Though iron only became widely available and workable during the industrial period, it has been in use for millenia. Small items made of iron, dating from around 4000 B.C., were made in Egypt and Sumer . The iron used for these probably came from meteorites, which made the metal significant to ancient people. During the 3rd millenium B.C., smelted iron came into use, mostly for weaponry, across Egypt , Mesopotamia , and the Meditteranean, and around 1200 B.C. it seems to have replaced bronze as the predominantly used metal, though this process took longer in Egypt , perhaps because of the availability of alternative metals there. This was wrought iron, a low-carbon, malleable metal that was painstaking to obtain, by burning iron and charcoal to form bloom, a spongy mixture from which ash and impurities had to be removed by beating and folding. Developments in iron-working continued around the globe during the last few centuries B.C. and into the middle ages. In China , cast and wrought iron were combined to make steel, and iron was also puddled - poured out into troughs and stirred - to remove carbon, rather than the tedious bloomery process. In India , crucible steel was already being made, by heating wrought iron, charcoal, and glass to melt the iron, causing it to absorb carbon. The Middle East also produced high-quality weapons from steel.
However, European metallurgists throughout the Middle Ages continued to expand bloomeries, building large furnaces whose bellows and hammers were powered by water-wheels. The cast iron produced was used mainly for cannonballs. Though blast furnaces, vertical furnaces into which hot air and iron were introduced that allowed slag to be siphoned off sideways, were made in the twelfth century in Sweden, they only came into play in England in 1496, at the commission of Henry VII . In the next two decades, England quickly became a leader in iron production; English cannon were considered superior to those made elsewhere. In 1619 a Dutch ironmaker discovered that English iron contained calcium, probably as residue from large limestone deposits. However, the best iron ore, with the least phosphorus content, was to be found in Sweden .
During this time, many methods of processing the ore into steel were experimented with at the time. in the early 17th century, cementation was used to carburize wrought iron: the iron was packed into stone boxes with charcoal and heated for several days until the iron absorbed the carbon. The scarcity of charcoal led Abraham Darby to smelt iron with coke at Coalbrookdale, though the process generated only inferior coal until it was refined by Darby's son. Along with John Wilkinson, his grandson built the Staffordshire Iron Bridge , the first in the world. Wilkinson also patented a borer that made precise holes in cast iron, suitable for cannon or valves in a steam engine. Known as the "great staffordshire ironmaster", Wilkinson made the first iron barge as well as the iron components of the Parisian waterworks system. In 1783, Henry Cort revisited the "rolling and puddling" process of decarburizing iron, and in the 1740s, Benjamin Huntsman applied the crucible steel process. While this produced strong steel, it was time-consuming and produced small quantities of product, and so remained too expensive for use on a large scale.
This problem was solved by Henry Bessemer of Sheffield , in 1855, with his purchase of William Kelly's patent for and the introduction of the Bessemer converter. Here, molten pig iron was heated in a large crucible while a hot air draft burned off the carbon. The heat given off by this combustion would keep the increasingly pure iron molten until the desired amount of carbon had exited the mixture as carbon dioxide. This process was successful, but was eventually surmounted. When Sir William Siemens invented the regenerative furnace (one in which bricks in the chamber where puddling was taking place heated incoming air) in 1867, he licensed it to Pierre and Emile Martin. The pair created the open-hearth process, which allowed the carbon content to be measured and for the puddling process to be used to effectively and precisely decarburize iron to steel. The process was improved still further when Percy Carlyle Gilchrist and Sidney Gilchrist Thomas lined converters with limestone to remove sulfur and phosphorus (which formed insoluble compounds with the limestone), making it possible to use ores containing these impurities for profitable steel production.
Steel posesses properties that made it the essential medium for constructing the infrastructure of the Industrial Revolution. At a molecular level, the metallic properties of iron make it workable and resistant to tension, while the intermediate amount of carbon added to the steel prevents the metallically-bonded iron atoms, which exist essentially as nuclei in a sea of electrons, to slide past each other and bend the metal. This combination makes steel a strong yet workable compound, without being as brittle as cast iron nor as soft as pig iron. Though in modern times a variety of other processes are used to control carbon content and maximize these qualities, the ability to produce large quantities of durable, malleable metal was intergral to the furthering of industrial production.
The physical landscape of the period was built largely from steel. Machines, which automated production of everything from thread to eventually steel itself were built on frames of this compound. George Fuller's innovative steel-cage system for buildings, which involved a unified steel framework to support the weight of tall buildings, created the multi-story factories which housed these machines as well as the skyscrapers in the cities that surrounded them, and the trains and tracks that brought people there.
“Steel Alloys and their Classifications.” December 2 2005 .
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