Controlling Optimal Efficiency During High-Frequency Welding of Steel Pipes

In high-frequency welding of steel pipes, welding power typically exceeds 100 kW, with 80% of this power consumed by coil heating, impedance, and heating of the press rolls and steel pipe. Therefore, minimizing power loss lies in optimally designing the coil, impedance, and mill configuration. In many cases, optimizing the welding process can reduce power loss by 50%, improving weld quality, reducing downtime, and increasing production.

1. High-Frequency Welding Mechanism
The mechanism of high-frequency welding is as follows: voltage is applied to the edges of the steel pipe opening, and an induction driving current is applied to the junction. As current flows through the junction, the metal is rapidly heated. Pressure from the squeeze rolls causes the heated and molten metal to form a thermally extruded bond, expelling impurities from the weld.

2. Why Choose High-Frequency Current?
Technically, when a low-frequency current passes through a resistive element, impedance is the primary dissipation. As the frequency increases, the magnetic field intensifies, and inductive reactance becomes the dominant factor in impedance, increasing with frequency. During welding, the induction coil, serving as the primary winding, should be appropriately small. Power can be adjusted through electromagnetic coupling. The magnetic losses caused by high frequency are related to the number of coil turns and the current. For example, welding a steel pipe with a 60Hz power supply requires a coil with hundreds of turns and thousands of amperes of current. High-frequency welding, on the other hand, typically requires only one to three turns and hundreds of amperes of current. Furthermore, high-frequency welding is more conducive to the occurrence of skin and proximity effects.

3. Operational Efficiency
The low efficiency of high-frequency welding can be attributed to poor positioning of the induction coil and resistor. When voltage is applied to the edge of a steel pipe, part of the current flows along the edge of the strip through the V-shaped zone, heating the edge. Another part of the current flows through the inner ring surface of the open pipe and returns through the outer ring surface, resulting in power loss. The amount of current flowing through the V-shaped zone and the inner ring surface depends on their impedance. Shortening the length of the V-shaped zone and keeping it close together can reduce the impedance of this path. Conversely, increasing the impedance increases heat loss. The length of the V-shaped zone has a greater impact on the efficiency of the heating zone than the frequency. Short working coils and larger pipe diameters can both increase the internal impedance of the pipe. Placing an impedance device inside the pipe also increases internal impedance.

4. Edge Condition
Burrs or irregular shapes on the strip edges in the V-zone increase welding heat loss, extrusion, and irregular weld beads, which can easily lead to weld defects. A through C represent poor edge conditions: the inner surfaces of the strips converge first in the V-zone, allowing a large amount of current to flow through the inner surfaces, causing excessive heating and melting of the metal in the weld area, resulting in a large number of weld beads. To achieve full penetration of the pipe wall, a high power consumption is necessary. D through F represent relatively good weld edge conditions, with the strip edges butted parallel in the V-zone.

5. V-zone Shape
From the perspective of maximizing efficiency, the V-zone should be as short as possible (to reduce conduction losses). However, in actual production, a certain size of squeeze roller is installed in the V-zone, positioning the induction coil away from the apex of the V-zone. Furthermore, pipe wall thickness also affects the length of the V-zone. The high-frequency current first heats the V-shaped zone at the junction of the two strip edges, creating a funnel-shaped heated zone. If the V-shaped zone is too short, the strip edges will be unevenly heated, resulting in incomplete welding, overburning, and decarburization of the weld zone. Heating of the V-shaped zone begins at the induction coil. It is generally inappropriate to measure the V-shaped zone length based on the distance from the end of the induction coil to the junction of the strip edges. The V-shaped zone length should be measured from the middle of the induction coil to the junction (generally 1.5 times the pipe diameter). However, the length of the V-shaped zone of large-diameter steel pipes and thin-walled steel pipes should be reduced, while the length of the V-shaped zone of welded steel pipes less than φ25.4mm (1") should be increased due to the structural size of the extrusion roller. The size of the convergence angle of the two sides of the 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 helps to reduce the magnetic flux in the impedance and solve the problem of limited space for impedance placement at a certain welding speed. However, too small a convergence angle can easily lead to "sparks" and aggravate the instability of the strip in the frame and the wear of the roller. Generally speaking, the optimal convergence angle for welding carbon steel is 3°~4°, and the higher convergence angle for welding stainless steel and non-ferrous metals is 3°~4°. The ideal confluence angle is 5° to 8°.

6. Impedance Device Placement
The impedance device's function is to provide high current impedance within the steel pipe, concentrating more current within the V-shaped region. Simultaneously, it concentrates the magnetic field according to the current in the working coil, concentrating the majority of the current energy within the V-shaped region of the steel pipe. A key parameter of the impedance device is the magnetic properties. Magnets with the highest possible flux density and oscillation characteristics, and the lowest possible electromagnetic losses, should be selected. However, these requirements are sometimes incompatible, requiring the operator to possess sufficient knowledge of welding operations and electromagnetic circuit design. The placement of the impedance device is crucial. If it is placed too close to the confluence point, while achieving high efficiency, it is also highly susceptible to damage. Therefore, the impeder is generally placed inside the steel pipe, leaving a gap equal to the pipe's wall thickness from the inner surface. Most small units install the impeller at the bottom of the pipe. This placement not only reduces welding efficiency but also makes it susceptible to dragging when the pipe is moved. The magnetic field of the impeder should extend from the center of the induction coil to the extrusion point. The minimum impeller length is the diameter of the extrusion roller plus the length of the induction coil. Some operators use the front end of the impeller as the optimal position, resulting in large weld beads. While this extends the life of the impeder, it also increases power consumption.

7. Induction Coil Design
Using low-voltage, high-current devices is safer in solid-state welding. Since the power consumption of the working coil is proportional to the current value, even with a low coil resistance, the power dissipation is still high. Therefore, to reduce resistance, induction coils are typically made by brazing oxygen-free copper plates with cooling tubes.

8. Welding Frequency
The high frequency used for welding steel pipes is between 80 and 800 kHz. While this frequency range has a minimal impact on the heating zone, it still has a direct impact on heating quality. Impedance has a greater impact on welding efficiency than frequency, but impedance losses increase with increasing frequency, making it more difficult to ensure cooling of the impedance element. High frequencies are suitable for the production of small-diameter steel pipes, while low frequencies are suitable for large-diameter pipes. The best approach is to have an adjustable welding frequency.