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History of ferrous metallurgy
The history of ferrous metallurgy began far back in prehistory, most likely with the use of iron from meteors. The smelting of iron in bloomeries began in the 12th century BC in India, Anatolia or the Caucasus. Iron use, in smelting and forging for tools, appeared in Sub-Saharan Africa by 1200 BC. The use of cast iron was known in the 1st millennium BC. During the medieval period, means were found in Europe of producing wrought iron from cast iron (in this context known as pig iron) using finery forges. For all these processes, charcoal was required as fuel.
Steel (with a smaller carbon content than pig iron but more than wrought iron) was first produced in antiquity. New methods of producing it by carburizing bars of iron in the cementation process were devised in the 17th century AD. In the Industrial Revolution, new methods of producing bar iron without charcoal were devised and these were later applied to produce steel. In the late 1850s, Henry Bessemer invented a new steelmaking process, involving blowing air through molten pig iron, to produce mild steel. This and other 19th century and later processes have led to wrought iron no longer being produced.
Additional recommended knowledge
Because meteorites fall from the sky, some linguists have conjectured that the English word iron (OE īsern), which has cognates in many northern and Western European languages, derives from the Etruscan aisar which means "the gods". Even if this is not the case, the word is likely a loan into pre-Proto-Germanic from Celtic or Italic. Krahe compares Old Irish, Illyrian, Venetic and Messapic forms). The meteoric origin of iron in its first use by humans is also alluded to in the Quran : "and We sent down Iron in which has incredible strength and many benefits for mankind". Iron was in limited use long before it became possible to smelt it. The first signs of iron use come from Ancient Egypt and Sumer, where around 4000 BC small items, such as the tips of spears and ornaments, were being fashioned from iron recovered from meteorites. However, their use appears to be ceremonial, and iron was probably an expensive metal, perhaps more expensive than gold. About 6% of meteorites are composed of an iron-nickel alloy, and iron recovered from meteorite falls allowed ancient peoples to manufacture small numbers of iron artefacts.
In Anatolia, smelted iron was occasionally used for ornamental weapons: an iron-bladed dagger with a bronze hilt has been recovered from a Hattic tomb dating from 2500 BC. Also, the Egyptian ruler Tutankhamun died in 1323 BC and was buried with an iron dagger with a golden hilt. An Ancient Egyptian sword bearing the name of pharaoh Merneptah as well as a battle axe with an iron blade and gold-decorated bronze haft were both found in the excavation of Ugarit (see Ugarit). The early Hittites are known to have bartered iron for silver, at a rate of 40 times the iron's weight, with Assyria.
Meteoric iron was also fashioned into tools in precontact North America. Beginning around the year 1000, the Thule people of Greenland began making harpoons and other edged tools from pieces of the Cape York meteorite. These artefacts were also used as trade goods with other Arctic peoples: tools made from the Cape York meteorite have been found in archaeological sites more than 1000 miles (1600 km) away. When the American polar explorer Robert Peary shipped the largest piece of the meteorite to the American Museum of Natural History in New York City in 1897, it still weighed over 33 tons.
Metallurgy in India
Archaeological sites in India, such as Malhar, Dadupur, Raja Nala Ka Tila and Lahuradewa in present day Uttar Pradesh show iron implements in the period between 1800 BC - 1200 BC. Early iron objects found in India can be dated to 1400 BC by employing the method of radio carbon dating. Spikes, knives, daggers, arrow-heads, bowls, spoons, saucepans, axes, chisels, tongs, door fittings etc. ranging from 600 BC to 200 BC have been discovered from several archaeological sites of India. Some scholars believe that by the early 13th century BC, iron smelting was practiced on a bigger scale in India, suggesting that the date the technology's inception may be placed earlier. In Southern India (present day Mysore) iron appeared as early as 11th to 12th centuries BC; these developments were too early for any significant close contact with the northwest of the country.
The beginning of the 1st millennium BC saw extensive developments in iron metallurgy in India. Technological advancement and mastery of iron metallurgy was achieved during this period of peaceful settlements. The coming years saw several advancements being made to the technology involved in metallurgy during the politically stable Maurya period.
