Modern industrial welding is a constructional method practised in workshops all over the world. Three principal welding systems are in use, the electric, the oxy-gas and the thermit process. Electric welding may call for temperatures as high as 7,500 degrees Fahrenheit
THE STEEL FRAMES FOR REINFORCING are being welded for a pipe line used in the Hetch-Ketchy Reservoir, San Francisco, California. Electric welding is divided into a number of processes, each with its own sphere of utility. In its essentials a welding machine comprises a pair of heavy copper electrodes. These may be clamps for welding bars or rods, or they may be pointed or cylindrical copper rods for use on plate or sheet metal work.
WELDING is one of the oldest processes of the blacksmith’s craft. Practised by Tubal Cain, it has been used through the ages to unite iron bars. Even to-day, wherever the blacksmith plies his trade, the bars are brought to a white hot state and are then beaten together to form a permanent weld.
Modern industrial welding, however, has little in common with the older craft. The heat now used causes the hardest steel to melt and flow as water. Steel melts at 2,768 degrees Fahrenheit. The temperature of the electric welding arc is between 6,500 and 7,500 degrees Fahrenheit. Not all welding, however, is carried out at these high temperatures; moreover the field of welding is shared by the oxy-acetylene flame. The whole wonderful process is the product of careful research by the mechanical and electrical engineer, the metallurgist and the chemist.
Welding to-day is not merely a matter of joining bars. It has become a constructional method which is carried out as a routine matter in the workshops of the world. With welding, which is applicable to all metals, is included the science of cutting by flame, an operation that can be carried out even under water.
There are three main welding systems in use: electric, oxy-gas, and the less common thermit process, in which a form of “incendiary bomb” is used.
Electric welding is divided into a number of different processes, each with its own sphere of utility. One of the most interesting of these systems is that of resistance welding, which depends on the heat generated by an electric current in overcoming resistance. The filament of an electric lamp glows white hot for this reason, and the heat generated by an electric fire is dependent on the same principle.
Electric resistance welding was discovered by accident. In the latter part of the last century, Professor Elihu Thomson was giving a lecture in the United States on electricity. At the end of one of the experiments, involving the use of a large spark-coil and some Leyden jars, Thomson found that two wires he had been holding end to end across the coil terminals refused to come apart. He had made the first resistance weld in history. From 1877 Thomson experimented with the newly discovered principle, and in 1886 he patented the first electric welding machine.
Resistance welding depends for its action on the passage of an electric current of low voltage (only half a volt in some instances) and high amperage. Alternating current is used, and its function is to heat the metal to a temperature at which the pieces can be joined by hammering or squeezing.
In what is known as “slow” resistance welding the heating is done electrically, but the uniting of the separate pieces of metal is accomplished with hammer and anvil. With fast resistance welding, on the other hand, we pass from the old to the new - and the machine takes control of the work. In its essentials a welding machine comprises a pair of heavy copper electrodes. These may take the form of clamps for welding bars or rods, or they may be pointed or circular-section copper rods for use on plate or sheet metalwork. The clamps are used for “butt welding” rods or wire end to end, and the points for the spot welding of metal plate. The action of the butt welding machine is simple. The rods to be joined are placed in the clamping jaws, which are then brought together so that the rod ends touch.
Current is then switched on and after an interval of time the machine jaws are brought close together by the action of a powerful lever. The partly molten ends of the two rods are thus forced tightly together to give a perfect weld. The electrodes are not welded to the rods by the current because, being copper, they offer little or no resistance to the current and so do not heat up. After a weld it is possible to touch the electrodes with a finger and not be burnt. In some types of welding machine, designed for continuous use, the electrodes are jacketed and cooled by a stream of water. Butt welding is used in making chains, wheel rims, window frames and many other articles.
The machines used for the spot welding of sheet metal are equipped with pointed or round copper electrodes These are often operated by a foot lever, but sometimes by compressed air. The electrodes are generally water cooled.
