Bernard E. Jones

Contact Welding.—The electric welding of iron strips and sheets is usually done by the Thomson process of “contact-welding.” In this process the metal is brought to a welding heat by passing a very large current through the joint to be welded, which, by virtue of its high resistance in relation to other parts of the circuit, develops great local heat. When the correct temperature for welding is reached, the joint is pressed together by mechanical means, and the current interrupted. In Fig. 71 the necessary arrangements for the welding of a steel rim are sketched. A is the iron core of an alternating current transformer and B the primary winding supplied with alternating current either from a works dynamo or a public supply, and controlled by a double-pole switch C. The transformer has a secondary winding consisting of a single copper strip of very heavy section D, in which secondary currents of low voltage but very large volume are induced. This winding D terminates in two heavy metal clamps E, one fixed and the other capable of movement by rack and pinion or screw, and the clamps must be shaped to the contours of the work F they are intended to hold, so as to fit well and present as little electrical resistance there as possible. The butt ends of the wheel rim are brought into contact, current switched on at the transformer primary, and immediately a very heavy secondary current passes round the “winding” D, generating intense heat at the junction of the metal rim G held in the clamps, where the electrical resistance is comparatively high. In a few moments the joint will arrive at welding heat and the screw feed is then operated, driving the joint together and completing the weld, except so far as a little hand dressing may be found necessary. Directly the weld is established the current is switched off at the mains and the job allowed to cool out. Nothing less than 5 kilowatts to 10 kilowatts is likely to be very satisfactory for hoops about ³/₄ in. by No. 16 gauge, and the current must be alternating. The primary voltage and frequency is immaterial, as the transformer can be wound to suit the circuit conditions whatever they may be.

Fig. 71.—Electric Contact Welding
of Steel Rim

The method of welding by resistance, that is, by raising locally the welding point to the temperature required by bringing the two surfaces into contact until their high resistance produces a welding heat and then squeezing them together, is by far the most manageable and satisfactory commercial process of the two electrical processes. It is adapted for “spot-welding” or producing local adhesions between metal plates after the manner of riveting, for butt or end-on welds, for seams, chains, rings, etc., and automatic welding machines are now made that can deal with no less than 1,500 welds and upwards per hour with semi-skilled labour, with the least possible percentage of failures and a very low cost for electrical energy. Alternating current is essential with this type of weld, and is used to energise a step-down transformer of special construction.

Arc Welding.

—Notwithstanding the superiority of the resistance welding process to most commercial work, particularly that of a small kind necessitating rapid repeat work, the “arc” method, which has been in use for many years, and was probably the first experimented with, has now become largely used on work where it was thought impossible to adapt it a few years ago. The system is extensively employed in iron and steel works, shipyards, and boiler works, and the class of work it is employed on varies from the dismantling of iron and steel buildings, by fusing and cutting through the structural ironwork and girders, to the filling up of blowholes in castings. The metal to be welded is connected to one pole, and the electrode handled by the operator forms the other pole, an arc being struck between the two. Broken castings and forgings can be satisfactorily repaired by running new fused metal round them.

Recently the arc system has been applied with success for making welds on tramway rails, the resistance of the welded joint being found very much lower than when made with the usual fishplates and bonded joints. Continuous current gives better results than alternating for the arc system, and a generator designed for use with this process has a “drooping characteristic,” that is, the volts at the terminals fall rapidly with an increase in the current output. In this way the current is automatically limited to some extent, when the short-circuiting effect of the operation comes into play. Successfully to weld by this process a current of 300 to 500 amperes at 80 volts is necessary, and every precaution has to be taken to protect the workman from the intense glare of the arc.

Thermit.—Thermit is an aluminium alloy whose combustion generates so much heat that the substance can be used for the welding of iron and steel. It is the patented invention of Dr. Hans Goldschmidt. With thermit as a means of melting and welding, and with the use of special clamps and devices, a number of operations, otherwise difficult, can be performed, and thermit has come into general use for repairing broken or defective parts. By the use of a portable jacket and clamp, the joints of gas, water, and steam pipes may be welded with the pipes in position; and the advantages of such a material to an engineer far removed from supplies and repair shop, as at sea, can hardly be enumerated. New journals have been welded to heavy rolls, broken pump-rods have been joined, and a number of structural parts successfully united by its aid.

Thermit is made up as follows: Iron oxide is intimately mixed up with about one-third as much in weight of powdered aluminium, according to the equation Fe₂O₃2Al = Al₂O₃2Fe. The metals to be united are placed together and surrounded by a little clay or similar substance and the mixture is placed all around the joint. The mixture is fired by a little magnesium, and the chemical change that follows creates such an intense heat that iron is readily welded. When the joint is cold and cleaned up it is hardly perceptible.

According to instructions published by Thermit, Ltd., it is of the utmost importance that the moulds, crucibles, etc., should be kept thoroughly dry, and they may advantageously be warmed before use to ensure the absence of any dampness. The parts to be welded must previously be brought to red heat, which is best effected by means of a gas and air flame. The proper design of the mould is of the utmost importance. It should have a runner and riser, and the metal should be allowed to flow between as well as around the ends of the pieces to be welded. The expense and inconvenience of making wooden patterns may be obviated by making the model of wax, ramming the sand round this, and subsequently melting out the wax.