High frequency welding technology of straight seam steel pipe

Straight seam high frequency welded steel pipe has the characteristics of relatively simple process and rapid and continuous production. It is widely used in civil construction, petrochemical, light industry and other departments. High frequency welding is based on the principle of electromagnetic induction and the skin effect, proximity effect and eddy current effect in the conductor of AC charge to locally heat the steel at the edge of the weld to the molten state. After being extruded by the roller, the butt weld can realize intergranular bonding, so as to achieve the purpose of weld welding. This paper summarizes the principle, process method and application of high frequency welding of straight seam steel pipe.

Straight seam high frequency welded steel pipe has the characteristics of relatively simple process and rapid and continuous production. It is widely used in civil construction, petrochemical, light industry and other departments. It is mainly used to transport low-pressure fluid or make various engineering components and light industrial products.

Production process flow of straight seam high frequency welded steel pipe

Straight seam welded steel pipe is a steel pipe made of long strip steel strip of certain specification rolled into circular pipe by high-frequency welding unit and welded by straight seam. The shape of steel pipe can be round, square or special-shaped, which depends on the sizing rolling after welding. The main materials of welded steel pipes are: low-carbon steel and low-alloy steel with σs≤300N/mm2, σs≤500N/mm2 or other steels. The production process of high-frequency welding of straight seam steel pipes is as follows:

20210924233312 77255 - High frequency welding technology of straight seam steel pipe

High frequency welding

High frequency welding is based on the principle of electromagnetic induction and the skin effect, proximity effect and eddy current thermal effect of AC charge in the conductor to locally heat the steel at the edge of the weld to the molten state. After being extruded by the roller, the butt weld can realize intergranular joint, so as to achieve the purpose of weld welding. High frequency welding is a kind of induction welding (or pressure contact welding). It has the advantages of no weld filler, no welding spatter, narrow welding heat affected zone, beautiful welding forming and good welding mechanical properties. Therefore, it is widely used in the production of steel pipes.
The high-frequency welding of steel pipe makes use of the skin effect and proximity effect of AC. after the steel (strip steel) is rolled and formed, a circular pipe blank with broken section is formed. One or a group of impedance (magnetic rod) is rotated near the center of the induction coil in the pipe blank. The impedance and the opening of the pipe blank form an electromagnetic induction loop. Under the action of skin effect and proximity effect, The edge of the opening of the pipe blank produces a strong and concentrated thermal effect, which makes the weld edge quickly heated to the temperature required for welding. After being extruded by the pressure roller, the molten metal realizes intergranular bonding, and a firm butt weld is formed after cooling.

High frequency welded pipe unit

The high-frequency welding process of straight seam steel pipe is completed in the high-frequency welded pipe unit. The high-frequency welded pipe unit is usually composed of rolling forming, high-frequency welding, extrusion, cooling, sizing, flying saw cutting and other components. The front end of the unit is equipped with a material storage looper, and the rear end of the unit is equipped with a steel pipe turnover frame; The electrical part is mainly composed of high-frequency generator, DC excitation generator and instrument automatic control device. Now take the φ165mm high frequency welded pipe unit as an example, the main technical parameters are as follows:

Finished welded pipe

  • Outer diameter of round pipe: φ 111-165mm
  • Square pipe: 50 × 50-125 × 125mm
  • Rectangular pipe: 90 × 50-160 × 60-180 × 80mm
  • Wall thickness of finished pipe: 2 – 6mm
  • Forming speed: 20 – 70m/min

High frequency sensor

  • Thermal power: 600kW
  • Output frequency: 200 – 250kHz
  • Power supply: three-phase 380V 50Hz
  • Cooling: water cooling
  • Excitation voltage: 750 – 1500V

High frequency excitation circuit

The high-frequency excitation circuit (also known as high-frequency oscillation circuit) is composed of a large electron pipe and an oscillation slot installed in the high-frequency generator. It uses the amplification of the electron pipe to feed back the anode output signal to the grid when the electron pipe is connected to the filament and anode to form a self-excited oscillation circuit. The excitation frequency depends on the electrical parameters (voltage, current, capacitance and inductance) of the oscillation channel.

High frequency welding process of straight seam steel pipe

Control of weld gap

The strip steel is sent to the welded pipe unit, rolled by multiple rolls, and the strip steel is gradually rolled up to form a circular pipe blank with opening gap. Adjust the reduction of extrusion roll to control the weld gap at 1 – 3mm, and make both ends of the welded junction flush. If the gap is too large, the proximity effect will be reduced, the eddy current heat will be insufficient, and the weld intergranular joint will be poor, resulting in incomplete fusion or cracking. If the gap is too small, the proximity effect will increase, the welding heat will be too large, and the weld will be burned; Or the weld is extruded and rolled to form a deep pit, which affects the surface quality of the weld.

