Key Points for Quality Control in High-Frequency Steel Pipe Welding

Many factors influence high-frequency welding quality, and these factors interact and constrain each other within the same system. A change in one factor influences the others. Therefore, when adjusting the high-frequency welding process, it's not enough to simply focus on local adjustments such as the frequency, power, current, or extrusion rate of the high-frequency welding machine. Such adjustments must be based on the specific conditions of the entire forming system, comprehensively considering all aspects related to high-frequency welding and eliminating all influencing factors one by one. Below, we break down the key factors in high-frequency welding into the following eight aspects:

1. Frequency

Frequency refers to the high frequency of the high-frequency welding machine during welding. High-frequency welding frequency significantly affects both the weld and its quality because it influences the distribution of current within the raw material. The primary impact of the selected frequency on welding is the size of the heat-affected zone (HAZ) of the weld. For welding efficiency, a higher frequency should be used whenever possible. A 100kHz high-frequency current can penetrate 0.1mm through ferritic steel, while a 400kHz one can only penetrate 0.04mm. This means the current density distribution on the steel plate surface is nearly 2.5 times higher at the 400kHz frequency. In production practice, frequencies between 300kHz and 400kHz are generally used when welding plain carbon steel. When welding alloy materials or steel plates thicker than 10mm, a lower frequency can be used because the skin effect of elements such as chromium, zinc, copper, and aluminum in alloy steel differs from that of plain carbon steel. The required welding frequency varies depending on the metallurgical structure and metal composition. Currently, international high-frequency manufacturers have adopted newer solid-state high-frequency technology. After setting a frequency range, this technology automatically adjusts the frequency based on material thickness, machine speed, and other factors during welding. Higher-end Sematur variable-frequency welding machines can even adjust the optimal frequency within a frequency range, with adjustable accuracy down to 1kHz.

2. Meeting Angle (VEE Angle)

The meeting angle is the angle between the two edges of the strip as they enter the extrusion point. Due to the proximity effect, when high-frequency current passes through the edge of a steel plate, a preheating zone and a molten zone (also known as a lintel) form along the edge. When the lintel is intensely heated, the molten steel within it rapidly vaporizes and explodes, creating a flash—the so-called "flash." The meeting angle (VEE angle) has the most direct impact on the molten zone (weld). A small meeting angle significantly increases the proximity effect, which helps increase welding speed. However, if the meeting angle is too small, the preheating and molten zones become longer. This elongated molten zone leads to an unstable flash process, making it more likely to form deep pits and pinholes after the lintel explodes, making extrusion and fusion difficult. If the meeting angle is too large, the molten zone becomes shorter, stabilizing the flash, but the proximity effect is weakened, significantly reducing welding efficiency and increasing power consumption. Furthermore, when forming thin-walled welded steel pipes, a large meeting angle can stretch the pipe edge, creating wavy wrinkles. In production, we generally adjust the contact angle within a range of 2° to 6°. When producing thin plates, the unit speed is higher, so a smaller contact angle is used during extrusion. When producing thick plates, the unit speed is slower, so a larger contact angle is used during extrusion. Here's a rule of thumb for reference: Contact angle * unit speed ≮ 100.

3. Welding Methods

There are two types of high-frequency welding: contact welding and induction welding.

A. Contact welding involves a pair of copper electrodes contacting the edges of the material being welded. The induced current has excellent penetration, and the two effects of high-frequency current (proximity effect and skin effect) are maximized due to the direct contact between the copper electrodes and the steel plate. Therefore, contact welding offers high welding efficiency and low power consumption. It is widely used in the production of high-speed, low-precision steel pipes and is generally required for the production of particularly thick welded steel pipes. However, contact welding has two disadvantages: First, the copper electrodes contact the steel plate, causing rapid wear. Second, due to the influence of the steel plate's surface flatness and edge straightness, contact welding has poor current stability, resulting in high burrs inside and outside the weld. Therefore, it is generally not used for welding high-precision welded steel pipes.

B. Induction welding involves wrapping an induction coil with one or more turns around the steel pipe being welded. Multiple turns provide better results than a single turn, but multi-turn coils are more difficult to manufacture and install. When using induction welding, a smaller distance between the induction coil and the steel pipe surface results in higher efficiency. However, this can easily cause discharges between the coil and the pipe. Furthermore, if the pipe is irregularly formed, too small a gap can easily damage the coil. Generally, a clearance of 5-8 mm is recommended. Since the induction coil does not contact the steel plate during induction welding, there is no wear. The induced current is relatively stable, ensuring weld stability and resulting in a smooth, high-quality weld seam.

4. Input Power

Controlling input power during high-frequency welding is crucial, and maintaining a stable input power is even more crucial. If the power is too low, the steel tube groove is insufficiently heated, failing to reach the welding temperature. This can cause defects such as cold welds, debonding, and pinch welds. If the power is too high, welding stability is compromised, and the temperature of the steel tube groove surface rises significantly above the required welding temperature, resulting in severe spattering, pinholes, slag inclusions, and other defects known as overburning defects. The input power for high-frequency welding should be adjusted based on the steel tube diameter, wall thickness, forming unit speed, and whether internal burrs are removed. Different forming methods, different machine equipment, different metal materials, different steel grades, and even different batches of raw materials can all have an impact. This requires comprehensive consideration, based on analysis from the production line, to develop a high-frequency welding process tailored to your specific equipment.

