Bessemer Process
The Bessemer process was the first inexpensive industrial process for the mass-production of steel from molten pig iron.
1855 The process is named after its inventor, Henry Bessemer, who took out a patent on the process in 1855.
A similar process was claimed to have been discovered in 1851 by William Kelly. The process had also been used outside of Europe for hundreds of years, but not on an industrial scale. The key principle is removal of impurities from the iron by oxidation through air being blown through the molten iron. The oxidation also raises the temperature of the iron mass and keeps it molten.
The process is carried out in a large ovoid steel container lined with clay or dolomite called the Bessemer converter. The capacity of a converter was from 8 to 30 tons of molten iron with a usual charge being around 15 tons. At the top of the converter is an opening, usually tilted to the side relative to the body of the vessel, through which the iron is introduced and the finished product removed. The bottom is perforated with a number of channels called tuyères through which air is forced into the converter. The converter is pivoted on trunnions so that it can be rotated to receive the charge, turned upright during conversion, and then rotated again for pouring out the molten steel at the end.
The oxidation process removes impurities such as silicon, manganese, and carbon as oxides. These oxides either escape as gas or form a solid slag. The refractory lining of the converter also plays a role in the conversion - the clay lining is used in the acid Bessemer, in which there is low phosphorus in the raw material. Dolomite is used when the phosphorus content is high in the basic Bessemer (limestone or magnesite linings are also sometimes used instead of dolomite) - this is also known as a Gilchrist-Thomas converter, after Sidney Gilchrist Thomas who had invented a process for dealing with the phosphorus with the help of his cousin, Percy Gilchrist[1].
In order to give the steel the desired properties, other substances could be added to the molten steel when conversion was complete, such as spiegeleisen (an iron-carbon-manganese alloy).
When the required steel had been formed, it was poured out into ladles and then transferred into moulds and the lighter slag is left behind. The conversion process (called the "blow") was completed in around twenty minutes. During this period the progress of the oxidation of the impurities was judged by the appearance of the flame issuing from the mouth of the converter: the modern use of photoelectric methods of recording the characteristics of the flame has greatly aided the blower in controlling the final quality of the product.
After the blow, the liquid metal was recarburized to the desired point and other alloying materials are added, depending on the desired product.
Before the Bessemer process Britain had no practical method of reducing the carbon content of pig iron. High quality steel was manufactured by the reverse process of adding carbon to carbon-free wrought iron, the most suitable iron being imported from Sweden. The manufacturing process, called the cementation process, consisted of heating bars of wrought iron together with charcoal for periods of up to a week in a long stone box. This produced blister steel. Up to 3 tons of expensive coke was burnt for each ton of steel produced. Such steel when rolled into bars was sold at £50 to £60 a ton. The most difficult and work-intensive part of the process was however the production of wrought iron in finery forges in Sweden.
This process was refined in the 1700s with the introduction of Benjamin Huntsman's crucible steel making technique, which added an additional three hours firing time, and additional massive quantities of coke.
In making crucible steel, the blister steel bars were broken into pieces and melted in small crucibles each containing 20 kg or so. This produced a higher quality crucible steel, and increased the cost. The Bessemer process reduced by about ½ the time to make steel of this quality, while requiring only the fuel needed initially to melt the pig iron. The earliest Bessemer converters produced steel for £7 a ton, although they priced it initially at around £40 a ton.
Both Bessemer and Huntsman were based in the city of Sheffield. Sheffield has an international reputation for steel-making, which dates from 1740, when Benjamin Huntsman discovered the crucible technique for steel manufacture, at his workshop in the district of Handsworth. This process had an enormous impact on the quantity and quality of steel production and was only made obsolete, a century later, in 1856 by Henry Bessemer's invention of the Bessemer converter which allowed the true mass production of steel. Bessemer had moved his Bessemer Steel Co to Sheffield to be at the heart of the industry. The city's Kelham Island Museum still displays one of the UK's last examples of a working Bessemer converter [from Workington, Cumbria] for public viewing.
