High frequency welding
Welding with currents of high frequency, most commonly used for the manufacture of welded stainless steel tubes of small and medium diameters. Such welding has a number of advantages:
• Production of pipes of various grades of steel — alloy, high alloy, carbon and non-ferrous metals for the same welding equipment
• Minimum consumption of electricity.
• Have the ability to achieve high speed welding up to 2−2,5 m/s without loss of quality of the weld.
• Use original blanks of different materials and obtaining a strong enough weld.
All this is achieved thanks to the significant concentration of current on the edges of the weld metal, which heats a very thin layer of metal. At a frequency of 450 kHz, the depth of heating of the edges at 20 °C is 0.05 mm. With the loss of magnetic properties (after 723 °C for steel) the depth increases to 0.9 mm at the same frequency and up to 2 mm at 70 kHz. The speed of welding and low power consumption is achieved thanks to the rapid warming up of thin layers of metal. To the edges of the workpiece current is supplied by contact or induction method.
The maximum current which is concentrated at the surface edges, causing rapid heating. When the edges closer together by means of rollers gripping the seam, forming a welding gauge, the current density increases. At the point where the edges converge, the temperature maximum. The convergence of hot edges and compression leads to the formation of the weld. If the diameter of the stainless pipe is minimal, then the shunt current heats the entire perimeter which leads to the need to increase power for welding. The negative side of the current supply contact is that the workpiece can only be used with the surface of high purity. But even in this case, the wear of contact, vertical oscillation of the edges, change the surface and inevitably leads to the violation of the welding performance. Can be burns on the surface of the pipe.
Eddy currents in the metal are excited by means of an annular inductor. The diameter of the stainless pipe are installed in the external inductors, and if the thickness of the pipe 200 mm and internal. Induced currents are concentrated at the ends of the edges. The higher the permeability, the higher the amperage and the heating of the steel. The advantages and disadvantages. Induction is good because regardless of the surface of the workpiece it provides a stable supply of energy, as well as the high quality of the weld. This advantage is made of the induction method is the most popular. Power inductors for TESA possible with as lamp generators (frequency up to 450 thousand Hz), and machinery (frequency of 10 thousand Hz). When welding pipes of steel and Nickel alloys up to Ø 100 mm, choose a frequency of 450 kHz, brass alloys — 70 kHz. When welding steel pipes, Ø>200 mm, the walls 6 to 20 mm lower frequency up to 10 kHz for deeper heating. The welding speed is reduced to 0.5−1 m/s to bring heating to the melting temperature at the point of welding. The presence of oxides, particularly refractory, it requires welding of pipes of medium diameter power up to 1600 kW. The possibility of using different modes of high-frequency welding is one of the important advantages of this method are applicable for welding alloy steels and non-ferrous metals.
High frequency welding lead to the formation as the outer and inner bead to 60% of the wall thickness of the pipe. This is considered a significant drawback. The outer Burr is removed using a deburrer, which is installed in-line mill. Internal Burr alert by shaping the edges, either cut or rolled and burned. This achieves higher quality and expanding the scope of the finished pipes. The most relevant domestic protonemata with cutter for continuous cutting. But the need for frequent replacement and the stop for the mill, making their use appropriate only in the production of limited quantities of pipes of higher quality.
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