Pig iron, also known ascrude iron, is anintermediate good used by theiron industry in the production ofsteel. It is developed bysmeltingiron ore in ablast furnace. Pig iron has a highcarbon content, typically 3.8–4.7%,[1] along withsilica and otherdross, which makes itbrittle and not useful directly as a material except for limited applications.[2]
The traditional shape of the molds used for pig ironingots is a branching structure formed insand, with many individual ingots at right angles[3] to a central channel or "runner", resembling a litter ofpiglets beingnursed by asow. 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 are insignificant when compared to the ease ofcasting and handling.
The expression “sweat like a pig” comes from the iron smelting process. As the iron cools, the surrounding air reaches its dew point, and beads of moisture form on the surface of the "pigs". This indicates that the iron has cooled enough to be safely handled.[5]
The Chinese were already making pig iron during the laterZhou dynasty (which ended in 256 BC).[6] Furnaces, such asLapphyttan in Sweden, may date back to the 12th century and some in theCounty of Mark to the 13th century, which is now part ofWestphalia, Germany.[7] It is unclear whether these northern European developments were derived from the Chinese ones or independent inventions. Wagner[8] has postulated a possible link via Persian contacts with China along theSilk Road and Viking contacts with Persia,[6] but there is a chronological gap between the Viking period and Lapphyttan. The step from bloomery to blast furnace is not big: all that is needed is to make the furnace higher and have more powerful bellows.
Smelting and producingwrought iron were known in ancient Europe and the Middle East, but it was produced inbloomeries bydirect reduction. Smallprills of pig iron dispersed inslag are produced in all iron furnaces, but the operator of a bloomery had to avoid conditions causing thephase transition 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 theMiddle Ages the prills were discarded with the slag.[9]
Traditionally, pig iron was worked intowrought iron infinery forges, laterpuddling furnaces, and more recently, it is worked intosteel.[10] 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 thoroughlyoxidized. An intermediate product of puddling is known asrefined pig iron,finers metal, orrefined iron.[11]
Pig iron can also be used to producegray iron. This is achieved by remelting pig iron, often along with substantial quantities of steel andscrap 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).
Pig iron was historically poured directly out of the bottom of theblast furnace through a channel into aladle car for transfer to thesteel mill in mostly liquid form; in this state, the pig iron was referred to ashot metal. The hot metal was then poured into asteelmaking vessel to producesteel, typically anelectric arc furnace,induction furnace orbasic oxygen furnace, where the excess carbon is burned off and thealloy composition controlled. Earlier processes for this included thefinery forge, thepuddling furnace, theBessemer process, and theopen hearth furnace.
Modern steel mills anddirect-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-kilogram (9–22 lb) piglets at discharge.
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