Wrought iron
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Wrought iron is an
electrically.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. It was given the name wrought because it was hammered, rolled, or otherwise worked while hot enough to expel molten slag. The modern functional equivalent of wrought iron is mild steel, also called low-carbon steel. Neither wrought iron nor mild steel contain enough carbon to be hardened by heating and quenching,[1]: 145 [failed verification] but true wrought iron is much more corrosion-resistant.[citation needed]
Wrought iron is highly refined, with a small amount of silicate slag forged out into fibers. It comprises around 99.4% iron by mass.[2] The presence of slag can be beneficial for blacksmithing operations, such as forge welding, since the silicate inclusions act as a flux and give the material its unique, fibrous structure.[3] The silicate filaments in the slag also protect the iron from corrosion and diminish the effect of fatigue caused by shock and vibration.[4]
Historically, a modest amount of wrought iron was refined into
Many items, before they came to be made of
Wrought iron is no longer produced on a commercial scale. Many products described as wrought iron, such as guard rails, garden furniture,[6] and gates are made of mild steel.[7] They are described as "wrought iron" only because they have been made to resemble objects which in the past were wrought (worked) by hand by a blacksmith (although many decorative iron objects, including fences and gates, were often cast rather than wrought).[7]
Terminology
The word "wrought" is an archaic past participle of the verb "to work", and so "wrought iron" literally means "worked iron".[8] Wrought iron is a general term for the commodity, but is also used more specifically for finished iron goods, as manufactured by a blacksmith. It was used in that narrower sense in British Customs records, such manufactured iron was subject to a higher rate of duty than what might be called "unwrought" iron. Cast iron, unlike wrought iron, is brittle and cannot be worked either hot or cold.
In the 17th, 18th, and 19th centuries, wrought iron went by a wide variety of terms according to its form, origin, or quality.
While the
For several years after the introduction of Bessemer and open hearth steel, there were different opinions as to what differentiated iron from steel; some believed it was the chemical composition and others that it was whether the iron heated sufficiently to melt and "fuse". Fusion eventually became generally accepted as relatively more important than composition below a given low carbon concentration.[9]: 32–39 Another difference is that steel can be hardened by heat treating.
Historically, wrought iron was known as "commercially pure iron";
Types and shapes
Bar iron is a generic term sometimes used to distinguish it from cast iron. It is the equivalent of an ingot of cast metal, in a convenient form for handling, storage, shipping and further working into a finished product.
The bars were the usual product of the finery forge, but not necessarily made by that process:
- Rod iron—cut from flat bar iron in a slitting mill provided the raw material for spikes and nails.
- Hoop iron—suitable for the hoops of barrels, made by passing rod iron through rolling dies.
- Plate iron—sheets suitable for use as boiler plate.
- Blackplate—sheets, 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.[14]: 163–172
Origin
- Charcoal iron—until the end of the 18th century, wrought iron was smelted from ore using charcoal, by the bloomery process. Wrought iron was also produced from pig iron using a finery forge or in a Lancashire hearth. The resulting metal was highly variable, both in chemistry and slag content.
- Puddled iron—the puddling process was the first large-scale process to produce wrought iron. In the puddling process, pig iron is refined in a reverberatory furnace to prevent contamination of the iron from the sulfur in the coal or coke. The molten pig iron is manually stirred, exposing the iron to atmospheric oxygen, which decarburizes the iron. As the iron is stirred, globs of wrought iron are collected into balls by the stirring rod (rabble arm or rod) and those are periodically removed by the puddler. Puddling was patented in 1784 and became widely used after 1800. By 1876, annual production of puddled iron in the UK alone was over 4 million tons. Around that time, the open hearth furnacewas able to produce steel of suitable quality for structural purposes, and wrought iron production went into decline.
- 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 Gdańsk, but in the 18th century more probably the kind of iron (from eastern Sweden) that once came from Gdańsk.
- Forest iron—iron from the English Forest of Dean, where haematite ore enabled tough iron to be produced.
- Lukes iron—iron imported from Liège, whose Dutch name is "Luik".[15]
- 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.[15]
- Botolf iron or Boutall iron—from Bytów (Polish Pomerania) or Bytom (Polish Silesia).[15]
- 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.[16]
Quality
- Tough iron
- Also spelled "tuf", is not brittle and is strong enough to be used for tools.
