In the process of arc welding, liquid metal, slag and gas interact with each other, which is the process of metal re smelting. But due to the welding condition Particularity, the process of welding chemical metallurgy is different from ordinary smelting process. First, welding metallurgy has high temperature, large phase boundary and fast reaction speed. When the air is invaded in the arc, the liquid metal will generate strong oxygen. There is also a large amount of metal evaporation, and the moisture in the air and the oil, rust and water in the workpiece and welding materials are high at the arc. The hydrogen atoms decomposed under the temperature can be dissolved in the liquid metal, resulting in the decrease of the ductility and toughness of the joint (hydrogen embrittlement), resulting in cracks. Secondly, the welding pool is small and the cooling is fast, which makes it difficult for all metallurgical reactions to achieve equilibrium. The chemical composition of the weld is uneven and the weld pool is uneven. When gas and oxide are not enough to emerge, it is easy to form holes, slag inclusions and other defects, and even produce cracks.