TOOL STEEL. This term includes steels used for machine tools, hand tools and cutlery. Prior to 1870 these were exclusively high carbon steels made by crucible or cementation process ; since then alloy steels have been introduced for cutting metals. Plain carbon steels of high quality are still pre-eminent for wood work ing, stone cutting and cutlery. The amount of carbon ranges from 0.6o to 1.30% the lower carbon content giving a moderate hard ness but great toughness. Sledges, chisels, picks, rock drills, axes, woodworking tools, files, cutlery and razors, each item needs greater and greater hardness and more ability to maintain a cut ting edge, with toughness progressively less important as the carbon content increases.
With carbon steels, if worked at high rates of speed, the friction between chip and tool-nose generates enough heat to "draw the temper" of the tool; that is, to soften it to a lesser hardness than the material being cut. This is avoided by using special alloys that temper very slowly, retain their hardness at high heats, or are intrinsically hard and require no heat treatment. Mushet steel (q.v.) is of the first category, and was introduced about 1870.
High speed steel (q.v.), which has the property of hardness at a red heat, is now widely used in machine shops for mass production of iron and steel parts. Stellite (although really not a steel) is an example of the third class of hard tool material. Another mate rial of the greatest hardness, introduced in 1928, is an alloy of tungsten carbide with up to 12% cobalt, that has already proven very useful for tough or abrasive substances like aluminium bronze, Hadfield's manganese steel and non-metallic electrical insulation.
Carbon steels owe their hardness after quenching to submicro scopic particles of hard iron carbide cemented together with the softer ferrite (pure iron). Mushet found that the metals tung
sten and chromium combined with high-carbon steel slowed down the softening reactions, so that a drastic quenching from a high heat was unnecessary. In 190o, Taylor and White disclosed the fact that, if Mushet's air-hardening tungsten-chromium steels were cooled from a sweating temperature and then tempered at a high heat, they would retain their hardness at a red heat.
Progress since then has come in working out the optimum com position in tungsten, chromium, carbon, and vanadium. Recently a satisfactory variant where most of the tungsten is replaced by cheaper molybdenum has been promoted as a "strategic" material in countries lacking ample supplies of tungsten. The revolution in machine shop practice was primarily due to the hardening and stabilizing effect of tungsten, chromium, and iron carbides in tool steel. More recently the addition of the metal cobalt in amounts up to 15% has been found to give a more durable cut ting edge at a red heat. Meanwhile Elwood Haynes had discovered stellite, a family of alloys essentially of tungsten, chromium and cobalt. This alloy soon established itself for machining hard cast iron ("semi-steel") and for rough turning medium steels. Variants of the composition have been used for wire-drawing dies. But this swing to hard metals seems to be a passing phase. The newest tool material is a hard carbide of tungsten with a softer cobalt metal as a binder. Developed in Germany in 1926 this was mar keted as "Widia" ; in America a well-known brand is called "Car boloy." Chemical compositions of commonly used American tool steels are within the ranges shown in table below. See IRON AND STEEL.
(E. E. T.)