Contents
Wrought iron
From Wikipedia, the free encyclopedia
Ferrite (α-iron, δ-iron)
Austenite (γ-iron)
Pearlite (88% ferrite, 12% cementite)
Bainite
Martensite
Ledeburite (ferrite-cementite eutectic, 4.3% carbon)
Cementite (iron carbide, Fe3C)
Steel classes
Carbon steel (≤2.1% carbon; low alloy)
Crucible steel
Alloy steel (contains non-carbon elements)
Stainless steel (contains chromium)
Tool steel (alloy steel for tools)
Other iron-based materials
Cast iron (>2.1% carbon)
Gray iron
Malleable iron
White iron
Wrought iron (contains slag)
Wrought iron is an iron alloy with a very low carbon content, in comparison to steel, and has fibrous inclusions, known as slag. This is what gives it a "grain" resembling wood, which is visible when it is etched or bent to the point of failure. Wrought iron is tough, malleable, ductile and easily welded. Historically, it was known as "commercially pure iron",[1][2] however it no longer qualifies because current standards for commercially pure iron require a carbon content of less than 0.008 wt%.[3][4]
Before the development of effective methods of steelmaking and the availability of large quantities of steel, wrought iron was the most common form of malleable iron. A modest amount of wrought iron was used as a raw material for manufacturing of steel, which was mainly to produce swords, cutlery and other blades. Demand for wrought iron reached its peak in the 1860s with the adaptation of ironclad warships and railways, but then declined as mild steel became more available.
Before they came to be made of mild steel, items produced from wrought iron included rivets, nails, chains, railway couplings, water and steam pipes, nuts, bolts, horseshoes, handrails, straps for timber roof trusses, and ornamental ironwork.[citation needed]
Wrought iron is no longer produced on a commercial scale. Many products described as wrought iron, such as guard rails, garden furniture[5] and gates, are made of mild steel.[6] They retain that description because they were formerly made of wrought iron or have the appearance of wrought iron. True wrought iron is required for the authentic conservation of historic structures.
Terminology
Wrought iron is so named because it is worked from a bloom of porous iron mixed with slag and other impurities. The word "wrought" is an archaic past tense form of the verb "to work". "Wrought iron" literally means "worked iron".
Wrought iron is a general term for the commodity, but is also used more specifically for finished iron goods, as manufactured by a blacksmith or other smith. It was used in this narrower sense in British Customs records, such manufactured iron being subject to a higher rate of duty than what might be called "unwrought" iron.
In the 17th, 18th and 19th centuries, wrought iron went by a wide variety of terms according to its form, origin, or quality.
Form
- Bar iron—iron in bars, which are the usual product of the finery forge, but not necessarily made by that process. These might be square or flat, and flat bars might be narrow or broad.
- Rod iron—cut from flat bar iron in a slitting mill to provide the raw material for nails.
- Hoop iron—suitable for the hoops of barrels, apparently made by passing rod iron through flat rolls.
- Plate iron—sheets of iron suitable for use as boiler plate.
- Blackplate—sheets of iron, perhaps thinner than plate iron, from the black rolling stage of tinplate production.
- Voyage iron—narrow flat bar iron, made or cut into bars of a particular weight, a commodity for sale in Africa for the Atlantic slave trade. The number of bars per ton gradually increased from 70 per ton in the 1660s to 75–80 per ton in 1685 and "near 92 to the ton" in 1731.[7]
Origin
- Charcoal Iron — Until the end of the eighteenth century, wrought iron was made using charcoal, by the bloomery process, in a finery forge or from the industrial revolution in a Lancashire hearth. The resulting metal was highly variable, both in chemistry and slag content.
- Puddled Iron — By the late eighteenth century there was a demand for wrought iron to be refined with coal as fuel. This resulted in ‘puddled iron’. The iron was kept separate from the fire in a reverberatory furnace to prevent harmful sulphur and phosphorous from entering the finished iron. Puddled iron, although also variable in its properties, was generally more consistent than the earlier irons, and the method lent itself to the production of far greater quantities. By 1876, annual production of puddled iron in the UK alone was over 4 million tons.
- Oregrounds iron—a particularly pure grade of bar iron made ultimately from iron ore from the Dannemora mine in Sweden. Its most important use was as the raw material for the cementation process of steelmaking.
- Danks iron — originally iron imported to Great Britain from Danzig (now Gdansk), but in the 18th century more probably the kind of iron (from eastern Sweden) that once came from Danzig.
- Forest iron — iron from the Forest of Dean, where haematite ore enabled tough iron to be produced.
- Lukes iron — iron imported from Liège, whose Dutch name is "Luik."[8]
- Ames iron or amys iron — another variety of iron imported to England from northern Europe. Its origin has been suggested to be Amiens, but it seems to have been imported from Flanders in the 15th century and Holland later, suggesting an origin in the Rhine valley. Its origins remain controversial.[8]
- Botolf iron or Boutall iron — from Butow (Pommerania) or Beuthen (Silesia).[8]
- Sable iron (or Old Sable) — iron bearing the mark (a sable) of the Demidov family of Russian ironmasters, one of the better brands of Russian iron.[9]
Quality
Defects
Wrought iron is redshort if it contains sulfur in excess quantity. It has sufficient tenacity when cold, but cracks when bent or finished at a red heat. It is therefore useless for welding or forging.
Coldshort iron, also known as coldshear, colshire or bloodshot, contains excessive phosphorus. It is very brittle when it is cold. It cracks if bent. It may, however, be worked at high temperature. Historically, coldshort iron was considered good enough for nails.
Nevertheless, phosphorus is not necessarily detrimental to iron. Ancient Indian smiths did not add lime to their furnaces; the absence of CaO in the slag, and the deliberate use of wood with high phosphorus content during the smelting, induces a higher P content (> 0.1%, average 0.25%) than in modern iron. There is more phosphorus as solid solution throughout the metal than in the slags (one analysis gives 0.10% in the slags for .18% in the iron itself, for a total P content of 0.28% in the metal). This high P content and particular repartition are essential factors in the formation of a passive protective film of “misawite” (d-FeOOH), an amorphous iron oxyhydroxide that forms a barrier by adhering next to the interface between metal and rust. From this technology recently rediscovered by metallurgists at IIT Kanpur through the study of the Iron Pillar of Delhi, rust-proof iron is at the last stages of being commercialized. This 1600 years-old rust-proof pillar is also of a remarkable strength, having withstood the impact of a cannon ball in the 18th century. Copper has a similar effect as phosphate regarding the formation of a passive protection film.[11][12][13] Furthermore, the presence of phosphorus (without carbon) produces a ductile iron suitable for wire drawing, for piano wire.[14]
History
Wrought iron has been used for many centuries, and is the "iron" that is referred to throughout western history. The other form of iron, cast iron, was not introduced into Western Europe until the 15th century; even then, due to its brittleness, it could only be used for a limited number of purposes. Throughout much of the Middle Ages iron was produced by the direct reduction of ore in manually operated bloomeries, although waterpower had begun to be employed by 1104.[15]
The raw material produced by all indirect processes is pig iron. It has a high carbon content and as a consequence it is brittle and could not be used to make hardware. The osmond process was the first of the indirect processes, developed by 1203, but bloomery production continued in many places. The process depended on the development of the blast furnace, of which medieval examples have been discovered at Lapphyttan, Sweden and in Germany.
The bloomery and osmond processes were gradually replaced from the 15th century by finery processes, of which there were two versions, the German and Walloon. They were in turn replaced from the late 18th century by puddling, with certain variants such as the Swedish Lancashire process. These too are now obsolete, and wrought iron is no longer manufactured commercially.