5. Steel Tube Groove

The groove of a steel tube billet refers to the cross-sectional shape. Generally, manufacturers directly feed high-frequency welding material after slitting, resulting in an "I"-shaped groove. When welding steel pipes with a thickness greater than 8-10mm, using an "I"-shaped groove requires melting the inner edge of the steel pipe billet due to the curved arc, resulting in high internal burrs. This can also lead to insufficient heating of the center and outer layers of the plate, compromising the strength of high-frequency welds. Therefore, when producing rear-wall steel pipes, it is best to plan or mill the steel pipe billet to create an "X"-shaped groove. Practice has proven that this groove is crucial for uniform heating and thus ensuring weld quality. Furthermore, the choice of groove shape also influences the adjustment of the angle of convergence. Weld joint design is a relatively weak link in welding engineering design, primarily because the groove design of many steel structures is not designed by welding engineers. Rigidly conforming to standards and resulting in poor process performance are common. The groove form plays a crucial role in controlling the internal quality of the weld and the manufacturing quality of the welded structure. Groove design must consider factors such as the base metal fusion ratio, welding space, welding position, and overall economic benefits.

6. Welding Speed

The forming speed of a welded steel pipe mill is constrained by the high-frequency welding and sawing speeds. Generally, the mill can operate at relatively high speeds, reaching 100 meters per second, and high frequency can also increase speed by increasing power. However, for steel plates thicker than 10 mm, domestic mills can only achieve forming speeds of 8-12 meters per second. Welding speed affects weld quality. Increasing the welding speed shortens the heat-affected zone (HAZ) and squeezes out oxides from the weld groove. Conversely, a very low welding speed widens the HAZ, resulting in larger weld burrs, thicker oxide layers, and poorer weld quality.

7. Impedance

The function of an impedance is to enhance the skin and proximity effects of high-frequency current. Impedances are generally made of M-XO/N-XO ferrites and are typically made of 10 mm x 120-160 mm φ magnetic rods, enclosed in a heat-resistant, insulating casing and cooled by water. The impedance should be configured to match the steel pipe diameter to ensure adequate magnetic flux. In addition to ensuring the magnetic permeability of the impeder and meeting the material requirements, the ratio of the impeder's cross-sectional area to the pipe diameter must be sufficiently large. In the production of high-precision, high-grade steel pipes, internal burr removal is required. Impeders can only be placed within the internal burr removal device, resulting in a significantly smaller cross-sectional area. In this case, a concentrated fan-shaped arrangement of magnetic rods is more effective than a circular arrangement. The distance between the impeder and the weld point also affects welding efficiency. The clearance between the impeder and the inner wall of the steel pipe is generally 6-15mm, with the upper limit being used for larger pipe diameters. The impeder should be placed concentrically with the pipe, with the distance between its head and the weld point being 10-20mm. Similarly, the higher value is used for larger pipe diameters.

8. Welding Extrusion Pressure

Welding pressure is also a key parameter in high-frequency welding. Theoretical calculations suggest a welding pressure of 100-300 MPa, but the actual pressure in this area is difficult to measure in actual production. It is generally estimated based on experience and converted into the amount of extrusion on the pipe edge. Different wall thicknesses require different extrusion rates. Typically, for pipes under 2mm, the extrusion rate is 0.5t to 1t for pipes 3-6mm to 6mm, 0.5t for pipes 6-10mm, and 0.3t to 0.5t for pipes 10mm and above. Gray speckle defects are common in the production of thick-walled steel pipes, such as API pipes. Gray speckle is a refractory oxide. To eliminate gray speckle, most manufacturers increase the extrusion pressure and weld allowance. For pipes over 6mm, the extrusion allowance is 0.8 to 1.0 of the material thickness, which is very effective.

Overview:

Common Problems with High-Frequency Welding and Their Causes/Solutions:

1. Weak welds, debonding, and cold laps;

Cause: Insufficient output power and extrusion pressure. Solution: A. Adjust the high-frequency welding machine power; B. Change the groove shape for thick steel pipe billets; C. Adjust the extrusion pressure.

2. Ripples on both sides of the weld seam;

Cause: Excessive convergence angle

Solution: A. Adjust the guide roller position; B. Adjust the solid bend forming section; C. Increase the welding speed.

3. Deep pits and pinholes in the weld seam

Cause: Overburning

Solution: A. Adjust the guide roller position and increase the convergence angle; B. Adjust the power; C. Increase the welding speed.

4. Weld burrs are too high

Cause: Excessive heat-affected zone

Solution: A. Increase the welding speed; B. Adjust the power.

5. Slag inclusions

Cause: Excessive input power, slow welding speed.

Solution: A. Adjust the power; B. Increase the welding speed.

6. External cracks in the weld seam

Cause: Poor base material quality; excessive extrusion pressure.

Solution: A. Ensure raw material quality; B. Adjust the extrusion pressure.

7. Overlap welding and lap welding

Cause: Poor forming accuracy.

Solution: Adjusting the Machine's Forming Die Rollers

High-frequency welding is a critical process in welded steel pipe production. Due to systemic factors and their interdependencies, it still requires extensive experience in production. Each machine has its own design and manufacturing differences, and each operator has unique habits.

We share this information to help you better understand the basic principles of high-frequency welding, so you can better integrate it with your own production practices and develop operating procedures tailored to your specific machine. At the same time, the welded steel pipe equipment industry is rapidly developing, with more equipment manufacturers investing in research and development, testing, and automation. We hope that in the near future, welded steel pipe production equipment will be able to automatically adjust according to product specifications and applications.