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https://www.hf-weld.com/resources/introduction-of-high-frequency-welding-machine.html

Unlike other welders, high frequency welding machine is not just a single weld. The high frequency welding machine can be used not only for the welding of various metal materials, but also for the processes of heat penetration, melting, heat treatment and the like. Welding is just one of its many uses.

The working principle is:
First of all, there is a set of unique electronic circuits in the high frequency welding machine, which converts the low-frequency alternating current (50Hz) input from the grid into high-frequency alternating current (generally above 20000Hz). After the high-frequency current is applied to the inductor (ie, the induction coil), it is converted into a high-frequency magnetic field by the principle of electromagnetic induction and acts on the metal object in the magnetic field.  An induced current proportional to the strength of the magnetic field is generated in the metal object by the eddy current effect. This eddy current is affected by the skin effect. The frequency is higher, and it is the more concentrated on the surface of the metal object. When the eddy current flows in the metal object, heat is generated by the current thermal effect principle by means of the internal resistance value. This heat is generated directly inside the object. Therefore, the heating speed is fast and the efficiency is high. It can melt any metal in an instant. Moreover, the heating rate and temperature are controllable.

High frequency welding machine is especially suitable for heat treatment quenching, annealing, metal through-heat forging, extrusion molding, solder welding, etc., such as high-frequency quenching heat treatment of crankshaft, connecting rod, piston pin, camshaft, valve, various gears in the gearbox, forks, various spline shafts, transmission half shafts, various small shaft crank pins, various rocker arms and rocker shafts.