Perhaps as early as 300 BC, although certainly by AD 200, high quality steel was being produced in southern India also by what Europeans would later call the crucible technique. In this system, high-purity wrought iron, charcoal, and glass were mixed in crucible and heated until the iron melted and absorbed the carbon. Iron chain was used in Indian suspension bridges as early as the 4th century.
Wootz steel was produced in India and Sri Lanka from around 300 BC. This early steel-making method employed the use of a wind furnace, blown by the monsoon winds. Also known as Damascus steel, wootz is famous for its durability and ability to hold an edge. It was originally created from a number of different materials including various trace elements. It was essentially a complicated alloy with iron as its main component. Recent studies have suggested that carbon nanotubes were included in its structure, which might explain some of its legendary qualities, though given the technology available at that time, they were probably produced more by chance than by design.
The iron pillar of Delhi, the capital city of India, is one of the world's foremost metallurgical curiosities, standing in the famous Qutb complex. The pillar—almost seven meters high and weighing more than six tonnes—was erected by Chandragupta II Vikramaditya. The pillar is made up of 98% wrought iron of pure quality, and is a testament to the high level of skill achieved by ancient Indian iron smiths in the extraction and processing of iron. It has attracted the attention of archaeologists and metallurgists as it has withstood corrosion for the last 1600 years, despite harsh weather.
Historians of metallurgy hold that Indian iron smelters had acquired an advanced and precise knowledge about the production of iron and steel, and the related details including the thermo-mechanical aspects and heat treatment. The Indians developed wootz which was popular in international markets. The Dutch carried wootz from South India to Europe, where it subsequently spread through mass production.
Will Durant wrote in The Story of Civilization I: Our Oriental Heritage:
About 1500 BC, increasing numbers of smelted iron objects (distinguishable from meteoric iron by the lack of nickel in the product) appear in Mesopotamia, Anatolia, and Egypt.
During the Early Iron Age (12th to 10th centuries BCE) iron came to replace bronze as the dominant metal used for tools and weapons across the Eastern Mediterranean (the Levant, Cyprus, Greece, Crete, Anatolia, and Egypt). Although iron objects are known from the Bronze Age across the Eastern Mediterranean, they occur only sporadically and are statistically insignificant compared to the quantity of bronze objects during this time.
The traditional explanation of the rise of iron was that the Hittites of Anatolia had mastered iron technology during the Late Bronze Age. They maintained a monopoly on ironworking, which allowed them to establish their empire based on iron's superiority over bronze. The invasions of the Sea Peoples at the end of the Late Bronze Age that brought an end to the Hittite empire broke up the monopoly, spreading the technological knowledge throughout the Eastern Mediterranean as a result of their migrations. This theory is no longer held in the mainstream of scholarship. One problem with it is that there is no archaeological evidence that the Hittites held a monopoly on iron during the Bronze Age. While there are some iron objects from Bronze Age Anatolia, the number is comparable to iron objects found in Egypt and other places of the same time period, and only a small number of these objects are weapons.
A more recent theory for the rise of iron has been that the collapse of the empires at the end of the Late Bronze Age disrupted the trade routes necessary for bronze production. Copper and, more importantly, tin were not widely available and needed to be transported over long distances. It is assumed that during the Early Iron Age this was not possible on a scale necessary to satisfy the needs of metalworkers. Since iron ore is more abundant naturally, metalworkers exploited this more universal metal. So, the rise of iron was the result of necessity due principally to the shortage of tin. The problem with this theory is that there is nothing archaeologically that would suggest a bronze or tin shortage in the Early Iron Age. Bronze objects are still abundant and these objects have the same percentage of tin as those from the Late Bronze Age.
Mesopotamia was fully into the Iron Age by 900 BC, central Europe by 800 BC. Egypt, on the other hand, did not experience such a rapid transition from the bronze to iron ages: although Egyptian smiths did produce iron artifacts, bronze remained in widespread use there until after Egypt's conquest by Assyria in 663 BC.
