Profit Pointer for the Analysis of Skelp Edge Buckling in Welded Tube and Pipe Roll Forming Processes

By Baicheng Wen, Roll-Kraft, Inc. Ohio, U.S.A.
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Abstract

The following is an analysis, using the finite element method, of skelp edge buckling in welded tube and pipe roll forming processes. Experience has shown that skelp edge buckling can occur during the formation of thin wall tube and pipe (large d/t ratios). As a result, the tube or pipe is poorly welded and the product is rejected. This analysis will help tube and pipe manufacturers understand the causes of edge buckling and to eliminate and prevent its occurrence.

I. Introduction

An increased demand for a wider variety of sizes and shapes in tube and pipe products, along with stringent requirements for high quality, characterizes todays tube and pipe making industry. In order to be competitive, it is important that manufacturers use high quality tooling, in conjunction with scientific management and operation techniques in the production process. These advanced designs and techniques are based on a thorough understanding of material deformation during the forming process. By applying this knowledge to tooling design and manufacturing procedures, many forming problems can be eliminated, resulting in the production of high quality tube and pipe product. Two common forming problems are skelp, or strip, edge buckling and surface marking. They often occur when forming tube and pipe with large diameter-to-thickness (d/t) ratios and using high strength/high stiffness materials, such as stainless steel and titanium. Skelp edge buckling often causes poor quality welds that lead to degrading, or scraping, of the finished tube or pipe. In some cases, this problem may become so severe that an unscheduled mill shutdown is required to remove the buckled product and readjust the mill. Many times, operators resort to a trial-and-error method of mill adjustment to correct skelp edge buckling problems. Due to the number of parameters that can be adjusted, this method is very inefficient and cannot guarantee success. By using the finite element method, which predicts the behavior of a material during the forming process, the causes of edge buckling are understood. This understanding has lead to the development of several solutions to eliminate it. These solutions can be used by both mill operators and design engineers. The operator will have a better insight into mill adjustments and set-ups that can be used to correct buckling problems. The engineer can use this information to design advanced tooling and operating techniques to prevent it.

II. Backround Information

Tube and pipe roll forming is a shaping process. A length of skelp, or metal strip, is formed, or shaped, transversely (width-wise) by a series of forming rolls with progressively different profiles. Several variables affect this forming process. Among these is mill layout, especially roll stand elevation. The roll stand elevations control the height of the skelp centerline trace, or forming curve, as it travels through the mill. The two most commonly used forming curves are level forming and downhill forming. Level forming utilizes equal roll stand elevations and produces a horizontal centerline trace as shown in FIGURE 1. Through most of the forming process, the length of the centerline trace and the length of the edge trace (the outside edges of the skelp) are not equal. This is the result of material stretching in the first forming stage (breakdown section) and material compression in the second forming stage (fin pass section). These two lengths, however, must be equal when the tube leaves the mill because the final product is straight. The length difference between the centerline trace and edge trace in level forming can cause production problems. For example, as the skelp passes through the fin section, the resultant longitudinal compressive stresses at the skelp edge can lead to edge buckling. The lengths of the edge and centerline traces can be equalized through the entire forming process by changing the mill set-up. To do this, curvature is introduced to the path of the centerline, increasing its length to match that of the edge trace. This concept is known as downhill forming.

As shown in FIGURE 2, the centerline trace follows a downhill path as it passes through the forming stages. This set-up is often used in mills that produce large size tube and pipe. Skelp edge buckling is the result of material instability at the edge of the skelp caused by axial compressive strain. The instability forms a wave at the edge of the skelp that is not removed by sub-sequent rolls. This wave actually looks like buckle in the material. Mill set-up plays an important role in skelp edge buckling. As described above, axial deformation varies according to the type of forming curve used in the mill. Adjusting the forming curve through changes in roll stand elevation can eliminate or cause skelp edge buckling.

