As forecast by Mushet, the essential constituent of the new steels is the metal, tungsten. But tungsten alone cannot give the desired property. Mushet, it will be remembered, was the metallurgist whose patents for the use of manganese in steel Bessemer was obliged to recognize to make his process a success, though the metal had earlier been used in crucible steel. The air-hardening property of Mushet’s steel was contributed by a happenstance combination of tungsten and this same metal, manganese. It later developed that tungsten and chromium were the best hardening elements and these have maintained their place, though refinements of the past few years have made use of vanadium, and, more recently, cobalt in addition. Usual amounts may be said to be tungsten 14 to 25 per cent, chromium 2 to 7 per cent, with vanadium ½ to 1½ per cent, and cobalt up to 4 per cent, perhaps. The carbon content is usually .6 to .8 per cent. Sometimes another comparatively rare metal, molybdenum, is used in high-speed steels in place of part of the tungsten, but its use does not seem to be on the increase.
Manufacturers differ considerably in formulas.
It will be noticed that at best there is left room for only 70 or 80 per cent of iron in the alloy. From certain standpoints, the high-speed steels might not at first thought be called “steels” at all since carbon seems to be of so little importance. They might be considered to be low carbon alloys somewhat similar to the newer “stellite” (an alloy from which tools are made), which contains little or no carbon and no iron but is made up mainly of cobalt and chromium. They fit in, however, with the general and very comprehensive scheme of classification of the iron-carbon alloys which has been developing over a period of twenty years and there is no doubt among metallurgists and metallographists that, as is the case with the alloy steels described above, they are iron-carbon alloys—in other words, steels—the properties of which have been greatly modified through the presence of the other elements. Carbon, therefore, is an essential, though it is much less in amount than in the carbon tool steels. The hardening and softening properties, also, very definitely classify these alloys with the “tool steels.” Stellite cannot be softened.
As with the carbon tool steels, most of the high-speed steels are made by the crucible method, though a small but increasing amount is of late being produced in the electric furnace. After careful pouring into small ingots and cooling, the ingots are removed from the iron molds and “topped” to remove any “pipe” or unsound portion. Then, if without defect and satisfactory as to analysis, they are slowly and carefully heated to forging temperature and are hammered out into bars. By this method they are taken nearly down to the final size desired. The bars are finished by rolling to size. After careful annealing they are ready for shipment to the tool maker.
When taking heavy cuts a tool of to-day may exert as much as ten tons’ pressure against the metal it is cutting and the advent of this wonderful material for tools necessitated the building of immensely heavier and stronger lathes and other machines, which, alone, were capable of giving them power to do their work. The high-speed steels, therefore, have revolutionized metal-cutting practice and shop methods and have very largely aided efficiency.
CHAPTER XVI
THE MECHANICAL TREATMENT OF STEEL
Molten steel is practically always poured into upright molds of cast iron which shape it into long slightly tapering blocks of metal of square or rectangular cross-section. After the ingot mold has been stripped off, the still red-hot ingot cannot well be taken directly to the rolls, for, while the exterior parts may have the proper temperature for rolling, the interior of the ingot may still be liquid. The ingot, throughout, should be uniform in temperature when it is rolled. It is therefore put into a closed pit or furnace of proper temperature where the center of the ingot can be cooling while the outer portions are kept hot or are reheated if necessary, until all is ready for the rolling operation.
It would take a “steel man” a long time to tell you all of the unfortunate things that can and do happen to such blocks or ingots of steel which influence their applicability to the purposes for which they are intended. You must have learned of the most serious of these—“pipes,” “cracks,” “segregation,” etc., through reports of investigations of broken railroad rails and accidents caused thereby. A word or two regarding these:
In the ingot mold the outside of the steel ingot is, of course, the first to solidify. It may be hours after the freezing of the outer crust before the interior is able to cool sufficiently that it, too, can set. As steel, like most other metals and alloys, occupies less space when “frozen” than it does when molten, there must occur a hollow space in the interior since the crust is solid and cannot contract much. This hollow space usually takes the form of a more or less elongated cavity extending along the axis of the upper quarter of the ingot. It is called a “pipe.”
