Under the condition of high-frequency straight seam welding pipe, the edge of the tube billet is heated from room temperature to welding temperature, during which the edge of the tube billet is completely exposed to the air without any protection, which inevitably reacts violently with oxygen and nitrogen in the air so that the nitrogen and oxide in the weld are significantly increased. According to the determination, the nitrogen content in the weld is increased by 20~45 times. Oxygen content is increased by 7~35 times. At the same time, a large number of beneficial alloy elements such as manganese and carbon burn and evaporate, resulting in the reduction of the mechanical properties of the weld. Thus, in this sense, the slower the welding speed, the worse the weld quality.

Not only that, the longer the heated tube blank edge is exposed to the air, that is, the slower the welding speed, it will cause the deep layer of non-metallic oxides, these deep non-metallic oxides in the subsequent extrusion crystallization process, is difficult to be all out of the weld after crystallization will remain in the form of non-metallic inclusion in the weld, forming an obvious fragile interface. Thus, the continuity of weld tissue is destroyed and the strength of the weld is reduced. The welding speed is fast, the oxidation time is short, the non-metallic oxide produced is less and limited to the surface, it is easy to be extruded from the weld in the subsequent extrusion process, and there will not be too much non-metallic oxide residue in the weld, the weld strength is high.

Crystallization stage

According to the principle of metal science, to obtain a high-strength weld, it is necessary to make the grain of the weld texture as fine as possible. The basic way of refinement is to form enough nuclei in a short time so that they contact each other before significant growth to end the crystallization process. This requires improving the welding speed, so that the weld quickly leaves the heating zone, in order to make the weld in a large degree of undercooling rapid crystallization; When the supercooling degree increases, the nucleation rate can be greatly increased, and the growth rate increased less, so as to achieve the purpose of refining the weld grain.

Therefore, no matter the heating stage of the welding process, or from the cooling after welding, it is on the premise of meeting the basic welding conditions, the faster the welding speed, the better the weld quality.