Steel framework

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 the qualities 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 5.1 percent carbon; ironically, alloys with higher carbon content than this are known as 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, like most metals, is not found in the Earth's crust in a native state. Since the rise of the cyanobacteria and their excretion of oxygen into the atmosphere, iron can be found only in oxide form, typically Fe₂O₃— the form of iron oxide found as the mineral hematite. 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 and tin both melt at just over 1000 °C, temperatures that could be reached with ancient methods that have been in use 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. Relatively pure iron at room temperature will tend to form the body-centered cubic ferrite form, which is fairly soft. At about 910 °C ferrite will transition to the denser, face-centered cubic austenite phase, which has considerably higher carbon solubility but is similarly soft and metallic. 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 stochiometeric 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 perlite 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. Martinsite 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, producign 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 in steel adds to the tensile strength and makes austenite more chemically stable, chromium increases the hardness, 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 re-processed 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. 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. 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 Egyptian ruler Tutankhamun died in 1323 BC and was buried with an iron dagger with a golden hilt. A battle axe with an iron blade and a gold-decorated bronze haft found in the excavation of Ugarit has been dated to about 1400 BC. The early Hittites sold iron to Assyria for 40 times its weight in silver.

Meteoric iron was also fashioned into tools in pre-contact 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 iron age

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 byproduct 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.

Iron did not, however, replace bronze as the chief metal used for weapons and tools for several centuries. 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 Cyprus and southern Greece, where iron artifacts dominate the archaeological record after 1050 BC. Mesopotamia was fully in the iron age by 900 BC, central Europe by 800 BC. The reason for this suden 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 to iron. Egypt, on the other hand, did not experience such a rapid transition between the iron and bronze 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 air is forced through a pile of iron ore and burning charcoal. The carbon monoxide produced by the charcoal reduces iron oxides to metallic iron, but the bloomery is not enough to melt iron. Instead, the iron collects in the bottom of the furnace as a spongy mass, or bloom, of metallic iron mixed with slag. The bloom was then reheated to soften the iron and melt the slag, after which the slag could worked out of the iron by repeatedly beating and folding it. The result of this time-consuming process was wrought iron, a malleable but fairly soft form of iron containing very little carbon.

This process resulted in a low-carbon steel we today refer to as wrought iron—so named due to the process of mechanically working the bloom. The vast majority of steel and iron up to and including the middle ages was in this form, meaning that a considerable amount of "working" needed to be done in order to produce metals suitable for weapons. With proper working conditions it was possible to allow almost no carbon to enter the mix, and if the carbon content was kept to about 0.1% the result was a useful metal. However if the carbon was allowed to mix into the iron, it would become brittle at around 3–4% carbon, a steel we know as cast iron because it is too brittle to work and can only be cast. Wrought iron and cast iron represent two extremes in carbon content, with "perfect" mixtures having about 1.5% carbon.

For some time the best steel implements were built by taking blocks of varying carbon content and hammering them together to make a single block of intermediate content. This process, today known as pattern welding, was widespread in Europe by about 500 AD although the secret was lost during the dark ages.

The first repeatably produced high quality steels were produced in India using a process known as the crucible technique. In this system ingots of bloom are broken into chunks and then heated in crucibles for long periods of time. Carbon leaks through the walls of the container into the iron mix inside, leaving the outer layers with higher carbon content than the middle. The resulting material could be worked to mix it together into a single batch of steel with an average carbon content close to optimal. This form of steel was traded throughout the middle east, known as pulad or wootz, and was also produced at a number of sites in Turkmenistan in later years. Pattern welding of wootz was widely used, although it appears other techniques were also used on wootz to develop Damascus steel, which also shows signs of pattern welding but most likely is based on an entirely different process.

For much of the world the crucible technique remained unknown, and the only way to achieve the required carbon content was to make thin pieces and allow it to diffuse in during heating. This allowed steel to be widely used in bladed weapons and smaller pieces such as arrowheads, but in general larger pieces were not possible. Steel also remained very expensive, requiring massive amounts of fuel (about 100 kg of charcoal for every 1 kg of iron) and long times to produce a quality product.

By the early 17th century blowers had improved to the point where European ironsmiths could directly smelt iron ore in the liquid state. Their methods typically used a bowl-shaped furnace into which the iron ore was piled and then covered with a thin layer of charcoal. Blowers would then ignite the charcoal and melt the ore. The iron oxide in the ore would react with the nearly pure-carbon charcoal to form metallic iron and carbon dioxide or carbon monoxide, which would be blown off. The result was high-quality iron without the need for heavy mechanical processing needed for irons smelted at lower temperatures.

Quenching, another poorly understood method, also became common during the middle ages. In Japan it evolved into a whole mythology that was carefully guarded by the master swordsmiths. For several centuries Japanese pattern welded steels were the best in the world, using manual processing and attention to detail that could not be bettered by automated processes until the 20th century. Quenching did become common to the point of being used universally by blacksmiths during the 17th century and later, who would repeatedly heat and quench their irons while adding carbon by placing the working material directly in a charcoal bed. Although the quality of such steels was not very repeatable, the methods could produce one-off batches of excellent quality. This knowledge of steel enabled swordsmiths to become gunsmiths and mass produce firelock rifles. At the beginning of the Edo period, the number of rifles were estimated to be over 100,000.

