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, also known as crude iron, is an intermediate product of the iron industry in the production of steel 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 is 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 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". [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 it was produced in bloomeries 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). [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.

Related Research Articles

Alloy Mixture or metallic solid solution composed of two or more elements

An alloy is an admixture of metals, or a metal combined with one or more other elements. For example, combining the metallic elements gold and copper produces red gold, gold and silver becomes white gold, and silver combined with copper produces sterling silver. Combining iron with non-metallic carbon or silicon produces alloys called steel or silicon steel. The resulting mixture forms a substance with properties that often differ from those of the pure metals, such as increased strength or hardness. Unlike other substances that may contain metallic bases but do not behave as metals, such as aluminium oxide (sapphire), beryllium aluminium silicate (emerald) or sodium chloride (salt), an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, opacity, and luster. Alloys are used in a wide variety of applications, from the steel alloys, used in everything from buildings to automobiles to surgical tools, to exotic titanium alloys used in the aerospace industry, to beryllium-copper alloys for non-sparking tools. In some cases, a combination of metals may reduce the overall cost of the material while preserving important properties. In other cases, the combination of metals imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength. Examples of alloys are steel, solder, brass, pewter, duralumin, bronze, and amalgams.

Metallurgy Domain of materials science that studies the physical and chemical behavior of metals

Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are called alloys. Metallurgy encompasses both the science and the technology of metals; that is, the way in which science is applied to the production of metals, and the engineering of metal components used in products for both consumers and manufacturers. Metallurgy is distinct from the craft of metalworking. Metalworking relies on metallurgy in a similar manner to how medicine relies on medical science for technical advancement. A specialist practitioner of metallurgy is known as a metallurgist.

Steel Metal alloy made by combining iron with other elements

Steel is an alloy of iron with typically a few tenths of a percent of carbon to improve its strength and fracture resistance compared to iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant need typically an additional 11% chromium. Because of its high tensile strength and low cost, steel is used in buildings, infrastructure, tools, ships, trains, cars, machines, electrical appliances, and weapons. Iron is the base metal of steel. Depending on the temperature, it can take two crystalline forms : body-centred cubic and face-centred cubic. The interaction of the allotropes of iron with the alloying elements, primarily carbon, gives steel and cast iron their range of unique properties.

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 that of 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-carbon alloys with a carbon content more than 2%.

Cast iron is a group of iron-carbon alloys with a carbon content more 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

Steelmaking is the process of producing steel from iron ore and/or scrap. In steelmaking, impurities such as nitrogen, silicon, phosphorus, sulfur and excess carbon are removed from the sourced iron, and alloying elements such as manganese, nickel, chromium, carbon and vanadium are added to produce different grades of steel. Limiting dissolved gases such as nitrogen and oxygen and entrained impurities in the steel is also important to ensure the quality of the products cast from the liquid steel.

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.

Open hearth furnace

Open-hearth furnaces are one of several kinds of furnace in which excess carbon and other impurities are burnt out of pig iron to produce steel. Since steel is difficult to manufacture owing 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 permitted the melting and refining of large amounts of scrap iron and steel.

Electric arc furnace

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

Bloomery Type of furnace once used widely for smelting iron from its oxides

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

A reverberatory furnace is a metallurgical or process furnace that isolates the material being processed from contact with the fuel, but not from contact with combustion gases. The term reverberation is used here in a generic sense of rebounding or reflecting, not in the acoustic sense of echoing.

Ironworks

An ironworks or iron works is an industrial plant where iron is smelted and where heavy iron and steel products are made. The term is both singular and plural, i.e. the singular of ironworks is ironworks.

Puddling (metallurgy)

Puddling is a step in the manufacture of high-grade iron in a crucible or furnace. It was invented in Great Britain during the Industrial Revolution. The molten pig iron was stirred in a reverberatory furnace, in an oxidizing environment, resulting in wrought iron. It was one of the most important processes of making the first appreciable volumes of valuable and useful bar iron without the use of charcoal. Eventually, the furnace would be used to make small quantities of specialty steels.

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 removing carbon from the molten cast iron through oxidation. Finery forges were used as early as the 3rd century BC in 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.

Osmond process

Osmond iron was wrought iron made by a particular process. This is associated with the first European production of cast iron in furnaces such as Lapphyttan in Sweden.

Ferrous metallurgy

Ferrous metallurgy, the metallurgy of iron and its alloys, began 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 in Sub-Saharan Africa. The use of wrought iron was known by the 1st millennium BC, and its spread defined the Iron Age. During the medieval period, smiths in Europe found a way of producing wrought iron from cast iron using finery forges. All these processes required charcoal 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.

Austempering

Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.

Lancashire hearth

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

Uddeholms AB

Uddeholms AB is a multinational producer of high alloyed tool steel with production in Hagfors, Sweden. Since 1991, the company is part of the Austrian Böhler-Uddeholm group which in turn is part of the voestalpine AG group since 2007. Uddeholms AB has 800 employees in Sweden. Globally, the Uddeholm group employs 3000 people.

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.