to the circumference of the bore; these are tested to ascertain the quality of the steel in the soft state.
It is found that the quality of the steel is greatly improved by forging, so long as this is not carried so far as to set up a laminar structure in the metal, which is thereby rendered less suitable for gun construction—being weaker across the laminae than in the other directions. It is then termed over-forged.
If the tests are satisfactory the forging is rough-turned and bored, then reheated to a temperature of about 1600° F., and hardened by plunging it into a vertical tank of rape oil. This process is a somewhat critical one and great care is observed in uniformly heating, to the required temperature, the whole of the forging in a furnace in close proximity to the oil tank into which it is plunged and completely submerged as rapidly as possible. In some cases the oil in the tank is circulated by pumping, so that uniformity of cooling is ensured; and, in addition, the oil tank is surrounded by a water jacket which also helps to keep it at a uniform heat. The forging is subsequently again heated to about 1200° F. and allowed to cool slowly by being placed in warm sand, &c. This last operation is termed annealing, and is intended to dissipate any internal stress which may have been induced in the forging by any of the previous processes, especially that of oil-hardening. After this annealing process a second set of test pieces, two for tensile and two for bending test, are cut from each end of the forging in the positions above mentioned; for guns of less than 3-in. calibre only half this number of test pieces is taken; and with hoops of less than 48 in. in length the test pieces are taken only from the end which formed the upper part of the cast ingot.
In all cases of annealed steel the test pieces of 2 in. length and 0·533 in. diameter must give the stipulated tests according to the character of the steel. For breech screws the steel is made of a harder quality, as it has to resist a crushing stress. These are the tests required in England, but they differ in different countries; for instance in France a harder class of carbon steel is employed for hoops, in which the tensile strength must not be less than 44·5 tons, nor the elastic limit less than 28·5 tons per square inch, neither must the elongation fall below 12%.
Fig.16.—Lathe used in Gun Construction.
Assuming that the tests of the annealed forging are satisfactory, the forging, which we will suppose to be a barrel, is tested for straightness and if necessary rectified. It is then rough-turned in a lathe (fig. 16) “to break the skin” (as it is termed technically) and so prevent warping during the subsequent operations. It is then bored out to nearly the finished dimension and afterwards fine turned on the exterior. In the meantime the other portions of the gun are in progress, and as it is far easier to turn down the outside of a tube than to bore out the interior of the superimposed one to the exact measurements required to allow for shrinkage, the interior of the jacket and other hoops are bored out and finished before the exterior of the internal tubes or of the barrel is fine turned. The process of boring is illustrated in fig. 17. The barrel or hoop A, to be bored, is passed through the revolving headstock B and firmly held by jaws C, the other end being supported on rollers D. A head E, mounted on the end of a boring bar F, is drawn gradually through the barrel, as it revolves, by the leading screw K actuated by the gear G. The boring head is provided with two or more cutting tools, and also with a number of brass pins or pieces of hard wood to act as guides, in order to keep the boring head central after it has entered the barrel. The revolving headstock B is driven by a belt and suitable gearing.
With wire guns the procedure is somewhat different. The wire is wound on to its tube, which has been previously fine turned; the exterior diameter of the wire is then carefully measured and the interior of the covering tube or hoop finished to suit. The covering hoop is allowed usually only a small shrinkage, or sometimes none, as it is simply intended as a protection to the wire and to give longitudinal strength; but in order to place it over the wire it must be heated and thus some little contraction always does take place on cooling. The heat to which these hoops are brought for shrinking never exceeds that used in annealing, otherwise the modifying effects of this process would be interfered with.
In the earliest modern type B.L. guns, the breech screw engaged directly with a screw thread cut in the barrel, which thus had to resist a large portion, if not all, of the longitudinal stress. This was also the system first adopted in France, but there are certain objections to it, the principal being that the barrel must be made of large diameter to meet the longitudinal stress, and this in consequence reduces the circumferential strength of the gun. Again, the diameter of the screw is always considerably larger than the breech opening, and so an abrupt change of section takes place, which it is always best to avoid in structures liable to sudden shocks. The thick barrel, however, gives stiffness against bending and, moreover, does not materially lengthen with firing; thin barrels on the other hand are gradually extended by the drawing out action of the shot as it is forced through the gun. In some large guns with excessively thin barrels this action was so pronounced as to entail considerable inconvenience. In the English system the breech screw is engaged either in the breech piece, i.e. the hoop which is shrunk on over the breech end of the barrel, or in a special bush screwed into the breech piece. This latter method suits the latest system of construction in which the breech piece is put on the barrel from the muzzle, while with the earlier type it was put on from the breech end.
With the earlier modern guns short hoops were used whenever possible, as, for instance, over the chase, principally because the steel in short lengths was less likely to contain flaws, but as the metallurgical processes of steel making developed the necessity for this disappeared, and the hoops became gradually longer. This has however, increased correspondingly the difficulties in boring and turning, and, to a much greater extent, those encountered in building up the gun. In this operation the greatest care has to be taken, or warping will occur during heating. The tubes are heated in a vertical cylindrical furnace, gas jets playing both on the exterior and interior of the tube. When sufficiently hot, known by the diameter of the tube expanding to equal previously prepared gauges, the tube is raised out of the furnace and dropped vertically over the barrel or other portion of the gun (fig. 18, Plate II.). In cooling it shrinks longitudinally as well as circumferentially, and in order to avoid gaps between adjoining tubes the tube is, after being placed in position, cooled at one end by a ring of water jets to make it grip, while the other portions are kept hot by rings of burning gas flames, which are successively extinguished to allow the hoop to shorten gradually and thus prevent internal longitudinal stress. A stream of water is also directed along the interior of the gun during the building up process, in order to ensure the hoop cooling from the interior. After the building up has been completed, the barrel is fine-bored, then chambered and rifled. The breech is then screwed either for the bush or breech screw and the breech mechanism fitted to the gun.
Fig. 17.—Boring.
In order to obtain additional longitudinal strength the outer tubes are so arranged that each hooks on to its neighbour from muzzle to breech. Thus, the chase hoop hooks on to the barrel by a step, and the succeeding hoops hook on to each other until the jacket is reached which is then secured to the breech piece by a strong screwed ring. In all the latest patterns of English guns
there is a single chase hoop covering the forward portion of the