The relationship between welding speed and weld quality

     The relationship between welding speed and weld quality should be understood dialectically and not neglected. Mainly manifested in the heating stage and crystallization stage.

    Heating stage: Under the condition of high-frequency straight seam welded pipe, the edge of the pipe blank is heated from room temperature to the welding temperature. During this period, the edge of the pipe blank is not protected and completely exposed to the air, which inevitably reacts violently with oxygen, nitrogen, etc. in the air, causing a significant increase in nitrogen and oxides in the weld seam. According to measurements, the nitrogen content in the weld seam increases by 20-45 times, and the oxygen content increases by 7-35 times; At the same time, alloying elements such as manganese and carbon that are beneficial to the weld seam are greatly burned and evaporated, resulting in a decrease in the mechanical properties of the weld seam. From this, it can be seen that in this sense, the slower the welding speed, the worse the quality of the weld seam. Moreover, the longer the edge of the heated billet is exposed to the air, the slower the welding speed, which can cause the formation of non-metallic oxides in deeper layers. These deep-seated non-metallic oxides are difficult to completely squeeze out of the weld seam during the subsequent extrusion crystallization process, and remain in the weld seam in the form of non-metallic inclusions after crystallization, forming a clearly fragile interface that destroys the continuity of the weld seam structure and reduces the strength of the weld seam. And the welding speed is fast, the oxidation time is short, and the non-metallic oxides produced are relatively small and limited to the surface layer. It is easy to be squeezed out of the weld seam during the subsequent extrusion process, and there will not be too much non-metallic oxide residue in the weld seam, resulting in high weld strength.

    Crystallization stage: According to the principles of metallurgy, in order to obtain high-strength welds, it is necessary to refine the grain structure of the weld as much as possible; The basic approach to refinement is to form enough crystal nuclei in a short period of time, so that they come into contact with each other before significantly growing and end the crystallization process. This requires increasing the welding speed to quickly remove the weld from the heating zone, in order to enable the weld to crystallize rapidly at a higher degree of undercooling; When the degree of undercooling increases, the nucleation rate can greatly increase, while the growth rate increases less, thus achieving the goal of refining the grain size of the weld seam. Therefore, whether viewed from the heating stage of the welding process or the cooling after welding, the faster the welding speed, the better the quality of the weld seam, provided that the basic welding conditions are met.