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Control of the Optimum Efficiency of High Frequency Welded Pipe Welding Process

In high frequency welding, welding power is generally over 100 KW, 80% of which is consumed by coil heating, impedance, roll pressing and tube blank heating.

Therefore, the key to minimizing power loss is to set up reasonable coils, impedance devices and rational configuration of rolling mills. In most cases, after optimizing the welding process, the power loss can be reduced by 50%, and the welding quality can be improved, the downtime can be reduced, and the output can be increased.

1 Operational efficiency

The reason for the low operating efficiency of high frequency welding is that the position of induction coil and impedance is not very good. When the voltage is applied to the edge of the billet, part of the current flows along the edge of the strip through the V-shaped zone while heating the edge of the strip; the other part of the current flows through the inner ring surface of the open tube body and returns to the outer ring surface, resulting in power loss.

 图一

The magnitude of the current flowing through the V-shaped region and the inner ring surface depends on their impedance, as shown in Figure 1. Shortening the length of V-shaped area and keeping its smaller distance can reduce the impedance of the circuit, and vice versa, increase the impedance and increase the heat loss. The length of V-shaped zone has greater influence on the efficiency of heating zone than frequency. Short working coil and larger diameter can increase the impedance of the tube. Placing an impedance in the tube (Fig. 1b) also increases the internal impedance. 

2 Edge state

The irregular shape or groove on the edge of V-shaped strip will increase the heat loss of welding, increase the amount of extruded metal and produce irregular beads, which will easily lead to welding defects. In Figure 2, A~C is in bad edge condition: the inner surface of strip converges in V-shaped zone first, and a large amount of current flows through the inner surface, which causes excessive heating and melting of the metal in the inner surface weld zone, resulting in a large number of beads. In order to penetrate the whole wall thickness, it will consume a larger power. The D~F in Figure 2 belongs to better welding edge state, that is, the strip edge is parallel butted in V-shaped zone.

图2

3 V-shaped shape

From the point of view of maximizing efficiency, the V-shaped region should be as short as possible (to reduce conduction loss). In actual production, a certain size of extrusion roll is installed in the V-shaped area, so that the induction coil is far away from the top of the V-shaped area. In addition, the thickness of pipe wall also affects the length of V-shaped zone. The high frequency current first heats the convergence point of both sides of the strip in the V-shaped zone, making the heating zone funnel-shaped. If the V-shaped zone is too short, the edge of the strip will be heated unevenly, which will lead to incomplete welding or overheating and decarbonization of the welding zone.

The heating of V-shaped zone of strip begins with its induction coil. It is improper to measure the length of V-shaped zone from the end of induction coil to the junction point of strip steel. The distance from the middle of induction coil to the junction point should be regarded as the length of V-shaped zone (usually 1.5 times the diameter of pipe). However, the length of V-shaped zone of large diameter and thin-walled pipe should be reduced, while the length of V-shaped zone of welded pipe less than 25.4mm (1″) should be increased due to the limitation of extrusion roll size.

The convergence angle of the two sides of V-shaped strip also affects the welding efficiency. A smaller convergence angle requires a smaller welding power, which not only concentrates the proximity effect, but also reduces the flux in the impedance and solves the problem that the space of the impedance is limited at a certain welding speed. However, too small convergence angle can easily lead to “fire” and aggravate the instability of strip steel in the frame and the wear of rollers. Generally speaking, the optimum convergence angle of welded carbon steel is 3 ~4 and that of welded stainless steel and non-ferrous metal is 5 ~8.

4 Installation of Impedance Device

The function of the impedance is to obtain large current impedance in the tube so that more current can be concentrated in the V-shaped region. 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 impedance is magnetism. Magnets with the highest flux density and oscillation and the lowest electromagnetic loss should be selected in use. But sometimes these requirements are incompatible, so operators are required to master certain knowledge of welding operation and electromagnetic circuit design.

The location of the impedance is very important. If it concentrates too much under the confluence point, it will be more efficient, but it will be easily damaged. Therefore, the impedance is usually placed in the tube, so that there is a gap equivalent to the thickness of the tube wall from the inner surface of the tube. The impedance of most small units is installed at the bottom of the pipe, which is not only inefficient for welding, but also easy to be dragged away when the pipe moves.

The magnetism of the impedance should extend from the center of the induction coil to the extrusion point. The minimum impedance length is the diameter of extrusion roll and the length of induction coil. Figure 3 shows the impedance setting. Some operators take the front end of the impedance as the best position, and a large number of beads appear during welding, which prolongs the life of the impedance, but increases the power consumption.

 图3

5 Design of induction coil

It is safe to use low voltage and high current devices in solid state welding. Because the power consumption of the working coil is the square of the current value, even if the coil has a smaller resistance, it will still have a larger power consumption. In order to reduce the resistance value, the induction coil is usually made by brazing the oxygen-free copper plate with the cooling pipe.

6 Welding frequency

The high frequency used for welded steel pipe is between 80 kHz and 800kHz. Although this frequency range has little influence on the heating zone, it has a direct impact on the heating quality.

The effect of impedance on the efficiency of welding process is greater than that of frequency, but the loss of impedance increases with the increase of frequency, so it is difficult to ensure the cooling of impedance.

High frequency is suitable for the production of small diameter pipes, while low frequency is suitable for the production of large diameter pipes. The best way is to adjust the welding frequency.


Post time: Feb-27-2019

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