Position control and purity improvement of induction coil of straight seam steel pipe

Control of the position of high-frequency induction coil of straight seam steel pipe: The excitation frequency of straight seam steel pipe is inversely proportional to the square root of capacitance and inductance in the excitation circuit, or 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, thereby achieving the purpose of controlling the welding temperature. For low-carbon steel, the welding temperature is controlled at 1250~1460℃, which can meet the requirements of 3~5mm penetration of pipe wall thickness. In addition, the welding temperature can also be achieved by adjusting the welding speed.

The high-frequency induction coil should be as close to the extrusion roller as possible. If the induction coil is far away from the extrusion roller, the effective heating time is long, the heat-affected zone is wide, and the weld strength decreases; on the contrary, the weld edge is not heated enough and the forming is poor after extrusion. The impedance is a special magnetic bar or a group of magnetic bars for welding pipes. The cross-sectional area of the impedance should usually be no less than 70% of the inner diameter cross-sectional area of the steel pipe. Its function is to make the induction coil, the edge of the weld of the tube billet, and the magnetic bar form an electromagnetic induction loop, produce a proximity effect, and the eddy current heat is concentrated near the edge of the weld of the tube billet so that the edge of the tube billet is heated to the welding temperature. The impedance is dragged in the tube billet with a steel wire, and its center position should be relatively fixed near the center of the extrusion roller. When the machine is turned on, due to the rapid movement of the tube billet, the impedance is greatly worn by the friction of the inner wall of the tube billet and needs to be replaced frequently.

After the two edges of the tube billet are heated to the welding temperature, the oil casing forms a common metal grain under the extrusion of the extrusion roller, which penetrates and crystallizes each other and finally forms a solid weld. If the extrusion force is too small, the number of common crystals formed is small, the strength of the weld metal decreases, and cracks will occur after being stressed; the weld will produce weld scars after welding and extrusion, which needs to be fixed. The method is to fix the tool on the frame and scrape the weld scars flat by the rapid movement of the welded pipe. The burrs inside the welded pipe are generally not. If the extrusion pressure is too large, the molten metal will be squeezed out of the weld, which not only reduces the strength of the weld, but also produces a large number of internal and external burrs, and even causes defects such as weld overlap.

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 unfused or incomplete penetration; when the input heat is insufficient, the edge of the heated weld exceeds the welding temperature, resulting in overburning or molten droplets, and forming a molten hole in the weld. The welding temperature is mainly affected by the high-frequency eddy current thermal power. According to the relevant formula, the high-frequency eddy current thermal power is mainly affected by the current frequency, and the 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, capacitance, and inductance.

The production process of straight seam welded pipe is simple, the production efficiency is high, the cost is low, and the development is rapid. The strength of the welded pipe is generally higher than that of the straight seam welded pipe. It can produce welded pipes with larger diameters with narrower billets, and can also produce welded pipes with different diameters with billets of the same width. However, compared with the straight seam pipe of the same length, the weld length increases by 30~100%, and the production speed is lower. Therefore, most of the smaller-diameter welded pipes use straight seam welding, while most of the large-diameter welded pipes use welding.

Welded pipe products are widely used in tap water projects, the petrochemical industry, the chemical industry, the power industry, agricultural irrigation, and urban construction, and are one of the 20 key products developed in my country. For liquid transportation: water supply and drainage. For gas transportation: coal gas, steam, liquefied petroleum gas. For structural use: as piling pipes, bridges; docks, roads, building structure pipes, etc.

The flattening and cracking of high-frequency welded pipes are caused by welding microcracks, hard and brittle phase inclusions, coarse granular structures, etc.

To better control the weld, the concept of welding inclusion crack index is proposed. It is mainly caused by insufficient welding strength, human shape, or ductility. When there are small inclusions that affect the impact toughness in the seam weld, the weld cracking may only occur when the two opposite pipe walls of the steel pipe are flattened too close to the iron box. To reduce weld cracking, improve weld toughness, and reduce weld inclusions. So how to reduce weld inclusions?

First, improve the purity of raw materials, reduce P and S content, and reduce the content of inclusions. Secondly, check whether the edge of the steel strip is bruised, rusted, or contaminated, which is not conducive to the discharge of molten metal and easily causes weld inclusions. Thirdly, uneven wall thickness, burrs, and bulges can easily cause fluctuations in welding current and affect welding.