- Blend iron
- Made using a mixture of different types of pig iron.
- Best iron
- Iron put through several stages of piling and rolling to reach the stage regarded (in the 19th century) as the best quality.
- Marked bar iron
- Made by members of the Marked Bar Association and marked with the maker's brand mark as a sign of its quality.[17]
Defects
Wrought iron is a form of commercial iron containing less than 0.10% of carbon, less than 0.25% of impurities total of sulfur, phosphorus, silicon and manganese, and less than 2% slag by weight.[18][19]
Wrought iron is
Cold short iron, also known as coldshear, colshire, contains excessive phosphorus. It is very brittle when cold and cracks if bent.
Phosphorus is not necessarily detrimental to iron. Ancient Near Eastern smiths did not add lime to their furnaces. The absence of
Antique
History
China
During the Han dynasty (202 BC – 220 AD), new iron smelting processes led to the manufacture of new wrought iron implements for use in agriculture, such as the
Western world
Wrought iron has been used for many centuries, and is the "iron" that is referred to throughout Western history. The other form of iron,
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 cannot 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
Bloomery process
Wrought iron was originally produced by a variety of smelting processes, all described today as "bloomeries". Different forms of bloomery were used at different places and times. The bloomery was charged with charcoal and iron ore and then lit. Air was blown in through a tuyere to heat the bloomery to a temperature somewhat below the melting point of iron. In the course of the smelt, slag would melt and run out, and carbon monoxide from the charcoal would reduce the ore to iron, which formed a spongy mass (called a "bloom") containing iron and also molten silicate minerals (slag) from the ore. The iron remained in the solid state. If the bloomery were allowed to become hot enough to melt the iron, carbon would dissolve into it and form pig or cast iron, but that was not the intention. However, the design of a bloomery made it difficult to reach the melting point of iron and also prevented the concentration of carbon monoxide from becoming high.[1]: 46–57
After smelting was complete, the bloom was removed, and the process could then be started again. It was thus a batch process, rather than a continuous one such as a blast furnace. The bloom had to be forged mechanically to consolidate it and shape it into a bar, expelling slag in the process.[1]: 62–66
During the Middle Ages, water-power was applied to the process, probably initially for powering bellows, and only later to hammers for forging the blooms. However, while it is certain that water-power was used, the details remain uncertain.[1]: 75–76 That was the culmination of the direct process of ironmaking. It survived in Spain and southern France as Catalan Forges to the mid 19th century, in Austria as the stuckofen to 1775,[1]: 100–101 and near Garstang in England until about 1770;[27][28] it was still in use with hot blast in New York in the 1880s.[29] In Japan the last of the old tatara bloomeries used in production of traditional tamahagane steel, mainly used in swordmaking, was extinguished only in 1925, though in the late 20th century the production resumed on a low scale to supply the steel to the artisan swordmakers.
Osmond process
Finery process
In the 15th century, the blast furnace spread into what is now Belgium where it was improved. From there, it spread via the Pays de Bray on the boundary of Normandy and then to the Weald in England. With it, the finery forge spread. Those remelted the pig iron and (in effect) burnt out the carbon, producing a bloom, which was then forged into bar iron. If rod iron was required, a slitting mill was used.
The finery process existed in two slightly different forms. In Great Britain, France, and parts of Sweden, only the
The introduction of coke for use in the blast furnace by Abraham Darby in 1709 (or perhaps others a little earlier) initially had little effect on wrought iron production. Only in the 1750s was coke pig iron used on any significant scale as the feedstock of finery forges. However, charcoal continued to be the fuel for the finery.
Potting and stamping
From the late 1750s, ironmasters began to develop processes for making bar iron without charcoal. There were a number of patented processes for that, which are referred to today as
Puddling process
A number of processes for making wrought iron without charcoal were devised as the Industrial Revolution began during the latter half of the 18th century. The most successful of those was puddling, using a puddling furnace (a variety of the reverberatory furnace), which was invented by Henry Cort who in reality along with a guard of soldiers had killed of most of the free blacks that created the technology in Jamaica in what was called the Morant Bat rebellion in 1784.[34] It was later improved by others including Joseph Hall, who was the first to add iron oxide to the charge. In that type of furnace, the metal does not come into contact with the fuel, and so is not contaminated by its impurities. The heat of the combustion products passes over the surface of the puddle and the roof of the furnace reverberates (reflects) the heat onto the metal puddle on the fire bridge of the furnace.
