By Baicheng Wen, Ph.D. Roll-Kraft, Inc. Ohio, U.S.A.
Today’s tube and pipe industry is characterized by customer demands for improved overall product quality. The advanced tooling and operating procedures required to make these high quality products are based on a thorough understanding of material deformation in the forming process. Therefore, in order to compete in this market, tube and pipe manufacturers need to understand this process and how it affects the quality of the final product. The application of this knowledge will eliminate forming problems and result in the production of tube and pipe products that meet customer specifications. The most common forming problems are strip edge buckling and surface marking, especially when making tube and pipe with large diameter-to-thickness (d/t) ratios and high strength/high stiffness materials such as stainless steel and titanium. Due to the characteristics of the roll forming process, surface marking on tube and pipe is inevitable; however, the use of advanced tooling and scientifically designed operating procedures can minimize surface marking and improve cosmetic appearance. On the other hand, severe surface marking can result from the use of improperly designed tooling and mill operating procedures. Roll forming welded tube and pipe requires transverse bending of a flat strip of material into a circular shape. This is accomplished by moving the strip past a series of tools with different surface profiles (rolls). Each pair of rolls turns at a constant rotational speed, but the tangential speed along the roll surface varies due to the difference in rotation radii (See Figure 1). This difference in speeds creates a sliding friction between the strip and the forming rolls and is the basic cause of surface marking. Most tube and pipe mills employ level forming (bottom line mills). With this type of mill setup, the root diameters of all the driven passes in the breakdown, fin and sizing sections are considered driven diameters. At these locations, the tangential speed of each roll is, more or less, equal to the traveling speed of the strip. However, due to the difference in rotation radii, a maximum relative sliding between the strip and forming roll surface occurs in the area near the outside diameter of each roll. This produces sliding friction that can result in surface marking.
In order to minimize surface marking during the forming process, consider a tooling design that can reduce both the sliding friction and localized high compression forces that cause it.
3.1 TOOLING DESIGNS
3.1.1 FLOATING FLANGE
Floating flanges are often used in driven passes as a means of reducing the relative sliding between the strip and the forming roll surface. A typical floating flange configuration is shown in Figure 2. The unique feature of this design is the bearing-supported flanges that rotate independent of the roll; therefore, the rotational speed of the flanges is controlled by the speed of the strip, not the speed of the roll. As a result, the relative sliding between the roll and strip, as well as the sliding friction, decreases and surface marking is reduced or eliminated. This design is particularly beneficial when forming large tube and pipe sizes. Remember, as the tube or pipe size increases (assuming constant throat diameter), the outside diameter of the rolls increases, which, in turn, increases relative slip and friction. It is recommended that floating flange rolls be used in the breakdown and fin sections of the mill. Most of the work to form the material is done in these two sections and the greatest transverse bending (compression) forces are produced there.
In addition to floating flanges, some tube mills use a four roll-type design. This setup is used in the fin pass and sizing sections with idle side rolls. As with floating flange rolls, the rotational speed of the idle rolls is controlled by the speed of the strip. This reduces surface sliding and sliding friction on the idle rolls.
3.2 FLANGE ANGLE AND CONTOUR CLEARANCE
In addition to alternate tooling designs, modifications can be made to the tooling to reduce surface marking. The most common alteration is machining a small flange angle, or contour clearance, on the roll surface near the flange. This clearance reduces surface compression and the resultant friction force, thereby minimizing marking. The amount of clearance required depends on the material being formed, the tube size and the mill configuration. In most cases, the visual-geometric method is used. It is based on the designed flowers at each pass in the forming process. The optimal angle, or clearance, allows the strip to be cleared out from the roll surface when strip enters the next forming pass. This is illustrated in Figure 3. This illustration shows flowers at three consecutive passes (two driven passes and one side roll pass) and the outline of the side roll. The incoming strip touches the bottom flange of the side roll first. This is an area where sliding friction is concentrated and surface marking can occur. To reduce this contact and compression of the strip, a flange angle, or clearance, is added to increase the gap between the strip and roll surface. Most flange angles are small and do not affect the forming of the tube or pipe.
