Part 90

_Steel_ is a compound of iron and carbon, in which the proportion of the latter seldom exceeds 1.7 per cent. It may be distinguished from iron by its fine grain, its susceptibility of hardening by immersing it when hot in cold water, and with certainty by the action of diluted nitric acid, which leaves a black spot on steel, and on iron a spot which is lighter colored in proportion as the iron contains less carbon. For the construction of cannon, steel may be divided into high and low steel, the difference being that the former contains more carbon than the latter. High steel is very hard and has great ultimate tenacity. It has but little extensibility within or without the elastic limit, and is therefore too brittle for use in cannon, unless used in such large masses that the elastic limit will not be exceeded by the explosive force of the powder. It melts at a lower temperature than wrought iron and is difficult to weld, as its welding temperature is but little less than that at which it melts. Low steel is often known as “mild steel,” “soft steel,” “homogeneous metal,” and “homogeneous iron,” and is made by fusing wrought iron with carbon in a crucible; after which it is cast into an ingot and worked under a hammer. As it contains less carbon than high steel, it has greater specific gravity. It can be welded without difficulty, although overheating injures it. It more nearly resembles wrought iron in all its properties, although it has much greater hardness and ultimate tenacity, and a lower range of ductility depending on its proportion of carbon. It has less extensibility within the elastic limit than high steel, but greater beyond it, or in other words, greater ductility. Its great advantage over wrought iron for general purposes is that it can be melted at a practicable heat, and run into large masses possessing soundness and tenacity. Its advantages for cannon are greater elasticity, tenacity, and hardness. Its tenacity when suitable for cannon is three times as much as cast gun iron, and one-half more than the best wrought iron. The principal varieties of steel are:

_Natural Steel._--This is made principally in Germany, and is used for making files and other tools. It is obtained by reducing the rich and pure kinds of iron ore with charcoal, and re-fusing the cast iron so as to bring it to a malleable state. The India steel, or Wootz, is a natural steel containing a small proportion of other metals.

_Blistered Steel._--This is prepared by exposing alternate layers of bar-iron and charcoal in a close furnace for several days. When taken out the bars are brittle in quality and crystalline in appearance. The purpose for which the steel is to be used determines the degree of carbonization. The best qualities of iron (Russian and Swedish) are used for the finest kind of steel.

_Tilted Steel._--This is blistered steel moderately heated and subjected to the action of a tilt-hammer, by which means its density and tenacity are increased.

_Shear Steel._--A blistered or natural steel refined by piling thin bars into fagots, and then rolling or hammering them into bars, after they have been brought to a welding heat in a reverberatory furnace. The quality is improved by a repetition of this process, and the steel is known accordingly by the names, half shear, single shear, double shear, etc.

_Cast Steel._--This is made by breaking blistered steel into small pieces, and melting it in close crucibles from which it is poured into iron molds. The ingot is then reduced to a bar by hammering or rolling with great care. Cast steel is the finest kind of steel, and is best adapted for most purposes; it is known by a very fine, even, and close grain, and a silvery homogeneous fracture. The most remarkable specimen of cast steel for tenacity which is on record was manufactured at Pittsburgh, Pa. It was tested at the Washington Navy-Yard, and found to sustain 242,000 pounds to the square inch. The strength of cast steel usually runs from 70 to 140,000 pounds.

_Bessemer Steel._--This steel is produced by forcing air into melted iron, by means of which the carbon and silicon of the crude cast iron is oxidized. The essential difference between this process and the ordinary puddling is mechanical, and consists in the intense and violent stirring of the Bessemerized iron, to which alone is due the production and maintenance of a temperature, without any other fuel than the carbon and silicon contained, that keeps the metal fluid so that it can be cast into homogeneous malleable ingots. When decarburation has been carried far enough, the current of air is stopped, and a small quantity of white pig-iron containing a large amount of manganese is dropped into the liquid metal. No very large cannon have yet been made wholly of Bessemer steel, but several small ones have, which have shown great endurance. Experiments at the Woolwich Arsenal have shown that the tenacity of this steel is more than doubled by hammering.

_Siemens-Martin._--In this process the ingredients of cast steel are melted together on the open hearth of a reverberatory furnace of special construction, and a certain proportion of manganese necessary to make a sound and practically malleable steel added. This steel is, however, little used in gun construction.

