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Steel

Steel is a metal alloy whose major component is iron, with carbon being the primary alloying material. Carbon acts as a hardening agent, preventing iron atoms, which are naturally arranged in a lattice, from sliding past one another. Varying the amount of carbon and its distribution in the alloy controls qualities such as the hardness, elasticity, ductility, and tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but is also more brittle. One classical definition is that steels are iron-carbon alloys with up to 1.5 percent carbon by weight; alloys with higher carbon content than this are known as cast iron. Currently there are several classes of steels in which carbon is replaced with other alloying materials, and carbon, if present, is undesired. A more recent definition is that steels are iron-based alloys that can be plastically formed (pounded, rolled, etc.).

Iron and steel

Iron ore pellets for the production of steel. Iron, like most metals, is not found in the Earth's crust in a native state. Iron can be found in the crust only in combination with oxygen or sulfur. Typically Fe2O3the form of iron oxide found as the mineral hematite, and FeS2Pyrite. Iron oxide is a soft sandstone-like material with limited uses on its own. Iron is extracted from ore by removing the oxygen by combining it with a preferred chemical partner such as carbon. This process, known as smelting, was first applied to metals with lower melting points. Copper melts at just over 1000 C, while tin melts around 250 C. Both temperatures could be reached with ancient methods that have been used for at least 6000 years (since the Bronze Age). Since the oxidation rate itself increases rapidly beyond 800 C, it is important that smelting take place in a fairly oxygen-free environment. Unlike copper and tin, liquid iron dissolves carbon quite readily, so that smelting results in an alloy containing too much carbon to be called steel.

Even in the narrow range of concentrations that make up steel, mixtures of carbon and iron can form into a number of different structures, or allotropes, with very different properties; understanding these is essential to making quality steel. At room temperature, the most stable form of iron is the body-centered cubic structure ferrite or -iron, a fairly soft metallic material that can dissolve only a small concentration of carbon (no more than 0.021 wt% at 910 C). Above 910 C ferrite undergoes a phase transition from body-centered cubic to a face-centered cubic configuration, called austenite or -iron, which is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04 wt% carbon at 1146 C). As carbon-rich austenite cools, the mixture attempts to revert to the ferrite phase, resulting in an excess of carbon. One way for carbon to leave the austenite is for cementite to precipitate out of the mix, leaving behind iron that is pure enough to take the form of ferrite, and resulting in a cementite-ferrite mixture. Cementite is a stochiometric phase with the chemical formula of Fe3C. Cementite forms in regions of higher carbon content while other areas revert to ferrite around it. Self-reinforcing patterns often emerge during this process, leading to a patterned layering known as pearlite due to its pearl-like appearance, or the similar but less beautiful bainite
.Perhaps the most important allotrope is martensite, a chemically metastable substance with about four to five times the strength of ferrite. Martensite has a very similar unit cell structure to austenite, and identical chemical composition. As such, it requires extremely little thermal activation energy to form. The heat treatment process for most steels involves heating the alloy until austenite forms, then quenching the hot metal in water or oil, cooling it so rapidly that the transformation to ferrite or perlite does not have time to take place. The transformation into martensite, by contrast, occurs almost immediately, due to a lower activation energy. Martensite has a lower density than austenite, so that the transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, these internal stresses can cause a part to shatter as it cools; at the very least, they cause internal work hardening and other microscopic imperfections.At this point, if its carbon content is high enough to produce a significant concentration of martensite, the metal resembles spring steel: extremely hard, but very brittle. Often, steel undergoes further heat treatment at a lower temperature to destroy some of the martensite (by allowing enough time for cementite, etc., to form) and help settle the internal stresses and defects. This softens the steel, producing a more ductile and fracture-resistant metal. Because time is so critical to the end result, this process is known as tempering, source of the term tempered steel. Other materials are often added to the iron-carbon mixture to tailor the resulting properties. Nickel and manganese in steel add to its tensile strength and make austenite more chemically stable, chromium increases the hardness and melting temperature, and vanadium also increases the hardness while reducing the effects of metal fatigue. Large amounts of chromium and nickel (often 18 and 8 %, respectively) are added to stainless steel so that a hard oxide forms on the metal surface to inhibit corrosion. Tungsten interferes with the formation of cementite, allowing martensite to form with slower quench rates, resulting in high speed steel. On the other hand sulfur, nitrogen, and phosphorus make steel more brittle, so these commonly found elements must be removed from the ore during processing. When iron is smelted from its ore by commercial processes, it contains more carbon than is desirable. To become steel, it must be melted and reprocessed to remove the correct amount of carbon, at which point other elements can be added. Once this liquid is cast into ingots, it usually must be "worked" at high temperature to remove any cracks or poorly mixed regions from the solidification process, and to produce shapes such as plate, sheet, wire, etc. It is then heat-treated to produce a desirable crystal structure, and often "cold worked" to produce the final shape. In modern steelmaking these processes are often combined, with ore going in one end of the assembly line and finished steel coming out the other. These can be streamlined by a deft control of the interaction between work hardening and tempering

