Elimination of Scratches on the Surface of Welded Pipes

Causes of scratch on welded pipe surface

The welded pipe formed by roller requires the strip to be bent horizontally into a circle, which is realized by a series of rollers with different passes to deform the strip. Each pair of rollers rotates at a certain speed, but the linear speed of the roller surface varies with the rotation radius. The sliding friction between the strip and the forming roller caused by the difference of linear speed is the fundamental reason for the scratch on the welded pipe surface.

Most welded pipe mills adopt horizontal forming method. For this kind of mill, the driving roll bottom diameter of roughing roll, guide ring roll and sizing roll is considered as driving diameter. The linear speed of these roll bottom diameter is basically equal to the conveying speed of strip. Due to different rotation radius, the relative sliding friction between strip and roll surface is large at the roll edge of each roll, which leads to surface scratch.

Friction affects the surface quality, friction will produce scratches. With the increase of friction, the positive pressure on the strip will increase and the deformation of the strip will increase, resulting in the strain of the roll on the strip and surface scratches.

Reasonable roll design and processing

1.Use floating flange roller

In order to reduce the relative sliding between the strip and the roll surface, the active roll uses the moving flange roll, a typical moving flange roll structure. Its design is that the moving flange is supported by the bearing, and its speed depends on the conveying speed of the strip, which is different from the speed of the roll. Using this kind of roll can reduce the relative sliding between the strip and the roll surface, so as to reduce or even remove the welded pipe surface It's a scratch.

2.Four roll forming

In addition to the use of floating flange roll forming, some welded pipe units use four roll forming. Four roll forming is used for guide ring roll and sizing roll. At this time, the horizontal roll uses active roll and the vertical roll uses passive roll. The rotation speed of vertical roll depends on the delivery speed of strip, which is conducive to reducing the relative sliding and sliding friction between strip and vertical roll surface.

3.Escape angle and clearance

In addition to changing the roll design, the roll modification can also reduce the surface scratch of welded pipe. The common method is to process an appropriate escape angle or clearance at the roll edge, which can reduce the surface pressure of the strip at this position, so as to reduce the surface scratch. The processing clearance depends not only on the material, pipe diameter and unit characteristics of welded pipe, but also on the roll shape of each roll in the forming process The escape angle or gap requires that the strip can escape from the roll surface. For welded pipes with different uses, such as high strength low alloy steel structural pipes, the surface quality is not as high as that of welded pipes that need to be repaired. In order to reduce the production cost, it is not necessary to spend too much effort to eliminate the surface scratch (if the scratch of the roll is within the standard requirements).

The surface treatment process can form a hard wear-resistant layer on the roll surface. The wear-resistant layer includes physical coating, chemical coating and thermal diffusion layer. The coating material generally contains titanium nitride and titanium carbide. The coating can increase the wear resistance of the roll and reduce the friction coefficient. However, the application of the roll in this field is limited by the deformation of the roll and the difficulty of repair The surface treatment of the roll can be considered in the design.

The scratch on the surface of welded pipe may be caused by simple reasons such as large surface friction or linear velocity difference on the surface of roll, but it is troublesome to analyze and eliminate the scratch. The whole forming process, including the design of roll, the installation and adjustment of roll and the reasonable use of lubrication system, needs to be analyzed in order to produce high-quality products.