In 1852, an American named William Kelley proved that the bits of other elements in melted pig iron could be burned out by a blast of air. Three years later, an Englishman named Bessemer discovered and patented the same process. This invention greatly increased the world’s steel production, and made possible the great era of railroad building that was so important during the latter half of the 19th century.
Pure iron is not as practical as that containing from 0.1 per cent to 01.0 per cent of carbon. This product, known as steel, can be permanently magnetized. Steel containing over 2 per cent carbon is brittle. Other metals are often alloyed with the steel to produce various characteristics. These additions are carefully controlled so that the desired percentages are obtained. For this reason, the impurities in each ton of pig iron—75 pounds of carbon, 25 pounds of silicon, 1 pound of sulfur, and 15 pounds of manganese must be modified in order to produce a quality-grade steel.
Two steps in the Bessemer Process
There are two distinct steps in the Bessemer process: 1) The impurities in the iron are almost completely removed by OXIDATION (the chemical union of oxygen with another substance—in this case, the impurities, such as carbon, silicon, etc.) 2) A definite amount of carbon is added to give the proper concentration, together with a small amount of manganese to absorb the oxygen.
In the first step, a large pear-shaped furnace, lined with silica of manganese brick is used. Its capacity is from 10 to 20 tons of molten pig iron. This vessel is called a converter, and may be tipped for filling and emptying. A powerful blast of air is blown through the molten metal to oxidize the impurities. The temperature of the iron is raised several hundred degrees during the process by the heat of the oxidation reaction, producing what is called the boil. The silicon is oxidized to silicon dioxide, SiO2; the carbon to carbon monoxide, CO; the manganese to manganese oxide, MnO; and a little of the iron to ferrous oxide, FeO. These metallic oxides then combine with the silica lining to form a slag—the waste product formed when the impurities are united with a material added for this reason. In this case, the material is oxygen; a material of this sort is called a flux.
After a blow of fifteen minutes, when the proper temperature has been obtained and the completed oxidation is indicated by a color change in the burning carbon monoxide, some spiegeleisen—a carbon-iron-manganese ALLOY is added. The blast is then restarted to insure uniformity. The manganese, which unites with any dissolved oxygen, prevents the formation of blow holes which would weaken the steel. The carbon is needed to produce the necessary concentration of this element in the steel. Small desired quantities of other alloys, called scavengers, such as ferrotitanium and ferrosilicon, as well as metallic aluminum may also be added to reduce any ferrous oxide and carbon monoxide present and also to remove nitrogen. The oxides formed are removed in the slag. Chromium, vanadium, tungsten, nickel and other elements are added to produce special steels. Some steel is then poured into a ladle to cool and be tested, while the remaining portion may be taken directly to the open hearth furnace or electric furnace for further refining.
Since the Bessemer process is so rapid, it is difficult to control it closely and the finished steel contains many impurities which seriously weakens it. Because phosphorous, in particular, is so difficult to remove, low grade ores containing it cannot satisfactorily be used in this converter. Bessemer steel containing phosphorous and sulfur lacks DUCTILITY, and cannot be made into bars, sheets, or wire. Steel containing sulfur is brittle when hot, while phosphorous makes cold steel brittle. Bessemer steel can only be used in products which are not subject to corrosion, stress, strain, and shock. It is often used as the flux in the open hearth furnace.
The Bessemer process varies slightly—depending upon the impurities within the pig iron. The modification is slight and consists of altering the lining materials, the method used for removing the oxidized slag, and in the time for controlling the adjustment of the composition.