History of iron and steelmaking

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 (see Iron: History). 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 artifacts. 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 artifacts 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
.
The name for iron in several ancient languages means "sky metal" or something similar. In distant antiquity, iron was regarded as a precious metal, suitable for royal ornaments.

The Iron Age

Iron axehead from Swedish Iron Age, found at Gotland, Sweden.Beginning between 3000 BC to 2000 BC increasing numbers of smelted iron objects (distinguishable from meteoric iron by their lack of nickel) appear in Anatolia, Egypt and Mesopotamia (see Iron: History). The oldest known samples of iron that appear to have been smelted from iron oxides are small lumps found at copper-smelting sites on the Sinai Peninsula, dated to about 3000 BC. Some iron oxides are effective fluxes for copper smelting; it is possible that small amounts of metallic iron were made as a by-product of copper and bronze production throughout the Bronze Age. 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. The early Hittites are known to have bartered iron for silver, at a rate of 40 times the iron's weigh, with Assyria. Iron did not, however, replace bronze as the chief metal used for weapons and tools for several centuries, despite some attempts. Working iron required more fuel and significantly more labor than working bronze, and the quality of iron produced by early smiths may have been inferior to bronze as a material for tools. Then, between 1200 and 1000 BC, iron tools and weapons displaced bronze ones throughout the near east. This process appears to have begun in the Hittite Empire around 1300 BC, or in Cyprus and southern Greece, where iron artifacts dominate the archaeological record after 1050 BC. Mesopotamia was fully into the Iron Age by 900 BC, central Europe by 800 BC. The reason for this sudden adoption of iron remains a topic of debate among archaeologists. One prominent theory is that warfare and mass migrations beginning around 1200 BC disrupted the regional tin trade, forcing a switch from bronze to iron. 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. 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. Wrought iron can be carburized into a mild steel by holding it in a charcoal fire for prolonged periods of time. By the beginning of the Iron Age, smiths had discovered that iron that was repeatedly reforged produced a higher quality of metal. Quench-hardening was also known by this time. The oldest quench-hardened steel artifact is a knife found on Cyprus at a site dated to 1100 BC.

Developments in China

Archaeologists and historians debate whether bloomery-based ironworking ever spread to China from the West. 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 1130C, hot enough to be considered a blast furnace. 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 BCAD 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.)Also during this time, 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.

India

Perhaps as early as 300 BC, although certainly by AD 200, high quality steel was being produced in southern India by what Europeans would later call the crucible technique. In this system, high-purity wrought iron, charcoal, and glass were mixed in crucibles and heated until the iron melted and absorbed the carbon. The resulting high-carbon steel, called ~HD'/ (puld) in Persian and wootz by later Europeans, was exported throughout much of Asia.

Middle East

By the 9th century, smiths in the Abbasid caliphate had developed techniques for forging wootz to produce steel blades of unusual flexibility and sharpness (Damascus steel). The secret of forging this kind of steel was lost, even in the Middle East, by around 1600, and only recently have metallurgists found methods for reproducing its properties.

