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
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
Tooling Design Consideration
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.
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.
Tooling Material Options
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
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
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
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
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 roll 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
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 titaniumnitride or
titanium carbide. These coating 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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