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Steel

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Steel is a metallic alloy whose major constituent is iron, with carbon being the primary alloying material. Carbon acts as a binding agent, locking the otherwise easily-moved iron atoms into a rigid matrix. Varying the amount of carbon and its distribution in the alloy controls the qualities of the resulting steel. With the increased carbon, steel is harder and has a much higher tensile strength than iron, but is also more brittle. One classical definition is that steels are iron-carbon alloys with up to 2.1% carbon.

Presently 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 no longer found in a natural state. Since the rise of the cyanobacteria and their dumping of oxygen into the atmosphere, iron can be found only in oxide form, typically FeO2 -- iron oxide. Iron oxide is a soft sandstone-like material with limited uses on its own. The secret to extracting useful iron is to "boil off" the oxygen by melting the iron oxide, which occurs at the fairly high temperature of 1535 C. 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 800C, it is important that smelting take place in a fairly oxygen-free enviornment.

Another important part of making quality steel is that the carbon-iron matrix can form into a number of different structures, some stronger than others. In its natural form steel will tend to form the body-centered cubic ferrite form, which is fairly soft. At about 910C ferrite will transition to the denser face-centered cubic austenite phase, in which the carbon has considerably higher solubility but is otherwise structually similar. While cooling, however, the mixture will take on one of several forms as it attempts to revert to the ferrite phase. One complex structure, known as cementite, forms when the over-saturated carbon precipitates out of the austenite to form a carbon-ferrite mixture. Cementite often forms in regions of higher carbon content while other areas form ferrite around it, leading to a patterned layering known as perlite due to its pearl-like microscopic structure, or the similar but unpattered bainite. Another important allotrope is martensite, a complex mixture of austenite and ferrite with about four to five times the strength of ferrite. Martensite is the most commonly desired steel allotrope, given its very high strength and resistant to breaking.

Key to producing a strong steel is to lock in the crystal structure in a strong state before it can revert to a softer one while cooling. This is accomplished by 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. However this process also introduces tiny cracks and imperfections into the metal structure, allowing the metal to break along those points. After quencing then, the metal is re-heated to a lower temperature and "worked" (via hammering or rolling) to force the cracks to close and produce a much stronger metal.

Other materials are often added to the iron-carbon mixture to tailor the resulting properties. Nickel in steel adds to the tensile strength, chromium increases the hardness, and vanadium also increases the hardness while reducing the effects of metal fatigue. On the other hand sulphur and phosphorous create gaps in the structure that leads to it being easily broken, these commonly found elements must be removed from the ore during processing.

Generally the production of steel proceeds in several steps. First the iron is smelted from the ore using a variety of methods, removing impurities in the process. It is then re-processed in order to add the correct amount of carbon and other alloying materials. Finally the mixture is cooled in a way to lock in the required structure, and then worked to remove mechanical defects. 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.

History of Iron and Steelmaking

Iron was in limited use long before it became possible to smelt it. With the lower temperature charcoal fires available via air blowing, between 1100 and 1200 C, iron would remain locked in the oxides. Although the iron does not liquify at these temperatures, it does soften considerably ("goes plastic") allowing it to be worked. A small amount of very pure iron periodically falls from the sky in meteorites, and was known in Sumeria as sky metal for this reason. Iron was considered a super-precious metal given the limited amounts available, King Tut was buried with an iron dagger in his hand for instance. The Innu of Greenland had been building tools from a single 30 ton meteorite for hundreds of years, until Admiral Robert Peary took it and had it shipped to the Natural History Museum in New York.

It was also known that iron was "trapped" in certain rocks (iron oxide), just as copper and tin were in others, but the technique of extracting it into a pure form remained a mystery. However it was possible to work fairly pure iron oxides at lower temperatures. The result is a mixture of bits of iron, iron oxide, slag and charcoal residue, known as iron bloom. By repeatedly pounding the bloom, folding it over, or twisting it, the partially melted slag could be forced to the surface and be broken off, leaving a better quality iron. This process was slow and time consuming, but with the fall of the tin trade in middle east, the easily worked bronze became unavailable in many parts of the world and iron became a primary metal for weapon making.

Timing and temperature of the working process needed to be tightly controlled. As the bloom is worked, carbon from the charcoal will mix into the bloom. In the 3-4% carbon range the iron becomes a brittle metal we now know as pig iron or cast iron (so named because it could only be formed by casting, it was too brittle to hammer or roll). The vast majority of steel and iron during 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 softer metal we now call wrought iron. 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 varing 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 500AD 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 100kg of charcoal for every 1kg of iron) and long times to produce a quality product. However a number of improvements, notably to the blowers, raised the operating temperatures of the smelters to the point where iron could be worked in a molten state. Although pure-smelting at melting temperatures improved the process considerably when it was introduced in the middle ages, the batch-to-batch repeatability remained a matter of luck.

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. The process 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 coal bed. Although the quality of such steels was not very repeatable, the methods could produce one-off batches of excellent 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 phosphorous content compared to most ores (notably those in England), which allowed for a finer and strong 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.

The introduction of steel as a common building material led from several key inventions in the 17th and 18th century, primarily in England. First of these was the introduction of coke to smelt the iron in place of charcoal. 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 gasses that resulted in a smelly, unappetizing beer. However they found that heating the coal in an oxygen-free enviornment 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 industur.It was later discovered that it was the sulphur content of common coal that led to both the smell and the poor quality iron, cooking it released it to the air.

The introduction of coke allowed for more widespread ironworking due to the cheapness of the fuel. 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 forcably 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 was the first low-carbon iron available in commercial quantities, and 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 next major innovation was then able to be introduced, the blast furnace. By blowing air into the middle of of the ore while it was still smelting, impurities in the iron would burn off, leaving fairly pure liquid iron which could then be poured out of the bottom of the furnace. The introduction of the steam engine to this process, powering massive blowers, 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 independantly rediscovered in England in the 18th century by Benjamin Huntsman at his workshop in Handsworth, Sheffield. In his process the wrought iron from the blast furnaces was re-heated in stone boxes for about a week at a time, resulting in a fairly even mix superior in quality to the similar wootz. However the crucible steel process remained an 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 a water-wheel powered, scythe-making works dating from Huntsman's times, which is still operated for the public, several times per year using crucible steel made on the Abbeydale site.

This problem of mass producting steel was finally solved by Henry Bessemer with the introduction of the Bessemer Convertor 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 convertor started with the pig iron from the blast furnace, which still contained considerable amounts of carbon. In the converter the temperature is carefully controlled until the iron is just above the melting point, and then oxygen is forced back into the mix. This ignites the carbon in the pig iron and burns it. As the carbon is burned off the melting point of the mixture increases, but the heat from the buring carbon provided the extra energy needed to keep the mixture liquid. 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 phosphorous, leaving a much more pure steel with far better qualities.

These three key inventions, coke, the blast furnace and the Bessemer Convertor, unlocked steel production. By the turn of the 20th century production had increased tremendously; 22 kilotonnes were produced in 1867, 500 in 1870, 1 million in 1880 and 28 million by 1900. Today worldwide production is around 500 million tonnes. The availablility 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 (invented by Harry Bearly of Sheffield, England in 1912) all of them based on the wide availability of inexpensive iron and steel and processes to alloy it at will.

Types of steel

Production methods

Steel producers

Main article Steel producers
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