Introduction
History of steel
The earliest known steel is about 4,000 years old and was excavated in Turkey. Roman, Iberian and Chinese civilizations used steel to construct weapons. However, these ancient civilizations had yet to master methods of producing steel and so its uses were limited and subject to very long production times. Steel has been produced on a small scale for thousands of years pre-100 AD
300 BC – 1700 AD was the era of the legendary Damascus Steel. The secrets of producing Damascus steel have been lost to time, but the artefacts it produced have not. While the history of Damascus steel can be traced back to India in around 300 BC, it was during the Crusades of the Middle Ages that it acquired its legendary status — Damascus steel could bend under pressure without breaking but could also hold its edge, and the civilisations that mastered its production were feared.
The mass-production of cheap steel only became possible after the introduction of the Bessemer process in 1855, named after its brilliant inventor, the British metallurgist Sir Henry Bessemer (1813-1898). He realised that molten iron could be combined with oxygen. By blasting air through the molten iron ore, the carbon content was reduced. At first, the carbon content was reduced too much, and further experimentation led to the addition of spiegeleisen — a compound of iron, manganese and carbon — to the Bessemer Process. The manganese helped remove the excess oxygen in the form of manganese oxide, and the carbon that was left behind was at the appropriate level to produce steel. Along with plastic injection moulding, the Bessemer Process was one of the most important manufacturing developments of the modern era.
One shortcoming of the initial Bessemer process, however, was that it did not remove phosphorus from the pig iron. Phosphorus makes steel excessively brittle. Initially, therefore, the Bessemer process could only be used on pig iron made from phosphorus-free ores. Such ores are relatively scarce and expensive, as they are found in only a few places (e.g. Wales and Sweden, where Bessemer got his iron ore, and upper Michigan). In 1876, the Welshman Sidney Gilchrist Thomas discovered that adding a chemically basic material such as limestone to the converter draws the phosphorus from the pig iron into the slag, which floats to the top of the converter where it can be skimmed off, resulting in phosphorus-free steel (This is called the basic Bessemer process, or the Thomas basic process.) This crucial discovery meant that vast stores of iron ore from many regions of the world could be used to make pig iron for Bessemer converters, which in turn led to skyrocketing production of cheap steel in Europe and in the U.S.
In the 1860s, a rival appeared on the scene: the open-hearth process, developed primarily by the German engineer Karl Wilhelm Siemens. This process converts iron into steel in a broad, shallow, open-hearth furnace (also called a Siemens gas furnace since it was fueled first by coal gas, later by natural gas) by adding wrought iron or iron oxide to molten pig iron until the carbon content is reduced by dilution and oxidation. Using exhaust gases to preheat air and gas prior to combustion, the Siemens furnace could achieve very high temperatures. Unlike the Bessemer converter, which makes steel in one volcanic rush, the open-hearth process takes hours and allows for periodic laboratory testing of the molten steel so that steel can be made to the precise specifications of the customer as to chemical composition and mechanical properties. The open hearth process also allows for the production of larger batches of steel than the Bessemer process and the recycling of scrap metal. Because of these advantages, by 1900 the open hearth process had largely replaced the Bessemer process The Bessemer Process and other steelmaking processes that had developed alongside it became obsolete in 1950 with the introduction of basic oxygen