Ironworking in medieval Europe

The middle ages in Europe saw the construction of progressively larger bloomeries. By the 8th century, smiths in northern Spain had developed a style that become known as a Catalan forge, a furnace about 1 meter (3 feet) tall, capable of smelting up to 150 kg (350 lb) of iron in each batch. In succeeding centuries, smiths in the Frankish empire and later the Holy Roman Empire scaled up this basic design, increasing the height of the flue to as tall as 5 meters (16 feet) and smelting as much as 350 kg (750 lb) of iron in each batch. These larger furnaces required more draft than could be provided by human power, and forging the large blooms that resulted was also beyond the capabilities of a single man. To this end, waterwheels were employed to power the bellows and hammers. Eventually, the scaling up of the bloomery reached a point where the furnace was hot enough to produce cast iron. Although the brittle cast iron may initially have been a nuisance to the smith, as it was too brittle to be forged, the spread of cannon to Europe in the 1300s provided an application for iron casting, cast iron cannonballs. The oldest known blast furnace in Europe was constructed at Lapphyttan in Sweden, sometime between 1150 and 1350. Other early European blast furnaces were built throughout the Rhine valley: blast furnaces were in operation near Lige (a city in modern-day Belgium) in the 1340s, and at Massevaux in France by 1409.The first English blast furnace was not built until 1496, when Henry VII commissioned a new ironworks at Newcastle, in a part of Sussex known as the Weald. Despite this late start, the production of English iron foundries rapidly grew, in no small part due to foreign craftsmen hired by Henry to bring the craft of iron casting to England. In 1543, William Levett, a Wealden ironmaster, and Peter Baude, a French craftsman in Henry VIII's employ, cast the Weald's first one-piece iron cannon. English iron cannons gained a reputation for being superior to, and less expensive than, the bronze cannons made elsewhere in Europe, and at least initially, efforts to copy them outside the Weald failed. The superiority of English cannons over Spanish ones has been credited as one factor in England's 1588 defeat of the Spanish Armada. In 1619, Jan Andries Moerbeck, a Dutch ironmaster, began importing Wealden iron ore for comparison to the ore available on the Continent. One difference he observed was that the English ore contained some calcareous material, and soon after, Dutch ironmasters introduced the use of limestone as a flux in the blast furnace. This practice improved the separation of slag from the cast iron and improved the quality of Continental cast iron.

Ironworking in early modern Europe

Also by the early 1600s, ironworkers in western Europe had found a means (called cementation) to carburize wrought iron without individually forging each piece. 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.For many years the best steels could be produced by buying expensive iron ore from Sweden. Although it was not understood at the time, Swedish ore had very low phosphorus content compared to most ores (notably those in England), which allowed for a finer and stronger crystal structure. Sales of Swedish ore generated considerable trade income, and local development helped the country become the industrialised nation it remains to this day.By the 18th century, deforestation in western Europe was making ironworking and its charcoal-hungry processes increasingly expensive. In 1709 Abraham Darby began smelting iron using coke, a refined coal product, in place of charcoal at his ironworks at Coalbrookdale in England. Although coke could be produced less expensively than charcoal, coke-fired iron was initially of inferior quality compared to charcoal-fired iron. It was not until the 1750s, when Darby's son refined the coking process to reduce the amount of sulfur in the coke that coke-fired furnaces became widespread. Another 18th-century European development was the reinvention of the puddling furnace. In particular, the form of coal-fired puddling furnace developed by the British engineer Henry Cort in 1784 made it possible to convert cast iron into wrought iron in large batches, finally rendering the ancient bloomery obsolete. Wrought iron produced using this method became a major metal in the English midlands' emerging toy industry. The combination of the blast furnace and the puddling furnace allowed iron to be produced at either end of the carbon spectrum, depending on the user's needs. As for alloys of intermediate carbon content (that is, steel), crucible steel was rediscovered in the 1740s by Benjamin Huntsman in Handsworth in England. In his process, wrought iron and cast iron were heated in small ceramic crucibles, melting together to form steel. While producing steel superior to cement steel, the crucible steel process remained relatively expensive in both time and fuel, and could not be used in any sort of modern industrial scale. The strong steels produced were however in high demand for specialty products such as cutlery and weapons. Sheffield's Abbeydale Industrial Hamlet has preserved a waterwheel powered, scythe-making works dating from Huntsman's times. It is still operated for the public, several times per year, using crucible steel made on the Abbeydale site.

Industrial steelmaking

The problem of mass-producing 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. 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.In 1867, the German-British engineer Sir William Siemens introduced an improved puddling furnace  the regenerative furnace  that used brick heat exchangers to preheat the incoming air and conserve fuel. The next year Pierre and Mile Martin, French ironmasters who had licensed Siemens' furnace design, developed a method for measuring the carbon content of molten iron. Thus, the decarburization could be stopped at the steel stage rather than proceeding all the way to wrought iron. This open-hearth process coexisted in industrial practice with the Bessemer process for many years, but eventually proved more economical and displaced it. Reasons for this include its ability to recycle scrap metal in addition to fresh pig iron, its greater scalability (up to hundreds of tons per batch, compared to tens of tons for the Bessemer process), and the more precise quality control it permitted.

