Wrought iron
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Ferrite (α-iron, δ-iron; soft) |
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Carbon steel (≤2.1% carbon; low alloy) |
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Cast iron (>2.1% carbon) |
Wrought iron is commercially pure iron. In contrast to steel, it has a very low carbon content. It is a fibrous material due to the slag inclusions (a normal constituent). This is also 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.
Examples of items that used to be produced from wrought iron include: rivets, chains, railway couplings, water and steam pipes, raw material for manufacturing of steel, nuts, bolts, horseshoes, handrails, straps for timber roof trusses, boiler tubes, and ornamental ironwork.
Wrought iron is no longer produced on a commercial scale.[1] Many products described as wrought iron, such as guard rails, are made of mild steel.[1] They retain that description because they were formerly made of wrought iron or have the appearance of wrought iron. True wrought iron is occasionally required for the authentic conservation of historic structures.
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[edit] 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. As irregular past-tense forms in English have historically been phased out over long periods of time, wrought became worked. Wrought iron literally means worked iron. Another theory is that "wrought" is the past tense of "wring".
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.
[edit] 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.[2]
[edit] Origin
- 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."[3]
- 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.[3]
- Botolf iron or Boutall iron — from Butow (Pommerania) or Beuthen (Silesia).[3]
- 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.[4]
[edit] Quality
- Tough iron — also spelt "tuf".
- Blend iron — made using a mixture of different types of pig iron.
- Best iron — in the 19th century, iron that had gone through several stages of piling and rolling, might reach the stage of being best iron.
- Marked Bar iron — iron made by members of the Marked Bar Association and marked with the maker's brand mark as a sign of its quality.
[edit] Defective quality
- 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.
- Iron is coldshort (or "coldshear" or "colshire" or "bloodshot"), if it contains phosphorus in excess quantity. 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.[5][6][7]
- Furthermore, the presence of phosphorus (without carbon) produces a ductile iron suitable for wire drawing, for piano wire.[8]
[edit] History
