Heat Treatment

■ have gone through a lengthy description I concerning heat treatment several times I before, in Vol. I and in Home Workshop I Prototype Firearms, to name just a cou-^^^ pie. I see no point in repeating it here. What is pertinent here is simply that we obtain a satisfactory degree of hardness and ductility for the component parts of this gun. Therefore, that is what we will concern ourselves with.

The upper and lower receivers, trigger housing, and barrel will not require any sort of heat treatment. These parts should simply be polished as desired, subjected to a suitable finish, and otherwise left alone.

The breech block, or bolt, should be heat-treated to a hardness consistent with a reading of 35 to 38 on the Rockwell "C" scale. This is done not only to prolong wearability, but also to prevent battering and deformation, especially in the bolt face and sear notch areas in the open-bolt version and in the area where the hammer contacts the bolt in the closed-bolt version. This part contains too much mass to heat satisfactorily with a torch; therefore, a furnace should be used.

Numerous machine shops in a given area will have furnaces suitable for this, and most will provide this service—even though it may seem like they are charging an exorbitant amount for it. Keep in mind, though, that it takes a considerable amount of electricity or gas to heat such a furnace, which must be paid for.

If you used SAE 4140 steel as I recommended, you should ask the heat treater to heat the part to between 1,525 and 1,625°F and quench it in oil. If then drawn at 900 degrees, it should have a hardness equal to C37 Rockwell, which is adequate. Most automobile axle material can be treated in a similar fashion. However, because we do not know the exact composition of the steel used here, try a small scrap first to determine the correct procedure. If the results are unsatisfactory, adjust the temperatures until you find the correct combination.

The small parts, including the sears, hammer, magazine latch, and trigger bar, should be somewhat harder than the bolt. Again assuming that 4140 was used as material, these parts should be heated to between 1,525 and 1,625°F

and quenched. They are then drawn at 800°F, which gives a hardness reading of C38. These parts, having little bulk, can be heat-treated with a torch if necessary.

In practice, you will need at least a quart of SAE 10 motor oil in a container with a large open top. Also required is an oxy-acetylene welding outfit capable of heating the material quickly. The small parts can be suspended from heavy steel wire, such as that coat hangers are made from, while heating.

The parts can be suspended singly directly over the oil container, using the heavy wire as a handle, and heated through use of an oxy-acety-lene torch adjusted to give the hottest flame possible. The material is heated to a bright, clear, glowing red, devoid of any yellowish tinge. This is the "cherry red" so often mentioned in connection with heat-treating activities. It should be held at this heat for a short time to ensure a complete saturation of heat. Then plunge it into the container of oil, which is at room temperature or slightly warmer. It should now be so hard a file won't touch it. If it is not, try a scrap of the same

Tourch Heat Treat Colors

Small parts can be heat-treated using a torch.

material at a slightly hotter temperature, and when the proper combination is found, apply it to the parts to be hardened.

The hardened part is then polished bright to allow observance of the color and placed on a fairly thick steel plate. The torch flame is then placed on the part and the plate it is resting on. It will begin to change color as it takes on heat, going from pale yellow to yellowish brown to purple to dark blue, which becomes progressively lighter as the temperature increases. When the part reaches a pale blue, the heat should be withdrawn and the part allowed to cool.

Another method that can be used on parts made from low-carbon steel such as 1018, or similar, involves the use of surface-hardening compounds such as Kasenit, Hard-N-Tuff, and others that are sold by gunsmith supply houses as well as welding supply stores. These compounds, when used as directed, impart a hard surface to the part so treated while retaining a soft core. The instructions that accompany all of these should be followed closely since use of each individual brand may differ slightly.

Using Torch

Small parts can be heat-treated using a torch.

As part approaches quenching color, move it closer to quenching bath.

Kasenit Surface Hardening CompoundMachine Guns Coloring Pages

When the desired temperature is reached, as deter-mined by color, drop the part into an oil bath.

The hardened part as removed from quenching bath.

After quenching, polish the part to a bright shine and heat to desired tempering color. Place it on a heavy plate, as shown, while heating to ensure an even temperature .

Low-carbon steel can be hardened using compounds such as Kasenit, as shown. Several other similar com-pounds are also available.

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