A huge vat of molten steel being poured into an open hearth furnace at the Jones and Laughlin Steel Company, Pittsburgh, May 1942.Initially, only ores low in phosphorus and sulfur could be used for quality steelmaking; ores rich in those elements yielded brittle metals little better than cast iron. This problem was solved in 1878 by Percy Carlyle Gilchrist and his cousin Sidney Gilchrist Thomas at the ironworks at Blaenavon in Wales. Their modified Bessemer process used a converter lined with limestone or dolomite, and additional lime was added to the molten metal as a flux. This added basic material removed phosphorus and sulfur from the steel as insoluble calcium or magnesium phosphates and sulfates. This development expanded the range of iron ores that could be used to make steel, especially in France and Germany, where high-phosphorus ores abounded. These developments increased the availability and decreased the price of steel; 22-thousand tonnes were produced in 1867, 500-thousand in 1870, 1-million in 1880 and 28-million by 1900. Today, worldwide annual production is around 850-million tonnes. This widespread availability of inexpensive steel powered the industrial revolution and modern society as we know it. It also led to the introduction of newer "niche" steels (such as stainless steel), all of them dependent on the wide availability of inexpensive iron and steel and the ability to alloy it at will.

Types of steel

Alloy steels were known from antiquity, being nickel-rich iron from meteorites hot-worked into useful products. In a modern sense, alloy steels have been made since the invention of furnaces capable of melting iron, into which other metals could be thrown and mixed. Carbon steel Damascus steel, which was famous in ancient times for its flexibility, was created from a number of different materials (some only in traces), essentially a complicated alloy with iron as main component. Stainless steels and surgical stainless steels contain a minimum of 10.5% chromium, often combined with nickel, to resist corrosion (rust). Some stainless steels are nonmagnetic. Tool steels HSLA Steel
 (High Strength, Low Alloy)  Advanced High Strength Steels Ferrous superalloys Though not an alloy, there exists also galvanized steel, which is steel that has gone through the chemical process of being hot-dipped or electroplated in zinc for protection against rust. In order to make steel not accidentally, but conscientiously, you obviously first need to make iron. In contrast to the noble metals like gold, silver or platinum (and the occasional find of pure copper), iron is never found as an element but practically always as an oxide. 
 
    However, in contrast to other metals found as oxides (especially Cu and Sn oxides needed to make bronze), the temperature of a "normal" fire is not sufficient to reduce iron oxide and to make the elemental iron liquid - the melting point of iron is Tm(Fe) = 1535 0C; far above the (1000 - 1100) 0C that the ancients could produce
.   

    For Copper (Cu), e.g., it is different - its melting point is Tm(Cu) = 1083 0C. Throw some copper minerals in a nice hot fire made with plenty of charcoal (producing CO which is great for reducing oxides), and liquid copper will result almost automatically.    

    This happened and was noticed probably a good 6000 years ago, when early potters tried to adore their pottery with nice green malachite - a copper mineral known in antiquity and used as a gem stone. What a surprise, when one day in a particularly hot fire, instead of decorated pots they found an ingot of pure - and then extremely precious - copper in their oven. Copper was otherwise only found in small quantities (much less frequent then the (then) ubiquitous gold) in mountain ranges and river beds. This was a decisive discovery for mankind: Precious and shiny metals could be made from dull stones. Things could be changed from one, seemingly immutable form into a completely different one - alchemy has its roots right here, and the yearning for "transmogrification" has never stopped since.   

    Early metal industry and the short-lived "copper age" began to be replaced rather soon by the bronze age (Cu + (5 - 10)% Sn and often some As); and the bronze age lasted more than 2000 years (it was not abruptly replaced by the iron age, but coexisted for about 1000 years). 

Up to the 16th century, Schleswig-Holstein was woodland. Then the trees were felled to produce charcoal (among other things). How that is done will be demonstrated by Stefan Brocke in the Loher woods.

    Here we first encounter the importance of impurities: A little bit of As an impurity atom makes bronze "harder", it doesn't deform so easily any more. Of course, nobody knew this. All that was probably known was that some sources of copper and tin ore, together with all kinds of tricks (including some magic or prayers, of course) produced superior bronze.    

    It is quite natural that tin and other metals were discovered shortly after the momentous discovery of copper smelting. Once you saw that precious copper could be made from some kind of rock, everybody not completely stupid would of course try what you could get with other rocks.   

    We also have the beginnings of an environmental disaster, because for metal smelting you need tremendous quantities of charcoal. First to obtain high temperatures, but, just as important, for reducing the metal oxide according to  MeO + C  Me + CO
                              
    About 100 kg charcoal are needed to smelt 5 kg of copper.   

