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on [is front face a coned copper ring which fitted into a coned scaling at the breech end of the powder chamber. The breech jtxlt was secured by means of a powerful breech*screw; a hole *2i made through the screw so that, in loading, the shell and cartridge could be passed through it after the breech block had been removed. After loading, the block was dropped into its •Mice and the breech screw turned rapidly so that it might jam tht block against its seating, and so prevent the escape of powder gis when the gun was fired. This gun was most successful, and a great number of guns of this type were soon introduced into the British army and navy.
They were employed in the China campaign of 1860, and utisfactory reports were made as to their scrviceablcness; but while the breech-loading system had obtained a firm footing on the Continent of Europe, there was a strong prejudice against it in England, and about 1864 M.L.K. guns were adopted. Breechloaders did not again find favour until about 1882, when a demand was made for more powerful guns than the M.L.R. In conseqwnce, M.L. guns having enlarged chambers for burning large charges of prismatic powder were experimented with by the Els«rick Ordnance Co. and subsequently by the War Office. The results were so promising that means were sought for further improvements, and breech-loading guns, having the Elswick cop obturation, were reintroduced.
Up to about 1850 the dimensions of canon had been proportioned by means of empirical rules, as the real principles underlying the construction of ordnance had been little understood. It was known of course that a gun was ^£.°f subjected to two fundamental stresses—a circumferential tension tending to split the gun open longitudinally, and a longitudinal tension tending to pull the gun apart lengthwise; 'the longitudinal strength of a gun is usually greatly in excess of any requirements. It is easy to demonstrate that any so-called homogeneous gun, i.e. a gun made of solid material and not built up, soon reaches a limit of thickness beyond which additional thickness is practically useless in giving strength to resist circumferential stress. This is due to the fact that the stress on the metal near the bore is far higher than that on the outer portion and soon reaches its maximum resistance which additional thickness of metal does not materially increase. The gun can, however, be arranged to withstand a considerably higher working pressure by building it up on the principle of initial tensions. The inner layers of the metal are thereby compressed so that the gas pressure has first to reverse this compression and then to extend the metal. The gun barrel supported by the contraction of the outer hoops will then be able to endure a gas pressure which can be expressed as being proportionaltotheinitialcom^ruiii>-iplustheec/CTui<ni,wbereasinthe old type solid gun it was proportional to the extension only. The first to employ successfully this important principle for all parts of a gun was Lord Armstrong (./..'.), who in 1855-1856 produced
a breech-loading field gun with a steel barrel strengthened by wrought iron hoops. In this system (fig. n) wrought iron coils were shrunk over one another so that the inner tube, or barrel, was placed in a state of compression and the outer portions in a state of tension—the parts so proportioned that each performs its maximum duty in resisting the pressure from within. Further, by forming the outer parts of wrought iron bar coiled round a mandril and then welding the coil into a solid hoop, the fibre of the iron was arranged circumferentially and was thus in the best position to resist this stress. These outer coils were shrunk over a hollow breech-piece of forged iron, having the fibre running lengthwise to resist the longitudinal stress. The several cylinders were shrunk over the steel inner tube or barrel. To obtain the necessary compression the exterior diameter of the inner portion is turned in a lathe slightly greater than the interior diameter of the outer coil. The outer coil is heated and expands; it is then slipped over the inner portion and contracts on cooling. If the strength of the two parts has been properly adjusted the outer will remain in a state of tension and the inner in a state of compression.