_Semi-Steel._--If in the process of puddling or decarbonizing cast iron the process be stopped at a particular time, determined by indications given by the metal to an experienced eye, an iron is obtained of greater hardness and strength than ordinary iron, to which the name of semi-steel, or puddled steel, has been given. The principal difficulty in its manufacture is that of obtaining uniformity in the product, homogeneity and solidity throughout the entire mass. It is much improved by reheating and hammering under a heavy hammer; but it has not been found a reliable material for even cannon of small caliber. The celebrated guns made by Mr. Krupp of Germany are of cast steel, made from puddled steel, and of peculiar character, combining great tensile strength with the property of stretching to a great extent without breaking. Sir Joseph Whitworth improves the qualities of steel for his more recent guns by casting it under hydraulic pressure.

_Chrome Steel._--An alloy of iron and chromium, which is not steel in the ordinary sense, but which possesses many of its characteristics. The tensile strength and resistance to crushing is much higher than ordinary cast steel. This material has been largely used in bridge-building, but has not yet been applied to cannon-making.

_Bronze_ for cannon (commonly called brass) consists of 90 parts of copper and 10 of tin, allowing a variation of one part of tin more or less; by increasing the proportion of tin, bronze becomes harder, but more brittle and fusible; by diminishing it it becomes too soft for cannon, and at the same time loses a part of its elasticity. Bronze is more fusible than copper, much less so than tin. It is harder, less susceptible of oxidation, and much less ductile than either of its constituents. Its fracture is of a yellowish color, with little lustre, a coarse grain, irregular, and often exhibiting spots of tin which are of a whitish color. The density and tenacity of bronze when cast into the form of cannon, are found to depend upon the pressure and mode of cooling. In consequence of the difference of fusibility of tin and copper, the perfection of the alloy depends much on the nature of the furnace and the treatment of the melted metal. By these means alone the tenacity of bronze has been carried up to 60,000 pounds. Bronze is but slightly corroded by the action of the gases evolved from gunpowder, or by atmospheric causes; but its tin is liable to be melted away at the sharp corners by the great heat generated in rapid firing. It is soft, and therefore liable to serious injury by the bounding of the projectile in the bore. This injury is augmented as the force of the rebound is increased by the elasticity of the metal. It was established by experiments of Maj. Wade of the U. S. Ordnance Corps more than twenty years ago that the tensile strength of bronze is related to its density. It has been discovered since that this density can be produced by artificial compression. Two men claim the honors of the invention--Gen. Uchatius of the Austrian army, and S. B. Dean, an American inventor. The methods are essentially the same. After the gun is cast, steel mandrels slightly conical in shape are driven through the bore by hydraulic pressure,--each being succeeded by one slightly larger,--thus enlarging the bore and compressing the metal surrounding it. It is claimed that the bronze is thus rendered harder and stronger, and the defects above cited in a large measure obviated. The term “steel bronze” or “bronze steel” has been applied to the metal so treated. Many guns have been made of it for the Austrian service,--the largest of which is a 6-inch breech-loader throwing a projectile of 85 pounds. This gun has proved itself slightly superior in power to the same sized Krupp gun of steel.

_Aluminium Bronze._--An alloy of 90 parts of copper and 10 of aluminium. It is harder than ordinary bronze; much stronger, being 100,000 pounds to the square inch; it does not tarnish readily. Its properties would seem to especially fit it for a gun metal. _Phosphor bronze_ is an alloy with very similar properties.

_Combined Metals._--Numerous trials have been made to improve the strength of cannon by combining two or more metals in such a way that the good qualities of one will counteract the defects of the others. But the only metals used to any extent are those described above. Steel is constantly gaining in favor as a cannon metal. It is now almost exclusively employed throughout Europe, and wherever the Krupp gun is used. The great perfection arrived at by Krupp and others in the manufacture of steel seems to place that metal above all others for gun construction, whilst the difficulty of handling large masses has been overcome by the enormous power of the machinery used. Steel is also sparingly employed both in the United States and England for converting smooth-bore guns into rifles according to the Palliser method, but experiments in the United States have shown that it is inferior to wrought iron for this purpose. See ORDNANCE, CONSTRUCTION OF.

Wrought and cast iron are much used in this way for cannon in both the United States and in England. In the former, all the larger cannon belonging to the official system (both siege and sea-coast) are made of the cast metal, whereas the Parrott gun and the new rifled pieces are a combination of both. (See ORDNANCE, CONSTRUCTION OF.) The metal chiefly employed in England is wrought iron, in combination with steel; the largest guns made at the Woolwich Arsenal are of this nature. Bronze, except as modified by the Austrians, has now nearly entirely gone out of use as a cannon metal. In France and the United States, field-pieces, mortars, and howitzers are still made of this material.

