The relationship between welding speed and weld quality

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The relationship between speed and weld quality should be understood in a dialectical manner, without neglecting either aspect. This is mainly reflected in the heating stage and the crystallization stage. During the heating phase, in the case of high-frequency straight-seam welded pipes, the edges of the pipe billet are heated from room temperature to the welding temperature. During this time, these edges are not protected in any way and remain completely exposed to the air, which inevitably leads to intense reactions with elements such as oxygen and nitrogen present in the air. As a result, the amount of nitrogen and oxides in the weld increases significantly; studies show that the nitrogen content in the weld rises by 20 to 45 times, while the oxygen content increases by 7 to 35 times ； At the same time, alloying elements such as manganese and carbon, which are beneficial to the weld, are extensively burned away and evaporated, resulting in a decrease in the mechanical properties of the weld. It can be seen that, in this sense, the slower the welding speed, the worse the quality of the weld. Moreover, the longer the edge of the heated tube billet remains exposed to air, that is, the slower the welding speed, the more non-metallic oxides will form in deeper layers. These deep-layer non-metallic oxides are difficult to be completely removed from the weld during the subsequent extrusion and crystallization process; they remain in the weld as non-metallic inclusions, creating a fragile interface that disrupts the continuity of the weld structure and reduces its strength. With a fast welding speed, the oxidation time is short; as a result, fewer non-metallic oxides are produced, and these are confined to the surface layer. They can be easily removed during the subsequent extrusion process, leaving no excessive amount of non-metallic oxides in the weld, thereby ensuring high weld strength. Crystallization stage: According to metallurgical principles, to obtain a weld with high strength, it is necessary to refine the grains in the weld structure as much as possible ； The basic approach to refinement is to generate a sufficient number of nuclei in a short period of time, so that they come into contact with each other before they can grow significantly, thereby ending the crystallization process. This requires increasing the welding speed so that the weld seam can leave the heating zone rapidly, thereby allowing it to crystallize quickly at a higher degree of supercooling ； As the supercooling degree increases, the nucleation rate can **increase**, while the growth rate increases to a lesser extent, thereby achieving the goal of refining the weld grains. Therefore, whether considering the heating phase during the welding process or the cooling after welding, provided that the basic welding conditions are met, the faster the welding speed, the better the weld quality.