High-frequency induction welded (HFI) pipe

High-frequency induction welded pipe production speed is high, the welding process is stable, good quality, has been widely used in welded pipe production. There are many factors affecting the quality of weld in the process of production, to obtain ideal welding effect, strip width, molding process and welding seam must be oriented, squeezing roller, induction coil, impedance factors such as control.

In order to obtain satisfactory result of welding, steel strip size to be as accurate as possible, in order to conform to the requirements of the standards, can generally be allowed wall thickness tolerance. Even if some strip width change can also lead to poor welding. Argued that under the condition of ideal welding, using only enough load force will strip edge extruding continuous welded together, and let the whole cross-section of steel strip produced only small fraction of the surface sag. This view is not correct. Although exact longitudinal shearing strip steel can meet the requirements of welding in the process of strip width, but in the unit coarse molding and forming period for dynamic changes caused by the unit itself may change the strip width of the welding points, for example, as each roll aperture on driving radius decreases, the welding point strip width may decrease. Due to roll stress on steel tube is derived from the molding machine drive, so the strip width reduction is should be based on roll applied stress, the strength and thickness of the strip. If the strip thicker, allows roll fully clamped and applied stress. Again, such as, roll gap when the unit has been adjusted to strip the minimum wall thickness range, and the strip steel is to allow maximum into the rough shape of wall thickness, it may cause strip thickness, width increases. Increase the value of the strip width and the strength of the steel strip, thickness, roll diameter and the distance between the two supporting bearings. As a result, the high frequency welded pipe unit installation accuracy is very important.

Molding technology and rolling rod type roller design is the key to the high frequency induction welding technology. Usually used for a long time of contact welding technology will strip bending into oval, electrode installation location close to the strip closed point, strip into the squeezing roller contact pressure of electrode and strips, in the current flows through the strip for the moment, and extrusion rod will be welded pipe completely surrounded, the formation of circular ring by welding, high frequency induction welding technology is within a certain distance before squeezing roller with induction coil around the pipe, when the sensor by high frequency current, outside the molding steel strip surface induced eddy current, the eddy current can shape along the openings in the plate edge into welding point to establish a closed loop, but also along the shunt forming within the strip surface. Sensors and forming of strip steel can also be seen as transformer primary coil and secondary coil, the strip steel as the core, refers to the secondary coil molding strip steel surface and load refers to the surface of strip steel. So useful along the shape of welding current loop to edge heating, useless within the current loop current along the molding steel strip surface, heat around and cause the heat losses. For this, at the time of induction welding, should try to increase the inner surface impedance, reducing the loss of diversion, and placed inside the tube billet resistance HangQi. When the induction welding, the welding point location is important. Strip edge to melt before entering the extrusion rod and high frequency current quickly through the form of a low impedance area, form a closed triangle vertex. High frequency induction welding, the weld joint is very important, if the strip around the first contact, so, this was mainly through the high frequency current, the strip edge cause overheating and melting structure, oxide inclusions, trends and large irregular porous inside burr, this kind of circumstance of welded pipe inside burr is more obvious.

Most of welded pipe unit has weld guide, the rectification of strip deviation. The device installed in essence between forming and welding points, so that the weld at the top of the welded pipe, easy to remove the burr. But weld guide cannot be used to eliminate the forming period of steel strip produced by defects, otherwise will cause the welding efficiency and weld quality decline. In practical use, to prevent or limit current flow guide plate, should choose appropriate raw materials. High strength steel to guide, guide plate should be installed far away from the position of the welding points in order to eliminate the influence of the spark, because once produced sparks, strip edge between the guide plate and will produce oxide inclusions in welding area. In addition, if the current flow guide plate, the welding power and welding efficiency will be affected. Is made of ceramic materials to guide pieces can be completely stop current to flow between the strip and guide plate. But a single ceramic material strength is not enough, so with high strength steel and ceramic composite materials guide plate, can increase the strength of the guide plate, and guaranteeing the electrical insulation of guide plate. Weld guide plate except for calibrating the welding point of the radial position, still can control the welding opening corner.

Squeezing roller has two function (1) to make its edge extrusion strip exert a certain force together, (2) by the roller roller type and the extrusion molding process to determine the shape of the welded pipe. In high frequency welding process, after squeezing roller is located in the closed welding point on the edge of the strip after contact, to change the outline of the edge or line is impossible, so it with auspicious cannot eliminate shape of strip deviation. Squeezing roller has a variety of regulating function, roll benchmark position can be obtained, to compensate the roll and bearing loss, squeezing roller pressure adjustment. The structure of form with two-high, four roller is the most complicated. Each has its advantages and disadvantages for each type. Two-high consists of two vertical roll can be adjusted, material control system can adjust the two roller at the same time to achieve the required extrusion or forging load. Every roll placed below a thin gaskets, roll diameter is large, usually spindle and bearing size is big, to withstand the shear stress and bending stress.

In order to obtain the highest in the high frequency induction welding welding efficiency, at the same time within a given input power to obtain the most narrow heat affect zone and the highest welding speed, if the reactance of induction coil can reduce, more energy can be converted into heat energy. Decreased the diameter of the coil can reduce its reactance, but the distance between the coil and the welded pipe has certain limitations, it can reduce the number of turns induction coil, which requires pipe welding machine output more current, and the actual only low current can be used. Some experts believe that only makes the value of the pipe welding machine, appropriate and welded pipe and gap between coil, appropriate to turn all of output power to work. In addition, in the process of welding attracted by the strong magnetic field induction coil of strip steel metal particles can be heated, coil spacer induction coil, lead to premature rupture, therefore, the induction coil should be kept clean, in order to prevent the parking for coil fault. And the induction coil, the impedance change of the position and size can also cause the change of the welding heat affected zone of strip cross-sectional contour. For example, the impedance is removed from the welding point, welded pipe internal heat affected zone will be widened. That is to say, the welded pipe outer surface heat affected zone width is controlled by the position and size of induction coil inner surface of heat affected zone width for the location and size of impedance control. If induction coil and impedance were removed welding points, the heat affected zone will be wider, welding after the large deformation of the surface of the sag and easily, at the same time, a large number of metal being squeezed out, the power consumption increased. In this case, the actual don’t reach the welding speed on the table shows the welding speed.

Post time: Oct-14-2019