High Speed Steels.—In recent years special alloy steels have been introduced for machine cutting tools which render much bet ter service than ordinary carbon steels. The action of these "high speed steels," which generally contain a considerable amount (sometimes up to 18 or 20 per cent) of tungsten, depends upon the fact that in ordinary carbon steels the hardness is rapidly re duced by rising temperature. When a tool is taking a heavy cut, its temperature rises rapidly and the amount of loading which can be applied to a tool made of carbon steel is limited by the com paratively low temperature which brings about softening. In the "high speed" steels the temperature which must be attained to cause softening is very much higher : the steels are for this reason said to possess "red hardness." They owe this valuable property to the peculiar manner in which the transformation temperatures are altered by the presence of the alloying elements.
In structural steel of lower carbon and alloy content, the pur pose of heat-treatment is not to produce a material of high hard ness but one possessing high, tensile strength combined with reasonable ductility and good behaviour in the notched-bar im pact test. For this purpose the steel is first "hardened" by quench ing, but this is more frequently done in oil than in water. The sub sequent tempering or "drawing" may be carried out at tempera tures which usually lie between 40o and 600° C. Two serious diffi culties have arisen in applying this heat-treatment particularly to larger masses of steel. Owing to their large heat content and the relative slowness with which heat flows from the interior to the exterior of a mass of steel, it is not possible to secure in the inner portions of a steel forging a rate of cooling sufficiently rapid to bring about hardening. The heat-treatment of large masses— and for this purpose pieces exceeding two inches in thickness must be regarded as "large"—thus presents grave difficulties in simple carbon and nickel steels. By the addition of suitable amounts of chromium, however, the transformations of the steel are rendered more sluggish so that even the moderately rapid rates of cooling which can be attained in large masses are sufficient to bring about hardening. Such steels can, therefore, be hardened and subse quently tempered, in relatively large masses. Here, however, the second difficulty arises. Steels of this type, especially the nickel chromium steels, exhibit the phenomenon known as "temper brit tleness" which makes itself felt most strongly under the notched bar impact test. When such steel has been tempered to show the desired degree of high ductility, it gives very low values under the impact test. This type of brittleness can be prevented by acceler
ating the cooling of the steel immediately after it has been tem pered. Rapid cooling, however, is difficult or impossible in large masses. Fortunately it has been discovered that the presence of a small percentage of molybdenum in the steel prevents the develop ment of temper-brittleness even if the material is slowly cooled after tempering.
The heat-treatment of non-ferrous metals and alloys was, until quite recently, confined to simple annealing at various tempera tures, for the purpose of softening metal hardened by cold working. To this must now be added forms of heat-treatment which, while they differ in detail, are closely analogous in principle to those applied to steel. The first non-ferrous alloy known to undergo hardening by heat-treatment is "duralumin," the well-known aluminium structural alloy discovered by Wilm in Germany about 1913. This alloy is treated by heating it for a short time to 480° C and then quenching it in water. Immediately after quenching, the material is quite soft—slightly softer than in the ordinary untreated condition. In the course of four or five days, however, at room temperature, or in less than an hour at about 19o° C, hardening sets in. Although this hardening is not comparable in its results with that of steel, it leads to a doubling of the strength and hardness of the material—a fact which has made the alloy ex tremely important in practice, particularly as the age-hardening at room temperature is not accompanied by any loss of ductility.
Some years after the first discovery of duralumin, the nature of the age-hardening process was discovered independently in England and in America and subsequently it was further recognized that a similar mechanism for potential hardening might exist in many other non-ferrous alloys. As a result, a series of aluminium alloys capable of hardening by heat-treatment has made its appearance— perhaps the most important of these is the British "Y" alloy which has the valuable property that it will respond to heat-treatment in the cast- state. In addition, still more recently, a number of cop per alloys have been developed both in America and Germany which undergo marked hardening after quenching from suitable temperatures, but in these cases the subsequent hardening only occurs on moderate heating. The most striking example of this type of copper alloy is that containing about 3% of beryllium. The Brinell hardness of this alloy rises from 8o to 400 with a rise of tensile strength from 14 to over 90 tons per sq. inch as a result of heat-treatment.