In high-frequency longitudinally welded pipes(ERW steel pipe), the manifestations of cracks include long cracks, local periodic cracks and irregular intermittent cracks. There are also some steel pipes that have no cracks on the surface after welding, but cracks will appear after flattening, straightening or water pressure testing. 

Causes of cracks

1. Poor quality of raw materials
In the production of welded pipes, there are usually large burrs and excessive raw material width problems.
If the burr is outward during the welding process, it is easy to produce continuous and long intermittent cracks.
The width of the raw material is too wide, the squeeze roll hole is over-filled, forming a welded peach shape, the external welding marks are large, the internal welding is small or not, and it will crack after straightening.

2. Edge corner joint state
The corner connection state of the edge of the tube blank is a common phenomenon in the production of welded tubes. The smaller the pipe diameter, the more severe the corner joint.
Inadequate forming adjustment is a prerequisite for corner joints.
The improper design of the squeeze roller pass, the larger outer fillet and the elevation angle of the pressure roller are the key factors that affect the angle joint.
Single radius cannot eliminate corner joint problems caused by poor molding. Increase the squeezing force, otherwise the squeeze roller will wear out and become elliptical in the later stage of production, which will aggravate the sharp peach-shaped welding state and cause serious corner connection.

The corner joint will cause most of the metal to flow out of the upper side, forming an unstable melting process. At this time, there will be a lot of metal splashing, the welding seam will be overheated, and the external burrs will become hot, irregular, large and not easy to scratch. If the welding speed is not properly controlled, "false welding" of the weld will inevitably occur.

The outer angle of the squeeze roller is large, so that the tube blank is not fully filled in the squeeze roller, and the edge contact state changes from parallel to "V" shape, and the phenomenon that the internal welding seam is not welded appears.

The squeeze roller is worn for a long time, and the base bearing is worn. The two shafts form an elevation angle, resulting in insufficient squeezing force, vertical ellipse and severe angle engagement.

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) of the tool Design and material, position of induction coil), inductor (material of coil, winding direction, position) and size and position of resistance.

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

①If the speed is too high or too low, it will cause low-temperature welding impermeability and high-temperature overburning, and the weld will crack after being flattened.
②When the squeezing force is insufficient, the edge metal to be welded cannot be completely pressed together, the impurities remaining in the weld are not easily discharged, and the strength is reduced.
When the extrusion force is too large, the metal flow angle increases, the residue is easily discharged, the heat-affected zone becomes narrow, and the welding quality is improved. However, if the pressure is too high, it will cause larger sparks and splashes, causing the molten oxide and part of the metal plastic layer to be extruded, and the weld will become thinner after being scratched, thereby reducing the strength of the weld.
Proper extrusion force is an important prerequisite to ensure welding quality.
③The opening angle is too large, which reduces the high-frequency proximity effect, increases the eddy current loss, and reduces the welding temperature. If welding at the original speed, cracks will appear;

If the opening angle is too small, the welding current will be unstable, and a small explosion (intuitively a discharge phenomenon) and cracks will occur at the squeezing point.

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

① The gap between the inductor and the tube blank is too large, resulting in a sharp drop in inductor efficiency;
If the gap between the inductor and the tube blank is too small, it is easy to generate electric discharge between the inductor and the tube blank, causing welding cracks, and it is also easy to be damaged by the tube blank.
② If the opening width of the inductor is too large, it will reduce the welding temperature of the butt edge of the tube blank. If the welding speed is fast, false welding and cracks are likely to occur after straightening.

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