Welding temperature control

The welding temperature is mainly affected by the high-frequency eddy current thermal power. According to formula (1), the high-frequency eddy current thermal power is mainly affected by the current frequency, and the eddy current thermal power is directly proportional to the square of the current excitation frequency; The current excitation frequency is affected by the excitation voltage, current, capacitance and inductance. The excitation frequency formula is:

  • 20210925000547 23772 - High frequency welding technology of straight seam steel pipe (1) 

In the formula:

  • F – excitation frequency (Hz);
  • C – capacitance in excitation circuit (f), capacitance = electric quantity/voltage;
  • L – inductance in the excitation circuit, inductance = magnetic flux/current

According to the above formula, the excitation frequency is inversely proportional to the square root of capacitance and inductance in the excitation circuit, or directly proportional to the square root of voltage and current. As long as the capacitance, inductance, voltage and current in the circuit are changed, the excitation frequency can be changed, so as to control the welding temperature. For low carbon steel, the welding temperature is controlled at 1250 – 1460 ℃, which can meet the penetration requirements of pipe wall thickness of 3 – 5mm. In addition, the welding temperature can also be realized by adjusting the welding speed.
When the input heat is insufficient, the heated weld edge cannot reach the welding temperature, and the metal structure remains solid, forming incomplete fusion or incomplete penetration; When the input heat is insufficient, the heated weld edge exceeds the welding temperature, resulting in overburning or droplet, which makes the weld form a melting hole.

Control of extrusion force

After the two edges of the pipe blank are heated to the welding temperature, common metal grains are formed under the extrusion of the extrusion roller, penetrate and crystallize with each other, and finally form a firm weld. If the extrusion force is too small, the number of common crystals will be small, the strength of weld metal will decrease, and cracking will occur after stress; If the extrusion force is too large, the molten metal will be extruded out of the weld, which will not only reduce the weld strength, but also produce a large number of internal and external burrs, and even cause defects such as welding overlap.

Regulation of high frequency induction coil position

The high-frequency induction coil shall be as close to the extrusion roll as possible. If the induction coil is far 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 enough and the forming is poor after extrusion.

The impedance device is one or a group of special magnetic rods for welded pipe

The cross-sectional area of the impedance device shall generally not be less than 70% of the inner diameter of the steel pipe. Its function is to form an electromagnetic induction circuit between the induction ring, the weld edge of the pipe blank and the magnetic rod to produce proximity effect. The eddy current heat is concentrated near the weld edge of the pipe blank to heat the pipe blank edge to the welding temperature. The impedance is dragged in the pipe blank with a steel wire, and its center position shall be relatively fixed near the center of the extrusion roll. During startup, due to the rapid movement of the pipe blank, the impedance is greatly worn by the friction of the inner wall of the pipe blank and needs to be replaced frequently.

The weld scar will be generated after welding and extrusion, which needs to be removed. The removal method is to fix the tool on the frame and scrape the welding scar by the rapid movement of the welded pipe. The burr inside the welded pipe is generally not removed.
Process examples
Now it is welded φ thirty-two × Take 2mm longitudinal welded pipe as an example to briefly describe its process parameters:

  • Strip steel specification: 2 × The 98mm bandwidth is expanded according to the pitch diameter and a small amount of forming allowance is added
  • 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: 50M/min
  • Parameter adjustment: timely adjust the output voltage and welding speed according to the change of welding line energy. After the parameters are fixed, they generally do not need to be adjusted.

Technical requirements and quality inspection of high frequency welded pipe

According to GB3092 welded steel pipes for low pressure fluid transportation, the nominal diameter of welded pipes is 6 – 150mm, the nominal wall thickness is 2.0 – 6.0mm, and the length of welded pipes is usually 4 – 10m, which can be delivered according to the fixed length or multiple length. The surface quality of steel pipe shall be smooth, and there shall be no folding, crack, delamination, lap welding and other defects. Minor defects such as scratch, scratch, weld dislocation, burn and scar that do not exceed the negative deviation of wall thickness are allowed on the surface of steel pipe. The wall thickness thickening at the weld and the existence of internal seam welding reinforcement are allowed.
Mechanical property test, flattening test and flaring test shall be conducted for welded steel pipes, and the requirements specified in the standard shall be met. The steel pipe shall be able to withstand a certain internal pressure. If necessary, 2.5MPa pressure test shall be carried out to keep it free of leakage for one minute. Eddy current flaw detection is allowed to replace hydraulic test. Eddy current testing shall be carried out in accordance with gb7735 eddy current testing methods for steel pipes. The eddy current flaw detection method is to fix the probe on the frame, maintain a distance of 3 – 5mm between the flaw detection and the weld, comprehensively scan the weld by the rapid movement of the steel pipe, and the flaw detection signal is automatically processed and sorted by the eddy current flaw detector to achieve the purpose of flaw detection.
The welded pipe after flaw detection shall be cut off with a flying saw according to the specified length and rolled off the line through the turnover frame. Both ends of the steel pipe shall be chamfered and marked. The finished pipe shall be bound and packaged in hexagon before leaving the factory.

Summary

Straight seam high frequency welded pipe technology has the characteristics of simple process and high efficiency. It has a wide application prospect in civil construction, petrochemical, light industry and other departments.

Source: Network Arrangement – China Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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