1 Misalignment defect of ERW pipe (lap welding)
The misalignment defect of ERW steel pipe, also called lap welding, refers to the misalignment of the two edges of the tube blank during welding. Even after removing the internal and external burrs, traces of misalignment can still be seen on the inner and outer surfaces of the steel pipe. The main hazard of the wrong side is to reduce the effective wall thickness of the steel pipe. According to the APISPEC5L standard, the wrong side of the electric welded steel pipe plus the residual burr shall not exceed 1. 5mm (0. 060 in). In addition, the wrong side will also affect the ultrasonic and X-ray inspection of steel pipes. During the use of the steel pipe, the misalignment will also become the starting point of the chemical corrosion of the steel pipe.
2 Causes of misalignment defects of ERW steel pipes
Misalignment caused by poor molding quality
ERW steel pipe needs to go through a series of rolls (rough forming, roll string, fine forming) to gradually roll the coil into a barrel shape, and the forming quality has a great influence on the welding quality. If the center of the steel strip is shifted during forming, it will cause the edge of the tube blank to be uneven and cause a misalignment. Usually, this misalignment is in one direction, which is easier to judge. Secondly, if the parameters of the forming roller are not set properly, before entering the extrusion roller, the height of the two edges of the strip steel changes: it is wavy, and after entering the extrusion roller, a small local misalignment occurs, and the direction of the misalignment is uncertain.
The effect of extrusion
The ERW steel pipe is welded by heating the edge of the strip and extruded by the extrusion roller. The extrusion force is one of the main parameters of the welding process and has a great influence on the quality of the ERW steel pipe. The extrusion force is small, the number of eutectics formed is small, and the weld strength is low. At the same time, non-metallic inclusions and formed oxide residues in the weld also affect the weld strength; if the extrusion force is too large, the metal that reaches the welding temperature will be All extruded, so that the strength of the weld is reduced. In addition, excessive extrusion force is easy to form a wrong side, and it is difficult for this kind of wrong side not to reduce the extrusion force.
Get adjusted. In general, the extrusion force is expressed and calculated by the extrusion amount of the circumference difference of the tube blank before and after the extrusion roller, and the extrusion amount is usually half of the wall thickness of the steel pipe.
The influence of rolls and tooling
(1) The influence of the roll
The rolls that affect the misalignment of ERW steel pipes are mainly fine-formed rolls and squeeze rolls. The wear of the fine-formed rolls is too large, the wear on both sides of the guide pieces of the fine-formed rolls is different, and the edge of the squeeze roll, especially the upper squeeze roll, is damaged. And the wear amount of the two squeeze rollers is not the same, all will produce wrong side, this kind of wrong side generally has a certain periodicity.
(2) Influence of roll bearing
Damage to the bearing of the extrusion roller causes the extrusion roller to not rotate or rotate unevenly, resulting in uneven extrusion force and misalignment. In addition, a large amount of bearing wear or a large bearing gap will also cause uneven rotation of the extrusion roller and cause misalignment.
(3) The influence of the installation quality of the roll tooling. If the precision forming roll and the side extrusion roll are not installed horizontally, the gap between the upper extrusion roll shaft and bearing and the gap between the shaft and the roll seat is too large, which will lead to the occurrence of misalignment.
3 Judgment and Confirmation of Wrong Side
The judgment and confirmation of the wrong side mainly depends on visual observation. When there is an obvious wrong side, it can be found after squeezing the roller. It can be judged from the following aspects.
(1) Observe the outer burr, the melt extruded by welding is not in the middle of the outer burr.
(2) The outer weld bead is not straight after the burr is removed, and the width and shape of the outer weld bead after scraping change.
(3) Observe the inner burrs, if there are intermittent splits in the inner burrs
Or cracking, there may be misalignment defects, as shown in Figure 3(b).
4. Adjustment and elimination of wrong side
When there is a wrong side, judge the cause of the wrong side according to the wrong side phenomenon, and take the following measures to adjust:
(1) After the strip steel enters the forming machine, it is necessary to ensure that the center of the strip steel does not deviate from the rolling center. When waves and bulges appear on the edge of the tube billet, reasonably adjust the fine forming diameter reduction and the reduction of the small row of rollers to reduce and eliminate the edge Waves and drum kits.
(2) Select reasonable process parameters, especially suitable welding extrusion force.
(3) Grind the rolls with excessive wear or different wear on both sides of the finely formed roll guide, and repair the squeeze rolls with edge damage.
(4) Check and replace bearings with large wear or large bearing clearance, and do a good job of lubricating the bearings.
(5) When changing lanes, ensure that the rolls are installed horizontally, and eliminate the excessive gap between the extrusion roll shaft and the bearing, as well as the shaft and the roll seat.
Misalignment is a common phenomenon in the production of ERW steel pipes, especially in the production of thin-walled pipes. When the wrong side occurs, firstly find out the reason of the wrong side, and then take corresponding measures to eliminate the wrong side. It should be noted that in normal production, due to the large bulge before and after the butt welding of raw materials, it is easy to produce misalignment when passing through the extrusion roller, and the misalignment can be eliminated by fine-tuning the upper extrusion roller.