It is made of a special kind of steel known as nickel-chrome gun steel, formed by adding certain proportions of the two rare metals nickel and chromium to the mixture of iron and carbon which we ordinarily call steel. The metal is made after the manner described in another chapter and is cast into the form of suitably-sized ingots which are afterwards squeezed in enormous hydraulic presses into the
rough shape required. Besides giving the metal the desired form this action has the effect of improving its quality. Since a gun is necessarily a tube it may be wondered why the steel is not cast straight away into that shape instead of into a solid block and the reason why that is not done is very interesting. It is found that any impurities in the metal—and it is impossible to make it without some impurities—collect in that part which cools last and obviously that part of a block which cools last is the centre. Thus the impurities gather together in the centre of the mass whence they are removed when that centre is cut away, whereas if the first casting were a tube they would collect in a part which would remain in the finished gun.
The ingot, then, is cast and pressed roughly to shape. Then it is put into a lathe where it is turned on the outside and a hole bored right through the centre.
But that is by no means all of the troubles through which this piece of steel has to pass. It undergoes a very stringent heat treatment, being alternately heated in a furnace to some precise temperature and then plunged into oil, whereby the exact degree of hardness required is attained.
Moreover, this is only one of the tubes which go to make up the gun, which is a composite structure of four tubes placed one over another with a layer of tightly wound wire as well.
First, there is the innermost tube, the whole length of the gun, then a second one outside that, usually made in two halves. Both are carefully made to fit,
and then the outer is expanded by heat to enable it to be slidden over the inner one, after which on cooling it contracts and fits tightly. Outside this second tube is wound the wire, or more strictly speaking tape, for it is a quarter of an inch wide and a sixteenth thick. It is so strong that a single strand of it could sustain a ton and a half. It is carefully wound on; first several layers running the whole length of the gun and then extra layers where the greatest stresses come, that is to say, near the breech, for that has to withstand the initial shock of the explosion. Altogether about 130 miles of wire go on a single gun.
The advantages of this form of construction are many. For one thing, a wire or strip can be examined throughout its whole length and any defect is sure to be found, whereas in a solid piece of steel, no matter how carefully it may be made, there may lurk hidden defects. Moreover, if a solid tube develops a crack anywhere it is liable to spread, whereas a few strands of wire may be broken without in any way affecting the rest. It has been found that even if a shell burst while inside one of these guns no harm is done to the men in the turret where it stands, a thing which cannot be said for guns composed entirely of tubes, so that the merit rests with the wire. A third advantage is that the wire can be wound on to the tube beneath it at precisely that tension which is calculated to give the best result, whereas in shrinking one tube on to another this cannot always be attained.
Over the wire there come two more tubes not
running the whole length but meeting and overlapping somewhat near the middle, so that at one point there are actually four concentric tubes besides the wire.