In order that it may be soft enough to work easily, tool steel must be annealed. The process consists in heating the metal to a uniform red heat and allowing it to cool slowly. Steel can generally be bought annealed more cheaply than it can be annealed when needed in the factory.
Annealing removes the strains, or the tendency of the steel to crack and spring when hardened. Strains are caused by rolling and hammering in the steel mill or forge shop. In order to remove the tendency to spring, the piece of steel should be machined somewhere near to size, sufficient stock being left to machine all over after the annealing. If the piece is to have a hole in it, such as a milling machine cutter blank, the hole should be drilled somewhat smaller than finish size - 1/16-inch is the amount generally allowed - and the piece turned in a lathe to remove all the outer surface which contains the marks of the hammer or rolls. The piece is now ready for annealing, which may be done in one of several ways.
For this method, it is necessary to have a furnace large enough to hold an iron box of a size sufficient to take the piece to be annealed. To do the work cheaply, enough pieces should be annealed at a time to fill one or more boxes, according to the capacity of the furnace.
The material used in packing the box is wood charcoal, which should be ground or pounded until the particles are about the size of a pea. A layer of charcoal covering the bottom to a depth of 1 inch is first placed in the box, then a layer of steel. The different pieces of steel should not come within 1/2 inch of each other, nor within 1 inch of the box at any point. The spaces between the pieces are filled with the charcoal, and they are covered to a depth of 1 inch. Another layer of steel may be put in, if the box is of sufficient size. When within 1 1/2 inches of the top, the remaining space is filled with charcoal, the whole tamped down, the cover put on, and the edges luted around with fire clay to prevent the direct heat of the fire entering the box.
Fig. 12. Diagram Showing Method of Annealing Tool Steel.
There should be several 1/4-inch holes drilled through the cover near the center, and through each of these, a piece of 2/16-inch wire should be placed, as shown in Fig. 12. The wires should extend to the bottom of the box and project about 1 inch above the top of the cover, so that they may be readily grasped by the tongs. These wires are to be drawn from the box and tested in order to determine the temperature of the contents. The box should be placed in the furnace, and after it has become thoroughly heated, one of the wires is drawn out by means of a pair of long tongs. If no such tongs are available, the legs of ordinary tongs may be lengthened by pieces of gas pipe. If the wire is not red hot, the heating process should be continued for 10 or 15 minutes longer. Then another wire is drawn, and the process kept up until a wire is drawn that is red the entire length. The work should be timed from the moment the box is heated through; this is shown by the wire.
The heat should be maintained a sufficient length of time to insure a uniform heat, which should not be allowed to go above a full red. The length of time the pieces remain in the fire depends somewhat on the size; for steel 2 inches or less in diameter, one hour after the box is heated through will do; larger pieces require a little longer time. After running for the necessary length of time, the heat should be shut off, and the boxes allowed to cool slowly; the pieces should be left in the box until cold.
This method of annealing gives satisfactory results with large pieces to be used for certain purposes, but for light, thin materials, its use is not advised, as the steel remains red hot for too long a period. When articles of this description are annealed, they should be heated to a low red, then placed in an iron box having two or three inches of hot ashes in the bottom, the hot ashes being used to prevent chilling.
When there are no facilities for annealing by the method above described, the steel may be heated to a uniform red and placed on a piece of board in an iron box, there being one or two inches of ashes under the board. A second piece of board should be placed on the steel and the box filled with ashes. The pieces of wood will smolder and keep the steel hot for a long time.
Another common method of annealing tool steel is to heat the piece to a red heat and bury it in ashes or lime. To secure satisfactory results the ashes or lime should also be heated, which can be accomplished by first heating a large piece of iron and then burying it in the contents of the annealing box. When the steel to be annealed is sufficiently heated, the piece of iron may be removed and the piece to be annealed put in its place, and thoroughly buried in order that it may take a long time in cooling. It should be allowed to remain in the ashes or lime until cold.
There is another method of annealing practiced in some shops, known as the water anneal, which answers in an emergency, but is not recommended for general use. The piece of steel is heated to a low red, making sure that the heat is uniform throughout. It should be removed from the fire and held in the air where no draft can strike it until not a trace of red can be seen when the piece is held in a dark place. It should then be plunged in water and allowed to remain until cold. Better results may be obtained if it is plunged in soapy water or oil.
In a previous paragraph it was stated that there are reasons for annealing steel other than to soften it. It may be necessary to impart some quality that can be given only by annealing; it may be necessary to toughen and strengthen a spindle or other piece, and at the same time, to have it workable. Use of the following method will secure such results.
The steel is heated red hot and plunged into oil where it is allowed to remain until cool; it is then heated again to a low red, removed from the fire, and allowed to cool in the air where no draft can strike it, and where no moisture can come in contact with it.
Steel annealed by this method is very tough, yet it can be bent to a greater degree than if annealed by any of the other methods.