Much of the material produced on a battlefield is ferrous in nature. The article looks at the chemistry and metallurgy of iron.
Iron is an abundant element in the universe; it is found in many stars, including the sun. Iron is the fourth most abundant element in the earth’s crust, of which it constitutes about 5% by weight, and is believed to be the major component of the earth’s core.
Iron is found distributed in the soil and water in low concentration. It is not found as a metal in nature but as ores and minerals which are abundant. The removal of elemental iron from the environment was one of the preconditions for the development of life on earth.
Iron is biologically significant as a component of haemoglobin, a red oxygen-carrying pigment of the red blood cells.
Iron rusts readily in moist air, forming a complex mixture of compounds that is mostly a ferrous-ferric oxide with the composition Fe3O4. Left undisturbed, the rust bloom can grow to many times the size of the object. X-ray is the recognised way to discover what, if anything, remains inside a lump of rust.
Iron ores are refined in the blast furnace. The product of the blast furnace is called pig iron and contains about 4% carbon and small amounts of manganese, silicon, phosphorus, and sulphur. About 95% of this iron is processed further to make steel, often by the open-hearth process or the Bessemer process, but more recently in the United States and other countries by the basic oxygen process or by an electric arc furnace. The balance is cast in sand moulds into blocks called pigs. It is further processed in iron foundries.
Cast iron is made when pig iron is re-melted in furnaces and poured into moulds to make castings. It contains 2% to 6% carbon. Scrap iron or steel is often added to vary the composition. Cast iron is used extensively to make machine parts, engine cylinder blocks, stoves and pipes.
Cast or 'grey' iron is produced when iron in the mould is cooled slowly. Part of the carbon separates out in plates in the form of graphite but remains physically mixed in the iron. Grey iron is brittle but soft and easily machined. White cast iron is made by cooling the molten iron rapidly and is harder and more brittle. The carbon remains distributed throughout the iron as iron carbide, Fe3C.
A malleable cast iron can be made by annealing white iron castings in a special furnace. Some of the carbon separates and becomes finely divided in the iron. A ductile iron may be prepared by adding magnesium to the molten pig iron - When the iron is cast, the carbon forms tiny spherical nodules around the magnesium. Ductile iron is strong, shock resistant, and easily machined.
Wrought iron is commercially purified iron. Pig iron is refined in a converter and then poured into molten iron silicate slag. The resulting semisolid mass is passed between rollers that squeeze out most of the slag. The wrought iron has a fibrous structure with threads of slag running through it; it is tough, malleable, ductile, corrosion resistant, and melts only at high temperatures. It is used to make rivets, bolts, pipes, chains, and anchors, and is also used for ornamental ironwork.
Steel is an alloy of iron, carbon, and small proportions of other elements. Iron contains impurities in the form of silicon, phosphorus, sulphur, and manganese. Steelmaking involves the removal of these impurities followed by the addition of desirable alloying elements.
Steel was first made by cementation, a process of heating bars of iron with charcoal in a closed furnace so that the surface of the iron acquired a high carbon content. The crucible method, originally developed to remove the slag from cementation steel, melts iron and other substances together in a fire-clay and graphite crucible. The famous blades of former times were made by these techniques.
Steel is often classified by its carbon content
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