    Besides shipbuilding, charcoal production is responsible for the disappearance of large parts of European forests (the disappearance of yew trees (which were ubiquitous in antiquity) from present day forests, by the way, is due to the middle age bow-and-arrow industry - nothing beats a yew bow!). Charcoal production was a major industry and the source of the many charcoaler ("Khler") stories in fairy tales and folklore.   

    Beside Cu and Sn, Pb, Hg, Ag, and of course Au, were known and produced on an industrial scale - especially by the romans. But the romans (and the Chinese, and the Indians, and the ...) had also Fe - but still no fire hot enough to melt it.   

    Early experience with the smelting and melting of other metals did not help in producing iron - it first came into use about 1000 years later than bronze. This must have been a kind of puzzle, because the ancients did know that iron existed. It was extremely rare and precious - because it fell from the sky in exceedingly small quantities.   

    King Tut, matter of fact, had a little iron dagger made from meteorite iron right on his breast - obviously his most precious object. In old Sumeria, iron was called "sky metal" and the pharaohs in old Egypt knew it as "black copper from the sky".   

    The Eskimos in Greenland, matter of fact, made their iron tools for hundred of years from a large (30 tons) meteorite.     

    Some American explorer (Admiral R. Peary) finally stole it (he wouldn't have expressed it that way, though) in the 1890s and had a hard time to transport it to the Natural History Museum in New York. Here it is: 

    So you do not get liquid iron - but you do get solid iron because reduction does take place - in a solid state reaction. What you get is an iron bloom ("Eisenblte" in German), a mixture of fine iron particles, unreacted iron oxide, slag and charcoal residue. Here is an actual picture of some ancient bloom (from around 600 AD; I actually "found" this myself (in some museum).

    The iron in the bloom was rather pure (and thus comparatively soft) because a solid state reaction produces only iron - carbon or other impurities have to diffuse in from the outside (if the iron would be liquid, it would just dissolve the dirt up to the solubility limit).  
 
    The early iron smiths (probably being Hethites of some form) could "wring" the iron from this bloom by separating the iron from the rest mechanically and repeatedly hammering together what was left at high temperatures (about 800 oC; some of the slag then is liquid and gets squeezed out) with, no doubt, proper prayers to the respective gods and many (magical) tricks.    

    What they finally obtained was "wrought iron" ("Schmiedeeisen"), i.e. a lump of rather pure iron consisting of small pieces welded together, with plenty of small inclusions (small, because of the hammering that breaks up large pieces of slag).   

    Extreme care was necessary - from the selection of the iron ore, the reduction process and the hammering business. If you were careless, the iron oxidized again (it really "burns" at temperatures in excess of about 800 oC), and if you kept your reduction process going too long, carbon diffuses in and you may end up with cast iron (C content about 3% - 4%; melting point as low as 1130 0C). Then you actually got it liquid - "casting" was possible - but cast iron is brittle and useless (for weapons, that is).   

    Somewhat later, with larger furnaces and increased experience, the bloom obtained may have contained some high-carbon melted parts on its top layer. It then consisted of a whole range of iron-carbon alloys - from rather pure wrought iron to cast iron with good steel - say 0,5 % - 1,5% carbon - in between. The art of the smith than included to pick the right pieces. This was a highly developed skill, we know about it especially from
Japan; but that does not mean that the Kelts or others did not do it just as well. But beware. The art of making iron and steel, developed over 2000 years in many civilizations, cannot be contained in a few lines, not to mention that very little is known about that story - iron, after all, rusts, and not much has been found that gives detailed knowledge about how the old romans, Indian, Chinese, etc. made their steel and iron products.   

    Nevertheless - the early smiths, starting with the Greek god Hephaistos (the roman Volcanos) and containing many fabulous figures like the Nordic "Wieland the smith" or "Mime" in Wagners "Ring des Nibelungen", could produce articles, especially swords, from the iron bloom that were much better than the customary bronze stuff (and than of course "Magical" swords). In other words, they sometimes succeeded in making good steel. What was their secret? It is rather simple - looking at it retrospectively: You need the proper concentration of C in the Fe bcc lattice at room temperature (some other impurities are helpful, too; while others - especially S and P - were harmful). Raising the about 0,1% C in wrought iron to an optimal 0,7 -0,9%, raised the hardness
 (or better the yield point) threefold! But if you got too much - say 2% - you were on the road to brittle cast iron not useful for swords.   

