The frequency of high frequency induction welding has a great influence on the welding because the high frequency affects the current distribution in the steel plate. The influence of frequency selection on induction welding is mainly the size of the weld heat affected zone. As far as welding efficiency is concerned, a higher frequency should be used as far as possible.High frequency current of 100KHz can penetrate 0.1mm of ferrite steel, while 400KHz can only penetrate 0.04mm, which means the current density distribution on the surface of the steel plate is nearly 2.5 times higher than the former. In production practice, the frequency of 350KHz~450KHz can be selected when welding common carbon steel materials. When welding alloy steel materials and thick steel plates of more than 10mm, the lower frequency of 50KHz~150KHz can be used, because the skin effect of chromium, zinc, copper, aluminum and other elements contained in alloy steel is different from that of steel to some extent. Foreign high frequency equipment manufacturers have now mostly adopted the new technology of solid state high frequency, which, after setting a frequency range, will automatically track and adjust the frequency according to material thickness and unit speed when welding.
Second, the Angle of convergence is the Angle when both sides of the steel tube enter the extrusion point. Due to the proximity effect, when high-frequency current passes through the edge of the steel plate, the edge of the steel plate will form a preheating section and a molten section (also known as the lintels). When the lintels are heated violently, the molten steel inside the lintels will be quickly vaporized and burst out, forming a flash of light. The size of the meeting Angle has a direct impact on the molten section. When the junction Angle is small, the proximity effect is significant, which is beneficial to improve the welding speed. However, when the junction Angle is too small, the preheating section and the melting section become longer, and the result of the melting section becoming longer makes the flash process unstable. After the lintels burst, pits and pinholes are easy to form, which makes it difficult to press. When the convergence Angle is too large, the melt segment becomes shorter and the flash is stable, but the proximity effect weakens, the welding efficiency decreases obviously, and the power consumption increases. At the same time, when forming a thin-wall steel tube, the meeting Angle is too large, which will lengthen the edge of the tube and produce wavy crease. At present, we generally adjust the junction Angle within 2°–6° in production. The production speed of a thin plate is relatively fast, and the smaller junction Angle should be used in extrusion forming. Thick plate production speed is slower, extrusion molding should use a larger Angle of convergence.