The relationship between welding speed and weld quality should be understood dialectically and should not be neglected. It is mainly reflected in the heating stage and the crystallization stage.
Heating stage
Under the condition of high frequency straight seam welded pipe, the edge of the tube blank is heated from room temperature to the welding temperature. During this period, the edge of the tube blank is completely exposed to the air without any protection, which is inevitably related to oxygen, nitrogen, etc. in the air. A violent reaction occurs, which significantly increases nitrogen and oxides in the weld. It is determined that the nitrogen content in the weld is increased by 20 to 45 times, and the oxygen content is increased by 7 to 35 times; at the same time, it is beneficial to the weld. Alloying elements such as manganese and carbon are burned and evaporated in large quantities, which reduces 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.
Not only that, the longer the edge of the heated tube blank is exposed to the air, that is, the welding speed is slow, which will cause the formation of non-metallic oxides in the deeper layers. It is completely extruded from the weld, and after crystallization, it remains in the weld in the form of non-metallic inclusions, forming an obvious fragile interface, thereby destroying the continuity of the weld structure and reducing the strength of the weld. The welding speed is fast, the oxidation time is short, and the non-metallic oxides produced are less and only limited to the surface layer. Oxide residue, high weld strength
Crystallization stage
According to the principle of metallography, in order to obtain high-strength welds, the grains of the weld structure must be refined as much as possible; and the basic way of refinement is to form enough crystal nuclei in a short time, so that they are not significantly When they grow up, they come into contact with each other to end the crystallization process. This requires that by increasing the welding speed, the welding seam can quickly leave the heating zone, so that the welding seam can be rapidly crystallized under a large degree of undercooling; when the degree of undercooling increases, the nucleation rate can be greatly increased, and the growth rate increases more than less, so as to achieve the purpose of refining the grain of the weld.
Therefore, no matter from the heating stage of the welding process or from the cooling after welding, on the premise of satisfying the basic welding conditions, the faster the welding speed, the better the weld quality.