III. Skelp Edge Buckling in Downhill Forming

>Downhill forming is established by setting roll stand elevations at progressively lower heights through the early forming stages of the tube or pipe. As shown in FIGURE 3, a typical downhill forming curve is characterized by (1) a steep decrease in elevation through the breakdown section and (2) a shallow decrease in elevation through the fin pass section (in some cases, a slight increase in elevation). Due to the steep downhill path of the forming curve, the skelp is axially bent at an angle that is based on the roll stand elevations and the radii of the forming rolls. This axial bending creates axial compressive deformations in the edges of the skelp which can lead to edge buckling. Skelp edge buckling in downhill formed ERW pipe was analyzed using the finite element method
(Reference 1). The analysis revealed that the buckling initiated with the yielding of the material at the point of maximum compressive strain. Also, the buckling developed with the formation of a plastic hinge. As a result, skelp edge buckling can be predicted by determining the distribution of axial compressive strain at
the edges of the strip and its localization under increasing deformation. For a specific mill set-up, the amount of axial bending in the skelp segment between two rolls stands can be estimated. This is easily done by analyzing the downhill forming curve. This analysis will indicate the amount of lift required at the end of the skelp to feed it into the next pass. This lift is represented by D and is illustrated in FIGURE 4. By considering this segment of skelp as a cantilever beam with one end fixed and the other lifted an amount D, the axial deformation can be predicted.

As an example, the lift values, D, for each skelp segment from a 16" mill, were estimated based on mill set-up. The axial deformations in these segments produced by axial bending were analyzed using the finite element method. FIGURE 5 shows the predicted displacements and axial strain distributions for the
skelp segment between the first and second breakdown rolls with lift values, D, between 43.17 mm and 77.16 mm. The results show that a visible edge buckle occurs in the skelp segment for lift values, D, equal to or greater than 60.75 mm. This buckle was initiated when the end lift reached 51 mm. It can be seen that as the deformation localizes, it causes plastic instability and a large transverse deformation (buckling). The mode and location of the skelp edge buckling predicted by this model are very similar to the buckling observed in the forming process. The results of this analysis indicate that downhill forming of tube and pipe can generate compressive edge strains at certain locations. Excessive downhill forming (steep forming curves) produces large axial bending and high compressive deformation at the skelp edge which can lead to edge buckling.

IV. Skelp Edge Buckling in Level Forming

As mentioned, downhill forming can create large axial bending that often results in edge buckling. Although axial bending does not exist in level forming, skelp edge buckling can still occur. Obviously, the causes of skelp edge buckling in level forming are different than those that produce it downhill forming. In level forming, the skelp edge is stretched in the early forming stages (breakdown section) and compressed in later stages (fin pass section) in order to produce a straight tube or pipe. The compression of the skelp edges in the fin pass section can cause edge buckling. As an example, a mill producing a 2" OD/0.025" wall thickness high yield tube was analyzed to predict skelp buckling in a typical level forming set-up. The stretching (elongation) of the skelp edge in the breakdown section is estimated from the difference between the lengths of the skelp edge trace and centerline trace. By tracing the edge line and centerline from the feed roll to the first fin pass, the total length difference between the two, represented by d, can be given as:

where i is 0 to N and N is the number of passes before the first fin pass. di is the length difference between the skelp edge trace and the centerline trace between two roll stands as shown in FIGURE 6.

Assuming that the cross-sectional planes of the skelp remain in the plane and parallel to each other, and ignoring material springback, the length of the edge line trace, le, and the centerline trace, lc, can be estimated from the following geometric relationship:

where lcd is the center-to-center distance between two roll stands. Hx and Hy are illustrated in FIGURE 7 and can be determined from the skelp flowers at these two roll stands. In the fin pass section, the skelp edge is compressed to compensate for the stretching (elongation) in the breakdown section. This compression at the skelp edge can cause skelp edge buckling. To model this edge compression, a section of skelp between two fin passes is analyzed using the finite element method.

The model of this section, with its finite element mesh, is illustrated in FIGURE 8. Only half of the section is modeled due to symmetry. The coming end is
fixed to reflect constraints from the forming rolls at that pass. At the other end, constraints are also applied in all rotations except displacement. At this end, radial displacements are applied to model transverse bending produced by the skelp entering the roll. Axial displacements, equal to the elongation, d, are applied to model compression. The displacements predicted from this model are shown in FIGURE 9. It can be seen that the largest radial displacement (buckling) occurs near the middle of the section between the two passes; however, it is offset from the center due to the difference in curvature at each end the segment. The mode and location of the skelp edge buckling predicted by this model are very similar to the buckling observed in the forming process.

