How to straighten the pipe after thermal expansion

Prerequisites for straightening: From the perspective of production practice, sometimes the straightening head cannot be straightened, and sometimes it can be straightened with a little adjustment, which shows that there are prerequisites for smooth straightening.

Prerequisites for straightening:
1. It is necessary to use a sizing machine. The sizing machine actually provides two (for the two-roller and four-roller straightening heads) fixed fulcrums for straightening, which match with the movable fulcrums of the straightening head, thus forming two straightening fulcrums in the same direction. The application point of the force and a force application point in the opposite direction act together on the pipe body to complete the straightening, so no matter it is two-roll, four-roll or eight-roll, it needs to be completed with the help of a sizing machine. Another meaning of adjusting with the help of a sizing machine is The magnitude and direction of the straightening force (forming rolling force) provided by the sizing machine need to be stable, which is why the straightness of the welded pipe will bend and flutter once the sizing machine is unstable.
2. The stress size and direction distribution on the welded pipe in the sizing machine need to be stable. The reason why the welded pipe can be straightened is that the straightening force applied by the straightening head and the direction of the straightening force and the internal stress of the welded pipe in the sizing machine achieve a short-term dynamic balance. As a result, once this balance is disturbed, such as changes in the thickness of the tube blank, changes in the flow of coolant, changes in welding heat, damage to the roll bearing, etc., the originally straight welded pipe will be bent immediately, and the force applied to the welded pipe needs to be changed by the straightening head. size and direction, and rebalance to restore the straightness of the welded pipe.
3. The position of the weld seam should be kept stable in the sizing machine. According to the welding principle and welding process, there is a welding heat affected zone at both ends of the weld seam. Since this area absorbs a large amount of heat energy during the welding process of the pipeline, it is During the cooling process of the welded pipe (after leaving the extrusion point), a large amount of shrinkage stress (compressive stress) will accumulate. This stress with the same magnitude and direction forces the welded pipe to bend to the side of the weld. The straightening force that is just right in both direction and direction is applied to a point that does not need the force (may not be so large, or may not be the direction of the force), thus disrupting the previous stress balance, turning the welded pipe from straight to straightening Before the head finds a new balance point, the welded pipe is always in a bent state. If the position of the weld seam is always unstable, the straightening head needs to constantly find a new balance point, and the welded pipe will continue to change between bending and straightness. Therefore, to If the straightness of the welded pipe is good, the position of the weld seam needs to be fixed first.