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.” [Illustration: PIPE AND BLOWHOLES IN AN INGOT OF STEEL ] Then, too, the metalloids of the steel do not always stay where they belong. Even if the steel has been of a uniform chemical composition when poured, the interior portions of the ingot after cooling will be found to have a greater amount of sulphur, phosphorus and carbon than parts which are nearer the surface. Such gathering together of constituents of the steel is known as “segregation.” With the development of the steel industry and the demand for greater and greater tonnages, ingots have been made larger and larger. Piping, segregation, etc., are very naturally accentuated in the large masses of steel.