Welding seam treatment of high frequency straight seam steel pipe

High-frequency straight seam steel pipe is based on the principle of electromagnetic induction and the skin effect, proximity effect and eddy current thermal effect of charges, thereby locally heating the steel at the edge of the weld to a molten state, and realizing butt welding through intergranular bonding extrusion, thereby achieving The purpose of seam welding.

1. Control of weld gap
The strip steel is sent to the welded pipe unit and rolled by multiple rollers. The strip steel is gradually rolled into a round tube blank with an open gap, and the compression of the extrusion roller is adjusted to control the welding gap between 1 and 3 mm. and make the welds flush. If the gap is too large, the proximity effect will be reduced, the eddy current will be insufficient, and the intergranular joint of the weld will be poor, 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. Otherwise, the weld after extrusion and rolling will form a deep pit, which will affect the surface quality of the weld.
2. Welding temperature control
The welding temperature is mainly affected by the high-frequency eddy current thermal power. According to the formula, high-frequency eddy current thermal power is mainly affected by the current frequency. The eddy current thermal power is proportional to the square of the current excitation frequency, which is affected by the excitation voltage and current. and the effects of capacitance and inductance.
When the input heat is insufficient, the heated weld edge cannot reach the welding temperature, and the metal structure remains solid, resulting in incomplete fusion or incomplete penetration.
3. Control of extrusion force
After the two edges of the tube blank are heated to the welding temperature, common metal grains are formed under the squeeze of the extrusion roller, allowing them to penetrate and crystallize with each other, and finally form a strong weld. If the extrusion force is too small, Then the number of eutectics formed will be reduced, the strength of the weld metal will be reduced, and cracks will occur after applying force; if the extrusion force is too large, the molten metal will be extruded from the weld, which will not only reduce the strength of the weld, but also A large amount of internal and external burrs will be produced.
4. 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 ring is far away from the pressure roller, the effective heating time is longer, the heat affected zone is wider, and the welding strength is reduced; on the contrary, the edge of the weld is not fully heated, resulting in poor molding after extrusion.
5. After welding and extrusion, the weld will produce weld scars, which need to be removed. The cleaning method is to fix the tool on the frame and rely on the rapid movement of the welded pipe to smooth the weld scars. The burrs inside the welded pipe are generally not removed.
High-frequency welding is a type of induction welding (or pressure contact welding) that requires no weld filler and no welding spatter. The welding heat affected zone is narrow, the welding shape is beautiful, and the welding mechanical performance is good. Therefore, it is widely used in the production of steel pipes.