��Popular Science Monthly
���The red hot steel about to be plunged into the oil tank for what is known as its "heat treatment." The tank extends down into the ground. The plunging of the hot steel into the oil suddenly cools it and hardens it and refines its grain or texture. After this it must be heated up again to temper it
��must be strong and that its life is short!
In the case of a "built-up" gun, as it is called when made of hoops or bands of steel, the outer tubes or rings are shrunk or sweated on — that is, they are heated so that they expand or swell a little, as all steel does when heated, and then while hot they are fitted over the inner part and allowed to cool and shrink, or contract. In so doing they fit very tightly on to the main tube. In making a wire-wound gun, the wire is wound or coiled around and around until more than one hundred miles of it has been wrapped around the big cannon. A 12-inch gun requires 117 miles of wire weighing about thirteen and one- half tons. Although the strength of the wire is such that it gives great resisting force to pressures exerted sideways, it does not bestow strength lengthwise. Therefore an extra thickness of metal must be put on the muzzle of the gun where the vibra- tion caused by the shell leaving the gun is the greatest.
The breech or back end of a gun is a very important part. Here the shell is inserted in a specially built chamber. After the shell is in place, the breech is closed by the shutting of a very complicated and strong door. It is fastened or fitted in the gun by extremely strong screws so that the charge will not burst the gun open at the back when it is fired.
Sometimes a shell explodes in the barrel of the gun. In a wire-wound gun the wire tends to prevent a grave disaster; it hinders the steel tubes from bursting into many pieces and flying in every direction. The solid gun is wholly built of tubes, while in the wire-wound gun there may be one or two tubes over which the wire is wound with the jacket tubes shrunk over the wire. A bush for the breech-ring is screwed into the rear end, which is also reinforced by a breech-ring outside.
Heat Treatment and What It Means
With all these precautions to make a big gun strong enough to withstand pres- sure, the result would not be successful except for the extreme care in making the steel and its "heat treatment." Steel in its crude state, or when originally cooled from its molten or liquid condition, is one mass of crystals relatively large and in- timately knit together. But when these crystals are large the steel is not as strong as when they are small and fine. The object of heat treatment is to render all