As mentioned in Section 2, friction force also plays a role in surface marking. The amplitude of this force is determined by (1) the amount of surface compression and (2) the coefficient of friction. The coefficient of friction is material-dependent and is based on the relative movement between the strip and the roll. Table 1 lists some representative friction coefficients. Notice that there is an order of magnitude (10x) difference in values between the coefficient of steel-on-steel and steel-on-plastic (for example, Teflon®). This suggests that surface marking should not occur if plastic tube is being made with steel tooling and vice versa. Experience has shown this to be true. Selecting a tool material with a lower coefficient of friction than that being used will reduce surface friction and, therefore, surface marking. Although the difference in coefficient values for various steel alloy son steel alloys may seem small, their effect on surface marking is noticeable. Table 2 lists coefficients of friction for several pairs of different materials. Notice that the friction coefficients of tool steel-on-stainless steel is greater than that of tool steel-on-mild steel. This suggests that forming stainless steel tube with tool steel rolls is more likely to cause surface marking than forming that tube with mild steel rolls. Likewise, forming mild steel tube with bronze tooling should produce less surface marking than forming that tube with tool steel rolls.
If possible, lubricants should be used during tube and pipe forming processes. Two major benefits are realized from using lubricants: (1) Tool life is extended and (2) the tube and pipe surface is protected from excessive marking and scratches. Lubricants reduce the friction coefficient between the tooling and the materials being formed, thus minimizing surface marking. To optimize the performance of a lubricant, choose one that is compatible with the material being formed to prevent surface staining and properly maintain the circulation system. A large number of materials, with a variety of finishes, are used today to produce tube and pipe products. Similarly, many different lubricants have been developed for use in tube and pipe mills. Careful research should be done to select the correct lubricant for the material being formed. The use of a suitable lubricant will increase productivity and also meet any applicable environmental regulations. Lubricants are not without their drawbacks. The two most common are “solid build-up” and “metal pick-up”. Solid buildup is the result of two phenomenons. The first is scavenging of foreign particles, such as scale and oxide that are generated at the surface of the materials during the forming process. The second is the precipitation of solids originally dissolved in the lubricant. These products can cause interference between the tube or pipe and the forming tools, resulting in surface marking. When most materials are formed, tiny metal particles are generated that break away from the surface. These are picked up and carried along by the lubricant stream. This is known as metal pick-up. Another source of metal particles are burrs on the edge of strip that result from the slitting process. While most of these burrs are too small to see, they break off during forming and are also picked up by the lubricant. These small particles become mixed with the lubricant, which contains other impurities, such as gear lubricant and roller bearing grease. This contaminated lubricant becomes trapped between the tool rolls and tube or pipe, causing a build-up of fine metal particles on the tooling. This buildup can cause surface marking. A properly maintained lubricating system can prevent both of these problems from occurring.
The causes of surface marking can be something simple, such as excess surface friction or a difference in tangential speed along the surface of the roll as discussed in Section 2.0. However, eliminating the surface marking should involve an analysis of the entire forming process. This includes tooling design, mill set-up and operation of the lubricating system. The result will be in tube and pipe products of the highest quality. Mill set-up plays an important role in tube and pipe roll forming. An incorrectly set mill can cause surface marking. For instance, a highly set side roll (higher than the designed metal line) can mark the tube along its bottom side. When marking does occur, as the above example shows, a check of all mill set-up parameters is highly recommended. Surface marking can also occur in the sizing sections or in an on-line reshaping section of the mill. In this case, the marking is located at the corner or near the tooling flange area. A different mechanism than those previously discussed is more than likely the cause. One of two possibilities exist: (1) the sizing or reshaping tooling was incorrectly designed, or (2) an excess amount of tubing material is being fed into the sizing section from the previous forming section. The only solution is to modify the tooling. It is important that the modified tooling allow the strip to flow smoothly through the sizing and reshaping sections and provide a reasonable amount of material for on-line reshape. In certain applications, such as some HSLA steel structural tube and pipe, surface appearance is not as important as other properties of the finished product. In order to reduce production costs, additional efforts to eliminate surface marking are not necessary as long as the finished tube or pipe is within specifications. Finally, there are several surface process methods which can be used to form a hard, wear resistant layer to the surface of tube and pipe rolls. They include chemical vapor deposition (CVD), physical vapor deposition (PVD) and thermal diffusion (TD). The coating materials are usually titanium nitride or titanium carbide. These coatings increase initial tooling wear resistance and reduce friction coefficients considerably; however, tooling distortion during application and rework difficulties severely limit their use with tube and pipe rolls. These surface processes become viable options only when surface marking is severe and the surface finish is absolutely critical to the final product.
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