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.

Friction forces also contribute to surface marking. They are generated by friction under high compression, according to the friction law stated below:

where is the coefficient of friction and P is normal compression. As the friction force increases, the amount of deformation in the strip also increases, causing the forming rolls to pick up material from the strip. The net result is 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.

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.

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 roll 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 vise versa. Experience has shown this to be true.

Selecting a tool material with a l ower coefficient of friction than that being used will reduce surface friction and, therefore, surface marking. Although the difference in coefficient values for various steel alloys-on-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 lubricant: (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 build-up 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 the 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 build-up can cause surface marking. A properly maintained lubricating system can prevent both of these problems from occurring.