Concurrent with the transition from bronze to iron was the discovery of carburization, which was the process of adding carbon to the irons of the time. Iron was recovered as sponge iron, a mix of iron and slag with some carbon and/or carbide, which was then repeatedly hammered and folded over to free the mass of slag and oxidise out carbon content, so creating the product wrought iron. Wrought iron was very low in carbon content and was not easily hardened by quenching. The people of the Middle East found that a much harder product could be created by the long term heating of a wrought iron object in a bed of charcoal, which was then quenched in water or oil. The resulting product, which had a surface of steel, was harder and less brittle than the bronze it began to replace. Quench-hardening was also known by this time.
Iron smelting at this time was based on the bloomery, a furnace where bellows were used to force air through a pile of iron ore and burning charcoal. The carbon monoxide produced by the charcoal reduced the iron oxides to metallic iron, but the bloomery was not hot enough to melt the iron. Instead, the iron collected in the bottom of the furnace as a spongy mass, or bloom, whose pores were filled with ash and slag. The bloom then had to be reheated to soften the iron and melt the slag, and then repeatedly beaten and folded to force the molten slag out of it. The result of this time-consuming and laborious process was wrought iron, a malleable but fairly soft alloy containing little carbon.
Early Developments in China
Archaeologists and historians debate whether bloomery-based ironworking ever spread to China from the Middle East. Around 500 BC, however, metalworkers in the southern state of Wu developed an iron smelting technology that would not be practiced in Europe until late medieval times. In Wu, iron smelters achieved a temperature of 1130°C, hot enough to be considered a blast furnace which could create cast iron. At this temperature, iron combines with 4.3% carbon and melts. As a liquid, iron can be cast into molds, a method far less laborious than individually forging each piece of iron from a bloom.
Cast iron is rather brittle and unsuitable for striking implements. It can, however, be decarburized to steel or wrought iron by heating it in air for several days. In China, these ironworking methods spread northward, and by 300 BC, iron was the material of choice throughout China for most tools and weapons. A mass grave in Hebei province, dated to the early third century BC, contains several soldiers buried with their weapons and other equipment. The artifacts recovered from this grave are variously made of wrought iron, cast iron, malleabilized cast iron, and quench-hardened steel, with only a few, probably ornamental, bronze weapons.
During the Han Dynasty (202 BC–AD 220), Chinese ironworking achieved a scale and sophistication not reached in the West until the eighteenth century. In the first century, the Han government established ironworking as a state monopoly and built a series of large blast furnaces in Henan province, each capable of producing several tons of iron per day. By this time, Chinese metallurgists had discovered how to puddle molten pig iron, stirring it in the open air until it lost its carbon and became wrought iron. (In Chinese, the process was called chao, literally, stir frying.) By the 1st century BC, Chinese metallurgists had found that wrought iron and cast iron could be melted together to yield an alloy of intermediate carbon content, that is, steel. According to legend, the sword of Liu Bang, the first Han emperor, was made in this fashion. Some texts of the era mention "harmonizing the hard and the soft" in the context of ironworking; the phrase may refer to this process. Also, the ancient city of Wan (Nanyang) from the Han period forward was a major center of the iron and steel industry. Along with their original methods of forging steel, the Chinese had also adopted the production methods of creating Wootz steel, an idea imported from India to China by the 5th century AD.
Chinese Water-Powered Bellows
The Chinese during the ancient Han Dynasty were also the first to apply hydraulic power (ie. a waterwheel) in working the inflatable bellows of the blast furnace. This was recorded in the year 31 AD, an innovation of the engineer Du Shi, Prefect of Nanyang. After Du Shi, Chinese in subsequent dynastic periods continued the use of water power to operate the bellows of the blast furnace. In the 5th century text of the Wu Chang Ji, its author Pi Ling wrote that a planned, artificial lake had been constructed in the Yuan-Jia reign period (424–429) for the sole purpose of powering water wheels aiding the smelting and casting processes of the Chinese iron industry. The 5th century text Shui Jing Zhu mentions the use of rushing river water to power waterwheels, as does the Tang Dynasty geography text of the Yuan-he Jun Xian Tu Chi, written in 814 AD.
In the 11th century, there is evidence of the production of steel in Song China using two techniques: a "berganesque" method that produced inferior, inhomogoneous steel and a precursor to the modern Bessemer process that utilized partial decarbonization via repeated forging under a cold blast.  By the 11th century, there was also a large amount of deforestation in China due to the iron industry's demands for charcoal. However, by this time the Chinese had figured out how to use bituminous coke to replace the use of charcoal, and with this switch in resources many acres of prime timberland in China were spared. This switch in resources from charcoal to coal was later used in Europe by the 17th century.
