Gas welding is a welding method which USES gas flame as heat source to melt base metal and filler metal.The most commonly used is oxyacetylene welding, that is, the use of acetylene (combustible gas) and oxygen (combustion gas) mixed combustion produced by oxyacetylene flame, to heat the melting workpiece and welding wire, condensation after the formation of a weld method. 

Acetylene USES pure oxygen to support combustion. Compared with air, it can greatly increase the flame temperature (up to about 3000℃).Compared with arc welding, the gas welding flame temperature is low, heat dispersion, heating speed is slow, so the productivity is low, the workpiece deformation is serious, the heat affected area of welding is large, the welding joint quality is not high.But the gas welding equipment is simple, the operation is flexible and convenient, the flame is easy to control, does not need the power supply.So gas welding is mainly used for welding thickness less than 3mm of low carbon steel sheet, copper, aluminum and other nonferrous metals and its alloys, as well as iron welding.In addition, it is also suitable for field work without power supply. 

Gas welding flame (oxyacetylene flame)

The flame formed by mixing oxygen and acetylene is called oxyacetylene flame.By adjusting oxygen valve and acetylene valve, the mixing ratio of oxygen and acetylene can be changed to obtain three different flames.Neutral flame, oxidizing flame and carbonizing flame. 

1. Neutral flame

When the ratio of oxygen to acetylene is 1 ~ 1.2, the resulting flame is called a neutral flame, also called a normal flame.It is composed of flame core, inner flame and outer flame. The flame core is near the welding tip and is white and bright.The second is the internal flame.Blue purple, the highest temperature here, about 3150℃, from the front end of the flame core 2 ~ 4mm, apply here to heat the workpiece and welding wire, the outermost flame, orange red. 

Neutral flame is a commonly used flame for welding low carbon steel, medium carbon steel, alloy steel, red copper, aluminum alloy and other materials. 

2. The carbonizing flame

When the volume ratio of oxygen to acetylene is less than 1, carbonation flame is obtained.Combustion is incomplete because there is less oxygen.The whole flame is longer than the neutral flame.Low temperature, excessive acetylene in carbonization flame, suitable for welding high carbon steel, cast iron and carbide materials.When other materials are welded with carbonized flame, the weld metal will be carbonized and become hard and brittle. 

3. The oxidizing flame

When the volume ratio of oxygen to acetylene is greater than 1.2, an oxide flame is formed.Due to more oxygen, burning violent, flame length significantly shortened, the cone of the flame core, the flame almost disappeared, and there is a strong filar sound, due to more oxygen in the oxide flame.Easy to make metal oxidation, so it is not widely used, only used for welding brass, to prevent the evaporation of zinc.