    Not being able too melt iron (and thus not being able to throw some magical stuff into the brew) the only way to get carbon (or on occasion N which also "works") into the Fe lattice was diffusion via the surface. What you needed to do was to "roast" you iron (possibly the whole sword) for the right time at the right temperature in a charcoal fire. Magic and praying helped - it did indeed: How do you keep track of the time without a watch? You utter a long prayer that you learned from your master - the right ones "worked"! The rest of the magical ritual was helpful in providing reproducible conditions.   

    Of course the old practitioners had no idea of what the really were doing; if they thought about it, they felt that were purifying the iron in the (more or less holy) fire. This erroneous believe (like so many others) goes back to the (from a materials science point of view somewhat questionable) philosopher Aristoteles who certainly asked the right questions about life the universe and so on, and is righteously famous for that. His answers, however, were invariably wrong - even in the few instances where he could have known better.   
    Well, we have made but the first step to steel. We now must make a few more steps for good homogeneous steel - or we delve into a fascinating world of its own, the various damascene techniques
, one of which is blending different kinds of steel into a compound material. More to that in the link.   
    Here we look first a bit on what happens in heating up and cooling down your material. We know, after all, that going up in temperature, iron changes at 910 0C from the bcc ferrite phase to the fcc austenite phase.   
    Carbon feels much more at home in austenite - its solubility
 is higher than in ferrite. If the smith kept his iron in a good fire very long, he now might have had a rather carbon rich austenite in the outer layers of his sword. So what happens upon cooling down?   

    Well, it depends. If the iron cools down s l o w l y, the carbon rich austenite will change to carbon rich ferrite. If there is more carbon in the austenite than the ferrite can dissolve, carbon will precipitate, forming a new Fe - C phase called cementite (with a quite complicated lattice). We now have cementite particles in fcc ferrite; usually in a very typical structure - both phases appear like a stack of plates. This kind of structure is called perlite because, looking at it under a microscope, it has a luster like pearls..   
    Perlite, the mixture of ferrite and cementite, however, is not much better than bronze as far as its mechanical properties are concerned. So you must prevent the phase change from austenite to perlite if you want to keep your sword "magic"! In other word, you must not allow enough time for the carbon atoms to diffuse around during cooling as would be necessary for forming precipitates. In other words: You must cool down rapidly (hopefully you did the proper exercise
 for calculating how fast you must cool down).   
    Here we have the next big trick - after making bloom, extracting wrought iron, and carburization: Quenching - often the big secret of master smiths (there is a whole Japanese mythology to this subject). The hot sword is stuck in a liquid for some time and thus quenched - and only very unimaginative smiths would have taken common water at room temperature for that.   

    If the cooling time was too short to allow Fe-C precipitate formation, we now have a supersaturation of C in the ferrite phase which then will have a strongly disturbed lattice structure. A kind of mixture between fcc and bcc phases will prevail which has its own name: "Martensite".    

    Now you did it: Martensite has the fivefold "strength" of wrought iron!   

    Unfortunately - if you got martensite at all, it tends to be brittle! Now the next bag of tricks is needed: Heat up your sword again - but keep the temperature moderate.    

    Some of the defects that make martensite brittle anneal out and its ductility goes up. Bang it (i.e. deform it plastically), and you produce dislocations (hey, that's were we started from some time back!). Now you are manipulating a second kind of defect for optimizing mechanical properties!   
    But now we stop (so does the smith)..   

    Anyway, if everything worked, you now have a very good (and of course magical) sword which was far superior to the bronze stuff of your opponents. In particular, you could make it longer without having to worry that it might break in battle (which was about the worst health hazard imaginable then).        And don't think that an increase in strength by a factor of 4 - 5 is not all that much. The old Gauls, Asterix and Obelix notwithstanding, were conquered by the Romans not least because their swords bent and needed straightening (over your knee) after a forceful blow - something the Roman swords did not need. (don't you believe all this roman propaganda!)   

    Well, making a good steel sword was lots of work, lots of knowledge, and lots of luck. Considering what could go wrong, it is quite remarkable that the old smiths actually did produce superior steel swords now and then. Of course, probably more often then not, only the outer layer was steel, while the inside was still soft wrought iron - the sword was made from compound materials, in fact.   

    This gives us (and possibly also the old smithies) the idea of doing that from the start: Weld together soft and hard layers, carefully picked from the bloom or made by carburization, and hope that the result will combine the positive properties of both materials. We are talking damascene techniques here.   

    However, the word "damascene techniques" is a collective identifier of several very different technologies. Most people associate it with a kind of compound technology where two different kinds of steel were put together in layers and then forged into a sword or whatever. While this is something that was done - especially by the Kelts and other North Europeans - it was not what the guys in Damascus did, the purported source of the famous damascene blades.   

