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 American Museum of Natural History in New York City .
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 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 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. 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. 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 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 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 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 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 the ore while it was still smelting, impurities in the iron would burn off, leaving fairly pure liquid pig 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 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 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 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 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 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 phosphorous, 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 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), all of them dependent on the wide availability of inexpensive iron and steel and the ability to alloy it at will.
Types of steel
- 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)
- Alloy steels
- Main article Steel producers
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