The encyclopedia · Engineering & Operations · Technical decision · 1856–1860
Bessemer blew air through molten iron to burn out the carbon
Henry Bessemer made cheap steel by forcing air through molten pig iron, using oxygen from the air to remove carbon and keep the metal hot.
Henry Bessemer
the move
Before the 1850s steel was used only for small, valuable objects like cutlery and tools; it was far too expensive for bridges, buildings, rails or cannons. The iron that was cheap enough for those jobs was wrought iron or cast iron, but neither could be made into proper steel economically. At the outbreak of the Crimean War, Bessemer was inspired, by a conversation with Napoleon III, to try to improve the quality of iron for artillery, and he set to work in January 1855.
The problem was the impurities in pig iron, chiefly excess carbon. The established method, puddling, removed them by hand-stirring a molten bath while burning fuel, which was slow, costly and labour-intensive. Bessemer's idea was to blow air through the molten metal instead. The oxygen in the air oxidises the carbon and other impurities, which leave as gas or slag, and the reaction, being exothermic, actually raises the temperature of the iron so it stays molten and can be poured without added fuel.
The early process had a critical flaw: without a way to know when to stop the blast, Bessemer could either leave too much carbon or burn the steel brittle. Robert Forester Mushet solved it by first burning off essentially all the carbon and impurities, then reintroducing a controlled amount of carbon and manganese by adding spiegeleisen. That gave a malleable, high-quality steel and made the process commercially reliable.
Bessemer took out a patent on the process in 1856. It was the first inexpensive industrial method for mass-producing steel from molten pig iron, and it cut the price of steel from about £40 a ton to £6–7 a ton, which is how steel became the material of the industrial age — railways, ships, buildings and bridges.
why it works
- Pig iron has excess carbon and impurities that make it brittle, and puddling removed them slowly at great fuel cost.
- Air blown through the molten metal supplies oxygen that oxidises the carbon, silicon and manganese.
- The oxidation is exothermic, so it keeps the metal molten and eliminates the need for extra fuel.
- Mushet's step of burning off carbon and then adding back a measured amount gave a controlled, workable steel.
what transfers
Use the impurity itself as a fuel: an oxidation that removes the contaminant can be exothermic enough to power the process, eliminating the separate heating step.
what came after
The process, patented in 1856, was the first inexpensive way to mass-produce steel from molten pig iron, cutting its cost from about £40 to £6–7 a long ton and hugely scaling up output. Bessemer earned millions in royalties, though William Kelly of Kentucky independently developed a similar method. Bessemer steel proved too brittle for many structural uses because nitrogen in the air weakened it, so open-hearth steel took over and basic oxygen steelmaking later superseded it, ending U.S. commercial production in 1968.
references
spotted an error? The archive wants to know.