Gas welding

In the gas or flame welding process, heat is supplied by the combustion of a suitable fuel gas mixed with air or pure oxygen. The most commonly used fuel gases are acetylene (C₂H₂), natural gas and hydrogen. Oxyhydrogen welding was the first gas welding process to be commercially developed. The maximum temperature developed by this process is about 2000⁰C.

Other combination of gases are

1.      Air hydrogen

2.    Air acetylene

3.    Oxyacetylene

‘oxyacetylene’ is the most popular combination of gases and develops the highest flame temperature of around 3200⁰C. Hence, gas welding now mainly refers to the oxyacetylene gas welding process.

Fig.

Oxy-acetylene welding

Oxygen and Acetylene are available in separate cylinders filled with regulators for controlling their flow and pressure. These gases are brought separately through rubber hoses to the welding torch and arc mixed in their proper proportions. The regulated mixture of gas coming out of the tip or nozzle at the end of torch possesses the desired characteristics after it is lighted.

Oxyacetylene welding

Principle of operation

A oxyacetylene weld is produced by heating with a flame obtained from the combustion of oxygen and acetylene, with or without the use of a filler metal. It is a form of fusion welding and, as a rule, no pressure is used.

There are two main divisions of oxy-acetylene welding:

1.  Welding by means of dissolved acetylene stored in cylinders (usually referred to as the “high pressure system”).

2. Welding by means of acetylene which is obtained from an acetylene generator (usually referred to as “generator system”).

The dissolved acetylene process, which is very convenient for job work and garages where there is no regular follow-through of welding work. On the other hand, the acetylene generator system is very convenient for work in a shop devoted entirely to welding.

‘oxygen’ is produced by electrolysis and liquefying air. Most of the commercial oxygen is made by liquefying air and separating oxygen from the nitrogen. It is stored in steel cylinders at a pressure of about 14 N/mm².

Acetylene is generated from the reaction of calcium carbide with water, in an acetylene generator.

C_aC₂ + 2H₂O —› C_a(OH)₂ + C₂H₂

(calcium carbide) (water) (slaked lime) (Acetylene gas)

Normally, acetylene cylinders are used. Acetylene cylinders are filled with a porous filter, saturated with acetone, in which the acetylene gas can be compressed safely. These cylinders hold about 9m³ of gas at pressures upto 1.8 N/mm².

For identification of cylinders from each other they are of different sizes and colours. Acetylene cylinders are painted ‘maroon’ and oxygen painted with ‘black’.

Precaution to be observed when using oxygen and acetylene cylinders:

1.      All valves must be closed when work is finished.

2.    There should be no leaks at the gland round the valve spindle.

3.    If a cylinder catches fire at the valve or regulator, shut the valve and tighten the joint properly before further use.

4.  If the cylinder becomes hot or fires internally, close the valve, disconnect the regulator, remove cylinder into open, then open the valve fully to allow gas to escape freely, and immerse cylinder into water.

5.     Store cylinders in upright position.

6.    Use trolley for handling cylinders to avoid damage to the valves.