Every nation has adopted this fundamental principle which governs all systems of modern gun construction. The winding, at a high tension, of thin wire or ribbon on the barrel or on one of the outer coils may be considered as having an exactly similar effect to the shrinking of thin hoops over one another. The American, Dr Woodbridge, claims to have originated the system of strengthening guns by wire in 1850; Brunei, the great railway engineer, also had similar plans; to Longridgc, however, belongs the credit of pointing out the proper mode of winding on the wire with initial tension so adjusted as to make the firing tension f'.fi the tension which exists when the gun is fired) of the wire uniform for the maximum proof powder pressure. Great success attended the early introduction of the coil system. Large numbers (about 3500) of breech-loading Armstrong guns from 2-5 in. to 7 in. calibre were manufactured for England alone; most of these had barrels of coiled iron, but solid forged iron barrels were also employed and a few h- Jjfl
were of steel. This manufacture continued r~ —= until 1867, when M.L. guns built up on the coil system (fig. 12) with the French form of rifling \\t~~—"~T~
were adopted; but as the knowledge of the HT~- r"^
proper treatment and the quality of the steel had improved, steel barrels bored from a solid steel forging were mostly used; the exterior layers were still iron hoops with the fibre of the metal disposed as in the original type. In order to cheapen manufacture the coils were thickened, by Mr Fraser of Woolwich Arsenal, so that a few thick coils were used instead of a number of thin ones (fig. 13).
In the Fraser system an attempt was made to obtain rigidity of construction and additional longitudinal strength by interlocking the various coils from breech to muzzle; this feature still exists in all designs adopted by the English government, but foreign designers do not favour it altogether, and many of their guns of the latest type have a number o( short independent hoops shrunk on, especially over the chase. Their view is that movements—such as stretching of the inner parts—are bound to take place under the huge forces acting upon the tubes, and that it is better to allow freedom for these to take place naturally rather than to make any attempt to retard them. On the other hand it cannot be denied that the rigid construction is
Fig. 13.—M.L. Gun Construction (Frascr). Fig. 14 shows the various stages of building up a B.L. gun and illustrates at the same time the principle of the interlocking system.
The steel barrels of the M.L. guns were forged solid; the material was then tested so as to determine the most suitable temperature, at which the oil hardening treatment should be carried out after the barrel had been bored. The bored barrel was simply heated to the required temperature and plunged vertically into a tank^of oil. The subsequent annealing process was not introduced until some years after; it is therefore not to be wondered at that steel proved untrustworthy and so was used with reluctance.
Since 1880 the steel industry has made so much progress that this material is now regarded as the metal most to be relied on. The long high-power gun*, however, require to be worked at a greater chamber pressure than the older B.L, guns, with which 15 tons or 16 tons per «qiMrc inch was considered the maximum. With the designs now produced 18*5 tons to 20 tons per square inch working pressure in the chamber Ib the general rule.
Fig. 14.—Modern B.L. Construction.
toughness and endurance under a suitable oil hardening and annealing process, the yielding stress being about 26, tons to 28 tons and the breaking stress from 45 tons to 55 tons per square inch, with an elongation of 16%. The tests for ordinary carbon gun steel are: "yield not less than 21 tons, breaking stress between 34 tons and 44 tons per square inch, and elongation 17 %."
The toughness of nickel steel forging* renders them much more difficult to machine, but the advantages have been so great that practically all barrels and hoops (except jackets) of modern guos are now made of this material.
The gun steel, whether of the carbon or nickel quality, used in England and most foreign countries, is prepared by the open hearth
method in a regenerative gas furnace of the Siemens- ^
Martin type (see Iron And Steel). The steel is run from Ttm*. the furnace into a large ladle, previously heated by gas, f0fsta& and from this it is allowed to run into a cast iron ingot mould of from 10 to 12 ft. high and 2 ft. or more in diameter. With very large ingots two furnaces may have to be employed. The external shape of these ingots varies in different steel works, but they are so arranged that, as the ingot slowly cools, the contraction of the metal shall not set up dangerous internal stresses. The top of the ingot is generally porous, and consequently, after cooling, it is usual for about one-third of the length of the ingot to be cut from the top and retnelted; a small part of the bottom is also often discarded. The centre of the larger ingots is also inclined to be unsound, and a hole is therefore bored through them to remove this part. In the Whitworth and Harmet methods of fluid compressed steel, thb porosity at the top and centre of the ingot does not occur to the same extent, and a much greater portion can therefore be utilized.