    As far as we know today, the "true" damascene technique actually worked with a famous kind of steel, so called "wootz" which was produced in India for maybe a 1000 years in a kind of closely guarded monopoly. Wootz was rich in carbon (about 2%; there was a secret carburization technique) and the trick was to precipitate the surplus carbon in a pattern of fine FeC3 precipitates.    
    A fascinating world unfolds behind the catch word "damascene technique", if you like you can browse the following links   
   
    Steel technology was not confined to the Mediterranean and the European North West. India may well have been at the apex of steel technology and China had its own technology centered around cast iron, used not so much for warfare but for civil objects like pots and pans.   

    And lets not forget the Haya, a people who lived in what is now Tanzania. They had a highly developed Fe technology and used it for beautiful sculptures, too. Their myths and fairy tales contain many stories relating to the making of iron, using a vocabulary that was heartily enriched with expressions relating to the making of humans.
   
    There is even some evidence - collected recently, that the old Africans had the highest temperatures of all, even reaching the melting point of iron some 2000 years ago (long before everybody else did)   
    Whatever happened whenever and wherever, during the millennia, and despite the many difficulties, iron and steel became common materials. At some time in the middle ages or Renaissance, the melting temperature could be reached, but the mass production of good steel still had to wait for the 19th century. Before, only "thin" objects - the paradigmatic "sword" or scimitar, saif, shamshir, tachi, tulwar, yatagan,.. - could be made by in-diffusion of carbon.   

    Charcoal was replaced in the 17th century with coal, but not without unpleasant surprises. Iron that was smelted with coal instead of charcoal was very brittle and completely useless. We now know, of course, that minute amounts of sulfur in the Fe lattice - it segregates in grain boundaries - are sufficient to make Fe brittle, and S, like other harmful impurities, is contained in regular coal in rather large concentrations.   

    The solution to this problem, surprisingly, did not come from the military related strata of society, but from the second most important enterprise dear to the hearts of men: beer brewing. Brewers had tried to use coal instead of charcoal for roasting the barley - and produced a stinking abominable brew. Thusly coke was invented: Roast coal in an environment deprived of oxygen - the stinky stuff will evaporate and what remains is clean carbon - called coke - which could not only be used to brew beer, but was also usable for the iron smelting industry.   
    The beginning of the industrial revolution was severely hampered by the lack of a large-scale process for the production of good steel. (Just imagine how the Si revolution would have fared without large dislocation free and rather perfect Si crystals). The (at least in German and French) paradigmatic Eisenbahn (chemin de fer in French), the rail road, needs rails; with regular wrought iron or cast iron the rails had to be renewed every 6 month because they deformed under the load (or cracked). Accidents were frequent and often catastrophic.   

    The production of large amounts of iron was common by then - the essential part was blowing large amounts of air into the fire with the aid of mechanical bellows powered by steam engines. The leading British production accounted for 2,5 million tons of iron in 1850, but the production of steel was still a cumbersome and expensive business, accounting for a few percent of the total production.   
    It was also known for sure since 1786 that steel had something to do with carbon; the first person suspecting this was one Tobern Bergmann
 in 1774 (other sources, however, refer to Vandemonte, Berthollet and Monge from France).    

    Still, all efforts to produce iron with the proper carbon content (and the right structure) "from scratch", were in vain. Sometimes things worked, sometimes they didn't - there was no large-scale, reliable, and reproducible process. And thus no big bridges, sky scrapers, safe railroads, big ships, efficient engines, and so on - one rarely reflects how much cheap steel changed the world!   
    This time, however, progress came from the military industrial complex. It became simply too embarrassing that the big canons (made from cast iron) had a tendency to explode. Something had to happen.   

    It was Henry Bessemer who was especially interested in good steel for big canons, because he had just invented a new kind of projectile that received some spin even from smooth bore guns (and thus was harder to destabilize during flight). Unfortunately, the canons couldn't take the additional pressure building up while the projectile was building up spin as well as speed- they exploded more than ever. So Bessemer was looking for large amounts of cheap steel.    
    He was then the first person (so it was believed for a while) who had the genius idea of making steel by getting carbon out of cheap, carbon rich cast iron, instead of using the cumbersome way of getting carbon into low-carbon wrought iron. The way to "drive out" the surplus carbon was to blast large amount of oxygen through the cast iron melt (which, by the way, definitely needed the steam engine; quite hard to do this through a reed). CO will form in the melt which not only burns off to CO2 upon hitting the air, but by doing this supplies the heat to increase the temperature of the melt because the melting point will go up with decreasing carbon content. If you stop at the right time, you will be able to adjust the carbon content of a large amount of iron to just the right value and thus produce large amounts of good steel.   
    Mr. Bessemer, who was not exactly unknown before (he already had some fame as the inventor of the "lead" pencil (which in reality contains graphite), after publishing his finding on Aug. 12th, 1856 became very famous - and very rich - quickly; everybody wanted his process. The London Times went as far as printing the whole paper two days later.   