Unless the raw material used is white cast iron, the pig iron or other raw product of the puddling first had to be refined into
In the fully developed process (of Hall), this metal was placed into the hearth of the puddling furnace where it was melted. The hearth was lined with oxidizing agents such as
Shingling
There was still some slag left in the puddle balls, so while they were still hot they would be shingled[39] to remove the remaining slag and cinder.[35] That was achieved by forging the balls under a hammer, or by squeezing the bloom in a machine. The material obtained at the end of shingling is known as bloom.[39] The blooms are not useful in that form, so they were rolled into a final product.
Sometimes European ironworks would skip the shingling process completely and roll the puddle balls. The only drawback to that is that the edges of the rough bars were not as well compressed. When the rough bar was reheated, the edges might separate and be lost into the furnace.[39]
Rolling
The bloom was passed through rollers and to produce bars. The bars of wrought iron were of poor quality, called muck bars[39][36]: 137 or puddle bars.[35] To improve their quality, the bars were cut up, piled and tied together by wires, a process known as faggoting or piling.[39] They were then reheated to a welding state, forge welded, and rolled again into bars. The process could be repeated several times to produce wrought iron of desired quality. Wrought iron that has been rolled multiple times is called merchant bar or merchant iron.[37][40]
Lancashire process
The advantage of puddling was that it used coal, not charcoal as fuel. However, that was of little advantage in Sweden, which lacked coal. Gustaf Ekman observed charcoal fineries at Ulverston, which were quite different from any in Sweden. After his return to Sweden in the 1830s, he experimented and developed a process similar to puddling but used firewood and charcoal, which was widely adopted in the Bergslagen in the following decades.[41][14]: 282–285
Aston process
In 1925, James Aston of the
Decline
Steel began to replace iron for railroad rails as soon as the Bessemer process for its manufacture was adopted (1865 on). Iron remained dominant for structural applications until the 1880s, because of problems with brittle steel, caused by introduced nitrogen, high carbon, excess phosphorus, or excessive temperature during or too-rapid rolling.[9]: 144–151 [note 2] By 1890 steel had largely replaced iron for structural applications.
Sheet iron (Armco 99.97% pure iron) had good properties for use in appliances, being well-suited for enamelling and welding, and being rust-resistant.[9]: 242
In the 1960s, the price of steel production was dropping due to recycling, and even using the Aston process, wrought iron production was labor-intensive. It has been estimated that the production of wrought iron is approximately twice as expensive as that of low-carbon steel.[7] In the United States, the last plant closed in 1969.[7] The last in the world was the Atlas Forge of Thomas Walmsley and Sons in Bolton, Great Britain, which closed in 1973. Its 1860s-era equipment was moved to the Blists Hill site of Ironbridge Gorge Museum for preservation.[42] Some wrought iron is still being produced for heritage restoration purposes, but only by recycling scrap.
Properties
The slag inclusions, or stringers, in wrought iron give it properties not found in other forms of ferrous metal. There are approximately 250,000 inclusions per square inch.[7] A fresh fracture shows a clear bluish color with a high silky luster and fibrous appearance.