The sound portion of the ingot is now heated in a reheating gas furnace, which is usually built in close proximity to a hydraulic forging press (fig. 15, Plate I.). This press is now almost exclusively used for forging the steel in place of the steam hammers which were formerly an important feature in all large works. The largest of these steam hammers could not deliver a blow of much more than some 500 ft. tons of energy; with the hydraulic press, however, the pressure amounts to, for ordinary purposes, from 1000 toniT to 5000 tons, while for the manufacture of armour plates it may amount to as much as 10,000 or 12,000 tons.
For forgings of 8-in. internal diameter and upwards, the bored out ingot, just mentioned, is forged hollow on a tubular mandril, kept cool by water running through the centre; from two to four hour* forging work can be performed before the metal has cooled down too much. Generally one end of the ingot is forged down to the proper size; it is then reheated and the other end similarly treated.
The forging of the steel and the subsequent operations have a very marked influence on the structure of the metal, as will be seen from the micro-photographs shown in the article Alloys, where (a) and (ft) show the structure of the cast steel of the actual ingot; from this it will be noticed that the crystals are very large and prominent, but, as the metal passes through the various operations, these crystals become smaller and less pronounced. Thus (c) and (<f) show the metal after forging; (r) snows the pearlite structure with a magnification of 1000 diameters, which disappears on the steel being oil hardened, and (f) shows the oil hardened and annealed crystals. At the Bofors Works in Sweden, gun barrels up to 24 cm. (9'5 in.) calibre have been formed of an unforged cast steel tube; but this practice, although allowing of the production of an inexpensive gun, is not followed by other nations.
After the forging is completed, it is annealed by reheating and cooling slowly, and test pieces are cut from each end tangt nti
ra the drcumferencc-of the bore; these are tested to ascertain the
qafctyof the steel in the soft state. It ti found that the quality of the steel is greatly improved by
Jofyiag, 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
i.-s 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 plucjin? it into a vertical tank of rape oil. This process is a somewhat critical »ne and great care is observed in uniformly heating, to the required temperature, the whole of the forging in a furnace ia dose proximity to the oil tank into which it is plunged and completely submerged as rapidly as possible. In some cases the c.l 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 nbsequently again heated to about 1200° F. and allowed to cool &rt\y by being placed in warm Sand, &c. This last operation is termed annealing, and is intended to dissipate any internal stress *hich may have been induced in the forcing 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 ten, are cut from each end of the forging in the positions above neationed; for guns of less than 3-in. calibre only half this number cf test pieces is taken; and with hoops of less than 48 in. in length tie te*t pieces are taken only from the end which formed the upper part of the cast ingot.
lo all cases of annealed steel the test pieces of 2 in. length and °~S33 in- diameter must give the stipulated tests according to the ciarjcter of the steel. For breech screws the steel is made of a harder quality, as it has to resist a crushing stress. These arc the testa required in England, but they differ in different countries; for instance in France a harder class of carbon steel is employed for hoops, in vhich the tensile strength must not be less than 44-5 tons, Dot the elastic limit less than 28-5 tons per square inch, neither must tike elongation fall below 12%.
Atsuming that the tests of the annealed forging are satisfactory, the forging, which we will suppose to be a barrel, is tested forstraight•ea and if necessary rectified. It is then rough-turned in a lathe >.V i'jj "to break the skin" (as it is termed technically) and so
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 arc 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, docs not materially lengthen with firing; thin barrets 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 steet 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 arc 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
warping during the subsequent operations. It is then torcd out to nearly the finished dimension and afterwards fine turned on the extenor. 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 cf the jacket and other hoops arc bored out and finished before the eiterior 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 hcadstock B ana firmly brid 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 revolve*, by the leading screw K actuated by the gear G. The boring head is provided with two or more
raised out of the furnace and dropped vertically over the barrel or other portion of the gun (fig. IS, 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 arc kept hot by rings of burning gas flames, which arc 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.