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Rh the practice of different craftsmen. The typical weld is the scarf. If, for instance, a bar has to be united to another bar or to an eye, the joint is made diagonally (scarfed) because that gives a longer surface in contact than a weld at right angles (a butt weld), and because the hammer can be brought into play better. Abutting faces for a scarfed joint are made slightly convex; the object is to force out any scale or dirt which might otherwise become entangled in the joint at the moment of closing and which would impair its union. The ends are upset (enlarged) previous to welding, in order to give an excess of metal that will permit of slight corrections being effected around the joint ("swaging") without reducing the diameter below that of the remainder of the bar. These principles are seen in other joints of diverse types, in the butt, the vee and their modifications. Joint faces must be clean, both chemically, i.e. free from oxides, and mechanically, i.e. free from dust and dirt, else they will not unite. The first condition is fulfilled by the use of a fluxing agent, the second by ordinary precautions. The flux produces with the oxide a fluid slag which is squeezed out at the instant of making the weld. The commonest fluxes are sand, used chiefly with wrought iron, and borax, used with steel; they are dusted over the joint faces both while in the fire and on the anvil. Mechanical cleanliness is ensured by healing the ends in a clean hollow fire previously prepared, and in brushing off any adherent particles of fuel before closing the weld. The scarf, the butt and the vee occur in various modifications in all kinds of forgings, but the principles and precautions to be observed are identical in all. But in work involving the use of rolled sections, as angles, tees, channels and joists, important differences occur, because the awkwardness of the shapes to be welded involves cutting and bending and the insertion of separate welding pieces ("gluts"). Welds are seldom made lengthwise in rolled sections, nor at right angles, because union is effected in such cases by means of riveted joints. But welding is essential in all bending of sections done at sharp angles or to curves of small radius. It is necessary, because a broad flange cannot be bent sharply; if the attempt be made when it is on an outer curve it is either ruptured or much attenuated, while on an inner curve it is crumpled up. The plater's smith therefore cuts the flanges in both cases, and then bends and welds them. If it is on an inner curve, the joint is a lap weld; if it is on an outer one, a fresh piece or glut is welded in. Gluts of rectangular section are used for cylindrical objects and rings of various sections. The edges to be united may or may not be scarfed, and the gluts, which are plain bars, are welded against the edges, all being brought to a welding heat in separate furnaces. The furnace tubes of boilers and the cross tubes are welded in this way, sometimes by hand, but often with a power hammer, as also are all rings of angle and other sections on the vertical web.

The temperature for welding is very important. It must be high enough to render the surfaces in contact pasty, but must not be in excess, else the metal will become badly oxidized (burnt) and will not adhere. Iron can be raised to a temperature at which minute globules melt and fall off, but steel must not be heated nearly so much, and a moderate white heat must not be exceeded. Welds in steel are not so trustworthy nor so readily made as those in iron.

Thermit Welding.—The affinity of finely powdered aluminium for metallic oxides, sulphides, chlorides, &c., may be utilized to effect a reduction of metals with which oxygen, sulphur or chlorine combine. C. Vautin in 1894 found that when aluminium in a finely divided state was mixed with such compounds and ignited, an exceedingly high temperature, about 3000° C, was developed by the rapid oxidation of the aluminium. He found that metals which are ordinarily regarded as in fusible were readily reduced, and in a very high degree of purity. These facts were turned to practical account by Dr If. Goldschmidt, who first welded two iron bars by means of molten iron produced by the process, to which the name of "thermit" is now commonly applied. The method has also been applied to the production of pure metals for alloying purposes, as of chromium free from carbon, used in the manufacture of chrome steel, of pure man ganese for manganese steel, of molybdenum, ferro-vanadium, ferro-titanium and others used in the manufacture of high speed steels.

Thermit as a welding agent is produced by mixing iron oxides with finely granulated aluminium, in a special crucible lined with magnesia. On ignition, the chemical reactions proceed so rapidly that the contents would be lost over the edges unless the crucible were closed with a cover. The result of the reaction is that two layers are produced, the bottom one of pure iron, the top one of oxide of alumina or corundum. If the contents are poured over the edge, the slag follows first, and is followed by the metal. But in welding the metal is poured first through the bottom upon the joint. It is practically pure wrought iron in a molten state, at 3000° C, or 5400° F. The heat is so intense that it is possible thus to burn a clean hole through a 1 in. wrought iron plate. The joints are prepared by abutting them, and provision is made with clamps to grip and retain them in correct positions. Often, but not always, the part to be welded is enclosed in a mould, into which the thermit is tapped from the crucible. The applications of thermit welding are numerous. A wide field is that of tramway rails, of which large numbers have been successfully welded. Steel girders have been welded, as also have broken and faulty steel and iron castings, broken shafts, broken stern posts (for which crucible? 6 ft. in height with a capacity of 7 cwt. have been constructed), and wrought iron pipes. Another application is to render steel ingots sound, by introducing thermit in a block on an iron rod into the mould, which prevents or greatly lessens the amount of piping in the head, due to shrinkage and occlusion of gases.

Electric Welding.—In electric welding and metal working the heat may be communicated to the metal by an electric arc, or by means of the electric resistance of the metal, as in the Thomson process. Arc welding is the older procedure, and it appears to have been first made use of by de Meritens in 1881 for uniting the parts of storage-battery plates. The work-piece was placed upon a support or table, and connected with the positive pole of a source of current capable of maintaining an electric arc. The other pole was a carbon rod directed by the hand of the operator so as first to make contact with the work-piece, and then to effect the proper separation to maintain the arc. The heat of the arc was partly communicated to the work and partly dissipated in the hot gases escaping into the surrounding air. The result was a fusion of the metallic lead of the storage-battery plate which united various parts of the plate. The process was somewhat similar to the operation of lead-burning by the hydrogen and air blowpipe, as used in the formation of joints in chemical tanks made of sheet-lead. The method of de Meritens has been modified by Bernardos and Olszewski, Slavienoff, Coffin and others.

In the Bernardos and Olszewski process the work is made the negative pole of a direct current circuit, and an arc is drawn between this and a carbon rod, to which a handle is attached for manipulating. As this rod is the positive terminal, particles of carbon may be introduced as a constituent of the metal taking part in the operation, making it hard and brittle, and causing cracks in the joint or filling; the metal may, in fact, become very hard and unworkable. The Slavienoff modification of the arc-welding process consists in the employment of a metal electrode in place of the carbon rod. The metal electrode gradually melts, and furnishes fused drops of metal for the filling of vacant spaces in castings, or for forming a joint between two parts or pieces.

In arc welding, with a current source at practically constant potential, a choking resistance in series with the heating arc is needed to secure stability in the arc current, as in electric arc lighting from constant potential lines. Little effective work can be done by the Bernardos and Olszewski method with currents much below 150 amperes in the arc, and the value in some cases ranges above 500 amperes. The potential must be such that an arc of 2 to 3 in. in length is steadily maintained. This may demand a total of about 150 volts for the arc and the choking resistance together. In the Slavienoff arc the potential required will be naturally somewhat lower than when a carbon electrode is used, and the current strength will be, on the other hand, considerably greater, reaching, it appears, in certain cases, more than 4000 amperes. In some recent applications of the arc process the polarity of the work-piece and the arc controlling electrode has, it is understood, been reversed, the work being made the positive pole and the movable electrode the negative. More heat energy is thus delivered to the work for a given total of electric energy expended.