Quite often when you study Merthyr’s industrial history, you will hear about ‘The Bessemer Process’. If, like me, it is a mystery to you, here is an excellent article (in two parts), courtesy of Alan Banks of the Wirral Model Engineering Society explaining the mysteries of the process.
The Bessemer Process
The Bessemer process was the first inexpensive industrial process for the mass-production of steel from molten pig iron. The process is named after its inventor, Henry Bessemer, who took out a patent on the process in 1855. The key principle is removal of impurities from the iron by oxidation with 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 on 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). 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 ironcarbon-manganese alloy).
Managing the process
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.
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. Steel was manufactured by the reverse process of adding carbon to carbon-free wrought iron, usually imported from Sweden.
The manufacturing process, called 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 long ton. The most difficult and work-intensive part of the process, however, was the production of wrought iron done 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 required additional large 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 higher quality crucible steel but increased the cost. The Bessemer process reduced to about half an hour the time needed to make steel of this quality while requiring only the coke needed to melt the pig iron initially. The earliest Bessemer converters produced steel for £7 a long ton, although it initially sold for around £40 a ton.
Sir Henry Bessemer described the origin of his invention in his autobiography. According to this book at the time of the outbreak of the Crimean War many English industrialists and inventors became interested in military technology and Bessemer himself developed a method for grooving artillery projectiles so that they could spin without the use of rifling in the bore of the gun. He patented this method in 1854 and began developing it in conjunction with the government of France. After a successful day of testing of his method at the Polygon in France he had a conversation with Claude- Etienne Minié who stated that a key barrier to the use of the larger, heavier spinning projectiles would be the strength of the gun and in particular “… he [Minié] did not consider it safe in practice to fire a 30-lb. shot from a 12-pounder castiron gun. The real question, he said, was; Could any guns be made to stand such heavy projectiles?”. This is what started Bessemer thinking about steel.
At the time steel was difficult and expensive to make and was consequently used in only small items like cutlery and tools. Starting in January 1855 he began working on a way to produce steel in the massive quantities required for artillery and by October he filed his first patent related to the Bessemer process.
To be continued……
Many thanks to the Wirral Model Engineering Society for this article