When taps, milling-machine cutters, and other tools having weak projecting portions are made from this steel, it is necessary to draw the temper in order to reduce the brittleness to a point where the tools will stand. To accomplish this, the surface is brightened and the temper drawn in the usual manner. The shanks of taps are plunged into red-hot lead, and allowed to remain there until they are red, when they are removed and buried in dry lime. The bodies of the taps are allowed to remain in the air. Annealing. High-speed steel is annealed by being packed in an iron box with dry fire clay or a mixture of lime and powdered charcoal, or some material that will exclude the air. A cover is placed on the box and luted with fire clay, which is allowed to dry before the box is put into the furnace. It is necessary to heat this steel more than ordinary steel, and to maintain the high heat longer. Generally it is heated to a yellow heat and allowed to remain at this temperature for a length of time that varies with the size of the pieces. For small pieces, 2 or 3 hours will suffice, but for extremely large blocks the high temperature must be kept up for 12 or 15 hours - after they are heated through. The steel should be allowed to cool slowly.
In many shops difficulty is experienced in hardening such articles as milling-machine cutters, forming tools for screw machines, and similar tools made from high-speed steel. The work can be done with uniformly satisfactory results if the tools are placed in an iron hardening box and surrounded with charred leather, a cover placed on the box and sealed, the whole being then put into the furnace and heated to a yellow heat. The articles should be kept in the furnace at this heat for several hours, the time depending on their sizes and shapes. For forming tools and milling-machine cutters of ordinary size 2 or 3 hours answer very well; smaller pieces should not be left in so long.
Fig. 23. Oven Furnace for Hardening Milling Cutters, Etc.
Courtesy of American Gas. Furnace Company.
New York City.
When the tools have been at the yellow heat for the proper length of time, they should be removed and plunged into a bath of raw linseed oil, and worked around in the oil until cool.