In the process of producing welded pipes with used HFW pipe equipments, how to ensure that the product quality meets the requirements of technical standards and the needs of customers, it is necessary to analyze the factors affecting product quality in the production process of steel pipes.

Through the statistics of unqualified products of a Φ76mm high-frequency welded steel pipe unit in a certain month, it is believed that the factors affecting the quality of steel pipe products in the production process include raw materials, welding process, roll adjustment, roll material, equipment failure, production environment and other reasons . seven aspects. Among them, raw materials account for 32.44%, welding process accounts for 24.85%, roll adjustment accounts for 22.72%, and the three together account for 80.01%, which are the main links. The four factors, such as roll material, equipment failure, production environment and other reasons, account for 19.99% of the impact on the quality of steel pipe products, which are relatively minor links. Therefore, in the process of steel pipe production, the three links of raw materials, welding process and roll adjustment should be controlled.

Control of Welding Seam Gap of Steel Pipe

After the steel strip enters the used HFW pipe equipments and is formed by the forming roller and the guide roller is oriented, a circular steel pipe blank with an opening gap is formed. Both ends of the welded joint remain flush. If the weld gap is controlled too large, the weld will be poorly welded and cause unfusion or cracking; if the weld gap is controlled too small, the weld will be burned due to excessive heat, and the molten metal will splash, which will affect the welding quality of the weld. 

Adjustment of the position of the high frequency induction coil

The induction coil should be placed on the same center line as the steel pipe, and the distance between the front end of the induction coil and the center line of the extrusion roller should be as close as possible to the specification of the steel pipe without burning the extrusion roller. If the induction coil is far from the squeeze roller, the effective heating time will be longer and the heat affected zone will be wide, which will reduce the strength of the steel pipe weld or not penetrate it; otherwise, the induction coil will easily burn the squeeze roller.

Control of Impedance Position

Impedance is one or a group of special magnetic rods for welded pipes. The cross-sectional area of the impedance should usually not be less than 70% of the cross-sectional area of the inner diameter of the steel pipe. Proximity effect is produced, and the eddy current heat is concentrated near the edge of the tube blank weld, so that the edge of the tube blank is heated to the welding temperature. The resistor should be placed in the heating section of the V-shaped area, and the front end should be at the center of the squeeze roller, so that its centerline is consistent with the centerline of the tube. If the position of the resistor is not well placed, the welding speed and welding quality of the welded pipe will be affected, and the steel pipe will be cracked.

High frequency welding process parameters - control of input heat

The heat input by the high-frequency power supply to the welded part of the steel pipe is called the input heat. When converting electrical energy into heat energy, the formula for the input heat is: Q=KI2 Rt (1) In the formula, Q—the heat input to the tube blank; K—energy conversion efficiency; I—welding current; R—circuit impedance; t—heating time.

Heating time: t=Lv (2), where L—the center distance from the front end of the induction coil or the electrode head to the squeeze roller; v—the welding speed.

When the heat input by the high frequency is insufficient and the welding speed is too fast, the edge of the heated pipe body cannot reach the welding temperature, and the steel still maintains its solid structure and cannot be welded, forming unfused or incompletely welded cracks; When the high-frequency input heat is too large and the welding speed is too slow, the edge of the heated pipe body exceeds the welding temperature, which is prone to overheating or even overburning, which will break down the weld and cause metal splashes to form shrinkage cavities. It can be seen from formulas (1) and (2) that the amount of high-frequency input heat can be controlled by adjusting the high-frequency welding current (voltage) or the welding speed, so that the weld of the steel pipe needs to be penetrated without being damaged. Weld through to obtain steel pipes with excellent welding quality.

When changing the steel pipe specifications, the entire set of rolls is generally replaced.

The method of roll adjustment is: use steel wire to draw a center line from the entrance to the exit of the used HFW pipe equipments, and adjust it so that the hole patterns of each frame are on a center line, and the bottom line of the forming meets the technical requirements. After changing the roll specifications, first make a comprehensive adjustment to the forming roll, guide roll, squeeze roll and sizing roll, and then focus on the adjustment of the closed hole pattern, guide roll and squeeze roll of the forming roll.

The function of the guide roller is to control the direction of the pipe seam of the steel pipe and the height of the bottom line of the pipe blank, ease the edge extension, control the rebound of the edge of the pipe blank, and ensure that the pipe seam is straight and not twisted into the extrusion roller. During the welding process, it is easy to cause welding defects such as torsion, lap welding and edge waves of the steel pipe seam.

The squeeze roller is the key equipment of the used HFW pipe equipments. Its function is to complete the pressure welding of the pipe body whose edge is heated to the welding temperature under the action of the extrusion force of the squeeze roller. In the production process, it is necessary to control the size of the opening angle of the extrusion roll. When the extrusion force is too small, the strength of the weld metal will decrease, and cracks will occur after being stressed; when the extrusion force is too large, the welding strength will be reduced, and the amount of external burrs will increase, which is likely to cause welding defects such as lap welding.

In the process of slow start of the used HFW pipe equipments, pay close attention to the rotation of the rolls in various parts, and adjust the rolls at any time to ensure that the welding quality and process dimensions of the welded pipe meet the specified requirements.

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