The Bessemer Process, and Its Adaptations
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The brilliant British inventor, Sir Henry Bessemer (1813-1898), was the dynamo behind the introduction of mass production steel. Reasoning that carbon in molten pig iron unites readily with oxygen, a strong blast of air through molten pig iron should convert it to steel by reducing its carbon content. In 1856 Bessemer designed his converter, which he filled with molten pig iron and blew compressed air through it. He found that the pig iron lost its carbon and silicon quickly and instead of freezing up from the blast of cold air, the metal became even hotter and so remained molten. However this process could not remove the phosphorous, so initially Sir Henry could only obtain steel using phosphorous -free ores which were expensive and in short supply.
Another British inventor, Robert Mushet, around the same time, showed that the air blast actually removed too much carbon and left too much oxygen behind in the molten metal. This made necessary the addition of a compound of iron, carbon, and manganese called spiegeleisen (or spiegel for short): the manganese removed the oxygen in the form of manganese oxide, which passed into the slag, and the carbon remained behind, converting the molten iron into steel. (Ferromanganese serves a similar purpose.) The blast of air through the molten pig iron, followed by the addition of a small quantity of molten spiegel, thus converts the whole large mass of molten pig iron into steel in just minutes, without the need for any additional fuel (as contrasted with the days, and tons of extra fuel and labour, required for puddling and cementation).
In 1876, the Welshman Sidney Gilchrist Thomas discovered that adding a chemically basic material such as limestone to the converter drew the phosphorus from the pig iron into the slag, which floats to the top of the converter where it can be skimmed off, resulting in phosphorus-free steel.(This is called the basic Bessemer process, or the Thomas basic process.) This crucial discovery meant that vast stores of iron ore from many regions of the world could be used to make pig iron for Bessemer converters, which in turn led to skyrocketing production of cheap steel in Europe and the U.S. In the U.S., for example, in 1867, 460,000 tons of wrought iron rails were made and sold for $83 per ton; only 2550 tons of Bessemer steel rails were made, fetching a price of up to $170 per ton. By 1884, in contrast, iron rails had virtually ceased to be made at all; steel rails had replaced them at an annual production of 1,500,000 tons selling at a price of $32 per ton. Andrew Carnegie’s genius for lowering production costs would drive prices as low as $14 per ton before the end on the century. (This drop in cost was accompanied by an equally dramatic increase in quality as steel replaced iron rails: from 1865 to 1905, the average life of a rail increased from two years to ten and the car weight a rail could bear increased from eight tons to seventy.)
The Bessemer process did not have the field to itself for long as over 100 inventors sought ways around the patents held by Henry Bessemer. In the 1860s, a rival appeared on the scene: the open-hearth process, developed primarily by the German engineer Karl Wilhelm Siemens.
But this will form the next chapter in the series!
This extract is based on information provided by Professor Joseph S. Spoer, Saint Anselm College
- 3rd September 2013
