Bessemer steel

The Bessemer process was the first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. 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.

Related decarburizing with air processes had been used outside Europe for hundreds of years, but not on an industrial scale. One such process has existed since the 11th century in East Asia, where the scholar Shen Kuo describes its use in the Chinese iron and steel industry. In the 17th century, accounts by European travelers detailed its possible use by the Japanese.

The modern process is named after its inventor, the Englishman Henry Bessemer, who took out a patent on the process in 1856. The process was said to be independently discovered in 1851 by the American inventor William Kelly, though there is little to back up this claim.

The process using a basic refractory lining is known as the "basic Bessemer process" or "Gilchrist–Thomas process" after the English discoverers Percy Gilchrist and Sidney Gilchrist Thomas.

The blowing of air through the molten pig iron introduces oxygen into the melt which results in oxidation, removing impurities found in the pig iron, such as silicon, manganese, and carbon in the form of 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 — clay linings are used when there is little phosphorus in the raw material - this is known as the acid Bessemer process. When the phosphorus content is high, dolomite, or sometimes magnesite, linings are used in the alkaline Bessemer limestone process. These are also known as Gilchrist-Thomas converters, after their inventor, Sidney Gilchrist Thomas. In order to produce steel with desired properties, additives such as spiegeleisen (a ferromanganese alloy), can be added to the molten steel once the impurities have been removed.

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