Page:EB1911 - Volume 20.djvu/220

Rh 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 were of steel. This manufacture continued until 1867, when M.L. guns built up on the coil system (fig. 12) with the French form of rifling were adopted; but as the knowledge of the 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 of 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 conducive to strength and durability, but it is essential that massive tubes of the highest quality of steel should be employed. . 12.—M.L. Gun Construction.

The actual building up of a gun entails operations which are exactly similar, whether it be of the M.L. or B.L. system; and the hardening treatment of the steel is also the same—the coiled iron hoops when welded, of course, received no such treatment.

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 guns, however, require to be worked at a greater chamber pressure than the older B.L. guns, with which 15 tons or 16 tons per square inch was considered the maximum. With the designs now produced 18·5 tons to 20 tons per square inch working pressure in the chamber is the general rule.

. 14.—Modern B.L. Construction.

A stronger material than ordinary carbon gun steel was consequently demanded from the steel-makers, in order to keep the weights of the heavier natures of guns within reasonable limits. The demand was met by the introduction of a gun steel having about 4% of nickel in addition to about 0·4% of carbon. This alloy gives great 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 forgings renders them much more difficult to machine, but the advantages have been so great that practically all barrels and hoops (except jackets) of modern guns 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 ). The steel is run from the furnace into a large ladle, previously heated by gas, 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 remelted; 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, this 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 tons 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 hours 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, where (a) and (b) 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 (d) show the metal after forging; (e) shows 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 tangentially