Although Du Shi was the first to apply water power to bellows in metallurgy, the first drawn and printed illustration of its operation with water power came in 1313 AD, in the Yuan Dynasty era text called the Nong Shu. The text was written by Wang Zhen (fl. 1290-1333 AD), who explained the methods used for the water-powered blast-furnace in previous times and in his era of the 14th century:
There was no fundamental change in the technology of iron production in Europe for many centuries. Iron continued to be made in bloomeries. However there were two separate developments in the Medieval period. One was the application of water power to the bloomery process in various places (as outlined above). The other was the first European production in cast iron.
The first blast furnaces in Europe
Cast iron development lagged in Europe, as the smelters could only achieve temperatures of about 1000 C; or perhaps they did not want hotter temperatures, as they were seeking to produce blooms as a precursor of wrought iron, not cast iron. Through a good portion of the Middle Ages, in Western Europe, iron was thus still being made by the working of iron blooms into wrought iron. Some of the earliest casting of iron in Europe occurred in Sweden, in two sites, Lapphyttan and Vinarhyttan, between 1150 and 1350 CE. Some scholars have speculated the practice followed the Mongols across Russia to these sites, but there is no clear proof of this hypothesis, and it would certainly not explain the pre-mongol datings of many of these iron-production centres. In any event, by the late fourteenth century, a market for cast iron goods began to form, as a demand developed for cast iron cannonballs.
Iron from furnaces such as Lapphyttan was refined into wrought iron by the osmond process. The pig iron from the furnace was melted in front of a blast of air and the droplets caught on a staff (which was spun). This formed a ball of iron, known as an osmond. This was probably a traded commodity by c.1200.
An alternative method of decarburising pig iron seems to have been devised in the region around Namur in the 15th century. This Walloon process spread by the end of that century to the Pay de Bray on the eastern boundary of Normandy before the end of that century, and to then to England, where it became the main method of making wrought iron by 1600. It was introduced to Sweden by Louis de Geer in the early 17th century and was used to make the oregrounds iron favoured by English steelmakers.
A variation on this was the German process. This became the main method of producing bar iron in Sweden.
Steelmaking in early modern Europe
In the early 17th century, ironworkers in Western Europe had found a means (called cementation) to carburize wrought iron. Wrought iron bars and charcoal were packed into stone boxes, then held at a red heat for up to a week. During this time, carbon diffused into the iron, producing a product called cement steel or blister steel (see cementation process). One of the earliest places where this was used in England was at Coalbrookdale, where Sir Basil Brooke had two cementation furnaces (recently excavated). For a time in the 1610s, he owned a patent on the process, but had to surrender this in 1619. He probably used Forest of Dean iron as his raw material, but it was soon found that oregrounds iron was more suitable.
The quality of the steel could be improved by faggoting, producing shear steel. However in the 1740s, Benjamin Huntsman found a means of melting blister steel, made by the cementation process in crucibles; this was cast usually as ingots as crucible steel. This is more homogeneous than blister steel.
Development of Water-powered bloomeries
Sometime in the medieval period, water power was applied to the bloomery process. It is possible that this was at the Cistercian Abbey of Clairvaux as early as 1135, but it was certainly in use in France by the early 13th century there and in Sweden. In England, the first clear documentary evidence for this is the accounts of a forge of the Bishop of Durham, near Bedburn in 1408, but that was certainly not the first such ironworks. In the Furness district of England, powered bloomeries were in use into the beginning of the 18th century, and near Garstang until about 1770.
The Catalan Forge was a variety of powered bloomery. Bloomeries with hot blast were used in upstate New York in the mid 19th century.
The transition to coke in England
Early iron smelting used charcoal as both the heat source and the reducing agent. By the 18th century, the availability of wood for making charcoal was limiting the expansion of iron production, so that England became increasingly dependent for a considerable part of the iron required by its industry, on Sweden (from the mid 17th century) and then from about 1725 also on Russia.
