The high-frequency welding process is the most widely used method for the production of welded pipes by high frequency welded pipe production line. It heats the metal by applying or inducing an electric current across the edge of the strip before the closing point of the open pipe, and presses the tube blank through squeeze rollers, The molten metal and inclusions are squeezed out of the weld pool to form a forged weld. But there is still a lot of room for improvement in this area. By designing and transforming the high-frequency welding process of the tube on the original basic configuration and operating it effectively, the welding process can be optimized, the welding efficiency can be improved, and the cost can be greatly reduced.

The factors that affect the efficient operation of the high frequency welded pipe production line mainly include: the edge state of the tube blank, the length and angle of the V angle, the position and length of the ferrite (magnetic bar), the position and length of the coil, the length of the opening angle, the type of impedance, the design of the coil and Welder frequency.

Reasonable configuration and design can greatly save electricity consumption, improve the quality of pipes and welds, reduce downtime and improve efficiency, and greatly reduce production costs.

Edge state

If the strip edges are not parallel, or if burrs or irregularities are formed during slitting and transportation, the welding efficiency will be greatly reduced. Because more metal needs to be heated before face-to-face welding can be done. Poor edge condition can also cause excess molten metal to be squeezed out, resulting in large and often irregular weld beads, which also invariably result in weld defects.

V-zone length and angle

From a pure efficiency point of view, the V-shaped region must be as short as physical conditions allow to reduce heat conduction energy losses and allow most of the induced current to flow in the V-shaped region. However, in the actual production of high frequency welded pipe production line, other factors must also be considered. For most equipment installations, the V-zone length is determined by the size of the squeeze rolls. The large squeeze roll side rolls force the coil to be positioned further from the ideal of the squeeze apex. Smaller squeeze rolls are of course more efficient, but often require regrinding or bearing replacement.

The wall thickness also affects the V-zone length. High-frequency welding tends to heat the strip corners first, which is why the known hourglass-shaped HAZ is formed. If the V-zone is too short, the heat distribution will be uneven at the edges of the strip, resulting in incomplete welds or overheating of the corners and possibly decarburization of the steel.

How to define the length of the V-shaped area

The edge of the strip is heated before the tube enters the coil, so the commonly defined method of measuring the length of the V-zone (from the exit end of the induction coil to the apex of the V-zone) is clearly wrong. A more practical approach is to measure the distance from the center of the induction coil to the apex of the V-shaped region. Using this calculation method, the V-zone length should be approximately equal to 1.5 times the diameter of the tube. Due to the constraints of the mechanical structure of the squeeze roller, this value should be reduced for large-diameter thin-walled pipes, and this value should be increased for pipes with a size below 25mm. Interestingly, as the V-zone length increases, the resistor must carry more magnetic flux relative to the tube diameter. In the case of small tubes, the relatively long V-zone length accentuates the importance of the resistor, since the size of the resistor is limited by the space inside the tube. This also ultimately limits the minimum size of pipe that can be most economically welded using the high frequency induction welding process.

Resistor Location and Length

The ferrite (bar magnet) in the resistor must be placed just above the actual solder joint. Since the actual welding point appears slightly ahead of the welding point, it is generally sufficient to place the resistor on the centerline of the squeeze roller. The resistor length should be 2.5 to 4 times the length of the V-shaped region. The centerline of the squeeze rolls should be at least 25% longer than the length of the resistor from the centerline of the last finishing frame. If the resistor extends back beyond the last finishing stand, another current path is created to allow a portion of the current to flow between the entry end of the line and the finishing stand. This creates arcing and increases energy loss.

Coil position and length

The diameter of the working coil must be sufficient to provide a reasonable working gap between the outer surface of the tube and the inner surface of the coil. For tubes from φ50mm to φ150mm, the inner diameter of the coil is usually 25% larger than the tube size. For pipes smaller than φ50mm, this ratio should be increased, and for pipes larger than φ160ram, the ratio should be decreased.

From a purely electrical point of view, the efficiency is highest when the coil diameter and length are equal. In the case of induction welding, this will result in an excessively long V-zone length, especially for large-diameter thin-walled pipes. To keep the V-zone length as short as possible, both the coil diameter and length need to be minimized. The position of the coil is usually determined by the diameter of the squeeze roll and the design of the welder, and generally cannot be changed: the length of the coil should not exceed the length required to deliver the current, and if welding pipes under 25mm in diameter, the length of the coil is usually much larger than its diameter, which is also Necessary to accommodate the number of coil turns and the current delivery capability required by the welder.

For maximum efficiency, both the coil length and the distance from the coil to the apex of the V-shape should be as short as possible.