The Bessemer process revolutionized the world. Prior to its widespread use, steel was far too expensive to use in most applications, and wrought iron was used throughout the Industrial Revolution. After its introduction, steel and wrought iron were similarly priced, and all manufacture turned to steel.
The Bessemer process was so fast (10-20 minutes for a heat) that it allowed little time for chemical analysis or adjustment of the alloying elements in the steel. Bessemer converters did not remove phosphorus efficiently from the molten steel; as low-phosphorus ores became more expensive, conversion costs increased. The process only permitted a limited amount of scrap steel to be charged, further increasing costs, especially when scrap was inexpensive. Certain grades of steel were sensitive to the nitrogen which was part of the air blast passing through the steel.
1856 An example of an early failure: 'BESSEMER'S PROCESS - COATBRIDGE. A party of gentlemen were invited to see an experiment in iron-making, according to Mr Bessemer's process, made on Monday, at Mr Jackson's works, Coatbridge. Accordingly, shortly after four o'clock they met around a cupola and furnace which had been erected for the purpose, and where about eight tons of iron were to be converted from the crude pig state into that of malleable iron. After a short delay the cupola was tapped, and the melted iron was admitted by a trough into the furnace, which was fitted up with air pipes through which the compressed air was carried in at a pressure of eight pounds, when a shower of sparks, stars, &c., rushed with immense force and in great volume from the top of the furnace. This continued for the space of twenty minutes, when the pressure upon the air was increased to ten pounds. No sooner was the air so pressed admitted to the furnace than the ebullition was increased to an astonishing degree, and forthwith commenced an exceedingly grand display of pyrotechny. Showers of bright but quickly vanishing stars poured out in enormous quantities, rising to a height of thirty feet in a beautiful curve, and falling in thick showers to be immediately extinguished, over a space of twelve or fourteen square yards. By and by masses of slag and scoria were thrown up from the boiling mass of iron, while the gorgeous and brilliant shower of stars rather increased than diminished. This continued for about other fifteen minutes, when the stars and sparks gradually diminished, and a volume of thick luminous smoke burst from the furnace, which was followed by a bright, faintly purplish flame, showing that the process was nearly completed. On the subsidence of the mass of thick white smoke and flame, a smell of burning sulphur was strongly perceptible, which soon changed to one having more the smell of iodine. The process, which occupied in all about forty minutes, being now complete, the furnace was tapped, and the purified iron ran white and limpid into moulds prepared for the purpose. The party then retired from the works for refreshment; and after some time a piece of the pig No. 4 iron upon which the experiment was tried, together with specimens of the iron bar after process, and specimens after once rolling, and after reheating and a second rolling, were exhibited to the party. The specimen of iron, after being purified, showed a bright silvery whiteness, with large crystals, but exceedingly brittle. The specimens of rolled iron preserved the same crystalline appearance on fracture, but in a state of greater compression, but without the slightest trace of fibre. We do not pretend to give an opinion of the merits of this wonderful process from the experiment which we had the greatest pleasure in witnessing. From what we saw of the iron, it appears to want every quality which would render it valuable for such purposes as malleable iron are usually applied, in fact, the specimens we examined were not malleable, and had nothing of that tenacity or ductility, properties which render iron valuable, and are so indispensable for the mechanical requirements of the present age. - Glasgow Herald.'[2]
1879 'IMPORTANT EXPERIMENT. A public test, at which the leading ironmasters of the Middlesborough district were present, took place yesterday, in Messrs. Bolckow, Vaughan, and Co.'s works, Middlesborough, of the new process for making steel direct from Cleveland ore. The metal was run direct from the furnaces into the ordinary crucible, and then placed in a Bessemer converter. In about twenty minutes the converter was tilted, and 1 1/2 tons of steel was made into ingots, which subsequent tests proved to be of superior quality, the phosphorus in the ore having been successfully eliminated.'[3]
See Also
Sources of Information
- [2] Wikipedia