Gas welding is a fusion welding process, in which a flame produced by the combustion of gases is employed to melt the metal. The molten metal is allowed to flow together thus forming a solid continuous joint upon cooling. By burning pure oxygen in combination with other gases, in special torches, a flame upto 33000 C can be attained. The gas is purchased in cylinder and connected through resulting valves and pressure gauges into flexible hoses attached to the nozzle.
The oxy-acetylene flame is used to pre heat the parts to be welded around the joint and also to melt the filler metal. A jet of oxy acetylene flame issuing from the nozzle of a burner is played on the junction of the two pieces to be welded. At the same time a filler rod is held in the zone of jet and its melt is deposited on the fused junction. A weld is obtained after the molten metal solidifies. The coating on the filler rod acts as a flux to keep the joint clean.
The filler metal or filler rod used must combine with the parts being joined. The melting point of the filler metal must be the same or lower than the melting point of the metal being joined.
The correct adjustment of the flame is very important for reliable works. When oxygen and acetylene are supplied to the torch in nearly equal volumes, a neutral flame is produced having a maximum temperature of 32000C. This neutral flame is desired for most welding operations. Neutral flame has little effect on the base metal and sound welds are produced when compared to other flames.
In a carbonizing flame or reducing flame excess of acetylene is present. The temperature of this flame is low. The excess unburnt carbon is absorbed in ferrous metals, making the weld hard and brittle. In between the outer blue flame and inner white cone, an intermediate flame feather exists, which is reddish in colour. The length of the flame feather is an indication of the excess acetylene present. Carbonizing flame is used for welding high carbon steels and cast iron, alloy steel and for hard facing.
In an oxidizing flame excess of oxygen is present. The flame is similar to the neutral flame with the exception that the inner white cone is some what small, giving rise to higher tip temperatures. Excess of oxygen in the oxidizing flame causes the metal to burn or oxidize quickly. Oxidizing flame is useful for welding some nonferrous alloys such as copper and zinc base alloys.
In a carbonizing flame or reducing flame excess of acetylene is present. The temperature of this flame is low. The excess unburnt carbon is absorbed in ferrous metals, making the weld hard and brittle. In between the outer blue flame and inner white cone, an intermediate flame feather exists, which is reddish in colour. The length of the flame feather is an indication of the excess acetylene present. Carbonizing flame is used for welding high carbon steels and cast iron, alloy steel and for hard facing.
In an oxidizing flame excess of oxygen is present. The flame is similar to the neutral flame with the exception that the inner white cone is some what small, giving rise to higher tip temperatures. Excess of oxygen in the oxidizing flame causes the metal to burn or oxidize quickly. Oxidizing flame is useful for welding some nonferrous alloys such as copper and zinc base alloys.
Advantages:
- The equipment is in expensive in complicated and it is easily portable.
- Useful for welding light metals such as automobile bodies and repair works.
- A large variety of material can be welded.
- Welds can be produced at reasonable cost.
- Compared to electric arc welding this provides greater flexibility with respect to heat impact and cooling rates.
Disadvantages:
- Gas welding equipment must always be handled carefully as in certain circumstances acetylene is explosive (when a flame is applied under pressure) as oxygen when used in an oily atmosphere (such as an olds dirty garage floor pit).
- A high temperature flame from a hand held torch is dangerous when handled carelessly.
- It is much slower than electric arc welding and does not concentrate the heat close to the weld. Thus, the heat treated area is larger, which causes more distortion.
- Highly skilled operators are required to produce a good weld.
- If electric arc welding is available gas welding is seldom used for work over 3.2mm thick.
- The process is not satisfactory for heavy section.
