Factors affecting the welding of erw pipe

Factors affecting the welding of erw pipe

Common problems of high frequency welding and their causes and solutions:

Welding is not strong, desoldering, cold stacking;

Reason: output power and pressure are too small;
Solution: 1. Adjust the power; 2. Change the groove shape of the thick tube blank; 3. Adjust the extrusion force

There are ripples on both sides of the weld;

Reason: The rendezvous angle is too large,
Solution: 1. Adjust the position of the guide roller; 2. Adjust the solid bending section; 3. Increase the welding speed

Welds have deep pits and pinholes;

Reason: overheated
Solution: 1. Adjust the position of the guide roller and increase the meeting angle; 2. Adjust the power; 3. Increase the welding speed

Weld burr is too high;

Reason: The heat affected zone is too wide
Solution: 1. Increase the welding speed; 2. Adjust the power;

Slag;

Reason: input power is too large, welding speed is too slow
Solution: 1 adjust the power; 2 increase the welding speed

Cracks outside the weld;

Reason: poor quality of parent material; too much extrusion force
Solution: 1. Guarantee the material; 2. Adjust the extrusion force

wrong welding, lap welding

Reason: poor forming accuracy;
Solution: adjust the forming die roll of the unit;

High-frequency welding is a key process in the production of welded pipes. Due to systematic influencing factors, we still need to explore experience in the first line of production. Each unit has its design and manufacturing differences, and each operator is also different. Habits, that is to say, crews, like people, have their own personalities. We provide these materials to you so that we can better understand the basic principles of high-frequency welding, so as to better combine our own production practices and summarize the operating procedures suitable for our own units.

The main factors affecting high-frequency welding are the following eight aspects:

frequency

The frequency of high-frequency welding has a great influence on the welding, because the high-frequency frequency affects the distribution of the current inside the steel plate. The influence of the selected frequency on welding is mainly the size of the heat affected zone of the weld. In terms of welding efficiency, a higher frequency should be used as much as possible. 100KHz high frequency current can penetrate 0.1mm of ferritic steel, 400KHz can only penetrate 0.04mm, that is, the current density distribution on the surface of the steel plate, the latter is nearly 2.5 times higher than the former. In production practice, the frequency of 350KHz~450KHz can generally be selected when welding ordinary carbon steel materials; when welding alloy steel materials, when welding thick steel plates over 10mm, a lower frequency such as 50KHz~150KHz can be used, because the alloy steel contains The skin effect of chromium, zinc, copper, aluminum and other elements is different from that of steel. Most foreign high-frequency equipment manufacturers have now adopted the new technology of solid-state high-frequency. After setting a frequency range, it will automatically track and adjust the frequency according to the thickness of the material and the speed of the unit during welding.


rendezvous angle

The meeting angle is the angle between the two sides of the steel pipe when they enter the extrusion point. Due to the proximity effect, when the high-frequency current passes through the edge of the steel plate, the edge of the steel plate will form a preheating section and a melting section (also known as a lintel). The blasting sprays out to form a flash, and the size of the meeting angle has a direct effect on the melting section.
When the convergence angle is small, the proximity effect is obvious, which is beneficial to improve the welding speed. However, if the convergence angle is too small, the preheating section and the melting section will become longer, and the melting section will become longer. As a result, the flashing process is unstable, and deep pits are easily formed after the lintel bursts. and pinholes, difficult to press fit.
When the rendezvous angle is too large, the melting section becomes shorter and the flash is stable, but the proximity effect is weakened, the welding efficiency is obviously decreased, and the power consumption is increased. At the same time, when forming thin-walled steel pipes, too large a meeting angle will elongate the edge of the pipe, resulting in wavy wrinkles. At present, in production, we generally adjust the meeting angle within 2°--6°. When producing thin plates, the speed is faster, and a smaller meeting angle is used when extruding; Larger meeting angle. Some manufacturers put forward an empirical formula: rendezvous angle × unit speed≮100, for reference.


Welding method

There are two types of high frequency welding: contact welding and induction welding.
Contact welding is that a pair of copper electrodes are in contact with the two sides of the welded steel pipe, the induced current has good penetration, and the two effects of high-frequency current are maximized due to the direct contact between the copper electrodes and the steel plate, so the welding efficiency of contact welding It has higher power consumption and lower power consumption, and is widely used in high-speed and low-precision pipe production. Contact welding is generally required in the production of particularly thick steel pipes. However, there are two disadvantages in contact welding: one is that the copper electrode is in contact with the steel plate and wears quickly; the other is that due to the influence of the flatness of the surface of the steel plate and the straightness of the edge, the current stability of the contact welding is poor, and the burr inside and outside the weld is relatively high. , is generally not used when welding high-precision and thin-walled tubes.
Induction welding is to wrap one or more turns of induction coils on the outside of the welded steel pipe. The effect of multi-turns is better than that of single-turns, but the production and installation of multi-turn induction coils is more difficult. The efficiency is higher when the distance between the induction coil and the surface of the steel pipe is small, but it is easy to cause discharge between the induction coil and the pipe. Generally, it is advisable to keep the gap between the induction coil and the surface of the steel pipe 5~8 mm. When induction welding is used, since the induction coil is not in contact with the steel plate, there is no wear, and the induced current is relatively stable, which ensures the stability of welding. The surface quality of the steel pipe during welding is good, and the weld seam is flat. When precision pipes are used, induction welding is basically used.


input power
 
Input power control during high frequency welding is very important. When the power is too small, the heating of the bevel of the tube blank is insufficient, and the welding temperature cannot be reached, which will cause unwelded defects such as virtual welding, de-soldering, and clip welding; when the power is too large, the welding stability will be affected, and the bevel surface of the tube blank will be heated. The temperature is much higher than the temperature required for welding, resulting in serious splash, pinhole, slag inclusion and other defects, which are called overburning defects. The input power during high-frequency welding should be adjusted and determined according to the thickness of the tube wall and the forming speed. Different forming methods, different unit equipment, and different material steel grades all require us to summarize from the first production line and compile suitable equipment for our own unit. high frequency process.


