Basic functions of welded pipe sizing

The sizing of high frequency straight welded pipe refers to rolling the welded pipe through the special pass roller, and adjusting the irregular size and shape of circular or special-shaped pipe to the finished pipe with regular shape and size meeting the standard requirements. The basic function of sizing process has four aspects.

1.Determine the basic size and shape of welded pipe

1.From circle to circle, the irregular pipe with undetermined diameter after the extrusion roll is adjusted to the finished pipe with qualified cross-section shape and size by adjusting the sizing round hole roll. The roundness of the measured pipe depends not only on the distribution of the actual tolerance zone, but also on the ovality of the pipe. The ovality is generally 80% of the limit deviation. In practice, although some welded pipes are not out of tolerance, but also on the ovality However, it exceeds the ovality tolerance or tolerance zone and approaches the limit value, which also needs to be adjusted.

2.round square, from round pipe to special-shaped pipe, through adjusting the special-shaped hole roller, the welded pipe with round cross section after extrusion roller is adjusted to special-shaped pipe with different cross-section shape and different size. In fact, no matter how complex the special-shaped pipe is, the adjustment process is carried out around the surface, angle and shape tolerance.

3.Square to square, in the process of direct forming, for the special-shaped pipe whose size and shape behind the extrusion roller do not meet the standard requirements, the shape and size tolerance can meet the requirements by adjusting the special-shaped roller.

2、Stress reduction

After forming, welding and cooling, the welded pipe becomes the welded pipe of undetermined diameter. In this pipe body, a large amount of longitudinal residual stress and transverse residual stress are accumulated. If the residual stress in the pipe is reduced without the sizing roll rolling, the bending of the welded pipe can not be carried out normally because of the stress. The bending of the welded pipe itself indicates that there are some defects in the pipe The longitudinal residual stress of inclination.

The mechanism of longitudinal residual stress reduction is that the welded pipe of undetermined diameter always warps along the weld seam. When the warped welded pipe is rolled by several straight flat and vertical sizing rolls, the welded pipe obtains an upward rolling force, and the warped weld seam changes from concave arc to straight line, and then elongates, which increases the tensile stress of the weld seam and reduces the compressive stress of the weld seam, thus reducing the vector generation of the residual stress of the weld seam At the same time, the concave arc at the back of the weld is straightened, which increases the compressive stress at the back of the weld and reduces the residual tensile stress at this part. The longitudinal residual stress of the welded pipe tends to be basically balanced in this increase and decrease. In this way, the longitudinal residual stress represented by the weld position and the back of the weld is very small, and the welded pipe after leaving the sizing roller is straight The same is true for the right bend.

3、Achieve basic straightness

In the production practice of welded pipe, there are two understandings of straightness: one is the straightness stipulated internationally, which is not more than 2 ‰ for round pipe and not more than 3 ‰ for special-shaped pipe, and the other is the serviceability straightness. The index requirements are agreed by the supplier and the demander. The former is applicable to "market goods" and the user is not fixed, while the latter is applicable to specific users who put forward requirements. No matter what kind of straightness is, it can only be rolled by sizing roll Only in this way can the internal stress of the pipe be balanced and the basic straightness of the welded pipe be realized.

4、Improving the surface quality of welded pipe

Make the weld smooth. After deburring, the weld surface is always connected with the outer circle of the welded pipe instead of being tangent. There are edges and corners when they are connected. The edges and corners can always be felt by hand at the weld of the pipe surface, which is not beautiful. After several sizing rolls are rolled, the edges and corners of the weld surface and the pipe surface can be removed to achieve smooth.

To reduce the surface indentation and scratch, from tube forming to welding, there are 20 or 30 rolls (more roll forming) rolling and high-temperature welding, no matter which link may leave scars and marks on the surface of welded pipe. However, after the sizing roll rolling, some scars and marks will become shallow and have no handle. To prevent the sizing section from producing scars, careful adjustment is required The symmetry of sizing pass and the correct application of rolling force ensure that the surface of welded pipe has no surface defects such as indentation and scratch.