TEMPERING, the art of imparting to metals, by means of heat treatment, a definite degree of hardness. The term is now applied almost exclusively to certain kinds of steel, which are used in the manufacture of tools. It is said that the ancients could harden and tem per copper; but this art, if it ever really existed, is now lost. The effects of thermal changes upon steel vary greatly with the quality of the steel and with the exact nature of the treat ment. It is necessary to distinguish clearly between and "temper ing.° Any steel (except those varieties that are alloyed with silicon, tungsten and certain other elements) may be annealed, but it is es sential that the steel shall contain a certain amount of carbon, in order that it may be ca pable of being hardened and tempered. If steel is raised to a red heat and is then allowed to cool very slowly, it becomes relatively soft, so that it can be filed and turned in a lathe. This process is called °annealing?' and it has usually been held that the slowness of the cooling is the essential thing in the softening process. There is excellent reason for believing, how ever, that the exact temperature to which the steel is exposed before it is cooled has a much greater influence than the rapidity of the cool ing. It has been shown, for example, by the researches of Brine!, Tschernoff, Le Chatelier, Heyn, Ridsdale, Stead and others that steel which has acquired a dangerously crystalline character from annealing for a long time at too low a temperature in a slightly oxidizing atmosphere, or from long continued heating at high tem peratures, may have its original structure and properties restored by the simple artifice of heating it to a certain critical temperature (which is about 1,600° F.), and then allowing it to cool; the rate of cooling, in this case, being a matter of comparative unimportance. Steels may sometimes be had which do not need special treatment to render them fit for use in certain classes of tools; the tool being ready for use after it has been forged and allowed to cool by natural exposure to the air. In general, how ever, a tool steel must receive special treatment in order to fit it for the work in hand; this treatment being given after the tool has been forged to shape. The process of tempering then
consists of two steps, the first of which consists in imparting to the cutting edge of the tool a degree of hardness that is too great for the work for which the tool is to be used, while the second step consists in (or °tem pering))) this hardness, until it attains a value that experience has shown to be satisfactory. The tempering of an ordinary tool may be de scribed as follows : The finished tool is heated to a bright red, care being taken to have the heat extend back some distance from the cutting edge. The cutting edge of the tool is then immersed in water to a slight depth and kept there until it has cooled sufficiently to remain wet when withdrawn from the water. By this means the steel is rendered exceedingly hard throughout the chilled part; that is, in the vicinity of the cutting edge. If it were used in this condition, however, the edge would be too brittle and would be likely to break in service. To reduce the hardness to the proper value, the tool, immediately after being with drawn from the water, is brightened up near the cutting edge with a piece of emery cloth, or in some similar manner, and the cleaned area is then watched while the heat from the un quenched part spreads toward the cutting edge. The oxidization of the steel, as the edge be comes hotter and hotter from conduction, causes a play of color to become visible, which serves as an index of the temperature. These colors run from the hot portion of the tool toward the quenched cutting edge. In the order in which they proceed, they may be described as pale yellow, straw yellow, brownish yellow, light purple, dark purple and blue. When the proper color reaches the cutting edge, the whole piece is again quenched, and the is com plete. The colors that are used for different implements are as indicated below: Very pale yellow (about 430° F.) : Steel engraving tools, turning tools, hammer faces, planer tools, wood-engraving tools.