Tube blank bevel

The groove of the tube blank is the shape of the section. Generally, the manufacturer directly enters the high-frequency welding after longitudinal shearing, and the groove is in the shape of "I". When the thickness of the welding material is greater than 8~10mm or more, if this "I"-shaped groove is used, because of the relationship of the curved arc, it is necessary to melt off the inner edge layer that the tube blank first contacts, forming a high inner edge layer. burrs, and it is easy to cause insufficient heating of the center layer and outer layer of the plate, which affects the welding strength of high-frequency welds. Therefore, in the production of thick-walled tubes, the tube blank is preferably edge-planed or edge-milled, so that the groove is in an "X" shape. Practice has proved that this groove is very important for uniform heating to ensure the quality of the weld.
The selection of the groove shape also affects the adjustment of the size of the meeting angle.
Welded joint interface design is a weak link in welding engineering design, mainly because many steel structural parts are not designed by welding engineers and technicians, and grooves with poor rigid sleeve standards and process performance are not uncommon. The groove form plays an important role in controlling the internal quality of the weld and the manufacturing quality of the welded structure. Groove design must consider the fusion ratio of base metal, welding space, welding position and comprehensive economic benefits. The lateral shrinkage value ΔB should be calculated as follows.
                      ΔB=5.1Aω/t+1.27d
In the formula, Aω——the cross-sectional area of the weld, mm?, t——the thickness of the plate, mm, d——the gap at the root of the weld, mm. After finding the relationship between ΔB and Aω, you can analyze and process the data according to the relationship table, optimize the design, and finally determine the rupture form of the butt weld of the rectangular pipe.


Welding speed

The forming speed of the welded pipe unit is restricted by the high-frequency welding speed. Generally speaking, the speed of the unit can be opened faster, reaching 100 m/s. The speed of the existing units in the world can even reach 400 m/s. Welding, especially induction welding, can only be formed under 60 meters per second, and steel plates exceeding 10 mm can be formed. The forming speed of domestic unit production can only reach 8 to 12 meters per second.
Welding speed affects welding quality. When the welding speed is increased, it is beneficial to shorten the heat-affected zone and extrude the oxide layer from the molten groove; on the contrary, when the welding speed is very low, the heat-affected zone becomes wider, which will produce larger welding burrs and thicken the oxide layer. , the weld quality deteriorates. Of course, the welding speed is limited by the output power and cannot be increased very high.
The domestic operating experience of the unit shows that the welding speed of 2-3 mm steel pipe can reach 40 m/s, the welding speed of 4-6 mm steel pipe can reach 25 m/s, and the welding speed of 6-8 mm steel pipe can reach 12 m/s. The welding speed of 10~16 mm steel pipe is below 12 m/s. The speed can be higher for contact welding and lower for induction welding.


Impedance

The function of the impedance is to strengthen the skin effect and the adjacent effect of the high-frequency current. The impedance is generally made of M-XO/N-XO type ferrite, and it is usually made into a magnetic field with a specification of Φ10mm×(120--160)mm. The rods are bundled in a heat-resistant, insulating casing, and the interior is cooled by water.
The setting of the impedance should be matched with the diameter of the pipe to ensure the corresponding magnetic flux. To ensure the magnetic permeability of the resistor, in addition to the material requirements of the resistor, it is also necessary to ensure that the ratio of the cross-sectional area of the resistor to the cross-sectional area of the pipe diameter is large enough. In the production of high-grade pipes such as API pipes, it is required to remove the internal burrs. The impedance device can only be placed in the inner burr knife body, and the cross-sectional area of the impedance device will be much smaller accordingly. Ring arrangement.
The distance between the resistor and the welding point also affects the welding efficiency. The gap between the resistor and the inner wall of the tube is generally 6~15 mm, and the upper limit is taken when the diameter of the tube is large; the resistor should be placed concentrically with the tube, and its head should be placed with the welding point. The distance between them is 10~20 mm. Similarly, when the diameter of the pipe is large, take the larger value.


Welding pressure

Welding pressure is also the main parameter of high frequency welding. Theoretical calculations suggest that the welding pressure should be 100~300MPa, 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 at the edge of the tube. Different wall thickness takes different extrusion amount, usually the extrusion amount below 2mm is: 0.5t~t when 3~6mm; 0.5t when 6~10mm; 0.3t~0.5t when more than 10mm .
In the production of API steel pipes, there are often gray spot defects in welds. The gray spot defects are refractory oxides. In order to achieve the purpose of eliminating gray spots, Baosteel and other manufacturers have adopted methods to increase the extrusion force and increase the welding allowance. The extrusion allowance of steel pipes above 6mm is 0.8~1.0 material thickness, and the effect is very good.

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