Smelting with coal (or its derivative coke) was a long sought objective. The production of pig iron with coke was probably achieved by Dud Dudley in the 1620s, and with a mixed fuel made from coal and wood again in the 1670s. However this was probably only a technological rather than a commercial success. Shadrach Fox may have smelted iron with coke at Coalbrookdale in Shropshire in the 1690s, but only to make cannon balls and other cast iron products such as shells. However, in the peace after the Nine Years War, there was no demand for these.
Abraham Darby and his successors
In 1707, Abraham Darby patented a method of making cast iron pots. His pots were thinner and hence cheaper than those of his rivals. Needing a larger supply of pig iron he leased the blast furnace at Coalbrookdale in 1709. There, he made iron using coke, thus establishing the first successful business in Europe to do so. His products were all of cast iron, though his immediate successors attempted (with little commercial success) to fine this to bar iron.
Bar iron thus continued normally to be made with charcoal pig iron until the mid 1750s. In 1755 Abraham Darby II (with partners) opened a new coke-using furnace at Horsehay in Shropshire, and this was followed by others. These supplied coke pig iron to finery forges of the traditional kind for the production of bar iron. The reason for the delay remains controversial.
New forge processes
It was only after this that economically viable means of converting pig iron to bar iron began to be devised. A process known as potting and stamping was devised in the 1760s and improved in the 1770s, and seems to have been widely adopted in the West Midlands from about 1785. However, this was largely replaced by Henry Cort's puddling process, patented in 1784, but probably only made to work with grey pig iron in about 1790. These processes permitted the great expansion in the production of iron that constitutes the Industrial Revolution for the iron industry.
In the early 19th century, Hall discovered that the addition of iron oxide to the charge of the puddling furnace caused a violent reaction, in which the pig iron was decarburised, this became known as 'wet puddling'. It was also found possible to produce steel by stopping the puddling process before decarburisation was complete.
The efficiency of the blast furnace was improved by the change to hot blast, patented by James Beaumont Neilson in Scotland in 1828. This further reduced production costs. Within a few decades, the practice was to have a 'stove' as large as the furnace next to it into which the waste gas (containing CO) from the furnace was directed and burnt. The resultant heat was used to preheat the air blown into the furnace.
Apart from some production of puddled steel, English steel continued to be made by the cementation process, sometimes followed by remelting to produce crucible steel. These were batch-based processes whose raw material was bar iron, particularly Swedish oregrounds iron. The problem of mass-producing cheap steel was solved in 1855 by Henry Bessemer, with the introduction of the Bessemer converter at his steelworks in Sheffield, England. (An early converter can still be seen at the city's Kelham Island Museum). In the Bessemer process, molten pig iron from the blast furnace was charged into a large crucible, and then air was blown through the molten iron from below, igniting the dissolved carbon from the coke. As the carbon burned off, the melting point of the mixture increased, but the heat from the burning carbon provided the extra energy needed to keep the mixture molten. After the carbon content in the melt had dropped to the desired level, the air draft was cut off: a typical Bessemer converter could convert a 25-ton batch of pig iron to steel in half an hour.
Finally, the basic oxygen process was introduced at the Voest-Alpine works in 1952; a modification of the basic Bessemer process, it lances oxygen from above the steel (instead of bubbling air from below), reducing the amount of nitrogen uptake into the steel. The basic oxygen process is used in all modern steelworks; the last Bessemer converter in the U.S. was retired in 1968. Furthermore, the last three decades have seen a massive increase in the mini-mill business, where scrap steel only is melted with an electric arc furnace. These mills only produced bar products at first, but have since expanded into flat and heavy products, once the exclusive domain of the integrated steelworks.
Until these 19th century developments, steel was an expensive commodity and only used for a limited number of purposes where a particularly hard or flexible metal was needed, as in the cutting edges of tools and springs. The widespread availability of inexpensive steel powered the second industrial revolution and modern society as we know it. Mild steel ultimately replaced wrought iron for almost all purposes, and wrought iron is not now (or is hardly now) made. With minor exceptions, alloy steels only began to be made in the late 19th century. Stainless steel was only developed on the eve of the First World War and only began to come into widespread use in the 1920s. These alloy steels are all dependent on the wide availability of inexpensive iron and steel and the ability to alloy it at will.
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