High-Frequency Welding: Techniques and Applications

High-Frequency Welding Techniques and Applications

High-Frequency Welding of Spiral Seam Pipes

When producing large-diameter pipes with high-frequency longitudinal seam welding, it is challenging due to the limitation of the strip width. The high-frequency spiral welding method, as shown in Figure 5-13, can weld large-diameter pipes with narrower strips, and use strips of the same width to weld pipes of different diameters.

During welding, the strip is continuously fed into the forming mill, spirally wrapped around the central axis into a cylindrical shape, and the edges form a butt seam as shown in Figure 5-13a or a lap seam as shown in Figure 5-13b, while forming the corresponding V-shaped meeting angle, then continuous welding is performed using high-frequency contact method.

The butt seam design is generally used for manufacturing thick-walled pipes, and the lap seam design is used for manufacturing thin-walled pipes. To avoid uneven heating of the butt end surfaces, the sides of the joint should be machined into a 60° to 70° bevel. The lap amount of the lap seam can be selected within the range of 2 to 5mm depending on the thickness of the strip.

With a 200kW high-frequency power source, large diameter spiral seam pipes with a wall thickness of 6 to 14mm and a diameter of up to Φ1024mm can be welded, and the welding speed can reach 30 to 90m/min. Since spiral pipes have a greater load-bearing capacity than straight seam pipes, they are often used in important scenarios such as oil and gas transportation.

Figure 5-13 Schematic Diagram of Off-Frequency Spiral Welded Pipe
a) Butt spiral seam b) Lap spiral seam
  • 1 – Finished pipe
  • 2 – Central axis
  • 3 – Electrode position
  • 4 – Welding point
  • 5 – Squeeze roller
  • HF – Commercial frequency power source
  • F – Squeeze force
  • n – Pipe rotation direction

High-Frequency Welding of Spiral Finned Tubes

To increase the heat dissipation surface area of various radiator tubes, common welding methods, especially high-frequency welding, are used to weld longitudinal heat sink fins or spiral heat sink fins to their surfaces, commonly known as fin (fin) tubes.

Finned tubes have a large heat transfer area, high efficiency, and are widely used in various heat exchange devices with them as the core components in power, chemical and refining devices. High-frequency welding to manufacture finned tubes has advantages such as high welding efficiency, narrow heat affected zone, good welding quality, and the use of dissimilar metal materials.

As shown in Figure 5-14, the finned tube is a thin curling piece of 0.3 to 0.5mm thickness wound at a certain pitch on the outer circle of the seamless steel tube, with the curling piece perpendicular to the surface of the outer circle of the steel tube.

During welding, the tube moves forward and rotates, the curling piece is fed to the tube wall at a certain angle, and is pressed onto the tube wall by the squeeze roller. When the curling piece and the contact point on the tube wall have high-frequency current, the edge metal of the meeting angle is heated and welded together by squeezing.

When welding spiral finned tubes, the problem of uneven heating due to the asymmetry of the connection surface can be solved by placing the contacts asymmetrically.

Figure 5-14 Schematic Diagram of Spiral Finned Tube Welding
1 – Tube 2 – Fin 3 – Contact Point
  • HF – High-Frequency Welding Power Source
  • n – Tube Rotation Direction
  • S – Fin Feeding Direction
  • F – Squeeze Force – Tube Movement Direction

The welding speed is very fast when high-frequency welding spiral finned tubes, usually 50 to 150m/min, and the diameter of the welded tube can be Φ16 to Φ254mm, with many suitable materials. Table 5-5 lists the high-frequency welding parameters for P91 heat-resistant steel pipe and 06Cr13 steel strip.

