In addition to the material factors, there are also some factors directly related to the welding process that affect the quality of high-frequency welded pipe, such as power frequency, junction angle, groove shape of pipe blank, placement position of contact, induction coil and impedance, input power, welding speed and welding pressure.
1. Selection of power frequency
High frequency welding can be realized in a wide frequency range. In terms of welding efficiency, increasing the frequency is conducive to the exertion of skin effect and proximity effect, the high concentration of electric energy on the surface of the connecting surface, and the rapid heating to the welding temperature, which can significantly improve the welding efficiency. Therefore, the selection frequency should be as high as possible. However, care should be taken to avoid the frequency band of radio transmission to avoid interference with broadcasting.
However, in order to obtain high-quality welds, the choice of frequency also depends on the material of pipe blank and its wall thickness. The best frequency required by different materials is different. Generally, the frequency of manufacturing non-ferrous metal pipes is higher than that of manufacturing carbon steel pipes. The reason is that the thermal conductivity of non-ferrous metals is higher than that of steel. It must be carried out at a welding speed higher than that of steel to make the energy more concentrated in order to realize welding. The optimum rate required varies with the thickness of the pipe wall. A large number of practices have proved that improper frequency selection either makes the heating on both sides of the weld too narrow or uneven in the thickness direction, or makes it too wide or oxidized, resulting in the reduction of weld strength. Therefore, only the frequency that can ensure the moderate heating on both sides of the weld and protect the uniform heating in the thickness direction can be selected; That's what matters. On the contrary, thicker tubes are usually used. When manufacturing common carbon steel pipe materials, the frequency of 350 ~ 450kHZ is mostly used; The frequency of 50KHz is used only when manufacturing special thick wall pipes.
2. Selection of convergence angle
The size of the confluence angle has a great influence on the stability of the flash process of high frequency welding, weld quality and welding efficiency. Generally, 2 ° ~ 6 ° is more appropriate. Small convergence angle and significant proximity effect are conducive to improve the welding speed. However, it should not be too small or too small. The flashing process is unstable, which makes it easy to form deep pits or pinholes that are difficult to press after lintel blasting. When the convergence angle is too large, the flash process is relatively stable, but the proximity effect is weakened, which reduces the welding efficiency and increases the power consumption. At the same time, it is difficult to form this angle. It is easy to wrinkle the edge of pipe blank.
3. Selection of groove shape of pipe blank
The shape of pipe blank groove has a great influence on the heating uniformity of groove surface and welding quality. I-shaped groove is usually used, because the groove surface can be heated evenly and the groove preparation is easy. However, when the thickness of the pipe blank is very large, if I-shaped groove is used, the central part of the groove cross section will be under heated, while its upper and lower edges are on the contrary, which appears to be over heated. In order to ensure the quality, x-groove shall be used. Practice has proved that when manufacturing thick wall pipe with high-frequency welding machine, using x-groove can heat the groove cross section evenly and the joint hardness after welding is also consistent.
4. Selection of placement position of contact, induction coil and impedance
(1) Contact position
In order to maintain high-efficiency welding, the contact shall be placed as close to the extrusion roller as possible. The distance between it and the connecting line between the centers of the two extrusion rollers is generally 20 ~ l50mm, the lower limit is taken when welding aluminum pipe, and the upper limit is selected when welding low-carbon steel pipe with wall thickness of more than 10mm. Typical contact position data.
(2) Position of induction coil
The induction ring shall be placed concentrically with the pipe, and the distance from its front end to the connecting line between the centers of the two extrusion rollers, as in the case of the contact, has a great impact on the welding efficiency and quality, and its value varies with the pipe diameter and pipe wall thickness.
(3) Impedance location
The impedance device shall be placed coaxially with the tube blank, and its head shall coincide with the central line of the two extrusion rollers or be 10 ~ 20mm away from the central line, so as to maintain high welding efficiency and avoid damage. When the gap between the impedance and the pipe wall is small, the efficiency can be improved, but it can not be too small, generally 6 ~ 15mm.
5. Selection of input power
Since the output power of the oscillator is directly proportional to the input power of the oscillator, and the input power is equal to the product of the screen voltage and screen current of the oscillation tube, and the size of the screen voltage and screen current can be adjusted and measured by the thyristor voltage regulator, relevant feedback elements and indicating instruments, the input power of the oscillator is generally used to measure the heating power output to the weld in production.
When the input power is small, due to insufficient heating of the groove surface of the pipe blank, the welding temperature cannot be reached, and there will be non welding defects. When the input power is too large, the heating temperature of the groove surface of the pipe blank will be too much higher than the welding temperature, resulting in overheating or overburning, and even weld breakdown, resulting in serious splashing of molten metal and the formation of pinholes or slag inclusion defects.
6. Selection of welding speed
Welding speed is also the main process parameter of welding. With the increase of welding speed, the extrusion speed of pipe blank groove surface will increase. This helps to squeeze out the liquid metal layer and oxide in the West that has been heated to melting, so as to easily obtain high-quality welds. At the same time, increasing the welding speed can also reduce the heating time of the groove surface, which can shorten the time of oxide formation and narrow the welding heat affected zone. On the contrary, not only the heat affected zone is wide, but also the thin layer of liquid metal and oxide formed on the groove surface will become thicker, produce large burrs and reduce the quality of weld.
However, when the input power is constant, the welding speed can not be improved indefinitely; Otherwise, the heating on both sides of the pipe blank groove will not reach the welding temperature, so it is easy to produce welding defects or cannot be welded at all.
Selection of welding pressure welding pressure is also one of the main parameters of high frequency welding. Since both sides of the groove of the pipe blank heated to the welding temperature are welded under the action of extrusion force, the pressure has an important impact on the weld quality. The connection pressure is generally l00 ~ 300mpa. At present, the domestic welded pipe unit has not adopted the device to directly measure this pressure; In production, the welding pressure parameters are replaced by the extrusion amount of joint pipe blank, and the extrusion amount is adjusted and measured by changing the spacing of extrusion rollers. The extrusion amount is also commonly expressed by the circumference difference of the pipe before and after the extrusion roller, and its specific value varies with the thickness of the pipe wall