V. Discussion and Conclusion
The analysis of edge buckling in downhill forming has shown that excessive downhill forming will create large axial bending during the forming process. This axial bending produces compressive deformations at the skelp edges and a yielding of the material. The result is local plastic instability that is known as skelp edge buckling. In level forming, the skelp edges are unevenly formed through the entire forming process. Skelp edges are initially stretched, then compressed in subsequent forming stages. This compression can also result in skelp edge buckling. The analysis presented in Sections III and IV demonstrate skelp edge buckling is likely to occur as skelp thickness decreases. This is due to the fact that the skelps resistance to bending and compression decreases, and the axial strain in the material increases, as its thickness decreases. This is consistent with the actual forming process in which thinner wall tubes and pipes are prone to edge buckling. The use of the finite element method and simplified geometry appears to explain the mechanism that causes skelp edge buckling. However, modeling the local skelp geometry within the rolls can provide improved accuracy. Springback of the skelp between roll stands occurs as the elastic deformation is released. Springback changes the skelp cross-section and its stiffness and can influence the occurrence of edge buckling. The modeling of springback in the tube and pipe forming process is it requires the consideration of more parameters in the forming process than currently analyzed and is beyond the scope of this paper. In general, current tube and pipe tooling design practice only considers transverse deformation of the material as illustrated in a typical roll flower. The finite element method described in this paper models skelp edge buckling during the forming process and provides data about the axial deformation of the skelp. This additional information should assist the design engineer in developing advanced tooling that can handle material deformation in both axial and transverse directions. Also, it will help select the most appropriate forming curve.

VI. Recommendations

The analysis of downhill forming revealed that excessive downhill forming (a forming curve that is too steep) is a cause of skelp edge buckling. Downhill forming can be advantageous if it is properly developed. The forming curve must be smooth and continuous. Therefore, it is recommended that downhill forming be gradually introduced through a series of roll stands. The numerical model from Reference 1 can be used to predict the critical end lifting value, Dcr. This value
can be used to set the mill stand elevations or adjust the mill set-up to eliminate edge buckling problems. The elevation of each roll stand should be set to a height at which the lifting of the skelp end is less than the critical value, Dcr. Skelp edge buckling in level forming is caused by compressive deformation at the skelp edge that results from material stretching in the previous forming stages. One solution to this problem is utilizing a degree of downhill forming in the early forming stages. This will prevent the large elongations at the skelp and the resultant material compression in the next forming stage that causes the buckling. The geometric method described in Section IV can be used to estimate the amount of downhill (slope of the forming curve) required for each roll stand. The goal is to equalize the length of the edge trace and centerline trace. The following equation can be used to estimate the downhill amount, Hd:

This downhill value, Hd, can be used to set up the mill with a downhill forming curve that will help eliminate edge buckling. Modifications must be made to this
equation, however, to take into account two important factors: (1) as the center-to-center distance between two roll stands increases, the stretching (elongation) of the edge line and Hd decrease; (2) stretching (elongation) of the edge line and Hd increase as tube size increases. The modified equation is:

where Ds and Lcd are the tube size and center-to center distance between the two roll stands, respectively. Ä( ) is a function of Ds/ Lcd. Its expression changes as the type of tooling design (flower) changes. It should be noted that this equation cannot be used when Hy is close to zero, since Hd approaches infinity. This would correspond to a situation in which the edge stretching (elongation) cannot be reduced by using downhill forming in a particular section. This discussion recommends that the above equation be used to determine the amount of downhill required in the first few breakdown rolls to prevent edge buckling. Subsequent roll stand elevations should be set at heights to produce a smooth transition to a level forming curve (or one that is slightly uphill) in the fin section. Alternatively, a downhill forming curve can be set up by taking measurements directly from the mill. Simply measure the length of the edge line trace and centerline trace between two roll stands. Set the roll stand elevations to produce a downhill forming curve in the breakdown section in which the length of the edge line trace approaches that of the centerline trace.

Reference

[1] B. Wen, et. al., Modeling of Skelp Edge Instabilities in the Roll Forming of ERW Pipe. Journal of Materials Processing Technology,
Volume 41, pp. 425-446 (1994)