High frequency electric resistance inductive or conductive welding without additional material (HFI or HF ЕRW), are two basic welding processes for the production of steel pipes in FZC 11 Oktomvri A.D. plants, besides the standard submerged arc welding with additional material (SAW).
High frequency welding is commonly most associated with welding of steel pipes, although it is applied in various other cases. Usually it uses AC with frequency from 100 to 800 kHz, or 100.000 to 800.000 cycles per second. This process is characterized by two effects: skin effect, or concentration of high-frequency current on the surface, and proximity effect when a high-frequency concentrates on the two adjacent parallel inner edges in the welded wedge, before the welding point.
These effects allow a welding process to take place with the least amount of heat released, and because of that the structure of the basic material changes in a relative small zone around the central – ferit line of welds. Scavenging melted material from inner and outer side of the weld is removed by scraping devices.
Thermal affected zone, with width of a few millimeters on both sides of the frontal connection point have a larger structure due to intensive water cooling. This undesirable structure is locally normalized by additional induction heating with inducers with medium frequency and naturally cooled air. In this way, the welded joint is at least equal to the basic material, or weld joint factor is v = 1,0.
