When hardening tools made from high-speed steel, it is necessary to vary the treatment to suit the particular class of tool. For instance, it is customary to heat ordinary lathe and planer tools nearly to the fusing point; in fact they are usually brought to a temperature that causes the edges and corners to drip, then placed in a strong blast of air, or dipped in cottonseed oil. When hardening reamers, taps, drills, milling-machine cutters, and other tools having slender projecting portions, or standard forms, it is not possible to heat them to such a temperature and preserve the shapes and slender portions, as they would be melted away; neither can they be cooled in an air blast, as the action of the air is to oxidize the slender portions and so to render the tool unfit for use. Most tools used in the lathe, planer, and similar machines, can be ground to shape after hardening, and the melting-away of the edges and corners does little or no harm; but taps, dies, reamers, and formed milling cutters must retain their shapes as they cannot be ground to form after hardening.

Lathe and planer tools of ordinary design may be heated in a fire of coke or well-coked blacksmith's coal, in an ordinary forge, although better results are obtained if they are heated in a furnace specially designed for high-speed steel; long slender articles, such as taps and reamers, give best results if heated in a furnace of the design shown in Fig. 22. This furnace is so constructed that the flame moves around the walls thus leaving a space at the center free from the direct flame. The tools are suspended from the top as shown and in the center, thus preventing oxidation of the steel.

Fig. 22. Cylindrical Hardening Furnace for High-Speed Steal.

Courtesy of American Gas Furnace Company, New York City.

Milling-machine cutters, punch-press dies, and many other forms of tools should be heated in an oven furnace as shown in Fig. 23. Tools heated in this form of furnace should not be placed on the bottom but on a piece of fire brick, as shown.

As the temperature in a furnace being used for heating highspeed steel is extremely high, cold tools should not be placed directly in the furnace, but should be preheated in an open fire or in a slow fire of some kind, brought to a red heat, and then put into the special furnace. The sudden and unequal expansion of a piece of cold steel when placed in contact with a very high temperature, would cause it to crack m various parts.