A detailed explanation of the welding process of large diameter straight seam steel pipe

The double wire automatic welding of straight seam steel pipe is a welding technology developed in recent years. In addition to the characteristics of semi-automatic single wire welding, it also has the welding characteristics of energy concentration and high deposition efficiency. The main and auxiliary wires are provided with welding current by a separate ordinary welding power source, which can be adjusted independently and can realize the optimal configuration of welding process parameters. It has static and dynamic characteristics, and the two separate power sources rely on the welding software to work together to supply power to the main and auxiliary wires. At the same time, the main and auxiliary wires are melted, and the metal is transferred to the weld to form a stable molten pool, which ensures the strength of the welded joint. It can not only use a conventional melting electrode welding power source to realize welding, but also reduce the equipment cost so that the welding heat is highly concentrated, the deposition speed is fast, the welding efficiency is high, and the deformation after welding is small, the labor intensity is low, and the welding seam of the straight seam steel pipe is effectively reduced. Microstructure properties, especially for the welding of high thermal conductivity materials, the effect of energy concentration is outstanding.

1. Control of weld gap
The strip is fed into the welded steel pipe unit, rolled by multiple rollers, and the strip is gradually rolled up to form a circular tube blank with an opening gap. , and make both ends of the weld flush. If the gap is too large, the proximity effect will be reduced, the eddy current heat will be insufficient, and the intergranular bonding of the weld will be poor, resulting in a lack of fusion or cracking. If the gap is too small, the proximity effect will increase, the welding heat will be too large, and the welding seam will be burned; or the welding seam will form a deep pit after extrusion and rolling, which will affect the welding seam 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, resulting in incomplete fusion or incomplete penetration; when the input heat is insufficient, the edge of the heated weld exceeds the welding temperature, resulting in Overburning or dripping, causing the weld to form a hole.

3. Control 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 from the extrusion roller, the effective heating time will be longer, the heat-affected zone will be wider, and the strength of the weld will decrease; on the contrary, the edge of the weld will be insufficiently heated, resulting in poor forming after extrusion.

4. Control of extrusion force
After the two edges of the large-diameter straight seam steel pipe, the blank is heated to the welding temperature, under the extrusion of the extrusion roller, common metal grains are formed to penetrate and crystallize each other and finally form a firm weld. If the extrusion force is too small, the number of common crystals formed will be small, the strength of the weld metal will decrease, and cracking will occur after being stressed; if the extrusion force is too large, the molten metal will be squeezed out of the welded steel pipe, not only The strength of the weld is reduced, and a large number of internal and external burrs will be generated, and even defects such as welding laps will be caused.

5. The impedance device is one or a group of special magnetic rods for welded steel pipes. The cross-sectional area of the impedance device 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 a proximity effect, 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 is dragged in the tube blank with a steel wire, and its central position should be relatively fixed close to the center position of the extrusion roller. When starting up, due to the rapid movement of the tube blank, the resistor is greatly lost 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 tool on the frame and scrape the welding scar by the rapid movement of the welded steel pipe.