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 became the industrial powerhouse it remains to this day. Swedish ore would be packed into stone boxes and heated for up to a week, slowly taking in carbon in a fashion very similar to wootz.

The introduction of steel as a common building material led from several key inventions in the 17th and 18th century, primarily in England. In 1709 Abraham Darby upset the ironmaking world with the introduction of a refined blast furnace. Unlike the bloom methods, his blast furnace mixed coke, a form of coal, with the iron ore in huge furnaces and smelted a huge batch at once. Coal had been tried as a smelting fuel on a number of occasions, but invariably produced a very brittle metal. The local brewing industry had similar problems, in that the coal gave off gases that resulted in a smelly, unappetizing beer. However they found that heating the coal in an oxygen-free environment led to a fuel that was not "smelly", which they called coke. Coke proved to be just as useful for smelting iron, causing a minor revolution in the industry. It was later discovered that it was the sulfur content of common coal that led to both the smell and the poor quality iron, cooking it released it to the air.

The blast furnace dramatically reduced the price of iron, not only because coal was less expensive than charcoal, but also because the furnaces were much larger and produced larger batches. On the other hand, the direct mixing of the coke with the ore meant that their product, pig iron, had much higher carbon content and was in the class of cast irons. In order to be processed using existing methods, the pig iron first had to be converted to a lower-carbon form.

At about the same time a new technique called the pudding furnace led to the introduction of large quantities of high-quality wrought iron. In this system workers would forcibly stir the molten pig iron produced from the smelters, with portions with lower carbon content sticking to their "rabbing bar", which could then be removed. Wrought iron produced using this method became a major metal in the English midlands emerging toy industry. However the pudding furnace shared one problem with earlier methods, it remained slow, manually intensive, and costly in terms of fuel.

The introduction of the steam engine to this process, powering massive blowers and hammers, allowed England to take the lead in iron production in the 19th century. England's steel industry, centered in Sheffield, led the world in production until the middle of the 20th century. The combination of the blast furnace and the pudding furnace allowed irons to be produced at either end of the carbon spectrum, depending on the user's needs.

Crucible steels were independently rediscovered in England in the 18th century by Benjamin Huntsman at his workshop in Handsworth, Sheffield. In his process, the wrought iron from the pudding furnaces was re-heated a dozen crucibles at a time. After reaching a high temperature, a small amount of pig iron was added, the "blister", whose high carbon content then mixed with the lower carbon wrought iron to form steel. The crucible steel process remained a relatively expensive technique in both time and fuel, and could not be used in any sort of modern industrial scale, although the strong steels produced were in high demand for specialty products such as cutlery and weapons. Sheffield's Abbeydale Industrial Hamlet has preserved a water-wheel 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.

This problem of mass producing steel was finally solved by Henry Bessemer with the introduction of the Bessemer Converter at his steelworks in Sheffield (an early example of which can still be seen at the city's Kelham Island Museum). Similar to the blast furnace in basic construction, the converter started with the pig iron from the blast furnace, which still contained considerable amounts of carbon. In the converter, the temperature was carefully controlled until the iron was just above the melting point, and then oxygen was forced back into the mix. This ignited the carbon in the pig iron. As the carbon was 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. The key "trick" was to stop the process when the temperature reached a particular point, which meant that the steel had a particular carbon content.

However the process proved more difficult in practice than in the lab. Using iron ores from England instead of the high quality ores from Sweden resulted in a brittle metal no better than cast iron in many cases. Several solutions to this problem were eventually discovered, notably adding chalk to the molten iron as suggested by Sydney Gilchrist Thomas and Percy Carlyle Gilchrist. The CaO of the burnt chalk reacted with the impurities in the English ore, namely phosphorus, leaving a much more pure steel with far better qualities.

These three key inventions, coke, the blast furnace and the Bessemer Converter, unlocked steel production. By the turn of the 20th century production had increased tremendously; 22 thousand tonnes were produced in 1867, 500 thousand in 1870, 1 million in 1880 and 28 million by 1900. Today, worldwide production is around 850 million tonnes. The availability of massive amounts 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, and hot-worked into useful items. Damascus blades, famous as the blades that the Saracens wielded against the crusaders, were probably smelted iron wire, mated wire obtained from meteorites, heated and worked to impart the properties of expensive "star metal" to cheaper wrought iron; an early attempt at alloying.

In a modern sense, alloy steels have been made since the advent of furnaces capable of melting iron, into which other metals may 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, and resist corrosion (rust). Some stainless steels are non-magnetic.
• Tool steels
• HSLA Steel (High Strength, Low Alloy)
• Ferrous superalloys

Production methods

• Crucible technique or puddling - the original steel making technique, developed in India as wootz, used in the Middle East as Damascus steel and independently redeveloped in Sheffield by Benjamin Huntsman in 1740, and Pavel Anosov in Russia in 1837.
• Bessemer process, the first commercial scale steel production process
• Open hearth furnace
• Basic oxygen steelmaking
• Electric arc furnace a form of secondary steelmaking from scrap, though the process can also use direct-reduced iron

See also

• Steel producers

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