Table 5-5 High-Frequency Welding Parameters for P91-06Cr13 Finned Tube

Steel TubeMaterial Specifications
/mm
P91
Φ73×9.56
Steel StripMaterial Specifications
/mm
06Cr13 17.5×1.5
Anode Voltage U/kV9.5×10.5
Anode Current I/A14~17
Grid Current I/A3~4
Grid Voltage U/kV85~100
Rotational Speed r (m/min)7.5~9
Top Forging Wheel Diameter D/mm60 ~ 63
Welding Head Height h/mm18~23
Pitch d/mm6

High-Frequency Resistance Welding of Structural Steel

High-frequency resistance welding is primarily used in the production of structural steel, such as T-shaped, I-shaped, H-shaped, and other profiles. Figure 5-15 shows light H-section steel with different cross-sections, produced by a foreign company using high-frequency resistance welding. The materials used are common non-alloy steel and high-strength steel.

Figure 5-15: Light H-Section Steel with Different Cross-Sections

The composition of the high-frequency resistance welding H-section steel unit is shown in Figure 5-16. The entire processing procedure from feeding to product output is mechanized, with a high production rate. The process is as follows: the steel billets of the top flange, web, and bottom flange are uncoiled at the three locations indicated by 1.

For the web, the edges need to be pre-thickened outside the thickening machine 3 (when no thickening treatment is performed, only 80%-85% of the web thickness can be welded). They are then sent with the top and bottom flanges, which have been leveled by the flange leveling machine 4, to the high-frequency resistance welding machine 5 for welding.

At this time, a V-shaped joining angle (40°~70°) is formed between the flange and the web. An electrode sliding contact then transmits a high-frequency current (with a voltage of 50~200V and a frequency of 450kHz) to one flange.

This current flows along the V-shaped area of the flange and web end face, then passes through the V-shaped area of the other end face of the web by a slide contact (electrode) on the web, forming a welding circuit. Finally, continuous squeezing and welding are performed through the squeezing roll.

It’s worth noting that, due to the thickness of the flange, adjusting the contact position to achieve appropriate heat distribution is crucial. After welding, burr removal and shaping are immediately carried out at location 6. After cooling at 7, longitudinal straightening and flange straightening are performed at the correction unit 8.

Non-destructive inspection of the weld seam is carried out at location 9. Finally, the product is cut to the required length at the flying saw 10.

High-Frequency Resistance Welding of Sheet (Strip) Material

High-frequency resistance welding can be used to connect shorter sheets or strips, as shown in Figure 5-17. The ends of the two strips or slab blanks to be welded are placed on a copper strip platform, and a certain pressure is applied to make them contact each other.

At the same time, a conductor is placed above the seam, with one end connected to one end of the strip platform and the other end connected to the output of the high-frequency power supply, along with the other end of the strip platform.

When the high-frequency current passes through, the seam area is heated to the welding temperature extremely quickly under the proximity effect, and then the welding is completed under substantial upsetting pressure.

Figure 5-16: Composition of High-Frequency Resistance Welding H-Section Steel Unit

1 – Uncoiler for Steel Coil, 2 – Flange Blank Conveying Device, 3 – Web Edge Thicker Machine, 4 – Flange Leveling Machine, 5 – High-Frequency Resistance Welding Machine, 6 – Deburring and Shaping Device, 7 – Cooling Cover, 8 – Correction Unit, 9 – Flaw Detection Device, 10 – Flying Saw.

By correctly choosing the frequency, the penetration depth of the current can be adjusted, allowing the weld seam to be evenly heated in the thickness direction. This method is suitable for butt joints of steel plates with a thickness of 0.6~5mm and a width of 76~900mm. For low-carbon steel with a thickness of 3mm and a butt weld seam length of 191mm, welding can be completed in only 1.1s.

This method is suitable for connecting coil ends and the strip and slab blanks required for manufacturing stamping parts. It can also be used for the direct production of parts, such as the welding of automobile wheel rim blanks.

Figure 5-17: Schematic Diagram of High-Frequency Resistance Welding of Sheet (Strip) Material

1 – Workpiece, 2 – Neighboring Conductor, 3 – Strip Seat, 4 – Seam, 5 – Current Path
HF – High-Frequency Power Source, F – Pressure

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