Oxy Acetylene Welding/Cutting

Gas welding is one of the oldest methods of welding and, for many years, was the most widely used method of metal-melting; however, its use is a lot less common today. Nevertheless, it is a versatile method, using simple and relatively cheap equipment. It is suitable for repair and erection work, for welding pipesltubes and structures with a wall thickness of 0.54 rnm in materials particularly prone to cracking, such as cast iron and non-ferrous metals. It is also widely used for cladding and hardfacing. The heat is generated by the combustion of acetylene in oxygen, which gives a flame temperature of about 3100 Deg. C. This is lower than the temperature of an electric arc, and the heat is also less concentrated. The flame is directed onto the surfaces of the joint, which melt, after which filler material can be added as necessary. The melt pool is protected from air by the reducing zone and the outer zone of the flame. The flame should therefore be removed slowly when the weld is completed. The less concentrated flame results in slower cooling, which is an advantage when welding steels that have a tendency to harden, although it does make the method relatively slow, with higher heat input and the added risk of thermal stresses and distortion. In addition to welding, gas flames are also often used for cutting, and are very useful for heating and flame straightening.

Acetylene (C2H2) is the fuel gas for gas welding. It consists of 92.3 % of carbon by weight, and 7.7 % of hydrogen. Its combustion in oxygen produces a higher combustion temperature than that of any other hydrocarbon gas. In addition, its flame is the most concentrated in comparison with other gases.

Oxygen is stored as a compressed gas or liquid. In bottles, it is usually stored at a pressure of 20 MPa. Large users usually receive the gas in liquid form. Make sure that all connections are clean and tight, in order to avoid leakage. Never apply oil or grease to connections.

The basic requirement for a good weld is that the size and type of the flame should be suited to the type of work. The size of the flame depends on the size of the torch nozzle and on the pressure of the gases flowing through it. This pressure should be maintained within certain limits. If it exceeds the normal pressure, there will be a considerable jet effect and the flame will become 'hard'. Below the correct pressure, the jet effect will be reduced and the flame
will be 'soft'. We distinguish between three different types of flames, depending on their chemical
effect on the melt pool: carburising, neutral, and oxidising.


Neutral flame
The normal flame is that which is used most. It is easily recognised by the three clearly distinguished combustion zones. The innermost zone, the cone, is a mixing zone and glows white. Acetylene is burning here, to form carbon monoxide and hydrogen which produce a colourless tongue around the cone. This second zone is chemically reducing, and so it reduces any metal oxides and keeps the melt pool clean. The outer, blue zone of the flame is where carbon monoxide and hydrogen are burning with oxygen from the air, forming the final combustion products of carbon dioxide and water vapour. It prevents oxygen in the air from coming into contact with the molten metal, and so acts as a shielding gas.


The carburising flame
If the proportion of acetylene in a neutral flame is increased, there is insufficient oxygen to burn the surplus acetylene in the core zone. The acetylene therefore continues to the second zone, where it appears as a highly luminous yellow-white flame. To some extent, the length of second zone indicates the amount of excess acetylene.


The oxidising flame
If the quantity of oxygen in the weakly reducing flame is further increased, the flame changes to an oxidising flame. The core length is reduced, and the flame takes on a violet tinge with low luminosity.