=Ordnance, Strains Upon.= The exterior form of cannon is determined by the variable thickness of the metal which surrounds the bore at different points of its length. In general terms, the thickness is greatest at the seat of the charge, and least at or near the muzzle. This arrangement is made on account of the variable action of the powder and projectile along the bore, and the necessity of disposing the metal in the safest and most economical manner. The pressure at different points may be approximately determined by calculation, or, more accurately, by experiment. In the latter method, the plan generally employed consists in boring a series of small holes through the side of a gun at right angles to its axis at known distances apart. A steel ball is projected from each hole in succession into a target, or ballistic pendulum, by the force of the charge acting through it, and the pressure at the various points is deduced from the velocities communicated to these balls. This method was adopted by Col. Bomford. Instead of the projectile a steel punch may be employed, which is pressed by the force of the charge into a piece of soft copper. (See PRESSURE-GAUGE.) The weight necessary to make an equal indentation in the same piece is then ascertained by a testing machine. The strains to which all fire-arms are subjected may be classified as follows: (1) The tangential strain which tends to split the piece open longitudinally, and is similar in its action to the force which bursts the hoops of a barrel. (2) The longitudinal strain which acts to pull the piece apart in the direction of its length. Its action is greatest at or near the bottom of the bore, and least at the muzzle, where it is nothing; these two strains increase the volume of the metal to which they are applied. (3) A strain of compression which acts from the axis outward to crush the truncated wedges of which a unit of length of the piece may be supposed to consist; this strain compresses the metal and enlarges the bore. (4) A transverse strain which acts to break transversely by bending outward the staves of which the piece may be supposed to consist. This strain compresses the metal on the inner and extends it on the outer surface. It is known that rupture will take place due to the tangential strain alone, when three times the pressure upon a unit of surface of the bore is greater than twice the tensile strength. Due to the longitudinal strain alone, rupture will take place in the direction of the length, when the pressure is greater than twice the tensile strength; and if the transverse strain alone is considered, rupture will take place when twice the pressure is greater than three times the tensile strength. It therefore appears that the tendency to rupture is greater from the action of the tangential force than from any other, and for lengths above two, or perhaps three calibers, the tangential resistance may be said to act alone, as the aid derived from the transverse resistance will be but trifling for greater lengths of bore; but for lengths of bore less than two calibers, this resistance will be aided by both the transverse and the longitudinal resistance. Every piece should therefore have sufficient thickness of breech to prevent splitting through the latter; after this point has been attained, any additional thickness of breech adds nothing to the strength of the piece. It therefore appears that a fire-arm is strongest at or near the bottom of the bore, and that its strength is diminished rapidly as the length of the bore increases to a certain point (probably not more than three calibers from the bottom); after which, for equal thickness of metal, its strength becomes sensibly uniform. The metals of which cannon are made being crystalline in structure, the size and arrangement of the crystals have an important influence on its strength to resist a particular force; and a metal will have the greatest strength with reference to a particular force when its crystals are small, and the principal faces are parallel to the straining force, if it be one of extension, and perpendicular to it, if it be one of compression. The position of the principal crystalline faces of a cooling solid is found to be perpendicular to the cooling surface; the result of this arrangement of crystals is to create planes of weakness where the different systems of crystals intersect. The effect of this law upon cannons, it has been discovered, is to render radial specimens more tenacious than those cut tangentially from the same gun. The manner and rapidity of cooling have also a great effect upon the ability of cannon to resist strains, and as all solid bodies contract their size in the operation of cooling, it follows that if the different parts of a cannon cool unequally, it will change its form, provided it be not restrained by the presence of a superior force. If it be so restrained, the contractile force will diminish the adhesion of the parts by an amount which depends on the rate of cooling of the different parts, and the contractibility of the metal. This is an important consideration in estimating the strength and endurance of cannon, particularly those made of cast iron. All such cannon cooled from the exterior (see ORDNANCE, CONSTRUCTION OF) are affected by two straining forces; the outer portion of the metal being compressed, and the interior extended, in proportion to their distances from the neutral axis or line composed of particles which are neither extended nor compressed by the cooling process. The effect of this unequal contraction may be so great as to crack the interior metal of cast iron even before it has been subjected to the force of gunpowder. The strain produced by the explosion of gunpowder is not distributed equally over the thickness of metal, but it varies inversely as the square of the distance from the centre; it therefore follows that the sides of a cannon are not rent asunder as by a simple tensile force, but they are torn apart like a piece of cloth, commencing at the surface of the bore. Hence it is that the effect of ordinary cooling is to diminish the strength and hardness of the metal of cannon at or near a point where the greatest strength and hardness are required, _i.e._, at the surface of the bore. The strains produced by unequal cooling increase with the diameter of the casting and the irregularity of its form. This explains the great difficulty found in making large cast-iron cannon proportionally as strong as small ones, and also how projections like bands, moldings, etc., injure the strength of cannon. It also explains why cannon made of “light” cast iron, or cast iron made more tenacious by partial decarbonization, are not so strong as cannon made of weaker iron; for it is well known that such iron contracts more than the latter in cooling, and therefore produces a greater strain of extension on the surface of the bore. Capt. Rodman of the U. S. Ordnance Department has proposed a plan for cooling cannon from the interior (see ORDNANCE, CONSTRUCTION OF), thereby reversing the strains produced by external cooling, and making them contribute to the endurance rather than to the injury of the piece. It is likely, however, that the strains produced by unequal cooling are modified by time, which probably allows the particles to accommodate themselves to a certain extent to their constrained position. In confirmation of this, great endurance has been frequently found in _old_ solid cast guns, as in the old 42-pounders tested about the beginning of the war, 1861-65.

