How to control the gap of welded pipe weld

The strip steel is fed into the welded pipe mill, rolled by multiple rollers, and the strip steel is gradually rolled up to form a round billet with open gap. The reduction of extrusion roller is adjusted to control the weld gap at 1 ~ 3mm, and the two ends of the weld joint are flush. If the gap is too large, the proximity effect will be reduced, the eddy current heat is insufficient, and the weld is not fused or cracked due to poor intergranular bonding. If the gap is too small, it will lead to failure With the increase of proximity effect, the welding heat is too large, which leads to the weld burning, or the deep pit formed after the weld extrusion and rolling, which affects the surface quality of the weld.

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 current frequency, eddy current thermal power is proportional to the square of current excitation frequency, current excitation frequency is affected by excitation voltage, current, capacitance and inductance, the excitation frequency formula is: F = 1 / [2 π (CL) 1 / 2], in the formula: f-excitation frequency（ C-capacitance in the excitation circuit (f), capacitance = quantity of electricity / voltage; L-inductance in the excitation circuit, inductance = flux / current. According to the above formula, the excitation frequency is inversely proportional to the square root of capacitance and inductance of the excitation circuit, or directly proportional to the square root of voltage and current. As long as the capacitance, inductance or voltage and current in the circuit are changed, the excitation frequency can be changed, so as to control the welding temperature. For low carbon steel, the welding temperature can be controlled When the temperature is 1250 ~ 1460 ℃, it can meet the requirement of 3 ~ 5mm penetration.

The control of extrusion force: after the two sides of the tube billet are heated to the welding temperature, under the extrusion of the extrusion roller, the common metal grains are formed, infiltrating and crystallizing with each other, forming a firm weld. If the extrusion force is too small, the number of common crystals is small, the strength of the weld metal is reduced, and cracking occurs after the stress. If the extrusion force is too large, the molten metal will be extruded out of the weld, not only The weld strength is reduced, and a large number of internal and external burrs will be produced, even resulting in weld defects.

The farther the induction coil is from the extrusion roll, the longer the effective heating time is, the wider the heat affected zone is, and the lower the weld strength is. On the contrary, the weld edge is not heated enough, which leads to poor forming after extrusion.

The impedance is one or a group of special magnetic rods for welded pipe, and the cross-sectional area of the impedance shall not be less than 70% of the cross-sectional area of the inner diameter of the steel pipe. Its function is to make the induction coil, the edge of the tube blank weld and the magnetic bar form an electromagnetic induction loop, which produces proximity effect. The eddy heat is concentrated near the edge of the tube blank weld, which makes the edge of the tube blank be heated to the welding temperature. The impedance device is dragged in the tube blank with a steel wire, and its center position should be relatively fixed near the center of the extrusion roll The dynamic impedance device is damaged by the friction of the inner wall of the tube blank and needs to be replaced frequently. The weld scar will be produced after welding and extrusion, which needs to be removed. The removal method is to fix the cutter on the frame, scrape the weld scar flat by the rapid movement of the welded tube, and the burr inside the welded tube is generally not removed.