The production of steel by this method furnishes employment for eight or nine hundred men daily in the Krupp works. The Bessemer process for converting iron into steel is also largely used there for making steel for certain purposes. All material used in the different classes of manufactures is subjected at every stage to extreme and exact tests; the standards being fixed with[Pg 43] reference to the purpose to which the metal is to be applied, and any material that proves faulty when suitably tested is rigorously rejected.
The guns originally manufactured by the Krupp firm were formed from solid ingots of steel, which were bored, turned, and fashioned as in the case of cast-iron smooth-bore cannon. With the development of the power of artillery, the greater strain caused by the increased powder-charges and by the adoption of rifling--involving enhanced friction between the projectile and the bore--had the result of demonstrating the weakness inherent in the construction of a gun thus made entirely from one solid forging, and that plan was eventually discarded. Artillerists have learnt that the strain produced by an explosive force operating in the interior of a cannon is not felt equally throughout the thickness of the metal from the bore to the exterior, but varies inversely as the square of the distance of each portion of the metal from the seat of effort. For example, in a gun cast solid, if two points be taken, one at the distance of one inch from the bore, and the other four inches from the bore, the metal at the former point will during the explosion be strained sixteen times as much as that at the distance of four inches. The greater the thickness of the material, the greater will be
the inequality between the strains acting at the points respectively nearest to and farthest from the interior. The metal nearest the seat of explosion may thus be strained beyond its tensile strength, while that more remote is in imperfect accord with it. In such a case, disruption of the metal at the inner surface ensues, and extends successively through the whole thickness to the exterior, thus entail-ing the destruction of the gun.
This source of weakness is guarded against by the construction of what is termed the built-up gun, in which the several parts tend to mutual support. This gun consists[Pg 44] of an inner tube, encircled and compressed by a long 'jacket' or cylinder, which is shrunk around the breech portion with the initial tension due to contraction in cooling. Over the jacket and along the chase, other hoops or cylinders are shrunk on successively, in layers, with sufficient tension to compress the parts enclosed. The number and strength of these hoops are proportionate to the known strain that the bore of the gun will have to sustain. The tension at which each part is shrunk on is the greater as the part is farther removed from the inner tube; the jacket, for example, being shrunk on at less tension than the outer hoops. The inner tube, on receiving the expansive force of the explosion, is prevented by the compression of the jacket from being forced up to its elastic limit; and the jacket in its turn is similarly supported by the outer hoops; and on the cessa-tion of the internal pressure the several parts resume their normal position.
This system of construction originated in England, and is now in general use. The first steel guns on this principle were those designed by Captain Blakely and Mr J. Vavasseur, of the London Ordnance Works. At the Exhibition of 1862, a Blakely 8.5-inch
gun, on the built-up system, composed wholly of steel, was a feature of interest in the Ordnance section. The plan devised by Sir W. Armstrong, and carried into effect for a series of years at Woolwich and at the Armstrong Works at Elswick, consisted in enclosing a tube of steel within a jacket of wrought iron, formed by coiling a red-hot bar round a mandrel. The jacket was shrunk on with initial tension, and was fortified in a similar manner by outer hoops of the same metal. The want of homogeneity in this gun was, however, a serious defect, and ultimately led to its abolition. The difference in the elastic properties of the two metals caused a separation, af-ter repeated discharges,[Pg 45] between the steel tube and its jacket, with the result that the tube cracked from want of support. Both at Woolwich and at Elswick (described on a later page), therefore, the wrought-iron gun has given place to the homogeneous steel built-up gun, which is also the form of construction adopted by the chief powers of Europe and by the United States of America.
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The failure of some of his solid-cast guns led Krupp, about 1865, to the adoption of the built-up principle. With few exceptions, the inner tube of a Krupp gun is forged out of a single ingot, and in every case without any weld. The ingot destined to form the tube has first to undergo a prolonged forging under the steam-hammers, by which the utmost condensation of its particles is effected.
It is then rough-bored and turned, and subsequently carefully tempered in oil, whereby its elasticity and tensile strength are much increased. It is afterwards fine-bored and rifled, and its powder-chamber hollowed out. The latter has a somewhat larger diameter than the rest of the bore, this having been found an improvement. The grooves of the rifling are generally shallow, and they widen towards the breech, so that the leaden coat of the projectile is compressed gradually and with the least friction. The jacket and hoops of steel are forged and rolled, without weld, and after being turned and tempered, are heated and shrunk around the tube in their several positions, the greatest strength and thickness being of course given to the breech end, where the force of explosion exerts
the utmost strain. The completed gun is mounted on its appropriate carriage, and having been thoroughly proved and tested and fitted with the proper sights, is ready for service. The testing range is at Meppen, where a level plain several miles in extent affords a suitable site for the purpose.
For many years all guns of the Krupp manufacture have[Pg 46] been on the breech-loading system, and he has devoted much time and ingenuity to perfecting the breech arrangements. The subject of recoil has also largely occupied his attention. In the larger Krupp guns the force of recoil is absorbed by two cylinders, filled with glycerine and fitted with pistons perforated at the edges. The pistons are driven by the shock of the recoil against the glycerine, which is forced through the perforations. In England a similar arrangement of cylinders, containing water as the resisting medium, has been found effective; and in America, petroleum is employed for the same purpose. The advantages of the use of glycerine are that in case of a leak it would escape too slowly to lose its effect at once, and it is also more elastic than water, and is less liable to become frozen.
The resources of Krupp's establishment are equal to the production of guns of any size that can conceivably be required. He has made guns of one hundred and nineteen tons weight. The portentous development of the size and power of modern ordnance is exemplified by these guns and the Armstrong guns of one hundred and eleven tons made at Elswick. Amongst the class of modern cannon, one of the most powerful is Krupp's seventy-one-ton gun. This, like all others of his make, is a breech-loader. Its dimensions are--length, thirty-two feet nine inches; diameter at breech end, five feet six inches; length of bore, twenty-eight feet seven inches; diameter of bore, 15.75 inches; diameter of powder-chamber, 17.32 inches. The internal tube is of two parts, exactly joined; and over this are four cylinders, shrunk on, and a ring round the breech. Its rifling has a uniform twist of one in forty-five. It cannot possibly be fired until the breech is perfectly closed. Its maximum charge is four hundred and eighty-five pounds of powder, and a chilled iron shell of seventeen hundred and eight pounds.[Pg 47]
Krupp's 15.6 Breech-loading Gun (breech open).
[Pg 48]
Krupp did much to promote the welfare and comfort of his workpeople. For their accommodation, he erected around Essen nearly four thousand family dwellings, in which more than sixteen thousand persons reside. The dwellings are in suites of three or four comfortable rooms, with good water-arrangements; and attached to each building is a garden, large enough for the children to play in. There are one hundred and fifty dwellings of a better kind for officials in the service of the firm. Boarding-houses have also
been built for the use of unmarried labourers, of whom two thousand are thus accommodated. Several churches, Protestant and Catholic, have also been erected, for the use of his workmen and their families. There have likewise been provided two hospitals, bathing establishments, a gymnasium, an unsectarian free school, and six industrial schools--one for adults, two for females. In the case of the industrial schools, the fees are about two shillings monthly, but the poorest are admitted free. A Sick Relief and Pensions Fund has been instituted, and every foreman and workman