What is Metal Rolling?
Metal Rolling is defined as a forming operation in which the metal is plastically deformed by passing it between the two or more opposite spinning rollers.
Metal rolling is a forming operation in which the thickness of the workpiece is reduced by the application of compressive forces.
Metal rolling is used in manufacturing of rails, section beams, thick plates for boilers, bridges, vessels, as well as thin sheets for car bodies, bushes, trains, and many other equipment.
Few important thing about metal rolling that you should know
- When the metal deforms during the rolling operation, it is subjected to a very high amount of compressive stresses due to the compression and friction between the two rollers and metal workpiece.
- The compressive forces between the rollers reduces the thickness of the metal and hence its grain structure is affected.
- During the metal rolling, the shape of the workpiece changes but its volume remains the same.
- As the volume of the metal remains same, the speed of the metal at the outlet is more as compared to that at the inlet.
- If there is no slip between the rollers and workpiece, then the surface velocity of the rollers and the velocity of material at the outlet will be same.
Types of Rolling
Various types of rolling used in industries are listed below (with images).
- Hot rolling
- Cold rolling
- Flat rolling
- Shape rolling
- Ring rolling
- Thread rolling
- Tube rolling or tube piercing
- Two high reversing rolling
- Two high non reversing rolling
- Three high rolling
- Four high rolling
- Cluster rolling
- Planetary rolling
- Tandem rolling
Let me explain each of these metal rollings with images.
#1) Hot rolling
Hot rolling is a metal rolling operation in which the metal is heated above its recrystallization temperature and its plastic deformation occurs when it is passed through the rollers.
Hot rolling of metals involves reheating of ingots, slabs, blooms or billets at the temperature range of 1200 °C to 1300 °C and then they are passed between two opposite spinning rollers.
The same heated metal piece is sometimes passed back and forth in the same roller with a roll gap being reduced gradually.
#2) Cold rolling
Cold rolling is a metal rolling operation which is carried out at (or near) the room temperature and the plastic deformation occurs when it is passed through the rollers.
In cold rolling operation, the temperature of the metal remains below its recrystallization temperature.
In cold rolling, the strength of the metals increases by around 20% due to strain hardening.
#3) Flat rolling
Flat rolling is a type of metal rolling operation in which the starting and ending material have a rectangular cross section.
The gap between the rollers is less than the thickness of the metal which is fed between the rollers.
This causes the metal to deform when it passes through the rollers.
#4) Shape rolling
Shape rolling is a type of metal rolling operation in which the forming of metal is done by using rollers of certain geometry.
The rollers are made of desired shapes which form the metal part into that specific shape.
By using shape rolling operations, cross sections like I-beam, U-channels, L-beams, rails, etc can be manufactured.
#5) Ring rolling
Ring rolling is a type of metal rolling in which the ring of smaller diameter is rolled into a precise ring of larger diameter with reduced cross section.
By using a steel ring rolling process, the flat steel rings as well as grooved steel rings or rings with different cross sections can also be manufactured.
#6) Thread rolling
Thread rolling is mostly used for manufacturing of screws, bolts and other fasteners that include threads.
The die surface has a shape of threads on it. These dies are rolled over the cylindrical metal surface to get the impression of the thread.
#7) Tube rolling or tube piercing
Tube piercing is a hot forming operation in which the long lengths of seamless tubes or pipes can be manufactured.
In order to shape the internal cavity of the tube, a mandrel is used.
#8) Two high reversing rolling
In two high reversing mills, the rotation of rollers occurs in forward direction as well as reverse direction.
Firstly, the rollers rotate in one direction and then they rotate in opposite directions so that the metal can pass several times in the back and forth direction through the rollers.
#9) Two high non reversing rolling
In two high non reversing mills, two rolls rotate only in a single direction.
As the name suggests, “non reversing” means that the reverse motion of the workpiece is not possible in this type of steel rolling mills.
However, the workpiece is carried back over to the inlet of the roller mill for passing it again through the rollers.
#10) Three high rolling
Three high rolling mills consist of three parallel rolls which are arranged one above the other (see below image).
As shown in the above diagram, the above two adjacent rollers rotate in opposite directions which forces the workpiece in the forward direction. Again this workpiece is fed between the middle and lower roller and finally the rolled workpiece is obtained.
#11) Four high rolling
Four high rolling mills consist of four rollers parallely arranged one above the other.
As shown in the diagram, two outermost rollers spin in opposite directions. Also the two inner rollers rotate in opposite directions.
The size of the inner rollers is kept smaller than the size of the outer rollers.
#12) Cluster rolling
Cluster rolling mills are special types of four high rolling mills in which the working rollers are backed up by two or more larger rollers.
Cluster rolling mills are generally used for rolling of hard materials.
#13) Planetary rolling
Planetary rolling mills are a special type of steel rolling mills in which the smaller diameter working rollers are arranged around the single larger roller (see below image).
The advantage of planetary rolling mills is that it avoids the sliding friction during the rolling operation.
In planetary rolling mills, generally the feed rollers are sometimes used to feed the metal piece to the planetary rollers.
#14) Tandem rolling
In tandem rolling mills, the metal is passed through several stands such that there occurs a gradual reduction of thickness in every passing.
Tandem rolling mills can also be used with a continuous casting process and this gives more advantages for manufacturing longer workpieces.