In 1868 appeared the first of a fifth group, now called special, or alloy, steel, made by R.F.Mushet in his small works in the Forest of Dean in Gloucestershire. More engineering products meant that more machining had to be done and this needed cutting tools. Before Mushet’s time the only means of cutting iron and steel was a tool made of carbon steel. This can be hardened by making it red hot and cooling it quickly; the obvious way was to plunge it into cold water. Such treatment makes the steel very hard and brittle; it has to be heated again to a lower temperature, and cooled once more to make it both hard and tough.
Carbon steel which has been hardened can be softened again by making it red hot and letting it cool naturally. When a carbon steel tool wore or was damaged, it was useful to be able to soften it, file it or machine it back to its original shape and reharden it. However, softening was a distinct disadvantage when it occurred accidentally. Heat is always generated during cutting and when iron or steel are cut the temperature can easily reach a point where the tool is softened. Then it is useless until it has been rehardened. Thus there is a limit to the speed or depth of cut which can be made by a carbonsteel tool.
Many improved machine tools had appeared in the first half of the nineteenth century. Driven by steam, they were strong and powerful, and capable of heavier work than their cutting tools could achieve. Mushet was asked to make somebody else’s patent tools and, finding them a failure, invented one of his own. He took advantage of the fact that iron and steel will unite with many other elements and experimented with tungsten. Mushet’s tungsten steel could be forged to shape and left to cool naturally in the air, when it became very hard and tough. It only needed grinding to a sharp cutting edge to be ready for use, and when it became blunt from use it was reground. The most useful feature, however, was that the new steel did not soften even at a dull red heat. Engineers and machinists welcomed it, although it was much more expensive than carbon steel. Tungsten steel is still used today, although its detailed composition is often a little different. The best engineers’ drills are usually made of a form of tungsten steel called High Speed Steel introduced in 1900.
Others followed Mushet with different alloys for special purposes. R.A. Hadfield, for example, used manganese, in 1887, to make a steel which was particularly tough and wear-resisting. It was used in such things as railway points and crossings, and in rock-crushing machinery. Over the years very many more alloy and special steels have been added to the list, all of them with some special application or group of applications. Stainless steel, in particular, has affected all areas of everyday life.
Stainless steel was invented by Harry Brearley, in Sheffield, in 1913. While experimenting with a better steel for rifle barrels he noticed that one of the steels he made was unaffected when he tried to treat it with acid so that he could examine it under the microscope. Attack by acid, or etching, is a form of corrosion. If the steel would not etch it would not corrode either, at least under many conditions where ordinary steel would. Some possible uses for the new steel suggested themselves at once, including cutlery.
Many foods contain weak acids which do not harm people but will stain and corrode ordinary steel. Brearley had some table knives made and found that they stood up to use very well, without losing their bright surface. His original experimental steel contained nearly 13 per cent of chromium and Brearley tried out other proportions, and also added different elements such as nickel. After his experiments had been suspended during the First World War, he and others went on to develop several kinds of stainless steel. One problem was that of hardening the steel: Brearley’s first knives were thick and blunt and he once said that he got a name for making knives that would not cut. Over time, several different kinds of stainless steel were developed for particular purposes. One of the commonest for table ware such as dishes and coffee pots contains about 18 per cent chromium and 8 per cent nickel, hence the figures 18/8 which are often found stamped on such goods. Other types, of different composition but all correctly called stainless steel, are used for such things as knives and razor blades, and surgeons’ scalpels. Still further varieties are used in industry to resist the heat of furnaces, or in other severe conditions.
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The Siemens-Martin process