V-shaped corner

The angle of the strip edge joint (approach angle or V-angle) also affects efficiency. Smaller angles require less welding energy because the proximity effect is more pronounced and more energy is concentrated on the strip surface. The small V-angle also reduces the magnetic flux of the resistor, which may be helpful when the saturation of the resistor is limiting welding speed. Too small a V-corner angle is detrimental as it may cause prearcing. In addition, it increases the mechanical instability of the strip due to roll bore diameter wear or bearing bending.

The ideal V-shaped angle of carbon steel is 3 to 4°. The welding effect of stainless steel and most non-ferrous metals is better when the V-angle is in the range of 5 to 8°.

Measuring V-angles is difficult, but there is a quick and simple way to share.

Use a small twist drill or dowel to slide along the V-shaped corner to the apex until it touches both sides of the open V-shaped corner, then measure the distance from the twist drill to the apex: take the diameter of the drill bit as the bottom edge, measure the distance as the height, and solve for the inverse The triangle is the V-shaped angle.

If the 3mm drill stops at 45mm from the vertex, the V-shaped angle is 90°-(arctan45/3), that is, 3.8°

Impedance Design

The main function of the impedance resistor is to increase the impedance of the eddy current path inside the tube, so that more energy can be concentrated in the welding V-shaped area. The impedance resistor will also concentrate the magnetic flux generated by the working coil current, so that more energy.

Resistor design and placement is one of the most important aspects of improving efficiency. Impedance resistors are critical components in induction welding. However, as a low-cost consumable, many welded pipe manufacturers choose and purchase only based on price. For a given welder power level, the choice of resistors can have up to 50% effect on welding speed. It can double the rolling line speed without increasing the welding power, or cut the power consumption in half at the same speed. Therefore, although the cost of the resistor is very low, the use value is huge. Other places can be saved, but the impedance must not be saved.

Unqualified naked rod form

The most important component in an impedance resistor is the ferrite. Some manufacturers use less expensive ferrites. This ferrite is designed for use with portable radio antennas, not for highly energy-dense and high-temperature welded pipes. Resistor ferrites should have the highest possible saturation flux density and amplitude permeability while having low electromagnetic losses to maintain reasonable cooling conditions. Sometimes these parameters conflict with each other, so choosing the right grade of ferrite requires deep knowledge of high frequency welder operation and magnetic circuit design.

The position of the resistor in the tube is extremely important. Since the magnetic flux can only enter the resistor through the V-shaped region (without penetrating the pipe wall), the resistor is most effective when placed close to the junction edge. However, this location is also the most prone to damage, so compromises are often required. A good way to do this is to position the resistor at one time the strip wall thickness below the upper surface of the inner wall of the tube. The resistor devices of most domestic rolling mills are placed at will, and even slide along the bottom. In this case, not only is the high frequency welded pipe production linethe least efficient, but it also wears prematurely due to friction with the moving pipe. Improper support of the resistor may also cause it to move up and down inside the tube, resulting in changes in welding temperature.

For maximum efficiency, the ferrites in the resistor should be distributed in the center of the coil up to a point slightly beyond the apex of the V-zone, and at least the same distance from the coil to the last finishing frame. The shortest length of the resistor is equal to the sum of the diameter of the squeeze roller and the length of the coil. A resistor that is too short will cause a sharp drop in welding efficiency. Some operators prefer to position the resistor in front of the optimum position to avoid damage to the resistor from molten steel droplets. Doing so can extend the life of the resistor, but at the expense of increased energy consumption. A better solution to the droplet problem is to understand its cause, and if possible eliminate the problem at the source, or use a more droplet-resistant resistor design.

Coil Design

Solid state welders are low voltage, high current devices with high reliability. But the coil current is 5 times higher than that of the vacuum tube welder. Since the energy loss of the working coil itself is proportional to the square of the current (energy = the square of the current × resistance), even a small resistance on the coil will greatly increase the energy consumption due to the heating of the coil itself. Proper coil design requires the designer to have extensive knowledge of welding machine matching requirements and induction welding processes. The most important thing when designing a coil is to follow the welder manufacturer's recommendations. In many cases, the cost of purchasing a coil from a welder manufacturer or other reliable source is far less than the cost of the energy loss associated with simply wrapping a copper tube around one.

Epilogue

As the profit margin of the welded pipe industry is getting smaller and smaller, the slight improvement in the efficiency of the production line is a big step forward on the basis of "everyone does it". This not only directly reduces energy costs, but also improves product quality and profitability. Through efficient operation and optimization, induction welding can easily achieve the same output with less downtime and lower maintenance costs.

Taishan does not refuse to fine soil, so it can be high; rivers and seas do not choose trickles, so they can be deep, and details often determine success or failure.

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