The welded pipe sizing of high frequency welded pipe equipment refers to rolling the welded pipe through a specific pass roll, and adjusting the round or special-shaped pipe with irregular size and shape to one with regular shape and size that meets the requirements of the standard. Finished tube.

The basic functions of the sizing process have four aspects: ① Determine the basic size and shape of the welded pipe. ②Reduce stress. ③ Reach the straightness of the entry. ④ Improve the surface quality of the welded pipe.

1.Determine the basic size and shape of the welded pipe

(1) Circle → Circle. Through the adjustment of the sizing round hole type roll, the irregular round pipe to be sized after the extrusion roll is adjusted into a finished round pipe with qualified cross-sectional shape and size. Measuring the roundness of a round tube depends not only on the distribution of the actual tolerance zone, but also on the ovality of the tube. It is generally specified that the ovality is 80% of the limit deviation. In practice, although some welded pipes are not out of tolerance, but they exceed the ovality tolerance, or the tolerance zone and are close to the limit value, they also need to be adjusted.

(2) Circle → Square (different). That is to say, from a round tube to a special-shaped tube, by adjusting the special-shaped hole-shaped roll, the welded tube with a round cross-section after the extrusion roll is adjusted to a special-shaped tube with different cross-sectional shapes and sizes, such as square tubes, Rectangular tube, oval tube, D-shaped tube, etc. In fact, no matter how complex the special-shaped tube, the adjustment process is nothing more than around the surface, angle, shape and tolerance.

①Surface: including plane and camber. It is required that there should be no waves, streaks and bamboo joints when viewed vertically. The camber must be smooth without edges and corners when viewed horizontally, and there should be no bumps on the plane.

②Angle: one refers to the shape, size and symmetry of the sharp corner at the intersection of the welded pipe surface and the surface, and the other refers to the angle between the welded pipe surface and the surface. Taking the rectangular tube as an example, if there is no special requirement, it is generally specified that the outer fillet r=1.5t, and the surface-to-surface angle β=90°±1°.

③Shape: Refers to the appearance of circular deformed welded pipes, such as square and rectangular pipes, which must look regular in shape, flat and sharp, with clear edges and corners, and are not allowed to appear rhombus, trapezoid, concave-convex, bending and twisting, etc.

④Tolerance: For square and rectangular tubes, including width, height, angle, straightness, diagonal, r angle, parallelism, weld position (if no special requirements), internal burr height and tube wall thickness, etc. All belong to the category that the sizing process needs to be controlled.

(3) Square → Square. In the direct forming process, for the special-shaped tubes whose size and shape do not meet the standard requirements after extrusion rolls, the shape and size tolerances can meet the requirements by adjusting the special-shaped rolls.

2.Reduce stress

The welded pipe produced by the high frequency welded pipe equipment will be formed, welded and cooled to become the welded pipe to be sized. In this pipe body, a large amount of longitudinal residual stress and transverse residual stress are accumulated. If part of the residual stress in the pipe is reduced without the shaping and rolling of the sizing roller, the welded pipe production can not be carried out normally just because of the bending caused by the stress.The bent welded pipe itself shows that there is a tendency for longitudinal residual stress in the pipe.

(1) Reduction mechanism of longitudinal residual stress. The welded pipe to be sizing is always upturned along the welding seam. When the upturned welded pipe is rolled by several flat and vertical sizing rollers arranged in a straight line, the welded pipe obtains an upward rolling force, so that the upturned welding seam part is rolled from the bottom. The concave arc becomes a straight line and is then elongated, which increases the tensile stress at the weld and reduces the compressive stress at the weld, thereby reducing the vector algebraic sum of the residual stress at the weld to achieve a basic balance; At the same time, the concave arc on the back of the weld is also straightened, compressed and shortened, which increases the compressive stress on 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 welding seam and the back of the welding seam is very small, and the welded pipe after the sizing roller is straight. The same is true for left and right bending.

(2) Reduction mechanism of transverse residual stress. There is a large amount of transverse residual tensile stress in the welded pipe to be sized. These transverse tensile stresses are not only caused by the welding and cooling process, but also left by the transverse deformation of the tube blank during the forming process, and the overall performance is tensile stress; full of transverse tensile stress to be determined After the diameter pipe is subjected to the radial rolling force exerted by the sizing roll, its circumference is slightly shortened, and the pipe wall obtains radial compressive stress, which offsets most of the transverse tensile stress in the welded pipe to be sizing. It is futile to try to completely eliminate the transverse tensile stress in the welded pipe through the sizing process, and there will be more or less residual transverse tensile stress in the cross section of the welded pipe after sizing. To completely eliminate the transverse tensile stress in the welded pipe as much as possible, it can be accomplished by the subsequent heat treatment process, which will not be repeated here.

As a proof, when producing high-strength pipes such as 16Mn or Q345 welded pipes, if the welding process is slightly improper, the welded pipes will automatically burst during the sizing process or just after leaving the sizing roll gap; and like Q195 welded pipes, Bursting only occurs when the welding process is seriously inappropriate. More often, when a welded pipe with a large residual transverse tensile stress is used as a conveying pipe, the internal pressure of the pipe and the residual transverse tensile stress are superimposed, resulting in the welded pipe sometimes bursting at a very low pressure.

3. To achieve basic straightness

In the practice of producing welded pipes by high frequency welded pipe equipment, there are two understandings of straightness. One is the straightness stipulated by the international regulations, the round tube is not more than 2‰, and the special-shaped tube is not more than 3‰; the other is the usability straightness, and the index requirements are negotiated by both parties. The former applies to "market goods" and the user is not fixed; the latter applies to specific users who make requests. No matter what kind of straightness, only the rolling of the sizing roller can balance the internal stress of the pipe and make the welded pipe reach the basic straightness.

4. improve the surface quality of welded pipes

The promotion effect of the sizing roller on the surface quality of the welded pipe is mainly manifested in three aspects:

(1) Promote the smoothness of the weld. After removing the outer burr, the weld surface and the outer circle of the welded pipe are always connected rather than tangent, and there are edges and corners when they are connected; the edges and corners can always be seen and felt by hand at the weld on the pipe surface, which is extremely unsightly. Only after several passes of sizing roll rolling can the edges and corners of the weld surface and the pipe surface be eliminated to achieve smoothness.

(2) Reduce surface indentation and scratches. From the forming of the tube blank to the completion of the welding, it is necessary to go through the rolling and high-temperature welding of 20 to 30 rollers (more rows of rollers are formed), and any link may leave scars and imprints on the surface of the welded tube. After rolling with sizing rolls, some of the scars and marks will become lighter and become untouched.

(3) Prevent the sizing section itself from producing scars. It is required to carefully adjust the symmetry of the sizing pass, apply the rolling force correctly, and ensure that the surface of the welded pipe is free from surface defects such as indentation and scratches.

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