Prevention of Weld Seam Cracking of High Frequency Welded Pipe

Posted by chase Liu on December 15th, 2022

Straight seam high frequency welded pipe (ERW pipe) is formed by forming hot-rolled coils through a forming machine. Then use the skin effect and proximity effect of high-frequency current to heat and melt the edge of the tube blank, and perform pressure welding under the action of the extrusion roller to realize the welded tube produced. In high-frequency longitudinal welded pipes, the cracks manifest in the form of full-length cracks, local periodic cracks, and irregular intermittent cracks. Some steel pipes have no cracks on the surface after welding, but cracks appear after flattening, straightening or hydraulic testing.

Factors Affecting Cracks

1. Poor raw material quality

(1) In the production of welded pipes, large burrs on the edges of raw materials and excessive width are common problems.

(2) If the burrs are outward during welding, it is easy to produce long intermittent cracks;

(3) The width of the raw material is too wide, the pass of the extrusion roller is too full, forming a welding peach shape, the outer welding scar is large, the inner welding rib is small or even no, and it cracks after straightening.

2. Edge corner joint state

(1) The fillet joint state of the edge of the tube blank is a phenomenon that often occurs in the production of welded pipes. The smaller the pipe diameter, the more serious the fillet joint.

(2) Insufficient forming adjustment is a prerequisite for corner joints.

(3) Improper pass design of the extrusion roller, large outer fillet on the pass, and elevation angle adjustment of the extrusion roller are the key factors affecting the corner joint.

(4) The single-radius extrusion roll pass can not eliminate the corner joint problem caused by poor forming. Increasing the extrusion force, or the wear of the extrusion roller in the later stage of production and the vertical ellipse shape will aggravate the pointed peach-shaped welding state, and serious corner joints will appear.

(5) The fillet joint will cause most of the metal to flow from the top, forming an unstable melting process. At this time, there is a large metal splash, and an overheated weld joint structure appears. The outer burr is high-heat, irregular, large in volume and difficult to scrape off, and the inner burr is extremely small. If the welding speed is slightly improperly controlled, "false welding" of the weld will inevitably occur.

(6) The outer fillet on the extrusion roller is large, so that the tube billet is not filled enough in the extrusion roller, the edge contact state changes, from parallel to "V" shape, and the inner weld is not welded.

(7) The extrusion roller shaft is subjected to long-term stress and wear, and the base bearing is worn, and the two shafts form an elevation angle, resulting in insufficient extrusion force, vertical ellipse and severe corner joints.

3. Unreasonable selection of process parameters

The process parameters of high-frequency welded pipe production include welding speed (unit speed), welding temperature (high-frequency power), welding current (high-frequency frequency), extrusion force (grinding tool design and material), opening angle (grinding tool design and material , induction coil position), inductor (coil material, winding direction, position) and the size and placement position of the resistor, etc.

(1) High-frequency (stable and continuous) power, welding speed, welding extrusion force and opening angle are the most important process parameters, which must be reasonably matched, otherwise the quality of the weld will be affected.

① If the speed is too high or too low, it will cause low-temperature weld impermeability and high-temperature overheating, and the weld will crack after being flattened.

② When the extrusion force is insufficient, the metals at the edges to be welded cannot be fully pressed, and the impurities remaining in the weld are not easy to discharge, and the strength of the weld is reduced. When the extrusion force is too large, the metal flow angle increases, the residue is easy to discharge, the heat-affected zone is narrowed, and the welding quality is improved. However, if the pressure is too high, it will cause a large spark splash, so that the molten oxide and part of the metal in the plastic layer will be squeezed out together, and the weld seam will be thinned after scratching, thereby reducing the strength of the weld seam and cracking. Appropriate extrusion force is an important prerequisite for weld quality.

③ If the opening angle is too large, the high-frequency proximity effect decreases, the eddy current loss increases, and the welding temperature decreases. If the original speed welding is maintained, cracks will be generated. If the opening angle is too small, the welding current will be unstable, and a small blast will occur at the extrusion point (intuitively, it is a discharge phenomenon), forming cracks.

(2) The inductor (coil) is the main part of the welding part of the high-frequency welded pipe, and the gap between it and the tube blank and the opening width have a great influence on the quality of the weld.

① The gap between the sensor and the tube billet is too large, so that the efficiency of the sensor drops sharply. If the gap between the inductor and the tube blank is too small, it is easy to cause discharge between the inductor and the tube blank, resulting in welding cracks, and it is also easy to be damaged by the tube blank.

② The opening width of the sensor is too large, which reduces the welding temperature of the butt joint edge of the tube blank. In the case of fast welding speed, it is easy to produce false welding and crack after straightening.

In the production of high-frequency welded pipes, there are many factors that cause weld cracking, and the prevention methods are also different. There are too many variables in the high-frequency welding process, and any link defect will eventually affect the quality of the weld. The above are some common factors.