The high-frequency induction coil of straight seam steel pipe should be as close to the extrusion roller as possible. The internal stress of the steel pipe is small and evenly distributed, which can effectively prevent stress corrosion cracking, with high dimensional accuracy and convenient on-site welding construction. 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 after extrusion is poor. When the input heat is insufficient, the heated weld edge cannot reach the welding temperature, and the metal structure remains solid, forming unfused or incomplete penetration; when the input heat is insufficient, the heated weld edge exceeds the welding temperature, resulting in overburning or molten droplets, so that the weld forms a molten hole. The impedance is a special magnetic rod or a group of welded pipes. The cross-sectional area of the impedance should usually be not less than 70% of the inner diameter cross-sectional area 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, resulting in a proximity effect. The eddy current heat is concentrated near the edge of the pipe blank weld so that the edge of the pipe blank is heated to the welding temperature. The process of pre-welding and then fine welding is adopted, the welding process is stable and the weld is high. The impedance of the straight seam steel pipe is dragged in the tube blank with a steel wire, and its center position should be relatively fixed near the center position of the extrusion roller.

The burrs inside the steel pipe are generally not. After the two edges of the tube blank are heated to the welding temperature, the oil casing is squeezed by the extrusion roller to form a common metal grain that penetrates and crystallizes each other, and finally forms a solid 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. If the extrusion pressure is too small, the number of common crystals formed is small, the strength of the weld metal decreases, and cracks will occur after force; the weld will produce weld scars after welding and extrusion, which is needed. The method is to fix the tool on the frame and scrape the weld scar flat by the rapid movement of the welded pipe. The steel pipe has a longitudinal weld, and the inner and outer welds are welded by submerged arc welding.

Methods for measuring the length of straight seam steel pipes:
1. Grating ruler length measurement: Two fixed-length grating rulers are set on the outside of the two ends of the straight seam steel pipe, and the rodless cylinder is used to drive the grating ruler close to the two ends of the steel pipe, and the interference phenomenon of light is used to measure the length of the straight seam steel pipe. The characteristic is high accuracy. However, the grating ruler is expensive and difficult to maintain and is very sensitive to dust and site vibration.
2. Camera length measurement: Camera length measurement uses image processing to measure the length of steel pipes. The principle is to install a series of photoelectric switches at equal distances on a certain section of the straight-seam steel pipe conveyor roller and add a light source and a camera on another section. When the steel pipe passes through this area, the length of the steel pipe can be determined according to the position of the image captured by the camera on the screen by a certain photoelectric switch. The characteristic is that online measurement can be realized, and the length data of the steel pipe can be obtained when it passes through the length measurement area without intervals. The disadvantage is that if a special light source is not used, the steel pipe will be interfered with by external light, and after using a special light source, the steel pipe will reflect light strongly due to the high brightness of the pipe end after chamfering, which is easy to cause reading errors.
3. Encoder length measurement: The principle is to install an encoder at the cylinder, use the cylinder to push the steel pipe to move on the roller, and install a series of photoelectric switches at equal distances on the other side. When the steel pipe is pushed by the cylinder and the end hits the photoelectric switch, the stroke of the cylinder is converted from the recorded encoder reading, so that the length of the steel pipe can be calculated. The characteristic is that the steel pipe needs to be raised when measuring the length. In addition, there are errors in the photoelectric switch detection, which may require full measurement.
4. Improved encoder length measurement: This method is an indirect measurement method. By measuring the distance between the two end faces of the steel pipe and their respective reference points, the length of the steel pipe is indirectly measured. A length measuring trolley is set at each end of the steel pipe, with the initial position being zero and the spacing being L. Then move the editor length to the walking distance (L2, L3) of each steel pipe end, L-L2-L3, which is the length of the steel pipe. This measurement method overcomes difficulties such as the large size of the steel pipe, the complex production site environment, and the inability of the measuring mechanism to pass through the lower support stand of the steel pipe. This method is easy to operate, with a measurement accuracy within ±10mm and a repeatability of ≤5mm.