    But point defects were fighting back. The industrial realization of the Bessemer process with large quantities of ore and coke yielded a big and very unpleasant surprise: Bessemer steel from large size production, in contrast to the Bessemer steel from "laboratory" experiments, was brittle and not fit for anything. Bessemer felt like "being hit by a flash of lightning from the blue sky"; the descend from the Olympic heights of top inventors to desperation was quick and brutal.   

    But Bessemer was a good materials scientist and engineer; if it worked once, it must work again. There must be reasons for what happened, and with diligence, one can find out what is going wrong. What had happened?   

    Well, Bessemers work, and the work of many others, supplied the (here much simplified) answer. Bessemer used Swedish iron ore for his experiments (you always use the best in lab experiments), while his industrial country fellows used English ore - and this stuff contained some phosphorous. The Bessemer process (possibly in contrast to the old-fashioned steel making process) did not remove the phosphorous, and small amounts of P are sufficient to render steel brittle. As we know now, P segregates in the grain boundaries and changes the local properties in a detrimental way.   

    Phosphorous had to be removed (if you lived in merry old England, out on a conquest to assemble an empire, you did not want to have your steel production depend on the supply of Swedish iron ore). Two cousins, Sydney Gilchrist Thomas and Percy Carlyle Gilchrist, found the way in 1875: Take (among other things) chalk stone for the lining of the Bessemer converter and even add some to the melt. The phosphorus would react with the CaO of the burnt chalk and end up in the slag which could be skinned form the liquid steel, or stuck to the lining.   

    There were plenty of other problems - on occasion, e.g., some oxygen remained in the steel and rendered it useless. Mr. Mushet, another Englishman coming to the add of his country, found the solution: Add some "Spiegeleisen" (an iron - manganese alloy found somewhere in Germany) and your problems are gone. The Mn reacts with the surplus O and forms slag. It also neutrlizes any sulfur n the miy, which would otherwise create real trouble.   
    So besides Bessemer, many people were involved in bringing large scale steel production to fruition. And, as it practically always will turn out with great inventions, somebody else did it before. In this case it was one Mr. Kelly from the USA, who had the "Bessemer" idea 10 years before Bessemer himself. While he made a mint over patent hassles, the name Bessemer remains attached to steel, and Kelly is quite forgotten as a materials scientist.   

    After the Bessemer process was sufficiently debugged, steel production took off and became supremely important strategically.    

    Siemens in Germany and Martin in France developed the "Siemens-Martin process" and so on and so forth. The world production of steel grew exponentially (like chips today): 22 kto in 1867, 500 kto in 1870, 1 Mto in 1880 and 28 Mto around the turn of the century. Today we are in excess of 500 Mto a year.   

    In 1970 politicians generally still believed, that the wealth of a nation (and thus its power to subdue others) was directly coupled to its steel production (and thus to the degree of the nations prowess in manipulating point defects in Fe).   
    You may feel now that we are talking chemistry here, and the typical urge of the chemist to produce pure substances. Nothing could be farther from the truth. We are exclusively discussing the dramatic influence of point defects on certain properties of a crystal lattice, like its resistance to the generation and movement of dislocations.         .   

    Polybius was the guy who wrote about those bending swords of the gauls. The gauls as all other celts, unfortunately, did not write anything.

    That the swords of the gauls / celts were inferior to those of the romans is about as believable as the wepaons of mass destruction in Iraq 2000 years later: It was and is propaganda, stupid!   

    It probably was the other way around. The celtish long sword made from damascene steel was far superior to the roman short sword, and eventually (around 300 AD) was adopted as the roman "spatha".   

    One is tempted to generalize: maybe the famous roman technology was mostly adopted from other folks? Be that as it may, the way the Romans used technology - based on discipline, organization and large-scale production - was unprecedented and instrumental in conquering most everybody.   

     Steel is used in organbuilding for making structural elements such as bracing and beams.

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