|Steels and other iron–carbon alloy phases|
|Other iron-based materials|
Crucible steel describes a number of different techniques for making steel in a crucible. Its manufacture is essentially a refining process in which another form of steel that had been consolidated by hammering, such as shear steel, is melted to give a more uniform material. Crucible steel has aroused considerable interest for well over a thousand years and there is a sizable body of work concerning its nature and production.
The crucible steel of the Industrial Revolution is referred to in this article as English crucible steel, which was developed for making watch springs. It was also used in other applications such as scissors, axes and swords.
The puddling process, developed in the 1840s, produced a satisfactory grade of wrought iron for many applications at a cost far below crucible steel; however, while puddled iron was suitable for structural applications, it did not replace steel in cutting tools. By the end of the 19th century new steel making processes such as Bessemer and the open hearth furnace were able to produce quality steel at a cost even below puddling, making the crucible process obsolete. The crucible process continued to be used for specialty steels, but is today obsolete. Basic oxygen furnaces and electric arc furnaces have largely replaced all previous ones.
- 1 Methods of crucible steel production
- 2 Ancient History of South Asian production
- 3 Early modern steel in Europe
- 4 Conclusion
- 5 See also
- 6 Notes
- 7 External links
Methods of crucible steel production
|This section may contain inappropriate or misinterpreted citations that do not verify the text. (October 2010)|
Various methods were used to produce crucible steel. According to Islamic texts such as al-Tarsusi and Abu Rayhan Biruni, three methods are described for indirect production of steel. The medieval Islamic historian Abu Rayhan Biruni (c. 973–1050) provides us with the earliest reference of the production of Damascus steel. He describes only three methods for producing steel. The first two methods have a long history in Central Asia and in the Indian subcontinent while the third is exclusive to South Asia. These three methods are generally considered to have originated from the Indian Subcontinent. The first method and the most common traditional method is solid state carburization of wrought iron. This is a diffusion process in which wrought iron is packed in crucibles or a hearth with charcoal, then heated to promote diffusion of carbon into the iron to produce steel. Carburization is the basis for the wootz process of steel. The second method is the decarburization of cast iron by removing carbon from the cast iron. Another indirect method uses wrought iron and cast iron. In this process, wrought iron and cast iron may be heated together in a crucible to produce steel by fusion. In regard to this method Abu Rayhan Biruni states: "this was the method used in Hearth". It is proposed that the Indian method refers to Wootz carburization method i.e. the Mysore or Tamil processes.
Variations of co-fusion process have been found preliminary in Persia and Central Asia but have also been found in Hyderabad, India called Deccani or Hyderabad process. For the carbon, a variety of organic materials are specified by the contemporary Islamic authorities, including pomegranate rinds, acorns, fruit skins like orange peel, leaves as well as the white of egg and shells. Slivers of wood are mentioned in some of the Indian sources, but significantly none of the sources mention charcoal.
Ancient History of South Asian production
|This section may contain inappropriate or misinterpreted citations that do not verify the text. (October 2010)|
Crucible steel is generally attributed to production centres in India and Sri Lanka where it was produced using the so-called "wootz" process and it is assumed that its appearance in other locations was due to long distance trade. Only recently it has become apparent that places in Central Asia like Merv in Turkmenistan and Akhsiket in Uzbekistan were important centres of production of crucible steel. The Central Asian finds are all from excavations and date from the 8th to 12th centuries AD, while the Indian/Sri Lankan material is as early as 300 BC.In addition India's superior iron ore has trace vanadium and other rare earths which unintentionally leads to the formation of carbon nano tubes in Indian crucible steel which was famous throughout the middle east for its ability to retain its edge even after prolonged usage.
South India/Sri Lanka
There are many ethnographic accounts of Indian crucible steel production, however, scientific investigations of crucible steel remains have only been published from four regions: three in India and one in Sri Lanka. Indian/Sri Lankan crucible steel is commonly referred to as wootz. It is generally agreed that wootz is an English corruption of the word ukko or hookoo. European accounts from the 17th century onwards have referred to the repute and manufacture of ‘wootz’, a traditional crucible steel made specially in parts of southern India in the former provinces of Golconda, Mysore and Salem. As yet the scale of excavations and surface surveys is too limited to link the literary accounts to archaeometallurgical evidence.