=Ordnance Department.= In the United States, was first established May 14, 1812, and was not provided for in the reduction of the army in 1815, but continued in the service. In 1821 the department was merged into the artillery, attaching to each regiment of artillery 1 supernumerary captain, and giving to each company 4 subaltern officers. The corps of ordnance was re-established April 5, 1832. The department consists of 1 brigadier-general, 3 colonels, 4 lieutenant-colonels, 10 majors, 20 captains, 16 first lieutenants, and 350 enlisted men. It is the duty of the senior officer of the ordnance department to direct the inspection and proving of all pieces of ordnance, shot, shells, small-arms, and equipments procured for the use of the armies of the United States; and to direct the construction of all cannon and carriages, and every implement and apparatus for ordnance, and all ammunition-wagons, traveling-forges, and artificers’ wagons; the inspection and proving of powder, and the preparation of all kinds of ammunition and ordnance stores. It is also the duty of the senior officer of the ordnance department to furnish estimates, and, under the direction of the Secretary of War, to make contracts and purchases for procuring the necessary supplies of arms, ordnance, and ordnance stores, etc. In the British service, the ordnance department was a distinct branch of the war department, originally for the supply of all warlike stores used in the naval or military service. The first master of ordnance was created in the time of Henry VIII., and the Tower of London was probably the depot of arms and military stores; Robert, earl of Essex, is said to have been the first master-general, in 1596. It does not appear that the ordnance department of the British service became especially military until the early part of the 18th century, after the organization of the Royal Artillery, in 1743, under the Duke of Montague as master-general. From this time the ordnance department was administered by a master-general and board, the latter being composed of a lieutenant-general of ordnance, surveyor-general, clerk of the ordnance, principal store-keeper, clerk of the deliveries, and treasurer. About 1763 the department became a construction board, with charge of all forts and fortresses, and directed the construction of all the fortifications and military store-houses, and barracks for the ordnance corps. The board was finally abolished as a separate department, the duties carried on by the commander-in-chief, and the various civil branches by separate offices under the secretary of state for war.

=Ordnance Office.= Before the invention of guns, this office was supplied by officers under the following names: the bowyer, the cross-bowyer, the galeater, or purveyor of helmets, the armorer, and the keeper of the tents. Henry VIII. placed under the management of a master-general, a lieutenant, surveyor, etc. The master-general was chosen from among the first generals in the service of the sovereign. The appointment was formerly for life; but since the restoration, was held _durante bene placito_, and not unfrequently by a cabinet minister. The letters patent for this office were revoked May 25, 1855, and its duties vested in the minister of war. The last master-general was Lord Fitzroy Somerset, afterwards Lord Raglan.

=Ordnance Projectile.= See PROJECTILE.

=Ordnance Sergeants.= In the U. S. service, are staff sergeants who are selected from the sergeants of the line of the army. Their duties consist in receiving and preserving the ordnance, arms, ammunition, and other ordnance stores at posts, under the direction of the commanding officer of the same. They must not be confounded with sergeants of ordnance, who are sergeants in the ordnance detachments at arsenals, etc.

=Ordnance Store-keeper.= In the British service, is a civil officer in the artillery who has charge of all the stores, for which he is accountable to the office of ordnance.

=Ordnance Store-keeper.= In the U. S. army, an officer of the ordnance department who holds the rank of captain. The grade has been abolished by act of Congress, and the duties appertaining to the office will be performed by other officers of the ordnance department.

=Ordnance Stores.= See ORDNANCE.