In high-frequency welding, the welding power is generally above 100 kW, of which 80% of the power is consumed in the heating of the coil, the impedance, and the heating of the pressure roller and the tube blank.
Therefore, the key to minimizing power loss is to set up reasonable coils, resistors, and proper configuration of the ERW tube mill. In most cases, the power loss can be reduced by 50% through an optimized welding process, which improves weld quality, reduces downtime, and increases production.
1. High frequency welding mechanism
The mechanism of high frequency welding is that the voltage passes through the edge of the open portion of the tube blank, and the current is brought to the meeting point by induction. The metal is rapidly heated as current flows through the junction. By the pressure of the squeezing roller, the molten portion of the metal is subjected to hot extrusion bonding, and the impurities are also extruded from the weld.
2. Why choose high frequency current
Technically, when a low frequency current passes through a resistive element, the impedance is primarily consumed. As the frequency increases, the magnetic field increases, and the inductive reactance becomes a major factor in the impedance and increases as the frequency increases. In the welding process, the induction coil as the primary winding should be appropriately small in size. The power can be adjusted by electromagnetic coupling, and the magnetic loss caused by the high frequency is related to the number of turns and the current of the coil. If a tube is welded with a frequency of 60 Hz, a coil of several hundred turns and a current of several thousand amps are required. For high-frequency welding, a coil of 1 to 3 turns and a current of several hundred amps are usually used. In addition, high-frequency welding is also conducive to the occurrence of “skin effect” and “proximity effect”.
The reason for the low efficiency of high frequency welding operations is that the position of the induction coil and the resistor is not very good. When a voltage is applied to the edge of the tube blank, a portion of the current flows along the edge of the strip through the V-shaped area to heat the strip edge; Another portion of the current flows from the inner ring surface of the open tubular body and returns through the outer ring surface, resulting in power loss.
The amount of current flowing through the V-shaped area and the inner ring surface depends on their impedance, as shown in Figure 1. Shortening the length of the V-shaped area and maintaining its small distance can reduce the impedance of the path, and vice versa, increase the impedance and increase the heat loss. The effect of the length of the V-shaped area on the efficiency of the heating zone is greater than the effect of the frequency. Short working coils and larger pipe diameters increase the internal resistance of the pipe. Placement of the resistor in the tube (Figure 1b) also increases the internal impedance.
In the V-shaped area, the edge of the strip has burrs or irregular shapes, which will increase the welding heat loss, increase the amount of extruded metal, and generate irregular beads, which may easily lead to welding defects. In Fig. 2, A~C is a bad edge state: the inner surface of the strip first meets in the V-shaped area, and a large amount of current flows from the inner surface, so that the metal in the inner surface weld zone is excessively heated and melted, and a large number of weld beads are generated. In order to make the entire wall thickness penetrate, it will consume a lot of power. However, D~F in Fig. 2 are in relatively good welded edge state, that is, the strip edges are parallelly butted in the V-shaped area.
From the point of view of maximizing efficiency, the V-shaped area should be as short as possible (to reduce conduction losses). In actual production, a certain size of squeeze roller is installed in the V-shaped area to make the induction coil away from the apex of the V-shaped area. In addition, the thickness of the tube wall also affects the length of the V-shaped area. The high frequency current first heats the confluence point on both sides of the strip in the V-shaped area, so that the heating zone appears as a funnel. If the V-shaped zone is too short, the strip edges will be unevenly heated and will result in incomplete or over-fired and decarburization of the weld zone.
The heating of the strip V-shaped area begins with its induction coil. Generally, it is not appropriate to measure the distance from the end of the induction coil to the junction of the steel strip as the V-shaped area. The distance from the middle of the induction coil to the junction point should be taken as the length of the V-shaped area (generally 1.5 times the diameter of the pipe). However, the length of the V-shaped area of the large diameter and thin-walled tube should be reduced, and the length of the V-shaped area of the welded pipe smaller than φ25.4 mm (1″) should be increased due to the size of the squeeze roller.
The size of the confluence angles on both sides of the strip in the V-shaped area also has an effect on the welding efficiency. The smaller junction angle requires less welding power, which not only concentrates the proximity effect, but also helps to reduce the magnetic flux in the resistor and solve the problem that the resistance placement space is limited at a certain welding speed. However, too small a confluence angle can easily lead to “ignition” and increase the instability of the strip in the frame and the wear of the roller shaft. In general, the best confluence angle of welded carbon steel is 3°~4°, and the ideal confluence angle of welded stainless steel and non-ferrous metal is 5°~8°.
The role of the resistor is to obtain a large current impedance within the tube so that more current is concentrated in the V-shaped area. At the same time, the magnetic field is concentrated according to the current of the working coil, so that a large amount of current energy is concentrated in the V-shaped area of the tube.
An important parameter of the resistor is magnetic. In use, a magnet with the highest magnetic flux density and oscillation and the lowest electromagnetic loss should be selected. But sometimes these requirements are mutually incompatible, so the operator is required to have a certain knowledge of welding operations and electromagnetic circuit design.
The placement of the impeder is important. If it is too concentrated under the meeting point, although it is more efficient, it is extremely vulnerable. Therefore, it is common to place the impeder in the tube so that it has a gap corresponding to the wall thickness of the tube from the inner surface of the tube. Most small unit’s resistors are mounted on the lower bottom of the tube. This placement is not only inefficiently welded, but is also easily towed away as the tube moves.
The magnetism of the resistor should extend from the center of the induction coil to the point of compression. The minimum resistor length is the diameter of the squeeze roller plus the length of the induction coil. Figure 3 shows the setting of the resistor. Some operators use the front end of the resistor as the optimum position, and a large number of beads appear during welding. Although the life of the resistor is extended, the power consumption is increased.
7.Induction circle design
It is safer to use low voltage, high current devices in solid state welding. Since the power consumption of the working coil is the square of the current value, even if the coil has a small resistance, there is still a large power consumption. Therefore, in order to reduce the resistance value, the induction coil is usually fabricated by an oxygen-free copper plate and a cooling tube.
The high frequency used for welding steel pipes is between 80 and 800 kHz. Although this frequency range has a small effect on the heating zone, it has a direct effect on the heating quality.
For the effect of the welding process efficiency, the impedance is greater than the frequency, but the loss of impedance increases with the increase of the frequency, so it is difficult to ensure the cooling of the resistor.
High frequency is suitable for the production of small diameter tubes, while low frequency is suitable for the production of large diameter tubes. The best way is to make the welding frequency adjustable.
Post time: Apr-15-2019