Under the working conditions of ERW pipes, the edge of the tube blank is heated to the welding temperature. During this period, the edge of the tube blank is exposed to the air without protection. It inevitably reacts violently with oxygen and nitrogen in the air, causing the welding Nitrogen and oxides in the seam have increased significantly. According to measurements, the nitrogen content in the weld has increased by 20 to 45 times, and the oxygen content has increased by 7 to 35 times. At the same time, alloy elements such as manganese and carbon are beneficial to the weld. A large amount of burning and evaporation leads to a decrease in the mechanical properties of the weld, so in this sense, the slower the welding speed, the worse the welding quality.
Not only that, the longer the edge of the heated tube blank is exposed to the air, that is, the slower the welding speed, will cause non-metal oxides to be produced in deeper places, which are difficult for the subsequent extrusion and crystallization process. Extrusion welds, after crystallization, remain in the welds in the form of non-metallic inclusions, forming a fragile interface, which damages the continuity of the weld structure and reduces the strength of the weld, but the welding speed is fast and the oxidation time is short. The non-metal oxides are less and limited to the surface layer, and are easy to be extruded from the weld in the subsequent extrusion process, and there will not be too many non-metal oxide residues in the weld, so the weld strength is high.
According to metallographic principles, in order to obtain high-strength welds, it is necessary to refine the grain size of the weld structure as much as possible. The basic way of refinement is to form enough crystal nuclei in a short period of time to make them grow up significantly. When contacting each other, the crystallization process ends. This requires that the welding seam quickly leave the heating zone by increasing the welding speed, so that the welding seam will crystallize rapidly under a large degree of undercooling. As the degree of undercooling increases, nucleation occurs. The rate of growth can be greatly increased, and the growth rate is increased less, thereby refining the weld grains. Therefore, whether from the heating stage of the welding process or the cooling after welding, the welding speed is higher if the basic welding conditions are met. The faster, the better the welding quality.
High-frequency welding is a key process in the production of welded pipes. Due to the systematic influencing factors and the mutual constraints of these factors, we also need to explore experience in production. Each unit has its design and manufacturing differences. Each operator There are different habits, and the welded pipe industry is also developing with each passing day. More and more equipment manufacturers are investing in R&D and experimentation for automation and standardization. It is expected that welded pipe production equipment can do it in the near future.