds of furnace where excess carbon and other impurities are burnt out of pig iron to produce steel. Since steel is difficult to manufacture due to its high melting point, normal fuels and furnaces were insufficient and the open hearth furnace was developed to overcome this difficulty. Compared to Bessemer steel, which it displaced, its main advantages were that it did not expose the steel to excessive nitrogen, was easier to control, and it permitted the melting and refining of large amounts of scrap iron and steel.

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An electric arc furnace (EAF) is a furnace that heats charged material by means of an electric arc.

Bloomery early form of iron smelter

A bloomery is a type of 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. The mix of slag and iron in the bloom, termed sponge iron, is usually consolidated and further forged into wrought iron. Blast furnaces, which produce pig iron, have largely superseded bloomeries.

Reverberatory furnace metallurgical furnace

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Puddling (metallurgy) metallurgical process

Puddling is a step in the manufacture of high-grade iron in a crucible or furnace. Known by the 1st century AD in the Han Dynasty of ancient China, it was advanced in Great Britain during the Industrial Revolution. There molten pig iron in a reverberatory furnace was stirred with consumable rods resulting in a less brittle, more purified steel. It was one of the most important processes of making the first appreciable volumes of valuable and useful bar iron. Eventually, the furnace would be used to make small quantities of specialty steels.

Foundry factory that produces metal castings

A foundry is a factory that produces metal castings. Metals are cast into shapes by melting them into a liquid, pouring the metal into a mold, and removing the mold material after the metal has solidified as it cools. The most common metals processed are aluminium and cast iron. However, other metals, such as bronze, brass, steel, magnesium, and zinc, are also used to produce castings in foundries. In this process, parts of desired shapes and sizes can be formed.

Finery forge

A finery forge is a forge used to produce wrought iron from pig iron by decarburization. The process involved liquifying cast iron in a fining hearth and removing carbon from the molten cast iron through oxidation. Finery forges were used as early as 3rd century BC, based on archaeological evidence found at a site in Tieshengguo, China. The finery forge process was replaced by the puddling process and the roller mill, both developed by Henry Cort in 1783-4, but not becoming widespread until after 1800.

Lapphyttan or Lapphyttejarn in Norberg Municipality, Sweden, may be regarded as the type site for the Medieval Blast Furnace. Its date is probably between 1150 and 1350. It produced cast iron, which was then fined to make ferritic wrought iron cake or bun-like blooms. These were then cut into lumps for trade. It is thought that they correspond to the iron pieces known as osmonds. Osmonds occur in English Customs records in the 1250s and seem to be alluded to in a commercial treaty with Novgorod in 1203. Lapphyttan is a part of Ecomuseum Bergslagen.

Ferrous metallurgy heavy industry that deals with the production of steel

Ferrous metallurgy is the metallurgy of iron and its alloys. It began far back in prehistory. The earliest surviving iron artifacts, from the 4th millennium BC in Egypt, were made from meteoritic iron-nickel. It is not known when or where the smelting of iron from ores began, but by the end of the 2nd millennium BC iron was being produced from iron ores from at least Greece to India, and more controversially Sub-Saharan Africa. The use of wrought iron was known by the 1st millennium BC, and its spread marked the Iron Age. During the medieval period, means were found in Europe of producing wrought iron from cast iron using finery forges. For all these processes, charcoal was required as fuel.

In metallurgy, refining consists of purifying an impure metal. It is to be distinguished from other processes such as smelting and calcining in that those two involve a chemical change to the raw material, whereas in refining, the final material is usually identical chemically to the original one, only it is purer. The processes used are of many types, including pyrometallurgical and hydrometallurgical techniques.

Vacuum arc remelting (VAR) is a secondary melting process for production of metal ingots with elevated chemical and mechanical homogeneity for highly demanding applications. The VAR process has revolutionized the specialty traditional metallurgical techniques industry, and has made possible incredibly controlled materials used in the biomedical, aviation, and aerospace fields.

Lancashire hearth

The Lancashire hearth was used to fine pig iron, removing carbon to produce wrought iron.

References

  1. Camp, James McIntyre; Francis, Charles Blaine (1920). The Making, Shaping and Treating of Steel (2nd ed.). Pittsburgh: Carnegie Steel Co. pp.  174. OCLC   2566055.
  2. SAMUEL THOMAS (September 1899). "REMINISCENCES OF THE EARLY ANTHRACITE-IRON INDUSTRY". TRANSACTIONS OF THE AMERICAN INSTITUTE OF MINING ENGINEERS (reprint by TheHopkinThomasProject.com). Archived from the original on 14 March 2014. Retrieved 5 December 2016.
  3. Glossary of Metalworking Terms. Industrial Press. 2003. p. 297. ISBN   9780831131289. Archived from the original on 2017-02-24.
  4. The Making, Shaping, and Treating of Steel: Ironmaking volume (PDF). AISE Steel Foundation. 1999. p. 18. Archived from the original (PDF) on 2016-03-04.
  5. 1 2 Wagner, Donald. Iron and Steel in Ancient China. Leiden 1996: Brill Publishers
  6. Several papers in The importance of ironmaking: technical innovation and social change: papers presented at the Norberg Conference, May 1995 ed. Gert Magnusson (Jernkontorets Berghistoriska Utskott H58, 1995), 143-179.
  7. https://www.persee.fr/doc/befeo_0336-1519_1995_num_82_1_2347
  8. R. F. Tylecote, A history of metallurgy (2nd edition, Institute of Materials, London, 1992).
  9. Rajput, R.K. (2000). Engineering Materials. S. Chand. p. 223. ISBN   81-219-1960-6.