Spot welding somewhat resembles riveting, but without the rivets. Riveting two plates together involves drilling holes in both plates and inserting a row of rivets, which are then burred or beaten over at the ends to hold the plates firmly together. In the spot welder the plates are temporarily clamped together and placed between the electrodes. The foot lever, acting directly or through the medium of a plunger operated by compressed air, brings the electrodes in close contact with the plates and at the same time the current is automatically switched on. On a circular section of the plates, equivalent to the area of a rivet, the metal in contact becomes welded. Continued pressure on the pedal automatically cuts off the current and the work is moved along to the next “spot” chosen for welding.
Spot welding is as strong as riveting, it can be done more quickly and provides a much lighter form of construction. The applications of spot welding range from the making of domestic utensils to the making of bonnets, petrol tanks and bodywork in the motor car industry.
Spot welding is sometimes continuous, with the spots touching or overlapping one another. This process forms a link with the further development known as seam welding. A seam-welding machine is provided with electrodes in the form of revolving copper disks. The work travels automatically with the disk-electrodes and the weld is produced as an unbroken seam. Large cans and drums are made in this manner and the process is used also in the manufacture of tubing.
Resistance welding is as unobtrusive as it is efficient and it is admirable for many types of work. It is, however, the use of the electric arc that gives to welding its more spectacular aspect. With the warning hiss of its tremendous power, and with the operator protected by special glass from sight-destroying glare, the blue-white flash of the arc presents yet another use to which man has put the force he calls electricity.
AN ELECTRIC ARC WELDER repairing a broken bearing in an aluminium crankcase of a motor vehicle. Other crankcases are in front of him waiting to be repaired. This illustration shows one cable from the welding machine connected with the electrode holder and the other cable clamped to the casting, thus completing the circuit. The feed rod in the welder’s left hand is being melted in the arc. The head, face and upper part of the body of the welder are protected against the action of the powerful rays from the dazzling electric arc. Electric arc welding is used chiefly for welding steel, but highly skilled welders using special feed rods are also able to weld some of the non-ferrous metals.
The electric welding arc, using a carbon electrode, was introduced by Nicholas de Benardos in the ’eighties of the last century. This form of welding was followed by the use of a metal electrode, introduced by Slavanoff in America in 1895 and by Kjellberg in Great Britain in 1907. Then came what is known as the quasi-arc system invented by A. P. Strohmenger of London, and in 1916 the plastic arc system was patented in America. Both direct and alternating current are used for arc welding according to circumstances or preference. Electric arc welding - which covers carbon-arc, quasi-arc and plastic-arc - can be done either by direct or by alternating current, but bare wire can be used with direct current only. Alternating current requires a flux-covered welding wire.
The apparatus required for electric arc welding is surprisingly simple. The principal part of the equipment is the electrode holder, resembling a dagger with the tip broken off. The hand guard is a circular plate of vulcanized fibre and from the handle trails the negative pole cable from the current supply. The positive pole cable is generally clamped to the work, but sometimes the pole positions are reversed The blade of the “dagger” is provided with a spring clip into which the electrodes can be inserted readily.
Hottest Flame Known
The electric arc is the hottest flame known and, although the visible rays of its light will dazzle all within range, it is the invisible infra-red and ultraviolet rays that are most harmful, not only to the eyes, but also to the skin of the hands and face.
For this reason, especially where the carbon arc is in use, the head must be protected with a helmet or screen provided with a window of coloured glass. Gauntlet gloves cover the hands and the body is protected with an apron of leather or woven asbestos. Where metal electrodes are used, a screen comprising a light glazed frame often takes the place of the helmet.
The carbon-arc welding process is carried out by touching the work (duly clamped to the positive cable) with the negative electrode, which is then withdrawn a little way, so “striking” the arc. Under the terrific heat the edges of the join begin to melt and the molten metal is then fed with a rod composed of its own material. In this manner seams can be welded, angles and rods can be joined and layers of metal can be added to the work where required.
A similar method is used in metal-arc welding, but in this instance the electrode itself serves as the filling rod that melts as the work progresses. The applications of electric arc welding in modern engineering practice are so numerous that any attempt at detailed enumeration is impossible. Electric arc welding has its uses in the erection of steel framed buildings, in repairing and joining tramway and railway lines and to an ever-increasing extent in shipbuilding.
