America's Munitions 1917-1918

by Benedict Crowell; United States. War Department

During all of these operations with the ingot, the steel is largely in the soft condition in which it left the forging press. As is well known, steel is capable of taking many degrees of "temper." Temper is an old term that no longer is quite descriptive of the condition desired or obtained, but it is sufficiently expressive of the condition desired for the purposes here. This condition is one of a certain degree of hardness—greater than that ordinarily carried by the soft steel—combined with the greatest obtainable degree of toughness. This combination of hardness and toughness produced to the proper degree resists the explosive power of the powder and also causes the wear on the gun in firing to be diminished and made as slight as possible.

To effect this combination of hardness and toughness it is necessary to take the bored and turned tubes of the guns and suspend them by means of a specially made apparatus in a furnace where they are heated for a period of perhaps eight hours to a temperature of approximately 1,500° F. or a bright-yellow color, uniform in every part of the piece.

After being subjected to this treatment for the time mentioned, the tube is then conducted by means of a traveling crane apparatus to a tank of warm water in which it is dipped and the heat rapidly taken from it down to a point of practically atmospheric temperature. This "quench" as it is called, produces the required degree of hardness called for by the ordnance officers' design; but the piece has not yet got the required degree of toughness. This toughness is now imparted to the hard piece by heating it once more in another furnace to a temperature of approximately 1,100° F., or a warm rosy red, for a period of perhaps 14 hours. From this temperature, the piece is allowed to cool naturally and slowly to the atmospheric temperature.

The ordnance inspectors at this point determine whether the piece has the required properties in a sufficient degree, by cutting from the tube a piece 5 inches long and ½ inch in diameter. The ends of this piece are threaded suitably for gripping in a machine. The piece is then pulled until the half-inch stem breaks. The machine registers the amount of force required to break this piece and this gives the ordnance engineer his test as to the degree of hardness and toughness to which the piece has been brought by the heat treatment processes just described.

A satisfactory physical condition having been determined by pulling and breaking the test pieces described, the whole forging is sent to the finishing shop where it is machined to a mirror polish on all its surfaces. The diameters are accurately measured and the forgings assembled into the shape of a finished gun.

In this process there is required a different kind of care and accuracy. Up until this time the care has been to provide a metal of proper consistency and quality. From this point forward the manufacture of a gun requires the machining and fitting of this metal into a shape and form so accurate that the full strength of the gun and the best accuracy of fire may be attained.

To explain how and why hoops are placed upon the gun tubing and how the various hoops are shrunk from the outside diameter of the gun will require a few lines.

Cannon are made of concentric cylinders shrunk one upon another. The object of this method of construction is twofold. The distinctly practical object is the attainment throughout the wall of each cylinder of the soundness and uniformity of metal which is more certainly to be had in thin pieces than in thick ones; the other object is more closely connected with the theory of gun construction.

When a hollow cylinder is subjected to an interior pressure the walls of the cylinder are not uniformly strained throughout their thickness, but the layer at the bore is much more severely strained than that at the outside. This can be readily seen if we consider a cylinder of rubber, for example, with a bore of 1 inch and an exterior diameter of 3 inches, which are about the proportions of many guns. If we put an interior air pressure on the cylinder until we expand the bore to 2 inches, the exterior diameter will not thereby be increased 1 inch. But supposing that it were increased as much as the bore, that is, 1 inch, we would have the diameter, and therefore the circumference, of the bore increased 100 per cent, and the circumference of the exterior increased 33⅓ per cent. That is, the layer at the bore would be strained three times as much as that at the exterior, and the interior layer would commence to tear before that at the exterior would reach anything like its limit of strength. The whole wall of the cylinder therefore would not be contributing its full strength toward resisting the interior pressure, and there would be a waste of material as well as a loss of strength. Let us now consider, instead of our simple cylinder, a built-up cylinder composed of two concentric ones, the inner one of a bore originally a little greater than 1 inch, and the outer one of exterior diameter a little less than three inches, originally; so that when the outer one is pressed over the inner one (its inner diameter being originally too small for it to go over the inner one without stretching) the bore of the inner one is brought to 1 inch, and the exterior of the outer one to 3 inches. We now have a cylinder of the same dimensions as our simple one, but in a different state; the layers of the inner one being compressed and those of the outer one extended.

If now we commence to put air pressure on the bore, we can put on a certain amount before we wipe out the compression of the inner layer, and bring it to a neutral state, and thereafter can go on putting on more pressure until we stretch the inner layer 100 per cent beyond the neutral state, as before; which would take just as much additional pressure as the total pressure which we employed with our simple cylinder. We have therefore gained all that pressure which is necessary to bring the inner layer of our built-up cylinder from its state of compression to the neutral state. If we have so proportioned the diameter of junction of our inner and outer cylinders and so gauged the amount of stretching required to get the outer one over the inner one that we have not in the process caused any of the layers of the outer one to be overstrained, the gain has been a real one, attained by causing the layers of the outer cylinder to make a better contribution of strength toward resisting the interior pressure. This is the theory of the built-up gun.