The high-frequency welding process is the most widely used method for the production of welded pipes by high frequency welded pipe production line. It heats the metal by applying or inducing an electric current across the edge of the strip before the closing point of the open pipe, and presses the tube blank through squeeze rollers, The molten metal and inclusions are squeezed out of the weld pool to form a forged weld. But there is still a lot of room for improvement in this area. By designing and transforming the high-frequency welding process of the tube on the original basic configuration and operating it effectively, the welding process can be optimized, the welding efficiency can be improved, and the cost can be greatly reduced.

The factors that affect the efficient operation of the high frequency welded pipe production line mainly include: the edge state of the tube blank, the length and angle of the V angle, the position and length of the ferrite (magnetic bar), the position and length of the coil, the length of the opening angle, the type of impedance, the design of the coil and Welder frequency.

Reasonable configuration and design can greatly save electricity consumption, improve the quality of pipes and welds, reduce downtime and improve efficiency, and greatly reduce production costs.

Principle of high frequency welding

High frequency welding is a type of resistance welding (ERW). A current applied (high frequency contact)) or induced (high frequency induction) across the edge of the strip flows along the edge of the strip to the junction and rapidly heats the metal before the closing point of the open tube. By applying pressure to the squeeze rollers, the heated metal will contact and form a thermal diffusion joint. Huge pressure can push molten metal and inclusions out of the weld pool. Therefore, this weld is produced by forging, unlike most other welding processes, which are the result of casting. Forge welding is the strongest welded structure available. one.

The real difference between high frequency contact welding and high frequency induction welding is:

For contact welding, the current is applied directly to the edge of the strip through the contact head, while in induction welding, the current is induced by the magnetic flux surrounding the coil. Both methods have their own strengths and weaknesses, but overall, induction welding seams are smoother and more consistent, but relatively less efficient.

Reason for choosing high frequency

If welding with a 50Hz power frequency power supply, most of the current will only flow on the back of the tube, heating the entire tube. Current always chooses the path with the least impedance (not necessarily resistance). For direct current and low frequency alternating current, there is basically no difference between resistance and impedance. From a technical point of view, at low frequencies, the impedance is mainly determined by the resistive element. As the frequency increases, the magnetic field generated by the current begins to affect the impedance, and the inductive reactance becomes the dominant factor in determining the impedance.

The current paths along the edge of the strip to the apex and the auxiliary current paths around the tube act as inductors, and their inductance increases with the step frequency, but the effect of frequency on the circumferential current path is more significant.

Another reason for the higher frequency of high frequency welded pipe production line is that it is best to keep the size of the coil small enough during the induction welding process. The coil and the tube together form a transformer. The coil acts as the primary winding and the tube acts as the single-turn secondary winding. The amount of energy coupled through the transformer depends on the strength of the magnetic flux and its rate of change (frequency). The higher the frequency, the more flux required. few. This reduces the number of coil turns and reduces the current. If a pipe is to be welded at the industrial frequency of 50Hz, hundreds of turns of coil are required to deliver thousands of amps of current. Typical high frequency welding coils are typically 1 to 3 turns and carry several hundred amps of current.

Higher frequencies also affect the behavior of the current at the V corner. As the frequency increases, the current tends to concentrate on the edge of the strip. The reason for this phenomenon is, on the one hand, the "skin effect" (see Figure 2), which makes current flow on the surface of the conductor at a very high frequency; on the other hand, the "proximity effect" (see Figure 3), which makes Currents in adjacent conductors are concentrated on adjacent surfaces.

Both of these effects are caused by the distortion and interaction between the current and the magnetic field. The combined effect of the skin effect and proximity effect results in the use of less current to heat less metal, increasing efficiency.

Efficient welding operation

The main reason for the inefficiency of high frequency welded pipe production line is the incorrect placement of the coils and impedances (magnet bars). When current is applied (or induced) to the edge of the strip, the current will flow in two main paths. The current flowing along the edge of the strip to the apex of the V-corner heats the strip to the welding temperature. Electricity also tends to flow inside the open tube, heating the entire tube, but that doesn't help the welding process. In induction welding, both parts of the current flow on the outer surface of the tube, forming a loop. Be aware that all current flowing on the inner surface of the tube will flow back through the outer surface, causing double the energy loss. Energy is proportional to the square of the current, so a small increase in current will result in a large consumption of energy.

The current flowing along the V-angle and inside the tube depends on the impedance of these two paths. Shortening and narrowing the V-shaped region reduces impedance: while a longer V-shaped region increases energy conduction losses by increasing the time it takes for heat to travel from the edge to the surroundings. It is important to realize that the length of the V-zone has a greater effect on the width of the heat-affected zone than the welding frequency.

Both shrinking the induction coil and increasing the tube diameter can increase the impedance inside the tube. Placing an impedance resistor inside the tube can further increase the inside impedance. Ideally, the impedance can be raised to the point where most of the current flows in the V-shaped region, but this is not easy to achieve with small diameter tubes due to the limited space inside the tube where the resistor can be placed. The internal burr removal device also takes up space where the resistor ferrite can be placed.