The known sites of crucible steel production in south India, i.e. at Konasamudram and Gatihosahalli, date from at least the late medieval period, 16th century. One of the earliest known sites, which shows some promising preliminary evidence that may be linked to ferrous crucible processes in Kodumanal, near Coimbatore in Tamil Nadu. The site is dated between the third century BC and the third century AD. By the seventeenth century the main centre of crucible steel production seems to have been in Hyderabad. The process was apparently quite different from that recorded elsewhere. Wootz from Hyderabad or the Decanni process for making watered blades involved a co-fusion of two different kinds of iron- one was low in carbon and the other was a high-carbon steel or cast iron. Wootz steel was widely exported and traded throughout ancient Europe, China, the Arab world, and became particularly famous in the Middle East, where it became known as Damascus steel.
Recent archaeological investigations have suggested that Sri Lanka also supported innovative technologies for iron and steel production in antiquity. The Sri Lankan system of crucible steel making was partially independent of the various Indian and Middle Eastern systems. Their method was something similar to the method of carburization of wrought iron. The earliest confirmed crucible steel site is located in the knuckles range in the northern area of the Central Highlands of Sri Lanka dated to 6th −10th centuries AD. In twelfth century the land of Serendib (Sri Lanka) seems to have been the main supplier of crucible steel, but over the centuries slipped back, and by the nineteenth century just a small industry survived in the Balangoda district of the central southern highlands.
A series of excavations at Samanalawewa indicated the unexpected and previously unknown technology of west-facing smelting sites, which are different types of steel production. These furnaces were used for direct smelting to steel. Because of their location on the western sides of hilltops for use of wind in the smelting process they are named west-facing. Sri Lankan furnace steels were known and traded between the 9th and 11th centuries and earlier, but apparently not later. These sites were dated to the 7th–11th centuries. The coincidence of this dating with the 9th century Islamic reference to Sarandib is of great importance. The crucible process existed in India at the same time that the west- facing technology was operating in Sri Lanka.
Early History of Central Asian production
Central Asia has a rich history of crucible steel production, beginning during the late 1st millennium AD. From the sites in modern Uzbekistan and Merv in Turkmenistan, there exists good archaeological evidence for the large scale production of crucible steel. They all belong in broad terms to the same early medieval period between the late 8th or early 9th and the late 12th century AD Contemporary with the early crusades.
The two most prominent crucible steel sites in eastern Uzbekistan carrying the Ferghana Process are Akhsiket and Pap in the Ferghana Valley, whose position within the Great Silk Road has been historically and archaeologically proved. The material evidence of the sites consists of large number of archaeological finds relating to steel making from 9th–12th centuries AD in the form of hundreds of thousands of fragments of crucibles often with massive slag cakes. Archaeological work at Akhsiket, has identified that the crucible steel process was of the carburization of iron ore. This process appears to be typical of and restricted to the Ferghana Valley in eastern Uzbekistan, and it is therefore called the Ferghana Process. This process lasts in that region for roughly four centuries..
Evidences of the production of crucible steel have been found in Merv, Turkmenistan, a major city on the 'Silk Road'. The Islamic scholar, al-Kindi (AD 801–866) mentions that during the ninth century the region of Khorasan, the area to which the cities Nishapur, Merv, Herat and Balkh belong, is a steel manufacturing centre. Evidence from a metallurgical workshop at Merv, dated to the ninth- early tenth century A.D., provides an illustration of the co-fusion method of steel production in crucibles, about 1000 years earlier than the distinctly different wootz process. The crucible steel process at Merv might be seen as technologically related to what Bronson (1986, 43) calls Heyderabad process, a variation of the wootz process, after the location of the process documented by Voysey in the 1820s.
Historical accounts and investigations
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In the first centuries of the Islamic period, there appear some scientific studies on swords and steel. The best known of these are by Jabir ibn Hayyan 8th century, al-Kindi 9th century, Abu Rayhan Biruni in the early 11th century, Murda al Tarsusi in the late 12th century, and Fakhr-i- Mudabbir 13th century. Any of these contains far more information about Indian and damascene steels than appears in the entire literature of classical Greece and Rome. The first European references to crucible steel seem to be no earlier than the Post Medieval period. European experiments with “Damascus” steels go back to at least the sixteenth century, but it was not until the 1790s that laboratory researchers began to work with steels that were specifically known to be Indian/wootz. At this time, Europeans knew of India's ability to make crucible steel from reports brought back by travellers who had observed the process at several places in southern India.
