The relationship between welding speed and weld quality

The relationship between welding speed and weld quality should be understood dialectically and neither should be neglected. It is mainly reflected in the heating stage and the crystallization stage.

1.Heating stage

Under the working conditions of high-frequency straight seam welded pipes, the edge of the tube blank is heated from room temperature to the welding temperature. During this period, the edge of the tube blank has no protection at all and is completely exposed to the air. This inevitably causes intense reactions with oxygen, nitrogen and other substances in the air, significantly increasing the nitrogen and oxides in the weld seam. It has been measured that the nitrogen content in the weld seam increases by 20 to 45 times as a result. The oxygen content is thus increased by 7 to 35 times. Meanwhile, a large amount of alloying elements such as manganese and carbon that are beneficial to the weld seam are 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.

Not only that, the longer the edge of the heated tube blank is exposed to the air, that is, the slower the welding speed, the more non-metallic oxides will be produced at a deeper level. These deep-level non-metallic oxides are difficult to be completely extruded out of the weld seam during the subsequent extrusion crystallization process. After crystallization, they remain in the weld seam in the form of non-metallic inclusions, forming a distinct fragile interface. Thereby destroying the coherence of the weld microstructure and reducing the strength of the weld. The faster the welding speed, the shorter the oxidation time, and the less non-metallic oxides produced, which are limited to the surface layer, can be easily extruded out of the weld seam during the subsequent extrusion process. There will also be no excessive non-metallic oxide residue in the weld seam, and the weld seam strength is high.

2.Crystallization stage

According to the principles of metallography, to obtain high-strength welds, it is necessary to refine the grains of the weld microstructure as much as possible. The basic approach to refinement is to form a sufficient number of crystal nuclei in a short period of time, so that they come into contact with each other before significantly growing and the crystallization process ends. This requires increasing the welding speed to make the weld seam quickly leave the heating zone, so that the weld seam can crystallize rapidly at a larger degree of subcooling. When the degree of undercooling increases, the nucleation rate can increase significantly, while the growth rate increases less, thereby achieving the purpose of refining the weld grain.

Therefore, whether viewed from the heating stage of the welding process or the cooling after welding, under the premise of meeting the basic welding conditions, the faster the welding speed, the better the quality of the weld seam.

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