How does the high-frequency welded pipe achieve welding through high-frequency current?
High-frequency welded pipe uses the skin effect and proximity effect of high-frequency current to concentrate the current in a narrow area at the edge of the steel strip. The current is conducted to the steel strip through the electrode, causing the edge to instantly heat to a molten state (about 1300-1400℃). Subsequently, pressure is applied through the extrusion roller to squeeze the molten metal together to form a weld. The entire process does not require filler material, and the welding speed can reach 20-100 meters/minute. The advantages of high-frequency welding are small heat-affected zone and weld strength close to the parent material.
What key equipment does the production line of high-frequency welded pipe include?
A typical high-frequency welded pipe production line includes an unwinder (unwinding steel coils), a leveler (eliminating steel strip bending), a forming machine (rolling the steel strip into a tube), a high-frequency welding device (including an oscillator and electrodes), an internal and external burr removal device (cleaning weld burrs), a cooling system, and a sizing machine (adjusting the pipe diameter accuracy). Modern production lines are also equipped with online detection equipment (such as ultrasonic flaw detection) to ensure quality. The automated control system can adjust the welding parameters in real time.
What is the difference between high-frequency welded pipe and submerged arc welded pipe (SAW)?
High-frequency welding (ERW) uses high-frequency current to directly heat the edge of the steel strip. It has a fast welding speed and does not require welding wire. It is suitable for thin-walled pipes (usually wall thickness <= 20mm). Submerged arc welding (SAW) burns under the flux layer through the arc, and requires filling welding wire. It is suitable for thick-walled large-diameter pipes (such as spiral welded pipes). The heat-affected zone of ERW is smaller, but the weld toughness of SAW is stronger and suitable for high-pressure environments. Both have their own emphasis in oil pipelines and building structures.
Why do the welds of high-frequency welded pipes need to be heat treated?
During high-frequency welding, the weld area will produce residual stress and hard and brittle structures (such as martensite) due to rapid heating and cooling, which may reduce toughness. Through online medium-frequency annealing or overall heat treatment, stress can be eliminated, grains can be refined, and the mechanical properties of the weld can be consistent with those of the parent material. For example, the API 5L standard requires that the hardness of the weld of the welded pipe shall not exceed 10% of the parent material. Heat treatment can also improve corrosion resistance and extend the life of the pipe.
What is the difference between the "cold rolling" and "hot rolling" processes of high-frequency welded pipes?
"Hot-rolled" high-frequency welded pipes use hot-rolled steel strips as raw materials, directly formed and welded, low cost but rough surface, often used for structural pipes. "Cold-rolled" high-frequency welded pipes use cold-rolled steel strips, with smooth surface and high dimensional accuracy, and need to be annealed to restore plasticity, and are mostly used in precision machinery or automotive parts. The tensile strength and fatigue resistance of cold-rolled pipes are better than those of hot-rolled pipes, but the cost is higher. The choice between the two depends on the purpose and budget.
