In the whole of this wide range of non-ferrous alloys the mechanism of hardening is the same ; the discovery of these alloys has, in fact, been based upon a theoretical study of their constitu tion. The hardening depends in every case upon the fact that the base metal can, at a high temperature, hold in solid solution a larger proportion of the alloying element than it can hold at lower temperatures. Quenching brings down to the ordinary temperature a solid solution which is, at that temperature, heavily super saturated. This solid solution is not hard so long as it remains in that super-saturated condition, but becomes hard as soon as break down or transformation begins, whether at room temperature or on slight heating. There is a close analogy between these phe nomena and what we believe to occur in the hardening of steel.
In regard to iron and steel, reference has been made to the "high speed" steels used for cutting tools. To these must be added two types of special alloys which furnish tools of still greater hardness and cutting power than the steels. The "stellite" type of alloy contains chromium, cobalt and tungsten. It can only be brought to the desired shape by casting, as it remains hard and non-ductile at all temperatures. The final shaping of the tool is done by grind ing. The tools can be used to cut rapidly and up to higher tempera tures than the steels, but are apt to break under a blow. Still harder and capable of resisting still higher temperatures are a group of substances whose most important constituent is tung sten carbide. Only thin strips or plates of these expensive sub stances are used as facings brazed to the front of a steel "tool" but with these tools it is possible to cut at fairly high speeds materials, such as toughened manganese steel, which resist all steel cutting tools. Even glass has—as a demonstration—been "ma chined" with one of these tools. The materials themselves are known under a variety of proprietary names such as "thoran," "widia," etc. Important advances have also been made in regard to steel for magnetic purposes. For permanent magnets tungsten steels are most widely used, but a Japanese discovery has shown that cobalt steels are markedly superior and these magnet steels are used where permanent magnets of minimum weight are important. The high cost of cobalt, however, sets a limit to the application of these steels. On the other hand, in the development of very soft magnetic steels, for transformer cores and for the inductive wind ing of submarine cables, special alloys have also been developed. Steel containing very little carbon with about 4% of silicon has a lower magnetic hysteresis loss combined with a higher electrical resistance than ordinary soft steel, and its extensive use has had wide effects in improving the economy of electric current trans formers. For the sheathing of submarine cables a magnetic ma terial having the highest possible permeability under very low magnetic fields is required. A remarkable material for this pur
pose is the American alloy of iron with about 79% of nickel. This alloy, known as "permalloy" and a British modification known as "mumetal" are, however, highly sensitive to slight plastic deformation and only attain their best properties after careful annealing.
The immense losses caused by corrosion have led to much re search on this subject both in regard to ferrous and non-ferrous metals. The addition of small amounts of copper to steel has been found to reduce corrosion in many circumstances, but very great resistance to corrosion has been attained by the addition to steel of considerable amounts of chromium. "Stainless steel" was devel oped in Sheffield by Brearley ; it contains about 13% of chromium and, with a moderate carbon content can be satisfactorily hardened and tempered. When properly hardened, tempered and polished, this steel resists corrosion extremely well, although there are cer tain conditions, such as prolonged exposure to sea-water, which cause attack. For purposes where softness and ductility are re quired, "stainless iron" has been developed with a very low carbon content. This is, however, being largely displaced by a type of rust-resisting steel of which "staybrite" and "anka" are typical. These contain still higher proportions of chromium than ordinary "stainless" steel, together with a high proportion of nickel. These steels retain the austenitic structure on cooling, whether fast or slow, and are thus soft, ductile and non-magnetic, but they can not be hardened by heat-treatment and are difficult to machine. They are extremely resistant to corrosion and chemical attack of all kinds and widely applied in chemical engineering and elsewhere.
The cost of these rust-resisting steels is, however. too high to allow of their use for general structural purposes so that methods of preventing or reducing corrosion in ordinary steel are still re quired. Apart from paints and other organic coatings, protection of steel by coating with another metal, such as zinc and tin, is widely used. Galvanized iron—really steel coated with a thin layer of zinc—is used in immense quantities. Perhaps the most striking development in protective coatings is the application of metallic chromium to this purpose. When properly deposited by electrolysis, this metal furnishes a coating of brilliant bluish-white colour which is incorrodible and untarnishable, while it is also so hard that it cannot be readily scratched. These promising proper ties have led to wide-spread attempts at immediate industrial ap plication and, while some success has been attained, many diffi culties and defects have been encountered. The process is there fore still in course of development but may well be expected to gain ground in the future.