Position Control And Purity Improvement of Straight Seam Steel Pipe Induction Coil

Adjustment of the position of the 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 the capacitance and inductance in the excitation loop, or directly proportional to the square root of the voltage and current. As long as the capacitance, inductance, voltage, and current in the loop are changed, the excitation frequency can be changed, thereby achieving control Welding temperature purpose. For low carbon steel, the welding temperature is controlled at 1250~1460℃, which can meet the welding penetration requirement of 3~5mm 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 as possible to the position of the squeeze roller. If the induction coil is far away from the extrusion roller, the effective heating time will be longer, the heat-affected zone will be wider, and the strength of the weld will decrease; on the contrary, the edge of the weld will not be heated enough and the shape will be poor after extrusion. The resistor is one or a group of special magnetic rods for welded pipes. The cross-sectional area of the resistor should usually not be less than 70% of the cross-sectional area of the inner diameter of the steel pipe. Its function is to form an electromagnetic induction loop with the induction coil, the edge of the pipe blank weld, and the magnetic rod. A proximity effect is generated, and the eddy current heat is concentrated near the edge of the tube blank weld, causing the edge of the tube blank to be heated to the welding temperature. The resistor is dragged inside the tube blank with a steel wire, and its center position should be relatively fixed close to the center of the extrusion roller. When starting up, due to the rapid movement of the tube blank, the resistor suffers from large losses due to friction on the inner wall of the tube blank and needs to be replaced frequently.

After the two edges of the tube blank are heated to the welding temperature, the oil casing is squeezed by the squeeze roller to form common metal grains that penetrate and crystallize with each other, eventually forming a strong weld. If the extrusion force is too small, the number of common crystals formed will be small, the strength of the weld metal will decrease, and cracking will occur after stress; the weld will produce weld scars after welding and extrusion, which is necessary. The method is to fix the tool on the frame and use the rapid movement of the welded pipe to smooth the weld scar. There are generally no burrs inside welded pipes. If the extrusion force 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 welding laps.

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 welding; when the input heat is insufficient, the heated weld edge exceeds the welding temperature, resulting in excessive welding. Burn or droplets will form 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, it can be seen that the 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; and the current excitation frequency is affected by the excitation voltage, Effects of 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 welded pipes is generally higher than that of straight seam welded pipes. Welded pipes with larger diameters can be produced from narrower billets, and welded pipes with different diameters can also be produced from billets of the same width. However, compared with straight seam pipes of the same length, the weld length is increased by 30~100%, and the production speed is lower. Therefore, smaller-diameter welded pipes are mostly made of straight seam welding, while large diameter welded pipes are mostly made of straight seam welding.

Welded pipe products are widely used in water supply projects, the petrochemical industry, the chemical industry, the electric power industry, agricultural irrigation, and urban construction. They are among 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 purposes: piling pipes, bridges; pipes for docks, roads, building structures, etc. Flattening and cracking of high-frequency welded pipes are caused by welding micro-cracks, hard and brittle phase inclusions, coarse-grained structures, etc. To better control the weld, the concept of welding inclusion crack index was proposed. Mainly caused by insufficient welding strength, shape, or ductility. When there are small inclusions in the seam weld that affect the impact toughness, cracking of the weld may only occur when the two opposite walls of the steel pipe are flattened close to the iron box. It is necessary to reduce weld cracking, improve the toughness of the weld, and reduce inclusions in the weld. So how to reduce inclusions in welds?

First, we must improve the purity of raw materials, reduce the content of P and S, and reduce the content of inclusions. Secondly, check whether the edge of the steel strip is bruised and whether it is stained with rust or dirt. These are not conducive to the discharge of molten metal and can easily cause inclusions in the weld. Thirdly, uneven wall thickness, burrs, and bulges can easily cause fluctuations in welding current and affect welding.