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Control Weld Gap of Welded Steel Pipe

When the factories manufacture welded steel pipe, the strip is fed into the welded pipe unit, rolled by the multi-way roll, the strip is gradually rolled up to form the circular tube with the opening gap, the pressing amount of the pressing roller is adjusted so that the gap of the weld is controlled at 1 ~ And make the ends of the weld flush. If the gap is too large, resulting in reduced proximity effect, insufficient eddy current heat, weld intergranular junction failure caused by non-fusion or cracking. If the gap is too small is caused by the adjacent effect increases, welding heat is too large, resulting in weld burns; or weld seam formed a deep pit after squeezing and rolling, which affects the surface quality of the weld.


1.The control of welding temperature


The welding temperature is mainly affected by the high-frequency eddy current thermal power. According to the correlation formula, the high-frequency eddy current heat is mainly affected by the current frequency, the eddy current thermal power is proportional to the square of the current excitation frequency, and the excitation frequency is excited by the excitation voltage, Current and capacitance, inductance effects.


The excitation frequency formula is:
F = 1 / [2π (CL) 1/2]
Where the f-excitation frequency (Hz); the capacitance (F) in the C-excitation circuit, the capacitance = the charge / voltage; the inductance, inductance = magnetic flux / current in the L-excitation circuit


In the above equation, the excitation frequency is inversely proportional to the capacitance in the excitation circuit, the square root of the inductance, or is proportional to the square root of the voltage and current. As long as the capacitance, inductance or voltage in the loop is changed, the current can change the magnitude of the excitation frequency Control the purpose of welding temperature. For low carbon steel, welding temperature control at 1250 ~ 1460 ℃, to meet the wall thickness of 3 ~ 5mm penetration requirements. In addition, the welding temperature can also be achieved by adjusting the welding speed.


When the input heat is insufficient, the edge of the heated weld does not reach the welding temperature, the metal structure remains solid, forming unmelted or not penetrated; when the input heat is insufficient, the edge of the heated weld exceeds the welding temperature, Burn or drop, so that the weld to form a melting hole.


2.The control of extrusion pressure


After the two edges of the tube are heated to the welding temperature, the common metal grains penetrate and crystallize under the extrusion of the squeeze roll to form a firm weld. If the extrusion pressure is too small, the number of common crystals is small, the weld metal strength decreases, the force will produce cracking; if the extrusion force is too large, will make the molten state of the metal is squeezed out of the weld, The weld strength, and will produce a lot of internal and external burrs, and even lead to welding joints and other defects.


3.The control of high-frequency induction coil position


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


4. The control of resistance


The resistance is a or a group of pipe dedicated magnetic bar, the cross-sectional area of the resistor should not be less than 70% of the cross-sectional area of the pipe diameter, the role is to make the induction coil, pipe edge and the edge of the magnet to form an electromagnetic induction Loop, resulting in proximity effects, eddy current heat concentrated in the vicinity of the edge of the pipe weld, so that the edge of the tube heated to the welding temperature. The resistor is dragged in a tube with a wire and its center position should be relatively fixed at the center of the squeeze roller. When the boot, due to the rapid movement of the tube, the resistance of the inner wall of the tube by the friction and wear and tear, need to be replaced frequently.


5.Clear the scar


The weld by welding and extrusion will produce scar, need to clear. The cleaning method is to fix the tool in the rack, the rapid movement of the welding pipe, scraping scrap scar. The burrs inside the pipe are generally not cleared.