The general process of tempering in described in "Metallurgy." Steel is hardened by cooling suddenly and softened by cooling slowly from a red heat. In hardening, care should be taken to plunge the tool into the water or oil vertically, so that there may be no uneven cooling of the sides. For iron working tools, plunge into the water only that portion of the tool that is to be tempered. This avoids brittleness in the supporting portions. It also enables enough heat to be stored in the metal to supply that needed for tempering:
As wood working tools are lighter, and as a partial cooling would be apt to cause warping or even cracking of the metal.it is customary to plunge them into the bath. The result is that they are cooled and hardened throughout. The reheating of these tools is done by means of an outside source, which must be capable of imparting a uniform temperature to all parts of the tool. If the beat so imparted is not uniform the tool will be warped. Where a large number of tools are to be tempered an anthracite coal fire is used. For this purpose a furnace similar to that illustrated in Fig. 55 is used. Where an anthracite fire is not available a hot iron can be adrantageoufcly employed. Take a piece of flat iron an inch or more in thickness and having a larger surface than that of the tool to be; tempered. Heat it to a bright cherry red and laying it on the anvil, hold the tool about half an inch above it. Hold by the edges as shown in Fig. 72, if possible. When the heat from the metal has raised the tool to the desired temperature as indicated by the color, planga it into the bath of water or oil. Do not plunge it in flatwise as held in Fig. 72. The cooling of the tinder side of the tool before the upper will cause warping. Put the tool into the bath edgewise.
Another method of heating large wood working tools, such as saw plates and the like, is to submerge them in a bath of molten lead. This communicates an absolutely even temperature to the plate. It also has the advantage that no overheating can occur. The color at which the desired temperature is attained for various classes of tools is given in "Metallurgy".
Annealing is a process of softening steel. It is done bo that it may be easily worked with hand or machine tools. It is accomplished by slowly cooling from a red heat. The usual method is to heat the piece red hot and then bury in lime or ashes. It is allowed to remain there until it is thoroughly cold. Twelve hours should be allowed for this work. For this purpose lime is better than ashes, as it is a poorer conductor of heat ; hence the piece cools more slowly.
Another and quicker way of annealing is by the water method. The steel is heated to a cherry red and allowed to cool in a dark place until the red just disappears. It is then quickly plunged into cold water. This method should never be used when there is time to wait for the other.