The Basic Process of High-frequency Welded Pipe Sizing

The sizing of high-frequency welded pipes (ERW steel pipe) refers to the rolling of welded pipes after welding by rollers of a specific pass, and the round or special-shaped pipes of irregular size and shape are adjusted to finished pipes with regular shapes and sizes that meet the requirements of the standard. The basic functions of diameter technology have four aspects.

1) Determine the basic size and shape of the welded pipe
1. Circle→circle, by adjusting the adjustment of the sizing round pass roll, the irregular round pipe to be sizing after the squeeze roll is adjusted to a finished round pipe with a qualified cross-sectional shape and size. The roundness of the measurement pipe is not only determined The distribution of the actual tolerance zone also depends on the ovality of the pipeline. The ovality is generally 80% of the limit deviation. In practice, although some welded pipes have no over-tolerance, they exceed the ovality tolerance or tolerance zone and are close to the limit value. , This also needs to be adjusted.

2. Round→square, from a round tube to a special-shaped tube. By adjusting the special-shaped pass roll, the welded pipe with a round cross section after the squeeze roll is adjusted to a special-shaped pipe with different cross-sectional shapes and different sizes. In fact, no matter how complicated the special-shaped tube is, the adjustment process is carried out around the surface, angle, and shape tolerances.

3. Square→square, in the direct forming process, for the special-shaped tube whose size and shape after the squeeze roll does not meet the standard requirements, the shape and size tolerance can meet the requirements by adjusting the special-shaped roll.

2) Reduce stress

After the welded pipe is formed, welded and cooled, it becomes a welded pipe to be sized. In this pipe body, a large amount of longitudinal residual stress and transverse residual stress are accumulated. If it is not subjected to the shaping and rolling of the sizing roll, part of the residual stress in the pipe will be reduced. It is just that the bending caused by stress will make the production of welded pipe unable to proceed normally, and the bent welded pipe itself shows that there is a tendency of longitudinal residual stress in the pipe.

The reduction mechanism of the longitudinal residual stress, the sizing welded pipe will always be upturned along the weld seam. When the upturned welded pipe is rolled by several straightly arranged flat and vertical sizing rolls, the welded pipe will obtain an upward rolling force, and the welded pipe will be upturned. The weld is changed from a concave arc to a straight line, and then elongated, which increases the tensile stress of the weld, reduces the compressive stress of the weld, thereby reduces the vector algebraic sum of the residual stress of the weld, and achieves a basic balance. The concave arc on the back of the weld is also rolled straight, which increases the compressive stress on the back of the weld and reduces the residual tensile stress in this part. The longitudinal residual stress of the welded pipe tends to be basically balanced during this increase or decrease. In this way, the welding The longitudinal residual stress represented by the seam position and the back of the weld is very small, and the welded pipe after leaving the sizing roll is straight, and the same is true for left and right bending.

3) Achieve basic straightness

In the production practice of welded pipes, there are two understandings of straightness. One is the internationally stipulated straightness, which is no more than 2‰ for round pipes and no more than 3‰ for special-shaped pipes. The other is straightness for use. The index requirements are determined by both parties. It was agreed that the former is suitable for "market goods" and the user is not fixed, and the latter is suitable for specific users who have requested. No matter what kind of straightness, only the rolling of the sizing roll can balance the internal stress of the pipe and achieve the basic straightness of the welded pipe.

4) Improve the surface quality of welded pipes

Make the weld smooth, the weld surface after removing the external burr is always connected to the outer circle of the welded pipe instead of tangent, there are edges and corners when they are connected, and the edges and corners can always be felt by hand at the weld on the pipe surface. It 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 smoothness.

Reduce surface indentation and scratches. From tube forming to welding, rolling and high temperature welding with 20 to 30 rolls (more roll forming will be carried out) and high temperature welding are required. No matter which part of the process, it may leave scars on the surface of the welded tube. However, after the sizing roll is rolled, some scars and imprints will become lighter and have no feel. To prevent the sizing section itself from scratching, it is necessary to carefully adjust the symmetry of the sizing pass and apply the rolling force correctly to ensure There are no surface defects such as indentation and scratches on the surface of the welded pipe.

Tips: In case ASTM A53 Grade B in ERW (electric resistance welded) pipe, the weld seam shall be done the heat treatment with a minimum 1000°F [540°C]. In this way the no untempered martensite remains.
In case ASTM A53 B erw pipe in cold expanded, then expansion should not exceed 1.5% of the required OD.