Welding technology and production method of large-diameter straight seam steel pipe

1. Control of the weld gap

The strip steel is sent to the welded steel pipe unit, after rolling by multiple rollers, the strip steel is gradually rolled up to form a round tube blank with an open gap, and the reduction of the squeeze roller is adjusted to control the weld gap to 1~3mm, And make the two ends of the weld flush. If the gap is too large, the proximity effect will decrease,

Insufficient eddy current heat, poor intergranular bonding of the weld, resulting in non-fusion or cracking. If the gap is too small, the proximity effect will increase, and the welding heat will be too large, causing the weld to burn; or the weld will form a deep pit after being squeezed and rolled, which will affect the weld surface.

2. Welding temperature control

When the input heat is insufficient, the edge of the heated weld cannot reach the welding temperature, and the metal structure remains solid, forming a lack of fusion or penetration; when the input of heat is insufficient, the edge of the heated weld exceeds the welding temperature, resulting in Overburning or dripping, so that the weld forms a molten hole.

3. Adjustment of the position of the high-frequency induction coil

The high-frequency induction coil should be as close as possible to the position of the squeeze roller. If the induction coil is far away from the squeeze roller, the effective heating time is longer, the heat-affected zone is wider, and the weld strength decreases; on the contrary, the edge of the weld is insufficiently heated, resulting in poor molding after extrusion.

4. Control of extrusion force

After the two edges of the large-diameter straight seam steel pipe, the billet is heated to the welding temperature, under the extrusion of the squeeze roller, a common metal crystal grain is formed to infiltrate and crystallize each other, and finally, a firm weld is formed. If the extrusion force is too small, the number of common crystals formed will be small, the weld metal strength will decrease, and cracks will occur after the force is applied; if the extrusion force is too large, the molten metal will be squeezed out of the welded steel pipe. The weld strength is reduced, and a large number of internal and external burrs are generated, and even defects such as weld overlap are caused.

5. Impedance is one or a set of special magnetic rods for welded steel 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. The magnetic induction loop produces the proximity effect, and the eddy current heat is concentrated near the edge of the tube blank weld so that the tube blank edge is heated to the welding temperature. The resistor is dragged inside the tube with a steel wire, and its central position should be relatively fixed close to the center of the squeeze roller. When starting up, due to the rapid movement of the tube blank, the resistance device is worn away due to the friction of the inner wall of the tube blank and needs to be replaced frequently.

6. Weld scars will be produced after welding and extrusion, which is required. The method is to fix the cutter on the frame and scrape the welding scar by the rapid movement of the welded steel pipe. Burrs inside welded steel pipes are generally not.