The quasi-arc process involves the use of a special electrode consisting of a steel wire core, with an aluminium wire alongside. Both wires are wrapped with prepared asbestos. In use the arc is struck as described above, but the electrode is then dropped to an angle. The true arc is then destroyed and what is known as a quasi-arc replaces it. Welding is then carried out under the surface of the slag formed by the melting of the asbestos. The function of the aluminium wire is to combine with the oxygen in the atmosphere and so prevent oxidization of the metal surface of the weld. The steel wire used in this process serves as a combined electrode and filling rod.
The plastic arc method of welding uses electric current at low voltage, about 20 volts only, and lends itself to extremely economical operation. There are other variations of the electric arc, all with their own advantages. A specially interesting development is atomic hydrogen welding. In this system a stream of hydrogen gas is directed on to the electric arc. The hydrogen is split up into atoms by the arc, and these combine again a short distance from the flame, releasing tremendous heat. This atomic hydrogen reduces any oxides on the welding surfaces, which are thus joined perfectly without oxidization.
OXY-ACETYLENE WELDING upon the cylinder block of a motor car engine. The operator is performing the delicate operation of welding a crack in a valve seating. In this process he works with a blowpipe which is fed with oxygen and acetylene - the two separate pipes can be clearly seen - and he uses the fierce flame to fuse into the casting the feed rod seen in his left hand. After the seat has been welded it is machined to match the other seats. The oxy-acetylene process is used principally for welding cast iron, but it is extensively used also for welding steel, aluminium and other metals.
A method of welding that uses neither electricity nor the oxy-acetylene blowpipe is the highly interesting thermit process. Thermit is a compound consisting principally of finely divided aluminium and oxide of iron. A special powder is used to ignite the mixture which, in some thirty seconds, becomes a white hot mass of molten steel that will burn its way through any substance. Hence its use, in a modified form, in incendiary bombs. The use of thermit can, however, be controlled, and for this purpose a crucible is used. The compound is placed in the crucible with a certain quantity of steel scrap where iron or steel is to be welded. The thermit is ignited by the powder and the resultant molten steel is poured into a sand mould built round the parts to be welded. The process is used for welding wrought-iron pipes, for repairing flaws in castings of various kinds and for similar purposes.
For the purpose of welding, the electric arc has its chemical counterpart in the flame of the oxy-acetylene or oxy-coal-gas blowpipe. The enormously high temperature obtainable by burning acetylene in oxygen was demonstrated by Le Chatelier in 1895, but the practical application of the discovery was not made until 1902 when the scientist Fouche introduced an oxy-acetylene blowpipe.
The temperature of the oxy-acetylene flame is about 6,000 degrees Fahrenheit, and apart from this tremendous heat Nature seems to have provided further conditions making its use ideal for welding. The hottest part of the flame, for instance, is surrounded by a protective coating of hydrogen which serves also to prevent the formation of oxide or scale during the welding process. Further, oxygen gas is easily and cheaply produced commercially and can be stored conveniently in cylinders. Acetylene, too, can be generated without difficulty and its storage gives little trouble.
The modern method of supplying acetylene to the workshop is to use cylinders containing porous material that has been treated with acetone. This substance, a liquid, is capable of absorbing 300 times its own volume of acetylene gas at a pressure of 180 lb. per square inch. In practice the cylinders contain acetone with 100 volumes of dissolved acetylene at a pressure of 150 lb.
The blowpipe, with its twin tubing leading off to the cylinders of oxygen and acetylene, forms an exceedingly handy and efficient device for working at the welding bench. The gas cylinders are provided with pressure gauges and with control and automatic regulator valves. The blowpipe calls for special comment as it is a far more complicated device than, for instance, the electrode holder of the electric welder.
There are two types of oxy-acetylene blowpipe, one for use with “generator” or low-pressure acetylene, the other for use with acetylene under pressure. Where the acetylene is used from a generator it is “pulled” by the action of the high-pressure oxygen as it rushes through a mixing chamber incorporated in the blowpipe. At the handle end are two tubes provided with taps for the oxygen and acetylene respectively. At the operating end is a head to which can be fitted a number of different-sized tips that are changed according to the work in hand.
