Gas Welding or Oxygen/Acetylene welding

Gas Welding

Oxygen/Acetylene welding, or “Gas Welding”, is a process which relies on combustion of oxygen and acetylene. When mixed together in correct proportions within a hand-held torch or blowpipe, a hot flame is produced with a temperature of about 3,200°C. The chemical action of the oxy/acetylene flame can be adjusted by changing the ratio of the volume of oxygen to acetylene, using the valves on the torch or blowpipe.

Equipment of Gas Welding or Oxygen/Acetylene welding

Oxy/acetylene equipment is portable and easy to use. It comprises oxygen and acetylene gases stored under pressure in steel cylinders. The cylinders should be fitted with regulators, to control the pressure and flow of gases. Flexible hoses are used to connect the regulators to the torch or blowpipe. Specially designed safety devices, called flame traps or “Flashback Arrestors” are fitted between the hoses and the regulators. Flashback arrestors prevent flames generated by a ‘flashback’ from reaching the cylinders.

Gas Welding or Oxygen/Acetylene welding
Gas Welding or Oxygen/Acetylene welding


Flashbacks

A flashback is a rapid, highpressure flame travelling back up the gas hoses, caused by welding or cutting at incorrect pressure settings, or from blockage or overheating of the nozzle (for example by operating with the nozzle too close to the material)

Flashbacks can result in extremely dangerous cylinder ignition and must be prevented by proper use of working Flashback Arrestors.

Lighting-Up Procedure

To begin oxygen/acetylene welding or cutting, open the cylinder valves slowly by means of the cylinder key(s). Do not open suddenly or there may be serious damage to the regulator and the possibility of an accident. Open the cylinder valve spindles one turn only. Open the fuel gas control valve on the blowpipe and adjust the regulator to give the correct working pressure (see chart on page 13), this ensures that any air or oxygen is purged from the hose. Repeat the procedure for the oxygen side.

Light the gas by means of a suitable sparklighter making sure that the sparklighter is held at right angles to the nozzle. Do not use liquid igniters (such as cigarette lighters) as the vapour/gas combination can be dangerous.

Reduce or increase the acetylene supply to the blowpipe valve until the flame just ceases to smoke.

Slowly increase the oxygen via the blowpipe control valve until the white inner cone in the flame is sharply defined with the merest trace of an acetylene haze. The blowpipe is now correctly adjusted.

WELDING TECHNIQUES

Leftward Welding

Leftward Welding is used on steel for flanged edge welds, for unbevelled plates up to 5.0mm (3/16in). It is also the method usually adopted for cast iron and non-ferrous metals. Welding is started at the right-hand end of the joint and proceeds towards the left.

The blowpipe is given a forward motion with a slight sideways movement to maintain melting of the edges of both plates at the desired rate and the welding rod is moved progressively along the weld seam – see fig. 

The sideways motion of the blowpipe should be restricted to a minimum.

Leftward Welding
Leftward Welding


Rightward and all position rightward welding

Rightward welding is recommended for steel plate over 5.0 mm (3/16 in) thick. Plates from 5.0 mm to 8.0 mm (3/16 in to 5/16 in) need not be beveled; over 8.0 mm (5/16 in) the edges are beveled to 30 degree to give an included angle of 60 degree for the welding V. Suitable for horizontal/vertical position.

Rightward Welding

The weld is started at the left-hand end and moves towards the right with the blowpipe flame preceding the filler rod in the direction of travel. The rod is given a circular forward motion and the blowpipe is moved steadily along the weld seam – See Fig.  This is faster than leftward welding and consumes less gas; the V angle is smaller, less filler rod is used and there is less distortion. The all-position rightward technique is a modification of the above and is particularly suitable for mild steel plate and pipe in the vertical and overhead position – See Fig. 

Rightward Welding
Rightward Welding


All-position rightward welding. Butt-welds in mild 5 – 8 mm (3/16 – 5/16 in) thick.

The advantages are that it enables the welder to obtain a uniform penetration bead and an even build-up, particularly in fixed position welding; the welder can work with complete freedom of movement and has a clear view of the weld pool and the fusion zone of the joint. 

Considerable practice is required to become familiar with this technique even by operators skilled in normal down hand rightward welding. An apparent undercutting of the plate surface at the edges of the weld bead is a fault to which this technique is prone but this can be controlled by appropriate manipulation of the rod and flame. The rod and blowpipe angle should be adjusted to give adequate control of the molten metal as in normal rightward welding.

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