Early history of steel-makingTen thousand years ago the technology of our ancestors was based upon the use of stone, three thousand years ago upon bronze, and two thousand years ago upon iron. Thus we refer to the Stone Age, Bronze Age,and the Iron Age. There is no corresponding period that archaeologists can specify as an age based on steel. Steel has been made in large quantities for only about 140 years, during which time it has become vital to our civilization. But the origins of steel lie in the remote past.
Plain or mild steel, as used in a host of applications, is an alloy of iron with a little carbon. More particularly, mild steel contains between 0.15 and 0.25 per cent carbon, partly in the form of iron carbide or cementite. Steel is much harder than pure iron and less brittle than cast iron (which contains more carbon than steel does); its strength, toughness, and springiness account for its great usefulness.
The first makers of iron tools and weapons on a large scale were the Hittites, and it was men of a subject tribe of the Hittites, the Chalybes, who first made steel in about 1400 BC. Chalybean steelsmiths in Asia Minor employed a cementation process; that is, they hammered hot but still unmolten iron together with charcoal until the iron became steel. During the hammering, carbon from the charcoal diffused into the iron, forming cementite – hence the name.
Molding and hardeningVariations on the cementation process continued in use for more than a thousand years, spreading from the Middle East into Europe and India. But it was in southern India that iron was melted for the first time and steel cast into molds. The Romans imported this steel in the shape of small, rounded cakes. They thought that it originated in China and called it "seric" (meaning Chinese) iron.
In Europe steel was sometimes made directly from an ore with the correct carbon content, but this was rare. More often steel resulted from the cementation process and, from the 8th century AD onwards, cementation steel began to began to be exported from the iron-rich parts of central Europe known as Styria and Carinthia. This steel had been further hardened by quenching it from red heat in water. It had taken a long time for this tempering to be learned, probably because the earlier known metals copper and bronze, quenched in this way, become softer. By the 15th century early printers were using steel punches in the manufacture of their type molds and, midway through the 17th century, tempered steel coach springs first added a little comfort to travel on Europe's rutted and pot-hole-strewn roads.
A great problem with early steel was the presence in it of slag waste from the ore, which made it difficult to manufacture large steel objects without structural weaknesses. This problem was solved by the mid-18th century as the result of the work of Turbern Bermann (1735–84), a Swedish metallurgist, and Benjamin Huntsman (1704–76). "Swedish steel", made from high-grade iron ore found in Sweden had a controlled carbon content and was free from slag, where Hunstman developed an improved form of the so-called crucible process that allowed him to make high-quality hardened steels for watch and clock springs. However, such high-grade steels were expensive to produce. In 1850, the entire output of steel in Britain was only 60,000 tons. Yet, 20 years later, steel was being produced in great quantities, at an average rate of one ton per minute, in batches made as cheaply as cat iron, by a new process that used Bessemer furnaces or "converters".
Mass production of steelThe secret of the Bessemer process, developed by Henry Bessemer (1813–1898) in 1856 from the invention of a bankrupt Kentucky steelmaker, William Kelly (1811–1888), was that excess carbon could be oxidized (combined with oxygen) by forcing bubbles of air through a mass of molten iron. Moreover, the carbon burned to carbon dioxide in the blast of air, so acting as a fuel. Once started, the process, the process continued without the addition of more fuel coal and was thus extremely economical.
Within five years the Bessemer process had a rival in the Siemens-Martin open-hearth process, in which iron, iron ore, and scrap steel are melted in such proportions as to drive of most of the carbon and oxygen as carbon monoxide. This inflammable gas is then burned "regeneratively" to pre-heat the blast. This efficient operation was, by 1900, producing more steel than even the Bessemer process.
The twentieth-century saw further revolutions in steelmaking, notably the continuous casting of steel and special steels such as those for making machine tools and turbine blades. The newer steels include stainless steels, which contain chromium, nickel, and sometimes molybdenum, and are made in an electric furnace. Most modern processes use a basic slag and a basic refractory furnace lining: acidic processes are incapable of removing phosphorus. When the impurities have been removed, desired elements are added in calculated proportions. The molten steel is cast as ingots which are shaped while still red-hot in rolling mills, or it may be cast as a continuous bar ("strand casting").
The properties of medium-carbon (0.25% to 0.45% carbon) and high-carbon (up to 1.7% carbon) steel may be greatly improved by heat treatment: annealing, casehardening, and tempering. Steel metallurgy is complex: unhardened steel may contain combinations of three phases – austenite, ferrite, and cementite – differing in structure and carbon content; hardened steel contains martensite, which may be thought of as ferrite supersaturated with carbon.
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