Gas welding is a process in which a fuel gas such as acetylene, hydrogen, natural gas, etc. is mixed with oxygen in proper proportion to burn to get a flame which, if found adequate for melting a given metal, can be used for fusing and joining different parts of it. The process is very versatile because it can weld most of the commonly employed engineering metals and their alloys. Brief description of the well known variants in this class of welding processes follows.
This is the best known gas welding process and was successfully employed first time in 1903 when a welding torch for the purpose was developed. In this process acetylene (C2H2) is mixed with oxygen in the welding torch and is then burnt at the torch tip to give a flame with a temperature of about 3300oC which can melt most of the ferrous and non-ferrous metals in common use.
Oxygen and acetylene are easily available in cylinders at temperature of about 1500 N/cm2 and 170 N/cm2 respectively. These are connected to the welding torch through hoses and are mixed and burnt to get different types of flames viz., neutral, oxidizing and carburising. The natural of the flame depends upon the ratio of the two gases. The neutral flame is most often used for the welding of most of the materials like low carbon steels, cast steel, cast iron, etc. The oxidising flame has higher proportion of oxygen than acetylene and is used for welding of manganese steel, brass and bronze whereas the carburising flame has higher proportion of acetylene in it and is used for welding aluminium, nickel, etc.
It is quite common to use filler material with gas welding through if need be, the process can be used without filler as well. Commercially, oxy-acetylene process is very widely used for welding of ferrous and non ferrous metals particularly for welding thin sections upto 6 mm. A large percent of the repair work is also done by oxy-acetylene welding.
Typical applications of the process include welding of root run in pipe and other multi-run welds,, light fabrications like ventilation and air-conditioning ducts and motor vehicle repairs.
Oxy-Acetylene Pressure Welding
In normal or non-pressure oxy-acetylene welding process no use of pressure is made to accomplish a joint. However, in slight variant of the usual process, the prices to be joined are heated by oxy-acetylene flame burner and brought to the pasty or semi-solid state. Axial pressure is then applied to achieve a joint which is normally of an upset weld type. No melting of the parts being welded is involved hence it is a solid-state welding process and is also called ‘Hot Pressure Welding’ or simply as ‘Gas Pressure welding’.
The ends of the work piece to be welded are prepared by cutting and cleaning. The pieces are then held in clamps under an axial pressure of about 3.5 to 7.0 n/mm2 for mild steel. An oxy-acetylene torch or a heating head of the desired configuration is made to surround the abutting (or projecting) ends and heated uniformly to about 1200oC. During the heating cycle the pressure is increased to the desired lengths of the two pieces. This is also prevents the possibility of local overheating or superficial melting.
Oxy-acetylene pressure welding can be used to weld all commercial metals which can be butt welded, however, presently it is used mainly to weld carbon and low alloy steels in the form of rods, rails, pipes, tubes, narrow strips, and structural shapes. In general, those materials which absorb their own oxides at elevated temperature, e.g. low carbon steels, are more easily welded by this process. However, this process finds only a limited use in the industry because it is uneconomical to use it for one-off work as well as due to lack of uniformity and consistency of components produced by the process.
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