From the mid- 17th century onwards, there are numerous vivid eyewitness accounts of the production of steel by European travellers to the Indian subcontinent. These include accounts by Jean Baptist Tavernier in 1679, Francis Buchanan in 1807, and H.W. Voysey in 1832. The 18th, 19th and early 20th century saw a heady period of European interest in trying to understand the nature and properties of wootz steel. Indian wootz engaged the attention of some of the best-known scientists. One was Michael Faraday who was fascinated by wootz steel. ‘It was probably the investigations of George Pearson in 1795 reported at the Royal Society, which had the most far- reaching impact in terms of kindling interest amongst European scientist in wootz. He was the first of these scientists to publish his results and, incidentally, the first to use the word "wootz" in print.
Another investigator, David Mushet, was able to infer that wootz was made by fusion. David Mushet patented his process in 1800. He made his report in 1805. But the first successful European process had been developed by Benjamin Huntsman some 50 years previously in the 1740s.
Early modern steel in Europe
It was possible to produce quality steel in Europe, by importing the highly valued Swedish iron. Although it was not understood at the time, the Swedish ore contained very low levels of common impurities, leading to higher quality irons and steels from otherwise identical techniques applied to other ores. Swedish bar iron was packed into stone boxes in layers with charcoal in between them and heated in a furnace for an entire week. The result was a bar of metal known as blister steel – the surface of the bars became uneven from a multitude of blisters (or blebs) – which varied in quality from one bar to the next and within each bar. A number of blister rods were then wrapped into a larger bundle and re-heated and hammer-forged to mix together and even out the carbon content, resulting in the final product, shear steel. Germany was particularly well invested in this process, largely due to being physically close to Sweden, and became a major steel exporter in the 18th century. The technique was the cementation process.
English crucible steel
A new technique was developed in England by Benjamin Huntsman, a clockmaker in search of a better steel for clock springs. It was only in 1740 after he moved to Handsworth near Sheffield, and after years of experimenting in secret he perfected his process. Huntsman's system used a coke-fired furnace capable of reaching 1,600 °C, into which ten or twelve clay crucibles, each holding about 15 kg of iron, were placed. When the pots are at a white heat they are charged with blister steel broken into lumps of about ½ kg, and a flux to help remove impurities. The pots are removed after about 3 hours in the furnace, impurities skimmed off, and the molten steel poured into ingots. Sheffield's Abbeydale Industrial Hamlet has preserved a water-wheel powered, scythe-making works dating from Huntsman's times, which is still operated for the public, several times per year using crucible steel made on the Abbeydale site.
Before the introduction of Huntsman's technique, Sheffield produced about 200 tonnes of steel per year based on Swedish wrought iron (see Oregrounds iron). The introduction of Huntsman's technique changed this radically; one hundred years later the amount had risen to over 80,000 tonnes per year – almost half of Europe's total production. This discovery enabled Sheffield to develop from a small township into one of Europe's leading industrial cities.
The steel was cast in a specialised workshop called a 'crucible furnace', which consisted of a workshop at ground level and a subterranean cellar. The furnace buildings varied in size and architectural style, gradually becoming larger towards the latter part of the 19th century as technological developments enabled multiple pots to be fired in one melt and gas was gradually introduced as a means of fuel to heat the crucibles. Each workshop had a series of standard features, such as rows of melting holes, teaming pits, roof vents, rows of shelving for the crucible pots and annealing furnaces to prepare each pot prior to firing. Additional ancillary rooms for the weighing each charge and for the manufacture of the clay crucibles were either attached to the workshop, or located within the cellar complex.
Crucible steel elsewhere
Another form of crucible steel was developed in 1837 by the Russian engineer, Pavel Anosov. His technique relied less on the heating and cooling, and more on the quenching process of rapidly cooling the molten steel when the right crystal structure had formed within. He called his steel bulat; its secret died with him. In the United States crucible steel was pioneered by William Metcalf.
Crucible steels remained the world's best, although very expensive, for some time. The introduction of the Bessemer process and open hearth steelmaking processes gradually replaced it, being able to produce steel of similar (or better) quality on a much larger scale more quickly and cheaply. The Bessemer process took a couple of decades of development until it was able to produce quality steel; however, nitrogen introduced with the blown air causes it to become brittle with age. The other part of the problem was lack of control of the final carbon content, which was finally solved by completely de-carburizing the iron and then adding back an alloy ingot containing the desired amount of carbon plus some manganese to tie up any sulfur. It was also found that iron ores other than hematite could be used by lining the converter with limestone, which reacted with the phosphorus in the iron.
The Bessemer process was eventually displaced by the open hearth process, which although requiring much more time to make a batch of steel was much easier to control for quality. Today's basic oxygen furnaces produce steel at a much faster rate than old style open hearth furnaces.
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- Merv, Turkmenistan
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- Wootz Steel: An advanced material of the ancient world