Tempering and Annealing.
For rock drills, cold chisels, milling and other tools it is necessary to use steel carefully tempered, so that brittleness is greatly reduced while considerable hardness and cutting power remain. Other changes of properties, as remarkable, follow upon subjecting steel to greater heat than that used for tempering. Says Professor Roberts-Austen:—“Three strips of steel identical in quality are taken. By bending one it is shown to be soft; if it is heated to redness and plunged in cold water it will become hard and will break on any attempt to bend it. The second strip, after heating and rapid cooling, if again heated to about the melting point of lead, will at once bend readily, but will spring back to a straight line when the bending force is removed. The third piece may be softened by being cooled slowly from a bright red heat, and this will bend easily and remain distorted. The metal has been singularly altered in its properties by comparatively simple treatment, and all these changes, it must be remembered, have been produced in a solid metal to which nothing has been added, and from which nothing has been taken away.”
It is the comparative slowness of cooling in oil, the greater slowness of cooling in air, that make these by far the best tempering processes, because the molecular re-arrangement, in which tempering consists, requires time. Often the critical temperature, at which a desired re-arrangement takes place, is declared by the metal losing all power of response to a magnet: this fact affords the steel-maker welcome aid; he has only to shut off heat as soon as his steel ceases to attract a magnet and plunge the steel into water in order to obtain the hardness he wishes.
The complex phenomena of heat treatment in steel manufacture are fully discussed by Professor H. M. Howe, in his “Iron, Steel and Other Alloys,” second edition, 1906.