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High frequency welding technology for straight seam steel pipe

Straight seam high frequency welded steel pipe has the characteristics of relatively simple process and fast continuous production, and is widely used in civil buildings, petrochemical industry, light industry and other departments. It is used for conveying low pressure fluid or making various engineering components and light industrial products.
1. Production process of straight seam high frequency welded steel pipe
The longitudinal welded steel pipe is rolled into a round tube by a high-frequency welding unit and welded by a straight seam. The shape of the steel pipe can be round or square or irregular. It depends on the sizing rolling after welding. The materials for welded steel pipe are mainly low alloy steel and low alloy steel or other steel products withσs≤300N/mm2、σs≤500N/mm2. 
2. high frequency welding
High frequency welding is based on the principle of electromagnetic induction and the skin effect, proximity effect and eddy heat effect of AC charge in the conductor, so that the steel at the edge of the weld is partially heated to the melting state. The intergranular joint is realized by the extrusion of the roller, thus the purpose of welding welding is achieved. High frequency welding (HF) is a kind of induction welding (or pressure contact welding). It is widely used in the production of steel pipe without the need of weld filling material, no welding spatter, narrow welding heat affected zone, beautiful welding molding, good mechanical properties and so on.
The high frequency welding of the steel tube is using the skin effect and the proximity effect of alternating current. After rolling, the steel (strip) is rolled and formed, forming a circular tube blank with a cross section, rotating near the center of the induction coil near the center of the induction coil, or a group of impedance devices (magnetic rods). The impedance device forms an electromagnetic induction back at the opening of the tube billet. Road, under the effect of skin effect and proximity effect, the edge of the opening of the tube produces a strong and concentrated heat effect, which makes the weld edge quickly heated to the temperature required by the pressure roller to be extruded. The metal of the molten state realizes intercrystalline joint and forms a solid butt weld after cooling.
3. high frequency welded pipe unit
The high frequency welding process of the longitudinal steel pipe is completed in the high frequency welded pipe mill. The high frequency welded pipe unit is usually composed of rolling molding, high frequency welding, extrusion, cooling, sizing, flying saw cutting and other components. The front end of the unit is equipped with a storage sleeve, and the back end of the unit is equipped with a steel tube rollover frame, and the electrical part mainly consists of high frequency generator, DC excitation generator and instrument automatic control device. Taking the 165mm high frequency welded pipe mill as an example, the main technical parameters are as follows:
3.1 welded pipe finished products
Diameter of circular tube: [Phi] 111~165mm
Square tube: 50 x 50~125 x 125mm
Rectangular tube: 90 x 50~160 x 60~180 x 80mm
Tube wall thickness of finished product: 2~6mm
3.2 molding speed: 20~70 M / min
3.3 high frequency inductor:
Thermal power: 600KW
Output frequency: 200~250KHz
Power: three phase 380V 50Hz
Cooling: water cooling
Excitation voltage: 750~1500V
4. High frequency excitation circuit
High frequency excitation circuit (also called high frequency oscillating circuit) is composed of large electronic tubes and oscillating channels installed in the high frequency generator. It uses the amplification of the electronic tube to feed the anode output signal to the gate and form a self excited oscillation circuit when the tube is connected to the filament and anode. The frequency of excitation depends on the electrical parameters of the oscillating channel (voltage, current, capacitance and inductance).
5. High frequency welding technology for straight seam steel pipe
5.1 Control of weld gap
The strip is sent to the welded pipe unit. Through the roll rolling, the strip is rolled up gradually, forming a circular tube with open gap, adjusting the pressing amount of the extrusion roll so that the weld gap is controlled in 1~3mm, and the two ends of the weld are flat. If the clearance is too large, the proximity effect will be reduced, the eddy heat is insufficient, and the intercrystalline bonding between the welds is poor, resulting in unfusion or cracking. If the gap is too small, the adjacent effect increases, the welding heat is too large and the weld is burned, or the weld is squeezed and rolled to form a deep pit, which affects the quality of the weld surface.
5.2 temperature control of welding
The welding temperature is mainly influenced by the high frequency eddy thermal power. According to the formula (2), it is known that the high frequency eddy thermal power is mainly influenced by the current frequency. The thermal power of the eddy current is proportional to the square of the current excitation frequency, and the current excitation frequency is influenced by the excitation voltage, current and capacitance and inductance. The excitation frequency formula is as follows:
F=1/[2 PI (CL) 1/2]… (1)
Type: f- excitation frequency (Hz); capacitance in the C- excitation loop (F), capacitance = power / voltage; inductance in L- excitation circuit = inductance / flux / current.
