The strip is fed into the welded pipe unit and rolled by multiple rolls, the strip is gradually rolled up to form a circular pipe billet with an open gap, adjust the press down amount of the extrusion roll to control the weld gap at 1~3mm and make both ends of the weld flush. If the gap is too large, the neighboring effect is reduced, the eddy current heat is insufficient, and the weld crystal is not fused or cracked. If the gap is too small, the neighboring effect increases, the welding heat is too large, resulting in weld burnout; or weld by extrusion, rolling the formation of deep pits, affecting the surface quality of the weld.
1、Welding temperature control
Welding temperature is mainly affected by high-frequency eddy current thermal power, according to the relevant formula, high-frequency eddy current thermal power is mainly affected by the current frequency, eddy current thermal power is proportional to the square of the current excitation frequency; and the current excitation frequency is affected by the excitation voltage, current and capacitance, inductance. The excitation frequency formula is
f=1/[2π(CL)1/2]
Where: f – excitation frequency (Hz); C – excitation circuit capacitance (F), capacitance = power / voltage; L – excitation circuit inductance, inductance = magnetic flux / current
The above formula shows that the excitation frequency is inversely proportional to the square root of the capacitance and inductance of the excitation circuit, or proportional to the square root of the voltage and current, as long as the capacitance, inductance or voltage and current in the circuit can be changed to change the size of the excitation frequency, so as to achieve the purpose of controlling the welding temperature. For low carbon steel, the welding temperature control at 1250 ~ 1460 ℃, can meet the tube wall thickness of 3 ~ 5 mm weld through the requirements. In addition, the welding temperature can also be achieved by adjusting the welding speed.
Insufficient heat input, the edge of the heated weld does not reach the welding temperature, metal tissue remains solid, the formation of unfused or unwelded; when the input heat is not enough, the edge of the heated weld exceeds the welding temperature, resulting in overburning or melt drops, so that the weld forms a melt hole.
2、Control of squeezing pressure
The two edges of the billet heated to welding temperature, in the extrusion roller extrusion, the formation of common metal grains penetrate each other, crystallization, and finally form a solid weld. If the squeezing pressure is too small, the number of common crystals formed is small, the strength of the weld metal decreases, the force will produce cracking; if the squeezing pressure is too large, will make the molten state of the metal is squeezed out of the weld, not only reduces the strength of the weld, and will produce a large number of internal and external burrs, and even cause defects such as welding lap seams.
3, high-frequency induction circle position control
High-frequency induction circle should be as close as possible to the location of the extrusion roller. If the induction circle is far from the extrusion roller, the effective heating time is longer, the heat-affected zone is wider, and the strength of the weld decreases; conversely, the edge of the weld is not heated enough, and the extrusion is poorly formed.
4, impedance is a welded pipe or a set of special magnetic rod, impedance of the cross-sectional area should usually be not less than 70% of the cross-sectional area of the inner diameter of the steel pipe, its role is to make the induction circle, the edge of the billet weld and the magnetic rod to form an electromagnetic induction loop, resulting in proximity effect, eddy current heat concentrated near the edge of the billet weld, so that the edge of the billet heated to welding temperature. The impedance is dragged inside the billet with a steel wire, the center of which should be relatively fixed close to the center of the extrusion roll. When the machine is turned on, due to the rapid movement of the pipe billet, the impedance is subject to the friction of the inner wall of the pipe billet and the loss is large, which needs to be replaced frequently.
5、The weld seam will produce welding scars after welding and extrusion, which need to be removed. The removal method is to fix the tool on the frame and rely on the rapid movement of the welded pipe to scrape the weld scars flat. The burr inside the welded tube is generally not cleared.