A fusion welding method that uses the heat produced by combustible gas in the combustion of oxygen and melts the base metal weld to achieve connection. Gas welding is a welding method using gas flame as heat source. The most widely used is oxygen acetylene flame with acetylene gas as fuel. Because the equipment is simple and easy to operate, the heating speed and productivity of gas welding are low, the heat affected zone is large, and the deformation is easy to cause. Gas welding can be used for welding many ferrous metals, nonferrous metals and alloys. Gas: acetylene, liquefied petroleum gas and so on. Acetylene, for example, burns at 3200 degrees when it burns in oxygen. There are three kinds of oxyacetylene flame. 1. neutral flame: the mixture ratio of oxygen and acetylene is 1 to 1.2. Acetylene is fully combusted and suitable for welding carbon steel and non-ferrous alloy. 2. carbon flame: oxygen and acetylene volume mixture ratio less than 1, acetylene surplus, suitable for welding high carbon steel, cast iron and high speed steel. 3. oxidizing flame: the volume ratio of oxygen to acetylene is greater than 1.2, and the excess oxygen is applied to brazing brass and bronze. The gas welding flame temperature is low, the heating speed is slow, the heating area is wide, the welding heat affected zone is wide, the welding deformation is large, and during the welding process, the protection of molten metal is poor and the welding quality is not easy to guarantee, so its application is very few. However, the gas welding has the advantages of no power supply, simple equipment, low cost, convenient movement and strong versatility, so it has practical value in the field of no power supply and field work. At present, it is mainly used for welding thin steel plates (thickness 0.5 ~ 3mm), welding of copper and copper alloys and welding of cast iron.