What are the influencing factors of high-frequency welding of steel pipes?

High-frequency welding is a new welding process that uses the skin effect and adjacent effect generated by high-frequency current to connect steel pipes and other metal materials. The emergence and maturity of high-frequency welding technology is a key process in the production of longitudinally welded pipes (ERW pipe). The quality of high-frequency welding directly affects the overall strength, quality level and production speed of the welded steel pipe.

Basic Principles of High Frequency Welding
The so-called high frequency is relative to the AC current frequency of 50Hz, and generally refers to the high frequency current of 50KHz~400KHz. When high-frequency current passes through a metal conductor, two peculiar effects will be produced: skin effect and proximity effect. High-frequency welding uses these two effects to weld steel pipes.

Main Factors Affecting High Frequency Welding of Steel Pipes

1. Frequency
The frequency of high-frequency welding has a great influence on welding, because the high-frequency frequency affects the distribution of current inside the steel plate. The influence of the selected frequency on welding is mainly the size of the heat-affected zone of the weld. In terms of welding efficiency, a higher frequency should be used as much as possible.

2. Meeting angle
The meeting angle is the angle between the two sides of the steel pipe when they enter the extrusion point. Due to the proximity effect, when the high-frequency current passes through the edge of the steel plate, the edge of the steel plate will form a preheating section and a melting section (also called a lintel). When the lintel section is heated violently, the molten steel inside is rapidly vaporized and blasted out to form a flash, and the size of the meeting angle has a direct impact on the melting section.
The proximity effect is significant when the meeting angle is small, which is beneficial to increase the welding speed. However, if the meeting angle is too small, the preheating section and the melting section will become longer, and the result of the longer melting section will make the flashing process unstable, and deep pits and pinholes will easily form after the lintel explodes, making it difficult to press.
When the meeting angle is too large, the melting section becomes shorter and the flash is stable, but the proximity effect is weakened, the welding efficiency decreases obviously, and the power consumption increases. At the same time, when forming thin-walled steel pipes, if the meeting angle is too large, the edge of the pipe will be elongated, resulting in wavy wrinkles.

3. Welding method
There are two ways of high frequency welding: contact welding and induction welding.
(1) Contact welding is a pair of copper electrodes in contact with both sides of the steel pipe to be welded, and the induced current has good penetration. The two effects of high-frequency current are maximized due to the direct contact between the copper electrode and the steel plate, so the welding efficiency of contact welding is high and the power consumption is low. It is widely used in the production of high-speed and low-precision pipes, and contact welding is generally required when producing particularly thick steel pipes.
(2) Induction welding is to wrap one or more turns of induction coils outside the welded steel pipe. The effect of multi-turns is better than that of single turns, but it is more difficult to manufacture and install multi-turn induction coils. The efficiency is higher when the distance between the induction coil and the steel pipe surface is small, but it is easy to cause discharge between the induction coil and the pipe. Generally, it is advisable to keep a gap of 5-8 mm between the induction coil and the steel pipe surface. When induction welding is used, since the induction coil is not in contact with the steel plate, there is no wear and tear, and the induced current is relatively stable, which ensures the stability during welding. The surface quality of the steel pipe during welding is good, and the weld seam is smooth. In the production of high-precision pipes such as API, induction welding is basically used.

4. Input power
Input power control during high frequency welding is very important. If the power is too small, the heating of the groove of the tube billet is insufficient, and the welding temperature cannot be reached, which will cause unwelded defects such as virtual welding, desoldering, and clamping welding. When the power is too large, it will affect the welding stability, and the heating temperature of the groove surface of the tube blank is much higher than the temperature required for welding, resulting in serious spattering, pinholes, slag inclusions and other defects, which are called over-burning defects. .

5. Tube groove
The groove of the tube blank is the cross-sectional shape. When producing thick-walled pipes, it is best to go through planing or milling to make the groove be "X" shaped. Practice has proved that this kind of groove has a great relationship with uniform heating to ensure the quality of the weld.

6. Welding speed
Welding speed affects welding quality. When the welding speed is increased, it is beneficial to shorten the heat-affected zone and facilitate the extrusion of the oxide layer from the molten groove. Conversely, when the welding speed is very low, the heat-affected zone becomes wider, larger welding burrs will be generated, the oxide layer will thicken, and the quality of the weld will deteriorate. Of course, the welding speed is limited by the output power and cannot be raised very high.

7. Resistor
The function of the impedance is to strengthen the skin effect and the adjacent effect of the high frequency current. The setting of the impedance should match the pipe diameter to ensure the corresponding magnetic flux. In order to ensure the magnetic permeability of the resistor, in addition to the material requirements of the resistor, it is also necessary to ensure that the ratio of the cross-sectional area of the resistor to the cross-sectional area of the pipe diameter is sufficiently large. When producing high-grade pipes such as API pipes, it is required to remove internal burrs. The impedance can only be placed in the inner burr bayonet body, and the cross-sectional area of the impedance will be much smaller. At this time, the effect of adopting the centralized fan layout of the magnetic rods is better than that of the ring layout.

8. Welding pressure
Welding pressure is also the main parameter of high frequency welding. Different wall thicknesses take different extrusion amounts, usually the extrusion amount below 2mm is: 0.5t~t when it is 3~6 mm; 0.5t when it is 6~10 mm; 0.3t~0.5t when it is more than 10 mm .