Adjustment of the position of the high frequency induction coil of the 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 to achieve control purpose of soldering temperature. For low carbon steel, the welding temperature is controlled at 1250~1460°C, which can meet the penetration requirements of the pipe wall thickness of 3~5mm. 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 squeeze roller position. If the induction coil is far away from the extrusion roller, the effective heating time is longer, the heat-affected zone is wider, and the strength of the weld seam decreases; otherwise, the edge of the weld seam is insufficiently heated, and the shape after extrusion is poor. The impedance is one or a group of special magnetic rods for welded pipes. The cross-sectional area of the impedance should usually not be less than 70% of the inner diameter of the steel pipe. The proximity effect is generated, and the eddy current heat is concentrated near the edge of the weld of the tube blank, heating the edge of the tube blank to the welding temperature. The impedance is dragged in the tube blank by 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 is worn out by the friction of 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 petroleum casing is squeezed by the extrusion roller to form common metal grains that penetrate and crystallize each other, and finally form a firm 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 cracks will occur after being stressed; the weld will produce weld scars after welding and extrusion. The method is to fix the tool on the frame, and the rapid movement of the welded pipe will scrape the weld scars. The burrs inside the welded pipe are generally not. If the extrusion force is too large, the metal in the molten state 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 laps.
When the input heat is insufficient, the heated weld edge cannot reach the welding temperature, and the metal structure remains solid, forming incomplete fusion or incomplete penetration; when the input heat is insufficient, the heated weld edge exceeds the welding temperature, resulting in overheating. Burning or dripping makes the weld form a molten hole. 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, The effect of current and 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 with narrower blanks, and welded pipes with different diameters can be produced with the same width of blanks. But compared with the straight seam pipe of the same length, the weld length is increased by 30~100%, and the production speed is lower. Therefore, most of the smaller diameter welded pipes adopt straight seam welding, and most of the large diameter welded pipes adopt welding.
Welded pipe products are widely used in water supply engineering, petrochemical industry, chemical industry, electric power industry, agricultural irrigation, and urban construction. They are 20 key products developed by our country. Used for liquid transportation: water supply and drainage. For gas transportation: gas, steam, liquefied petroleum gas. For structural purposes: as piling pipes, as bridges; pipes for wharves, roads, building structures, etc.
The flattening and cracking of high-frequency welded pipes is caused by welding microcracks, hard and brittle phase inclusions, and coarse grained structures.
In order to control the weld well, the concept of welding inclusion crack index is proposed. It is mainly caused by insufficient welding strength, shape or ductility. When there are small inclusions in the seam weld that affect the impact toughness, weld cracking may occur only when the two opposite walls of the steel pipe are flattened close to the iron box. To reduce weld cracking, improve weld toughness, reduce weld inclusions. So how to reduce the inclusions in the weld?
Firstly, the purity of raw materials should be improved, the content of P and S should be reduced, and the content of inclusions should be reduced. Secondly, check whether the edge of the steel strip is scratched, whether it is stained with rust or dirt, these are not conducive to the discharge of molten metal, and it is easy to cause weld inclusions. Again, uneven wall thickness, burrs and bulges are likely to cause fluctuations in welding current and affect welding.
Post time: May-12-2023