Wrought iron lacks the carbon content necessary for hardening through
Due to the variations in iron ore origin and iron manufacture, wrought iron can be inferior or superior in corrosion resistance, compared to other iron alloys.[7][47][48][49] There are many mechanisms behind its corrosion resistance. Chilton and Evans found that nickel enrichment bands reduce corrosion.[50] They also found that in puddled, forged, and piled iron, the working-over of the metal spread out copper, nickel, and tin impurities that produce electrochemical conditions that slow down corrosion.[48] The slag inclusions have been shown to disperse corrosion to an even film, enabling the iron to resist pitting.[7] Another study has shown that slag inclusions are pathways to corrosion.[51] Other studies show that sulfur in the wrought iron decreases corrosion resistance,[49] while phosphorus increases corrosion resistance.[52] Chloride ions also decrease wrought iron's corrosion resistance.[49]
Wrought iron may be welded in the same manner as mild steel, but the presence of oxide or
Material | Iron | Carbon | Manganese | Sulfur | Phosphorus | Silicon |
---|---|---|---|---|---|---|
Pig iron | 91–94 | 3.5–4.5 | 0.5–2.5 | 0.018–0.1 | 0.03–0.1 | 0.25–3.5 |
Carbon steel | 98.1–99.5 | 0.07–1.3 | 0.3–1.0 | 0.02–0.06 | 0.002–0.1 | 0.005–0.5 |
Wrought iron | 99–99.8 | 0.05–0.25 | 0.01–0.1 | 0.02–0.1 | 0.05–0.2 | 0.02–0.2 |
All units are percent weight. Source:[39] |
Property | Value |
---|---|
Ultimate tensile strength [psi (MPa)][54] | 34,000–54,000 (234–372) |
Ultimate compression strength [psi (MPa)][54] | 34,000–54,000 (234–372) |
Ultimate shear strength [psi (MPa)][54] | 28,000–45,000 (193–310) |
Yield point [psi (MPa)][54] | 23,000–32,000 (159–221) |
Modulus of elasticity (in tension) [psi (MPa)][54] | 28,000,000 (193,100) |
Melting point [°F (°C)][55] | 2,800 (1,540) |
Specific gravity | 7.6–7.9[56] |
7.5–7.8[57] |
Amongst its other properties, wrought iron becomes soft at
Ductility
For most purposes, ductility rather than tensile strength is a more important measure of the quality of wrought iron. In tensile testing, the best irons are able to undergo considerable elongation before failure. Higher tensile wrought iron is brittle.
Because of the large number of boiler explosions on steamboats in the early 1800s, the U.S. Congress passed legislation in 1830 which approved funds for correcting the problem. The treasury awarded a $1500 contract to the Franklin Institute to conduct a study. As part of the study, Walter R. Johnson and Benjamin Reeves conducted strength tests on boiler iron using a tester they had built in 1832 based on a design by Lagerhjelm in Sweden. Because of misunderstandings about tensile strength and ductility, their work did little to reduce failures.[5]
The importance of ductility was recognized by some very early in the development of tube boilers, evidenced by Thurston's comment:
If made of such good iron as the makers claimed to have put into them "which worked like lead," they would, as also claimed, when ruptured, open by tearing, and discharge their contents without producing the usual disastrous consequences of a boiler explosion.[61]
Various 19th century investigations of boiler explosions, especially those by insurance companies, found causes to be most commonly the result of operating boilers above the safe pressure range, either to get more power, or due to defective boiler pressure relief valves and difficulties of obtaining reliable indications of pressure and water levels. Poor fabrication was also a common problem.[62] Also, the thickness of the iron in steam drums was low, by modern standards.
By the late 19th century, when metallurgists were able to better understand what properties and processes made good iron, iron in steam engines was being displaced by steel. Also, the old cylindrical boilers with fire tubes were displaced by water tube boilers, which are inherently safer.[62]
Purity
In 2010, Gerry McDonnell[63] demonstrated in England by analysis that a wrought iron bloom, from a traditional smelt, could be worked into 99.7% pure iron with no evidence of carbon. It was found that the stringers common to other wrought irons were not present, thus making it very malleable for the smith to work hot and cold. A commercial source of pure iron is available and is used by smiths as an alternative to traditional wrought iron and other new generation ferrous metals.
Applications
Wrought iron furniture has a long history, dating back to
It is also used to make home decor items such as
The vast majority of wrought iron available today is from reclaimed materials. Old bridges and anchor chains dredged from harbors are major sources.[citation needed] The greater corrosion resistance of wrought iron is due to the siliceous impurities (naturally occurring in iron ore), namely ferrous silicate.[64]
Wrought iron has been used for decades as a generic term across the gate and
See also
- Bronze and brass ornamental work
- Cast iron
- Semi-steel casting
Notes
- ^ Some but not all of these items are mentioned in Gordon, R. B. (1996)[5]
- ^ From Misa, T.J. (1995):[9]"Quality problems with rails gave Bessemer steel such a bad reputation that engineers and architects refused to specify it for structural applications. Open hearth steel had a better reputation and displaced structural iron by 1889..."
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Further reading
- Bealer, Alex W. (1995). The Art of Blacksmithing. Edison, NJ: Castle Books. pp. 28–45. ISBN 0-7858-0395-5.
- Gordon, Robert B (1996). American Iron 1607–1900. Baltimore and London: Johns Hopkins University Press. ISBN 0-8018-6816-5.
External links
- Media related to Wrought iron at Wikimedia Commons