35. As actually made, manganese steel contains about 12% of manganese and 1.50% of carbon. Although the presence of 1.50% of manganese makes steel relatively brittle, and although a further addition at first increases this brittleness, so that steel containing between 4 and 5.5% can be pulverized under the hammer, yet a still further increase gives very great ductility, accompanied by great hardness—a combination of properties which was not possessed by any other known substance when this remarkable alloy, known as Hadfield’s manganese steel, was discovered. Its ductility, to which it owes its value, is profoundly affected by the rate of cooling. Sudden cooling makes the metal extremely ductile, and slow cooling makes it brittle. Its behaviour in this respect is thus the opposite of that of carbon steel. But its great hardness is not materially affected by the rate of cooling. It is used extensively for objects which require both hardness and ductility, such as rock-crushing machinery, railway crossings, mine-car wheels and safes. The burglar’s blow-pipe locally “draws the temper,” i.e. softens a spot on a hardened carbon steel or chrome steel safe by simply heating it, so that as soon as it has again cooled he can drill through it and introduce his charge of dynamite. But neither this nor any other procedure softens manganese steel rapidly. Yet this very fact that it is unalterably hard has limited its use, because of the great difficulty of cutting it to shape, which has in general to be done with emery wheels instead of the usual iron-cutting tools. Another defect is its relatively low elastic limit.
36. Chrome steel, which usually contains about 2% of chromium and 0.80 to 2% of carbon, owes its value to combining, when in the “hardened” or suddenly cooled state, intense hardness with a high elastic limit, so that it is neither deformed permanently nor cracked by extremely violent shocks. For this reason it is the material generally if not always used for armour-piercing projectiles. It is much used also for certain rock-crushing machinery (the shoes and dies of stamp-mills) and for safes. These are made of alternate layers of soft wrought iron and chrome steel hardened by sudden cooling. The hardness of the hardened chrome steel resists the burglar’s drill, and the ductility of the wrought iron the blows of his sledge.
Vanadium in small quantities, 0.15 or 0.20%, is said to improve steel greatly, especially in increasing its resistance to shock and to often-repeated stress. But the improvement may be due wholly to the considerable chromium content of these so-called vanadium steels.
37. Tungsten steel, which usually contains from 5 to 10% of tungsten and from 1 to 2% of carbon, is used for magnets, because of its great retentivity.
38. Chrome-tungsten or High-speed Steel.—Steel with a large content of both chromium and tungsten has the very valuable property of “red-hardness,” i.e. of retaining its hardness and hence its power of cutting iron and other hard substances, even when it is heated to dull redness, say 600° C. (1112° F.) by the friction of the work which it is doing. Hence a machinist can cut steel or iron nearly six times as fast with a lathe tool of this steel as with one of carbon steel, because with the latter the cutting speed must be so slow that the cutting tool is not heated by the friction above say 250° C. (482° F.), lest it be unduly softened or “tempered” (§ 29). This effect of chromium, tungsten and carbon jointly consists essentially in raising the “tempering temperature,” i.e. that to which the metal, in which by suitable thermal treatment the iron molecules have been brought to the allotropic γ or β state or a mixture of both, can be heated without losing its hardness through the escape of that iron into the α state. In short, these elements seem to impede the allotropic change of the iron itself. The composition of this steel is as follows:—
| The usual limits. | Apparently the best. | |
| Carbon | 0.32 to 1.28 | 0.68 to 0.67 |
| Manganese | 0.03 to 0.30 | 0.07 to 0.11 |
| Chromium | 2.23 to 7.02 | 5.95 to 5.47 |
| Tungsten | 9.25 to 25.45 | 17.81 to 18.19 |
39. Impurities.—The properties of iron and steel, like those of most of the metals, are profoundly influenced by the presence of small and sometimes extremely small quantities of certain impurities, of which the most important are phosphorus and sulphur, the former derived chiefly from apatite (phosphate of lime) and other minerals which accompany the iron ore itself, the latter from the pyrite found not only in most iron ores but in nearly all coal and coke. All commercial iron and steel contain more or less of both these impurities, the influence of which is so strong that a variation of 0.01%, i.e. of one part in 10,000, of either of them has a noticeable effect. The best tool steel should not contain more than 0.02% of either, and in careful practice it is often specified that the phosphorus and sulphur respectively shall not exceed 0.04 and 0.05% in the steel for important bridges, or 0.06 and 0.07% in rail steel, though some very prudent engineers allow as much as .085% or even 0.10% of phosphorus in rails.
40. The specific effect of phosphorus is to make the metal cold-short, i.e. brittle in the cold, apparently because it increases the size and the sharpness of demarcation of the crystalline grains of which the mass is made up. The specific effect of sulphur is to make the metal red-short, i.e. brittle, when at a red heat, by forming a network of iron sulphide which encases these crystalline grains and thus plays the part of a weak link in a strong chain.
41. Oxygen, probably dissolved in the iron as ferrous oxide FeO, also makes the metal red-short.
42. Manganese by itself rather lessens than increases the malleableness and, indeed, the general merit of the metal, but it is added intentionally, in quantities even as large as 1.5% to palliate the effects of sulphur and oxygen. With sulphur it forms a sulphide which draws together into almost harmless drops, instead of encasing the grains of iron. With oxygen it probably forms manganous oxide, which is less harmful than ferrous oxide. (See § 35.)