It is known that the excitation frequency is inversely proportional to the square root of the capacitance and inductance in the excitation loop, or is proportional to the square root of the voltage and current. As long as the capacitance, inductance, or voltage and current in the loop are changed, the excitation frequency can be changed to control the welding temperature. For low carbon steel, the welding temperature is controlled at 1250~1460 C, which can meet the requirement of 3~5mm penetration. In addition, the welding temperature can also be achieved by adjusting the welding speed.
When the input heat is insufficient, the weld edge of the welded seam can not reach the welding temperature, and the metal structure remains solid to form the non fusion or non weld penetration. When the input heat is insufficient, the heated weld edge exceeds the welding temperature, resulting in overheating or droplets to make the weld formation melting hole.
Control of 5.3 extrusion pressure
After the two edges of the tube are heated to the welding temperature, the common metal grains are formed to penetrate and crystallize under the extrusion of the extrusion rollers, and finally a solid weld is formed. If the pressure is too small, the number of the common crystal is small, the strength of the weld metal decreases and the force will crack. If the extrusion pressure is too large, the molten metal will be squeezed out of the weld, which not only reduces the strength of the weld, but also produces a large number of internal and external burrs, so as to cause welding joints and other defects.
5.4 control of the position of high frequency induction coil
The high frequency induction coil should be as close as possible to the position of the extrusion roll. If the induction coil is far away from the extrusion roll, the effective heating time is longer, the heat affected zone is wider and the weld strength decreases; on the contrary, the weld edge is not heated and the forming is bad after extrusion.
The 5.5 impedance is a special magnetic rod for a welded pipe. The section area of the impedance is usually not less than 70% of the section area of the inner diameter of the pipe. The effect is to make the induction coil, the weld edge of the tube billet and the magnetic rod to form an electromagnetic induction circuit, which produces the adjacent effect. The heat of the eddy current is concentrated near the edge of the weld seam, so that the edge of the tube is heated. To the welding temperature. The impedance device is driven by a steel wire in the tube blank, and its central position should be relatively fixed near the center of the extrusion roller. Because of the rapid movement of the tube blank, the impedance of the impedance tube is greatly damaged by the friction of the inner wall of the tube and needs to be replaced frequently.
5.6 weld seam will be welded and pressed to produce weld scar and need to be removed. The cleaning method is to fix the tool on the rack and scrape the weld scar by the fast movement of the welded pipe. The burrs inside the welded pipe are generally not cleared.
5.7 process examples
Taking the welded 32 2mm welded pipe as an example, the process parameters are briefly described.
Strip specification: 2 * 98mm bandwidth expanded according to medium diameter and small amount of molding allowance.
Steel material: Q235A
Input excitation voltage: 150V excitation current: 1.5A frequency: 50Hz
Output DC voltage: 11.5kV DC current: 4A frequency: 120000Hz
Welding speed: 50 m / min
Parameter adjustment: adjust the output voltage and welding speed according to the change of welding line energy. When the parameters are fixed, it is generally not to be adjusted.
6. Technical requirements and quality inspection of high frequency welded pipe
According to the standard of GB3092 “welded pipe for low pressure fluid transportation”, the nominal diameter of the welded pipe is 6~150mm, the nominal wall thickness is 2.0~6.0mm, the length of the welded pipe is usually 4~10 meters, and the length of the pipe can be made at the length or the length of the ruler. The surface quality of the steel pipe should be smooth, and there are no defects such as folding, cracking, delamination, lap welding and so on. There are minor defects such as scratches, scratches, weld mismatches, burns and scarring, which are not more than negative wall thickness. Allow thickening of weld thickness and internal seam welds.
Welded steel pipes should be tested in mechanical properties, flattening tests and flaring tests, and meet the standard requirements. The steel pipe should be able to bear certain internal pressure. If necessary, carry out the 2.5Mpa pressure test, and maintain no leakage for one minute. Eddy current testing is allowed instead of hydraulic pressure test. Eddy current inspection is carried out according to the GB7735 “eddy current inspection method for steel pipe” standard. The method of eddy current flaw detection is to fix the probe on the frame, to keep the distance between the flaw detection and the weld 3~5mm, to scan the weld by the fast motion of the steel tube, and to achieve the purpose of the flaw detection by the automatic treatment and the automatic separation of the eddy current flaw detector.
After welding, the welded pipe is cut off according to the required length, and the bottom line is turned off. The ends of the steel pipes should be chamfered, printed and marked, and the finished products can be packed with hexagonal bundled packages and left the factory.
7. Summary
The technology of straight seam high frequency welded pipe has the characteristics of simple process and high efficiency. It has wide application prospects in civil buildings, petrochemical industry, light industry and other departments.
Source: China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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