chapter  9
26 Pages

Gas welding, cutting and plasma arc

The first successful oxy-acetylene welding equipment was developed in France at the turn of the century (1901-3). The equipment was introduced to the commercial industries a year later and it immediately proved successful. Acetylene was first discovered in 1836, but was not produced successfully on a commercial basis until many years after this when the use of calcium carbide was first discovered. Oxygen was discovered many centuries ago, but its preparation on a commercial basis was not started until 1892. Gas welding processes are so called because the

welding heat is provided by a flame produced by the combustion of a mixture of gases. A variety of gases are used commercially, but an oxy-acetylene mixture is the most common because of its high flame temperature and because the gases are convenient to handle. Oxygen forms 20 per cent of the atmosphere, the

rest being nitrogen and a small percentage of rare gases such as helium, neon and argon. To obtain the oxygen in a state that makes it usable for welding, it is necessary to separate it from the other gases. Oxygen is produced commercially either by the electrolytic process or by a separation method known as the liquid-air process. Unlike oxygen, acetylene is not a natural but a man-made gas. It is a hydrocarbon gas produced by the action of water on calcium carbide. Acetylene is a colourless gas with a very distinctive nauseating odour. It is highly combustible when mixed with air or oxygen. It burns in ordinary air with a luminous smoky flame, but in pure oxygen it burns with an intensely hot non-luminous flame. Although it is stable under low pressures, it becomes

very unstable and dangerous if compressed to any great pressure. Hence acetylene for welding purposes is dissolved in a liquid chemical known as acetone, which is capable of absorbing up to 25 times its own volume without changing the nature of the gas itself, and then stored in steel cylinders. When acetylene and oxygen are mixed in the

correct proportions and ignited, the resulting flame reaches a temperature of 3200°C. This is intense enough to melt all commercial metals so completely that metals to be joined actually flow together to form a complete bond without mechanical pressure or hammering. Except on very thin material, extra metal in the form of a wire rod is usually added to the molten metal in order to strengthen the seam slightly. If the weld is performed correctly, the section where the bond is made will be as strong as the base metal itself. Metals which can be welded with oxy-acetylene flame include iron, steel, cast iron, copper, brass, aluminium and bronze, and many alloys may be welded; it is possible also to join some dissimilar metals, e.g. steel and cast iron, brass and steel, copper and iron, brass and cast iron. The oxy-acetylene flame is also employed for cutting metal, case hardening and annealing. The welding industry grew very rapidly, and by

the year 1907 the use of oxy-acetylene equipment was very popular in all parts of Europe. Today this type of welding is no longer used in the construction but only in the repair of motor vehicles. A casual inspection of a modern car reveals little evidence of welding, yet no modern car could be assembled without it. In body repair work welding techniques are used for frame straightening, reinforcing frame

members and welding in new or patching old panel assemblies. It is often necessary to repair panels where the metal has partially disintegrated through rust, or has been torn or otherwise broken. At one time this type of repair was effected by either riveting a patch over the rusted or torn parts, or renewing the part concerned. With the development of small portable gas welding equipment, however, the edges of the material can be heated locally until they melt, and then fused or welded together.