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 Fe2 O3 2 Al = Al2 O3 2 Fe. 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.
CHAPTER XVI
Oxy-Acetylene Welding
Of late years oxy-acetylene welding and cutting have made great strides, and have placed at the disposal of the metal-working trades a means of doing many things that hitherto were impossible. Purified acetylene and oxygen, both under pressure, are supplied to a special blowpipe or torch. The flame in its hottest part has a temperature of about 4,000° C. and is therefore sufficiently high to melt any metals with which it may be brought into contact. The torch is fitted with all necessary adjustments to vary the supply of either of the gases, and constitutes a handy tool with which the intelligent worker soon acquires great dexterity. In a special form of the torch there is a means of introducing a further supply of compressed oxygen, which makes it possible for the blowpipe flame to cut its way rapidly through thick metal, the particles of which are actually consumed in the path of the oxygen.
It is out of the question in a single chapter of a handbook covering such a large scope as the present work to do more than indicate some of the uses to which the oxy-acetylene torch or blowpipe may be put. This chapter is obviously no attempt whatever at providing a complete working guide to oxy-acetylene welding. All that will be here attempted is to present a brief description of a typical outfit and some notes on working the blowpipe, and then to give some practical instruction from the pen of an oxy-acetylene welder on the treatment of copper, aluminium and cast-iron. It may here be pointed out that there is now a large number of firms specialising in the manufacture of oxy-acetylene welders’ appliances, and most of them publish illustrated catalogues which anyone proposing to equip himself for oxy-acetylene welding would do well to obtain.
The source of the oxygen used in welding is now always cylinders, which are obtainable in various sizes, either by purchase or on hire from the gas-compressing companies, to whom they have to be returned for recharging. The acetylene also can be had in the compressed form, but in this case the gas is not simply compressed into steel cylinders because, if it were, any simple shock would be likely to cause explosion. The acetylene is therefore dissolved in liquid acetone, the cylinders containing some porous substance such as fossil meal, which is saturated with the acetone and the acetylene then pumped in. These also can be bought or hired from the gas-compressing companies. A tremendous amount of welding is done, however, with acetylene generated on the spot, and there are on the market quite a number of approved appliances that can be recommended, the best form of generator being that in which the calcium carbide is dropped into the water instead of the water dripping into the charge of calcium carbide. It is essential that the gas be purified before use. As most people doubtless know, acetylene is one of the hydro-carbon series of gases and is evolved by the action of water on calcium carbide, a substance which is one of the products of the electric furnace.