Welding with the oxy-acetylene flame is similar to that done with the electric arc, but there are certain differences in the methods used. A filling-rod, to suit the material operated on, is necessary, and it is usual to coat this rod with a chemical flux to keep the weld free from oxidization. Oxygen is welcome as an aid to the acetylene flame, but is highly undesirable in the weld. The flux may also be applied to the weld in the form of paste. The oxy-acetylene flame is in common use in welding metals for which the heat of the electric arc is too fierce. One of the most important applications of the oxy-acetylene process is in the welding of aluminium and its alloys, now extensively used in the motor car and aircraft industries.
METAL-SPRAYING PISTOL at work on building up a worn shaft, by the process of metallization. By this process molten metal can be sprayed on to any other substance. The pistol is fitted with a handle and a nozzle from which the molten spray issues. Attached to the body of the pistol are three flexible pipes—one for oxygen, one for acetylene or coal gas and one for compressed air—and a roll of wire from a drum. The wire is composed of the metal to be used in the form of a spray.
Aluminium, however, and metals such as lead and zinc, all with low melting points, are often welded by using coal gas instead of acetylene in the oxygen blowpipe. The temperature of the oxy-coal gas flame is lower than that of the oxy-acetylene flame.
Oxygen has yet another property of vital use to the engineer, in addition to its power as a welding medium. In 1889 Thomas Fletcher of Warrington showed that when an iron plate was brought to a white heat it was possible to cut holes in it with a stream of oxygen. Wrought iron (as well as steel and other metals) burns in oxygen; that is, the iron and oxygen combine to form iron oxide. The cutting blowpipe is now one of the most valuable tools of the engineer.
Acetylene or coal gas is mixed with the oxygen in the blowpipe to render the metal white hot, after which the pressure of the “fuel” gas melts the iron or other oxide as it is formed, while the oxygen performs the cutting. The metal does not melt away, but is burnt away to form an oxide, and this oxide is melted. It is unnecessary to raise the temperature of steel or wrought iron to melting point; the lower melting point of the oxide is sufficient.
A remarkable property of the oxygen cutting flame is its ability to cut a clean edge that requires little subsequent machining. Many ingenious machines have been devised to hold the oxygen blowpipe and guide it in the same manner as any other type of cutting tool.
It is in the railway shops that oxygen cutting machines may be seen at their best. For the cutting of boiler plates in steel and copper they are valuable aids in speeding up production. Locomotive frames, huge slabs of steel over an inch thick, are cut to the most complicated contours by the oxygen flame. Two frame plates are laid on trestles over which swing the arms of the cutting machines. The arms carry a templet of thin plate steel, the contour of which is an exact replica in miniature of the contour to be cut in the pair of plates below.
Below each main templet arm is a double swinging gate carrying at the end an oxygen blowpipe directed downwards. Attached to the gate above the blowpipe is an electric motor with a vertical magnetized shaft. When the templet has been duly set in the required direction to coincide with the frame plates, the oxy-gas flame is started and current is switched on to the motor. The magnetized shaft of the motor clings to the edge of the templet, the contour of which it follows while revolving. The gate moves the oxygen flame along, and the frames are cut to the size and shape required.
Glove of Molten Steel
The subject cannot be dismissed without reference to the marvels of metallization, the spraying of molten metal on to any other substance. This process is carried out by a special pistol weighing about 3½ lb. The pistol, which resembles a paint-spraying pistol, is fitted with a handle and a nozzle from which the molten spray issues. Attached to the body of the pistol are three flexible pipes, one for oxygen, the second for acetylene or coal gas and the third for compressed air. A fourth attachment is the end of a wire from a drum. The wire is composed of the metal to be used in the form of spray.
The oxy-gas equipment supplies the necessary heat, and the compressed air has a dual purpose. First it works a small turbine running at 12,000 to 40,000 r.p.m., attached to a miniature winch that draws in the wire and feeds it to the oxygen burner. The other portion of the air supply sprays the molten metal from the nozzle of the pistol.
The most delicate of substances can be coated without injury. By this means the human hand could be provided with a glove of molten steel without burning the skin. The principal uses of the metallization process are to apply a non-corrosive covering to iron and steel and to build up worn metal components, adding layer upon layer of metal, to the thickness desired. A worn shafting can be built up by revolving it slowly in a lathe and spraying continuously with a metal-spraying pistol held in the tool slide-rest.