Chapter Nine Hardening And Tempering

After all the pistol's parts are shaped to their finished dimensions, they must be hardened and tempered. The theory connected with tempering is quite simple, even though considerable confusion exists concerning it. In practice, though, the tempering/hardening procedures can be quite difficult to properly accomplish. When steel is heat-treated, certain changes in the metal's molecular structure result which alter some of its physical properties. Therefore, the most crucial aspect of this process is temperature. Steel heated insufficiently will not harden properly, while steel heated too much will actually burn.

To further clarify these concepts, I will briefly define the results obtained by heat treatments, which are: annealing, hardening, and tempering.

Annealing is the process of softening metal by heating it to a high temperature and allowing it to cool slowly. In most cases steel is annealed by slowly and completely cooling it from a cherry red heat. Ideally, this is done by putting the steel in a cast iron box and covering it with some material such as sand, ashes, or fire clay. The box is heated in a furnace to the required temperature, after which the box and contents are removed, and cooled slowly enough to prevent any hardening. The material placed around the steel serves to exclude air, preventing oxidation. The sand also retains heat, allowing the metal to cool more slowly. A somewhat crude annealing method is the wood fire method described in Chapter Seven.

Hardening is the process of raising the temperature of the steel up to its decalescence point and then quenching it in a suitable cooling medium.

In practice, the steel to be hardened is heated slightly above its decalescence point. This insures that the temperature of the steel is not below its decalescence point, and also allows for a slight loss of heat while transferring the steel from the heating source to the quenching bath.

A tempering furnace is the best way to harden steel. Such furnaces are designed specifically to heat steel evenly to high temperatures for tempering purposes. They are also shielded for protection of those working with them.

In the event no furnace is available, find another suitable source of heat. Large parts of even thickness can be heated with an acetylene torch. But the torch method is not suitable for thin and irregularly-shaped parts. The thin pieces are likely to burn when heated with a flame. To heat a small part safely, place it on a large piece of iron or steel, and heat the bottom piece to the desired temperature. The small part to be hardened will soon reach the required temperature, and can then be removed and quenched.

Parts of irregular shape, having both thick and thin sections, are liable to overheat in the thin areas before the thicker portions reach the proper temperature. For this reason, a lead bath is the best way to heat these parts. To prepare one, melt lead in a cast iron pot and cover its surface with powdered charcoal to prevent oxidation. The parts to be hardened are immersed in the lead bath and heated until they reach a red heat, then quenched.

Despite the irregularities of this system, it has many advantages, and may be used for hardening ortempering. Baths, whether salt, lead or oil, hold temperatures more uniformly than any other method, and are easier to maintain at specific temperatures. Parts also heat more rapidly in such baths. Just be certain that the bath's temperature is uniform before using it. Cold steel can not be immersed in a heated bath, due to the danger of cracking it. The steel must be preheated to around 300 degrees F. before placing it in the bath. Preheating is not necessary if the bath is at a comparatively low temperature when the steel is placed in it. The bath is then heated to the desired temperature. This last procedure is most commonly done with oil or salt baths, which remain liquid at low temperatures.

When parts are taken from an oil bath, immerse them in a tank of caustic soda, then water, to remove any oil adhering to the part.

After hardening, the parts are ready for tempering. That is because the hardening process has not only made the parts extremely hard, but also brittle.

Tempering is the process of reheating previously hardened steel, then quenching it, resulting in tougher, less brittle steel.

The worst part of it is the tempering process softens the metal as well as toughening it. Toughness and also softness obtained in tempering depend on the degree of heat to which the metal is raised. The higher the heat used, the less brittle it will be and also less hard.

Tempering hardened steel is best done in a specially prepared tempering bath; a metallurgist's high temperature thermometer is used in conjunction with the bath.

Another, only slightly less satisfactory method, is based on observing the steel's color while it is being heated.

Heated steel becomes covered with a very thin oxidation film that changes color as the temperature rises. Commonly, this color variation is used as an indication of the steel's temperature and its corresponding temper.

Heated sufficiently, the oxide film will pass from a very pale yellow through brown, blue and purple. At the time the desired color appears, the steel is quenched in water or brine.

The color scale of temperatures, while standard for many years, is regarded as indicating only a rough approximation of the steel's temperature. Also, this color scale varies for different steels.

Keep in mind that the methods and following color chart in this chapter apply to carbon steel only. Certain alloy steels require entirely different methods of heat treatment.


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