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of heavy pressure to force the heated surfaces together. With iron and steel the method secures a very strong weld and the heating is confined closely to the weld itself. Moreover, there is a saving of time and often of energy.

Percussion Welding. If an electric condenser of large capacity be discharged by wire terminals of relatively small section made to approach each other in line, the discharge occurs with a flash of light at or before actual contact, depending on their potential difference. With sufficient capacity of condenser the restricted areas of the op- posed ends of the discharge wires are brought superficially for an instant to a high temperature, and if immediately pressed into firm contact will weld or unite. In percussive or percussion welding the condenser (or, better, a polarization battery of limited capacity) is charged from any suitable source of electric energy and its terminals attached to the work pieces, which are then brought into percussive contact, as by arranging to have one of them fall toward the other from an appropriate height determined by experiment. The per- cussion may be assisted by a weight or spring suitably adjusted. The discharge occurs as above described, and the heated opposed surfaces are brought instantly together by the forcible impact. A weld may thus be obtained between the pieces. The rise of tem- perature is confined almost entirely to the thin layer of metal forming the joint. The heating effect is thus more local than in any other form of welding. It is applicable to small work and it extends to a con- siderable degree the practical possibilities of electric welding. The stored energy of an electro-magnetic circuit may also be employed for the instantaneous discharge demanded by percussion welding.

Electric Arc Welding. Stimulated in large measure by the need of rapid ship construction in the World War, and the modern exten- sion of electric supply, that form of fusion welding in which the electric arc is employed has in the past few years grown rapidly in importance and extent of application. Many forms of arc-welded joint in steel structures have already been to a degree standardized. The arc terminal applied to the work (usually the negative electrode when direct current is used) is a wire or rod of mild steel, mounted in a suitable holder manipulated by the operator, upon whose skill the perfection of the work largely depends. These electrode wires ordinarily vary in diameter according to the scale of the work or current strength used, and range from ^ in. to & in. or more. As the welding wire is rapidly consumed in the operation of fusing a joint, it is constantly fed forward. Automatic arc welders have been devised and in these the arc separation is controlled automat- ically and the wire also fed automatically from a reel. In operation the arc voltage may be from 10 to 20 volts and the current traversing the arc may be from 80 to 200 amperes or more. The welding is attended by much sputtering and projection of fused and super- heated globules of iron from the end of the wire electrode toward the cooler and heavier masses of the work pieces. In fact, the deposition of metal on the work is possibly due to a jet of iron vapour from the electrode wire, carrying fused iron globules as a result of explosive boiling of the iron. This action would be a natural consequence of the central area of the end of the electrode wire being at the highest temperature, as it loses heat by radiation less readily than the outer surface of the wire at the arc. This central area reaches a temp- erature of about the boiling point of iron. The temperature of the arc is so high that the surface of the work pieces, however mas- sive such pieces may be, is penetrated and fused so that incorporation of the metal of the work and that from the welding electrode wire takes place. The welding may be regarded as a progressive filling or plaster- ing action by condensed iron vapour and fused iron. The operation is facilitated by coating the electrode wire lightly with mineral films, such as lime, which probably act by furnishing volatile material which adds to the stability of the arc. Depending on the strength of current in the arc and the skill of the operator, from I Ib. to 2 Ib. of metal per hour may be deposited in effecting the welds, and about 80 % of the metal of the wire used enters the welds, the remaining 20% being vapourized, burned into oxide, or scattered in small globules. When plates of over -fg in. in thickness are to be butt- welded they should be bevelled before abutting them, so that a groove of not less than 60 flare shall be provided, to be filled with the fused metal (see fig. 3.). Where the plates meet at an angle, as in fig 4, the fused metal is deposited either at a or b, or both.

FIG. 3.

FIG. 4.

Arc welding can be carried on even upon the under side of the work (such as a boiler or tank in situ). In this case the electric arc is at the upper end of the welding wire, and the disadvantageous position results in the rate of forming the welds being about 60 / of that in ordinary work. The actual rate at which seams can be

made in arc welding naturally depends upon the thickness 1 of the plates to be united, the kind of joint to be made and other condi- tions. With automatic machines on small work it may rise to about 2 ft. per minute, while in heavy work by hand operation it may not exceed 2 in. per minute. Ordinary arc welds on steel may possess a tensile strength of as high as 50,000 Ib. per sq. in., but there is almost negligible elongation. Cast iron is amenable to arc welding when proper precautions are taken. Likewise bronze and copper may be arc-welded, a favourable condition for which is preheating of the work pieces. Arc welding has usually been done by the use of direct current, and special dynamo generators are constructed for supplying the current, such generators having been designed with regulating characteristics suitable to welding. The alternating-current arc is, however, adaptable to welding, provided the frequency is not too low. Arc welding covers a large field of application, constantly extending. It is employed in the construction of tanks, and is espe- cially useful in caulking the seams of tanks which must retain oil or thin liquids without leak. It is revolutionizing the fabrication of many structures of iron and steel, and is much used for repair work. It is readily applicable to joining broken pieces and to re- placing metal worn away in use, of which the restoration of rail surfaces of tramways in situ is now a familiar instance. It is generally found to be less costly in application than the other forms of fusion welding, such as that by the use of oxygen blowpipe or thermit welding. (E. T.)

(2) GAS-TORCH WELDING. Gas-torch welding is variously known as " autogenous " welding, " oxy-acetylene blowpipe " welding, " hot gas flame " welding, " fusion " welding, and other terms which are more or less inaccurate, general, and con- fusing. The gas combinations more commonly used for torch or blowpipe welding are either oxygen-acetylene or oxygen-hydro- gen. Of these two, oxy-acetylene is in more general use for welding, while oxy-hydrogen, on account of its longer flame, is generally used to supply heat for steel-cutting torches. The oxy-acetylene flame has a maximum heat under ideal conditions of about 3,400 C., and oxy-hydrogen about 2,000 C.

FIG. i. Principle of the low pressure or injector type of gas torch.

The use of a blowpipe or torch in some form was known to the ancients, but the high-temperature gas flame is a development of the last quarter of a century, and especially the past ten years. The application of the oxy-acetylene torch to metallic welding dates experimentally from 1901 and commercially from 1903; Edmond Fouche, Paris, who did considerable experimenting in conjunction with Ficard, is generally credited with making the first really practical torch. The early torches used both oxygen and acetylene under high pressure, but this proved too danger- ous, and a low-pressure or injector type was next used. Follow- ing this was the Gauthier-Ely positive or medium pressure torch, which used both gases under moderate and independent pressure. The injector and the positive-pressure types are the ones now in commercial use. The development of the latter is largely due to Augustine Davis and Eugene Bournonville.

FIG. 2. Principle of the medium or positive-pressure type of gas torch.

The fundamental principle of the low-pressure or injector type of torch is shown in fig. I. The acetylene enters at A and the oxygen