Bessemer process

A Bessemer converter was a large, tilting vessel lined with a refractory material. Molten pig iron was charged into it, and air was forced through openings near the bottom. Oxygen in the blast reacted with carbon and other impurities in the iron, forming gaseous oxides and slag. These oxidation reactions released enough heat to keep the charge molten without an external fuel furnace. When refining was complete, the converter could be tilted to pour out the steel. The process was fast compared with earlier production methods, although operators had to control the reaction and adjust the resulting metal to achieve useful steel properties.

Henry Bessemer presented his steelmaking method publicly in 1856 and patented it that year. William Kelly had independently experimented with air-blown molten iron in the United States, and received a US patent in 1857. The two inventors' work formed part of the shift toward pneumatic steelmaking. Once technical and commercial difficulties were overcome, converters enabled steel to be produced in far greater quantities and at lower cost than had previously been practical. The process spread internationally and was adopted by steelmakers serving expanding railway, construction and manufacturing markets.

The original converter used an acidic refractory lining and could not effectively remove phosphorus from the iron. It therefore depended on suitable low-phosphorus pig iron, limiting the ores and supplies that could be used. In 1878, Sidney Gilchrist Thomas and Percy Gilchrist developed a basic-lined converter that allowed phosphorus-rich iron to be treated with a lime-rich slag. The open-hearth process later offered greater flexibility for steel production, and twentieth-century basic oxygen steelmaking used oxygen rather than an air blast. These developments displaced the original Bessemer process, though its converter design and rapid oxidation principle remain important in the history of industrial metallurgy.

By reducing the cost and time required to make steel, the Bessemer process helped make steel more widely available for railways, bridges, buildings, ships and machinery. Its impact was not the work of a single inventor alone: it depended on suitable iron supplies, practical converter design, metallurgical adjustments, investment and the growth of markets able to use large volumes of steel. The process is consequently remembered as a major enabling technology of nineteenth-century industrialization, as well as one stage in the continuing development of steelmaking.