ME 318
Manufacturing Techniques
Rolling
Objective
To perform rolling process on an lead bar in order to observe the change in both the crosssectional area and the general shape.
Theory
1. Definition
Flat rolling or Rolling is defined as the reduction of the cross-sectional area of the metal
stock, or the general shaping of the metal products, through the use of the rotating rolls [1].
It allows a high degree of closed-loop automation and very high speeds, and is thus capable
of providing high-quality, close tolerance starting material for various secondary sheet
metal working processes at a low cost [1].
2. Schematic Drawing of Rolling Process
Figure 1. Rolling Process [2]
The rolls rotate as illustrated in Figure 1. to pull and simultaneously squeeze the work
between them. The basic process shown in Figure 1 is flat rolling, used to reduce the
thickness of a rectangular cross section.
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Figure 2. Various configurations of rolling mills: (a) two high, (b) three high, (c) four high,
(d) cluster mill, and (e) tandem rolling mill [2].
Various rolling mill configurations are available to deal with the variety of applications
and technical problems in the rolling process. The basic rolling mill consists of two
opposite rotating rolls and is referred to as a two-high rolling mill (Figure 2a). In the threehigh configuration Figure 2(b), there are three rolls in a vertical column, and the direction
of rotation of each roll remains unchanged. To achieve a series of reductions, the work can
be a passed through from either side by raising or lowering the strip after each pass. The
equipment in a three-high rolling mill becomes more complicated, because an elevator
mechanism is needed to raise and lower the work [2].
Roll-work contact length is reduced with a lower roll radius, and this lads to lower forces,
torque, and power. The four-high rolling mill uses two smaller diameter rolls to contact the
work and two backing rolls behind them. Another roll configuration that allows smaller
working rolls against the work is the cluster rolling mill.
To achieve higher throughput rates in standard products, a tandem rolling mill is often
used. This configuration consists of a series of rolling stands. With each rolling step, work
velocity increases, and the problem of synchronizing the roll speeds at each stand is
significant [2].
3. General Overview of Process
The primary objectives of the flat rolling process are to reduce the cross-section of the
incoming material while improving its properties and to obtain the desired section at the
exit from the rolls. The process can be carried out hot, warm, or cold, depending on the
application and the material involved. The rolled products are flat plates and sheets.
Rolling of blooms, slabs, billets, and plates is usually done at temperatures above the
recrystallization temperature (hot rolling). Sheet and strip often are rolled cold in order to
maintain close thickness tolerances.
Basically flat rolling consists of passing metal between two rolls that revolve in opposite
directions, the space between the rolls being somewhat less than the thickness of the
entering metal. Because the rolls rotate with a surface velocity exceeding the speed of the
incoming metal, friction along the contact interface acts to propel the metal forward. The
metal is squeezed and elongated and usually changed in cross section. The amount of
deformation that can be achieved in a single pass between a given pair of rolls depend on
the friction conditions along the interface. If too much is demanded, the rolls will simply
skid over stationery metal. Too little deformation per pass results in excessive cost.
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Rolling involves high complexity of metal flow during the process. From this point of
view, rolling can be divided into the following categories [3]:
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Uniform reduction in thickness with no change in width: Here, the deformation is
in plane strain, that is, in the directions of rolling and sheet thickness. This type
occurs in rolling of strip, sheet, or foil.
Uniform reduction in thickness with an increase in width: Here, the material is
elongated in the rolling direction, is spread in the width direction, and is
compressed uniformly in the thickness direction. This type occurs in the rolling of
blooms, slabs, and thick plates.
Moderately non-uniform reduction in cross section: Here, the metal is elongated in
the rolling direction, is spread in the width direction, and is reduced non-uniformly
in the thickness direction.
Highly non-uniform reduction in cross section: Here, the reduction in the thickness
direction is highly non-uniform. A portion of the rolled section is reduced in
thickness while other portions may be extruded or increased in thickness. As a
result, in the width direction metal flow may be toward the center [3].
Hot Rolling
The distinctive mark of hot rolling is not a crystallized structure, but the simultaneous
occurrence of dislocation propagation and softening processes, with or without
recrystallization during rolling. The dominant mechanism depends on temperature and
grain size. In general, the recrystallized structure becomes finer with lower deformation
temperature and faster cooling rates and material of superior properties are obtained by
controlling the finishing temperature [1].
Hot rolling offers several advantages [1]:
1) Flow stresses are low, hence forces and power requirements are relatively low, and
even very large workpieces can be deformed with equipment of reasonable size.
2) Ductility is high; hence large deformations can be taken (in excess of 99%
reduction).
3) Complex part shapes can be generated.
The upper limit for hot rolling is determined by the temperature at which either melting or
excessive oxidation occurs. Generally, the maximum working temperature is limited to
50°C below the melting temperature. This is to allow the possibility of segregated regions
of lower melting material [4].
Cold Rolling
Cold rolling, in the everyday sense, means rolling at room temperature, although the work
of deformation can raise temperatures to 100-200°C. Cold rolling usually follows hot
rolling. A material subjected to cold rolling strain hardness considerably. Dislocation
density increases, and when a tension test is performed on this strain-hardened material, a
higher stress will be needed to initiate and maintain plastic deformation; thus, the yield
stress increases. However, the ductility of the material – as expressed by total elongation
and reduction of area – drops because of the higher initial dislocation density. Similarly,
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ME 318
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strength coefficient rises and strain-hardening exponent drops. Crystals (grains) become
elongated in the direction of major deformation [1].
Cold rolling has several advantages [1]:
1) In the absence of cooling and oxidation, tighter tolerances and better surface finish
can be obtained.
2) Thinner walls are possible.
3) The final properties of the workpiece can be closely controlled and, if desired, the
high strength obtained during cold rolling can be retained or, if high ductility is
needed, grain size can be controlled before annealing.
4) Lubrication is, in general, easier.
Rolling Problems and Defects
The main problem during rolling process is the calibration of rollers. This calibration faults
may occur in case of used bearings and may affect the thickness of parts. A simple
classification is as here below:
a. Lengthwise Occurring Defects
Change of rollers speed
Material temperature
Roller temperature
Inlet thickness
Material properties
Eccentric and conical rollers
Used bearings
b. Transversally Occurring Defects
Parallel position of rollers
Surface geometry of rollers
References
[1] Shey, John A., Introduction to Manufacturing Processes, 2nd Edition, McGraw-Hill
Book Company, New York, 1987.
[2] Groover M.P., Fundamentals of Modern Manufacturing, John-Wiley and Sons, New
York, 1999.
[3] Rolling Process, http://www.cemr.wvu.edu.
[4] Dieter, G.E., Mechanical Metallurgy, SI Metric Edition, McGraw-Hill Book Company,
London, 1988.
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