Profit Pointer for Selecting Advanced Tooling for High Quality Tube and Pipe
By Baicheng Wen, Ph.D. Roll-Kraft, Inc. Ohio U.S.A.
The tube and pipe industry faces many challenges in today’s marketplace. First of all, end users are requesting tube products in a wider variety of shapes and sizes. Secondly, applications that require special materials are becoming more common. And finally, improved product quality is being demanded from every manufacturer. To meet these challenges, it is necessary to use high quality tooling and advanced manufacturing techniques. This is especially true for producing tube products for special applications. These special applications include (1) the use of high strength materials such as stainless steel, titanium and high strength steel, (2) forming thin wall tube and pipe (high d/t ratios) and (3) reshaping round tube into various shapes and sizes. The design of advanced tooling and the evolution of production techniques are based upon an understanding of material deformation during the forming process. With the aid of computer-based numerical analysis tools, such as the finite element method, tooling engineers can predict the behavior of a metal as it is formed. From this information, tube and forming rolls can be designed, and mill setup modified, to minimize or totally eliminate forming problems during production. Advanced design methods and tooling are necessary in order to address several problems that are inherent with manufacturing tube and pipe for special applications. The most significant of these is material springback. This is especially true when using high strength materials. Strip edge buckling is common when forming thin wall tube. To eliminate it, axial compressive deformation in the strip must be minimized. Finally, maintaining accurate cross-sectional geometry and material properties is critical when reshaping tube and pipe. By using advanced designs, the tooling engineer is able to overcome these problems to ensure the production of high quality tube and pipe.
II. Tube and Pipe Tooling Designs
Several factors play a major role in tooling design. These include the material to be formed, the mill setup used in the forming process and the mill configuration. When designing tooling for a specific application, these elements must betaken into account. (Note: A detailed tooling analysis can be found in Reference 1.) Of particular importance to the tooling engineer is the type of material that is being formed. This has a significant impact on the design of the breakdown and fin section rolls where most of the strip is formed. The four most popular designs used in the breakdown section are (1) conventional, (2) standard edgeform, (3) versatile edgeform and (4) "W", or reverse bending. Versatile edgeform and reverse bending breakdown sections are recommended when forming high strength materials. Typical strip flowers for these two designs are shown in Figures 1a and 1b, respectively. A large reduction in material springback and a better presentation of the strip edges into the fin pass are two major advantages to using versatile edgeform and reverse bending breakdown sections. Of these two designs, versatile edgeform is preferred for several reasons. First, this design forms more strip into the edgeform radius. Also, it reduces the possibility of kinking at the strip center that can be caused by reverse bending (see Figure 1b). Finally, strip marking is greatly reduced, resulting in an improved cosmetic appearance.
The primary function of the fin pass is to prepare the strip edges for welding. Four major designs are used. They are (1) round design, (2) flat oval design, (3) high oval design and (4) finless design. Of the four, the flat oval design is often recommended for high strength materials and high diameter-to-thickness ratio tubes. Several benefits are realized by using a flat oval fin pass. They include reduced material springback near the strip edges, elimination of the vertical peak at the bottom of strip that can be caused in the breakdown section and consistent, high quality welds. The major drawback to this design is the forming of a nonround tube when it enters the welder. As a result, more work is required in the sizing section. When a versatile edgeform breakdown section is utilized, a round design fin pass section can be used to avoid this problem. Since strip edges are adequately formed using this breakdown design, a round tube is produced in the fin section and less work is required in the sizing section. Accurate cross-sectional geometry is critical to reshaping round tubes into various tubular products. The first step in obtaining this geometry is calculating the circumferential shrinkage and the reduction in cross-sectional area required to produce the desired shape. This is done by evaluating the deformation and formability of the material. Next, the size of the round tube needed is established. Finally, the amount of work to be done in each pass is determined. It is important to remember that the strip is work hardened (material hardness and yield strength increase) as it passes through each stand. As a result, the tube becomes more difficult to form as it approaches its final shape. Therefore, it is suggested that most of the work be done in the first few passes in the reshaping section.
This problem can be avoided by changing the mill setup to make use of a downhill forming curve, as shown in Figure 2b. Using this setup, a downhill bottom line is obtained as the strip passes through the breakdown section. The difference in length between the edge lines and bottom line is reduced compared to that seen in level forming. As the strip passes through the fin section, the edge line length is equal to the bottom line length. This setup minimizes edge line stretching and axial compressive deformation, thus reducing strip edge buckling. The correct downhill forming curve should be calculated based on the type of material being formed and the size of the tube. It is important to avoid over downhill forming as it can cause axial bending which also leads to strip edge buckling. After the tooling and mill are designed, the mill is set up for production. Setup specifications can be found on a setup chart, such as the one shown in Figure 3. This chart lists the rolls, spacer length, side roll metal line, rim clearance, etc. for each pass, driven and idle, on the mill. Setting the mill to these specifications is essential to forming the material as designed.
IV. Tooling Maintenance
To consistently produce high quality tube and pipe products, it is essential that the tooling be properly maintained. Tooling should be measured for wear and proper setup on a regular basis and reworked as needed. It is strongly suggested that this information be recorded in a tooling maintenance file and used to predict rework and maintenance intervals. Also, other critical tooling information, such as replacement, rechroming and sidegrinding dates, should be recorded. This data can be used to readjust the mill to maintain the correct metal line, make shims for universal and lower side roll stands and new spacers for side ground rolls and calculate the speed for multiple drive mills. Finally, the information recorded in a properly organized maintenance file will ensure faster mill setup, longer tooling life and higher quality products. Tooling maintenance is discussed in detail in Reference 2.
V. The Continuation of Excellence
The final ingredient needed for producing high quality tube and pipe is a supplier with the expertise to manufacture the tooling you need. This is especially true for the advanced tooling needed to make tube and pipe for special applications. Such a supplier not only manufactures tooling, but can scientifically design it. Numerical methods that are based on advanced material deformation theory, such as finite element analysis, as well as computer aided design (CAD), are the foundation of their design process. Their design engineers are trained in metallurgy, material deformation mechanics, mechanical design and manufacturing processes. Additionally, they employ advanced manufacturing techniques and equipment, including CNC machinery, that ensures the production of quality, cost-effective tooling. Finally, they should offer a wide range of technical support that includes mill setup, tooling design, troubleshooting, tooling maintenance and rework services. A tooling supplier with these capabilities and range of services is a supplier you can trust to make the high quality, advanced tooling you need to manufacture pipe and tube to meet your customer’s needs.