The relationship between welding speed and weld quality should be understood dialectically and cannot be ignored. It is mainly reflected in the heating stage and the crystallization stage.
1. Heating stage
Under the condition of high-frequency straight seam welded pipe (erw pipe), the edge of the tube blank is heated from room temperature to the welding temperature. During this process, the edge of the tube blank is completely exposed to the air without any protection, which must be violent with oxygen and nitrogen in the air. reaction, resulting in a significant increase in nitrogen and oxides in the weld. It is determined that the nitrogen content in the weld increases by 20-45 times, and the oxygen content increases by 7-35 times. At the same time, a large number of alloying elements such as manganese and carbon that are beneficial to the weld are burned and evaporated, resulting in a decrease in the mechanical properties of the weld. Therefore, 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 slower the welding speed, the deeper the non-metallic oxide will be produced. It is difficult for these deep non-metallic oxides to be completely squeezed out of the weld during the subsequent extrusion crystallization process. After crystallization, they will remain in the weld in the form of non-metallic inclusions, forming an obvious brittle interface, thereby destroying the microscopic weld. The continuity of the organization reduces the strength of the weld. However, the welding speed is fast, the oxidation time is short, and the generated non-metallic oxides are few and limited to the surface layer, so it is easy to be squeezed out of the weld in the subsequent extrusion process, and there will not be too many non-metallic oxide residues in the weld. Therefore, the weld strength is high.
2. Crystallization stage
According to the principles of metallography, in order to obtain high-strength welds, the grains of the weld microstructure must be as fine as possible. The basic way of refining is to form enough nuclei in a short period of time so that they come into contact with each other before they grow significantly, and then the crystallization process ends. This requires that by increasing the welding speed, the weld can quickly leave the heating zone, so that the weld can crystallize rapidly under a large degree of undercooling; when the degree of undercooling increases, the nucleation rate can be greatly improved, and the growth rate can be slightly increased , so as to achieve the purpose of refining the grain of the weld.
Therefore, on the premise of satisfying the basic welding conditions, whether it is the heating stage of the welding process or the cooling stage after welding, the faster the welding speed, the better the quality of the weld.
Tips: ASTM A53 Grade B is more popular than other grades. These pipes can be bare pipes without any coating, or it may be Hot-Dipped or Zinc-Coated and manufactured by Welding or by a Seamless manufacturing process. In Oil and Gas, A53 grade pipes are used in the structural and non-critical applications.