Brine may be used for this purpose; it is even better
than plain water. As soon as the hissing stops, remove the work from the
water or brine and plunge in oil for complete cooling.
[Illustration: Figure 6.--Cooling the Tool for Tempering]
In hardening high-speed tool steel, or air hardening steels, the tool
should be handled as for carbon steel, except that after the body reaches
a cherry red, the cutting point must be quickly brought to a white heat,
almost melting, so that it seems ready for welding. Then cool in an oil
bath or in a current of cool air.
Hardening of copper, brass and bronze is accomplished by hammering or
working them while cold.
_Tempering_ is the process of making steel tough after it has been
hardened, so that it will hold a cutting edge and resist cracking.
Tempering makes the grain finer and the metal stronger. It does not affect
the hardness, but increases the elastic limit and reduces the brittleness
of the steel. In that tempering is usually performed immediately after
hardening, it might be considered as a continuation of the former process.
The work or tool to be tempered is slowly heated to a cherry red and the
cutting end is then dipped into water to a depth of 1/2 to 3/4 inch above
the point (Figure 6). As soon as the point cools, still leaving the tool
red above the part in water, remove the work from the bath and quickly rub
the end with a fine emery cloth.
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