The Ultimate Guide to Choosing Welding Parameters

The main factors affecting the quality of high-frequency welded pipes are the power source frequency, convergence angle, tube blank groove shape, electrode contact head, and the location of the induction coil and impedance device, input power, welding speed, and welding pressure (extrusion force), etc.

1. Power Source Frequency

High-frequency welding can be implemented within a broad frequency range. Increasing the frequency enhances the skin effect and proximity effect, which helps concentrate the electric energy on the surface layer of the joint and rapidly heats it to the welding temperature, significantly improving the welding efficiency.

However, to obtain high-quality welds, the choice of frequency mainly depends on the material and wall thickness of the tube blank. The optimal frequency required for different materials varies; generally, the frequency for manufacturing non-ferrous metal pipes is higher than that for manufacturing non-alloy steel pipes.

This is because non-ferrous metals have a higher thermal conductivity than steel and must be welded at a greater speed to concentrate the energy for welding. The optimal frequency required also varies with different pipe wall thicknesses.

To ensure moderate heating width on both sides of the seam and uniform heating in the thickness direction, thin-walled pipes usually choose a higher frequency, while thick-walled pipes choose a lower frequency. The frequency of 350~450kHz is often used for welding non-alloy steel pipes, and a frequency of 50kHz may be used when welding thick-walled pipes.

2. Convergence Angle

The size of the convergence angle significantly impacts the stability of the high-frequency welding flash process, weld quality, and welding efficiency. Usually, an angle of 2°~6° is suitable. If the convergence angle is too small, the flash process will be unstable, leading to defects such as deep pits or pinholes that are difficult to compress.

If the convergence angle is too large, the proximity effect weakens, reducing the welding efficiency and increasing power consumption.

3. Tube Blank Groove Shape

To achieve uniform heating of the groove face and easy preparation, an I-shaped groove is usually used. However, when the tube blank thickness is large, the I-shaped groove will cause insufficient heating of the central part of the groove cross-section and excessive heating on its upper and lower edges.

In this case, an X-shaped groove should be used to ensure uniform heating across the cross-section and consistent hardness of the weld after welding.

4. Location of Electrode Contact Head, Induction Coil, and Impedance Device

(1) Electrode Contact Head Position

To maintain high-efficiency welding, the electrode contact head should be positioned as close as possible to the extrusion roller. The distance range from it to the line connecting the centers of the two extrusion rollers is 20~150mm. The lower limit is used when welding aluminum pipes, and the upper limit is used when welding low-carbon steel pipes with a wall thickness of more than 10mm.

The typical requirements for the electrode contact head installation position are shown in Table 5-1.

Table 5-1: Electrode Contact Position Installation (Low Carbon Steel)

Tube Outer Diameter D/mmΦ16Φ19Φ25Φ50Φ100
Distance to the Line Connecting the Centers of the Two Squeezing Rolls L/mm2525303032

(2) Induction Coil Position

In high-frequency induction welding, the induction coil should be placed concentric with the pipe. Its front end should also be as close as possible to the line connecting the centers of the two extrusion rollers. This value varies with the pipe diameter and wall thickness and can be selected according to Table 5-2.

Simultaneously, the relation between the induction coil width ‘a’ and the tube blank diameter ‘D’ is: a = (1.0~1.2)D; the gap ‘h’ between the induction coil inner diameter and the tube blank surface is 3~5mm.

Table 5-2: Induction Coil Position (Low Carbon Steel)

Tube Outer Diameter D/mmΦ25Φ50Φ75Φ100Φ125Φ150Φ175
Distance to the Line Connecting the Centers of the Two Extrusion Rollers L/mm4055658090100110

(3) Impedance Device Position

The impedance device should be placed coaxially with the tube blank. Its head should coincide with the line connecting the centers of the two extrusion rollers or be 10~20mm away from this line to maintain high welding efficiency and avoid damage.

The cross-sectional area of the impedance device should be 75% of the tube blank’s inner circular cross-sectional area, and the gap between it and the inner wall of the pipe is 6~15mm. A smaller gap can improve efficiency, but it should not be too small.

5. Input Power

The power required for welding depends on the material and wall thickness of the pipe. The power needed for welding aluminum pipes is higher than that for steel pipes, and thick-walled pipes require more welding power than thin-walled pipes.

When welding a given pipe, if the input power is too low, the heating of the tube blank groove face will be insufficient, and the welding temperature will not be reached, resulting in incomplete welding defects. If the input power is too high, the groove face will be overheated or burnt, causing the weld to burn through and even leading to severe metal spattering, resulting in pinhole or inclusion defects.

6. Welding Speed

Welding speed is a primary welding parameter. Increasing the welding speed will also increase the extrusion speed of the tube blank groove face, facilitating the expulsion of the melted metal layers and oxide on both sides, making it easier to obtain high-quality solid-phase welds.

At the same time, it can reduce the heating time of the groove face, thereby shortening the time for oxide formation and narrowing the heat-affected zone. Conversely, not only will the heat-affected zone broaden, but the liquid metal and oxide layers formed on the groove face will also thicken, and larger burrs will occur, leading to a decline in weld quality.

With a fixed input power, the welding speed cannot be increased indefinitely; otherwise, the ideal welding temperature will not be reached, resulting in failed welding. The welding speed of high-frequency resistance welding for pipes of different wall thicknesses is shown in Table 5-3.

Table 5-3: Welding Speeds for High-Frequency Resistance Welding of Pipes with Different Wall Thicknesses

Wall Thickness /mmWelding Speed (mm/s)
SteelAluminum
0.7545005000
1.525003000
2.515001800
48751120
6.4500620

Welding Pressure

Welding pressure is one of the main parameters in high-frequency welding, usually suitable between 100~300MPa. In production, the amount of tube blank extrusion can represent the welding pressure, which is controlled by adjusting the distance between the squeezing roll wheels.

The squeezing amount usually varies with the tube wall thickness, and the empirical values in Table 5-4 can be referred to for selection.

Table 5-4 Empirical Values of Tube High-frequency Welding Extrusion Amount

Tube Wall Thickness δ/mm≤1.01.0~4.04.0~6.0
Extrusion Amount /mmδ2/3δ1/2δ

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