High frequency welding
Welding with high frequency currents is most often used for the manufacture of electrically welded stainless pipes of small and medium diameter. Such welding has a number of advantages:
• Manufacture of pipes from various grades of steel — alloyed, highly alloyed, carbonaceous, and also non-ferrous metals, on the same welding equipment
• Minimum power consumption.
• It is possible to achieve high welding speeds — up to 2−2.5 m / s without loss of weld quality.
• Use of initial blanks of various materials and obtaining a sufficiently strong weld seam.
All this is achieved due to a significant concentration of current on the edges of the weld metal, which heats up 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. Fast welding and low power consumption are achieved due to the rapid heating of thin layers of metal. To the edges of the workpiece, the current is supplied by a contact or induction method.
The maximum current that concentrates on the surface of the edges causes a rapid warm-up. When the edges are brought together by means of rollers that compress the seam forming a welding gauge, the current density increases. At the point where the edges meet, the temperature is maximum. Convergence of the heated edges and compression leads to the formation of a welded seam. If the diameter of the stainless pipe is minimal, the shunt current heats the entire perimeter, which leads to the need to increase the input power for welding. The negative side of the contact current input is that the workpiece can only be used with a high purity surface. But even in this case, contact wear, vertical edge oscillation, surface change is inevitable and leads to a violation of the welding regime, a decrease in productivity. There may be burns on the surface of the pipe.
Eddy currents in the metal are excited by means of ring inductors. By the diameter of the stainless pipe, external inductors are installed, and if the pipe is thicker than 200 mm — and internal. Induction currents are concentrated at the ends of the edges. The higher the permeability, the higher the current and the heating of the steel. Advantages and disadvantages. The induction method is good in that, regardless of the surface of the workpiece, it provides a stable supply of energy, as well as a high quality of the welded seam. This advantage made the induction method the most sought after. Power supply of inductors to TESA is possible with the help of both lamp generators (with a frequency of up to 450,000 Hz) and with machine (with a frequency of 10,000 Hz). When welding pipes of steel and nickel alloys to Ø 100 mm, choose the frequency of 450 kHz, brass alloys — 70 kHz. When welding steel pipes, Ø> 200 mm, with walls of 6−20 mm, reduce the frequency to 10 kHz for deeper heating. At the same time, the welding speed is reduced to 0.5−1 m / s in order to bring the heating to the melting point at the welding point. The presence of oxides, in particular refractory, requires for welding pipes of average diameters of power up to 1600 kW. The possibility of applying different modes of high-frequency welding is one of the important advantages of this method, applicable both for welding alloy steels and non-ferrous metals.
High-frequency welding leads to the formation of both external and internal beads up to 60% of the thickness of the pipe walls. This is considered a significant drawback. The outer grate is removed with a grinder, which is installed in the line of the mill. The formation of the inner bead is prevented by shaping the edges, either cut off, or rolled up and burned. At the same time, higher quality is achieved and the scope of finished pipes is expanded. The most applicable are internal grinders having one cutter for continuous cutting. But the need for frequent chisel change and stopping for this mill makes it expedient to use them only in the production of limited lots of pipes of increased quality.
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