Short Answer:

Rolling is a metal forming process in which a metal workpiece is passed through one or more pairs of rotating rolls to reduce its thickness and make it uniform. The process uses compressive forces to deform the metal plastically and produce sheets, plates, rods, or other sections of desired shape and size.

Rolling is widely used in the manufacturing industry for producing structural steel, rails, sheets, and other metal products. It can be performed at high temperature (hot rolling) or at room temperature (cold rolling) depending on the required properties and accuracy of the final product.

Detailed Explanation:

Rolling

Rolling is one of the most important and widely used metal forming processes. In this process, the metal is passed between two or more rotating rolls that apply high compressive forces. These forces cause the metal to deform plastically, reducing its thickness and making its cross-section uniform. The rolls rotate continuously in opposite directions, and the metal workpiece moves forward between them due to friction. Rolling is mainly used to produce flat products like plates and sheets or long products like bars and rods.

The rolling process is classified based on temperature, type of rolls used, and direction of metal flow. It is an efficient and economical process for shaping metals and improving their mechanical properties through grain refinement and uniform structure.

Types of Rolling

Rolling processes are classified mainly into hot rolling and cold rolling, depending on the temperature at which the operation is performed.

1. Hot Rolling

Hot rolling is carried out above the recrystallization temperature of the metal. At this temperature, the metal becomes soft and can be deformed easily without cracking. The main purpose of hot rolling is to reduce the thickness of large ingots or billets into smaller and more workable forms like sheets, rods, or beams.

Characteristics of Hot Rolling:

• Metal is heated above its recrystallization temperature.
• Large reductions in thickness can be achieved.
• Surface finish is rough, and further processing is often required.
• Produces fine and uniform grain structure due to recrystallization.
• Reduces internal stresses and porosity in the metal.

Applications:
Used in manufacturing structural beams, railway tracks, plates, and large metal sheets used in construction and heavy machinery.

2. Cold Rolling

Cold rolling is performed below the recrystallization temperature, usually at room temperature. In this process, the metal is hardened due to strain hardening, resulting in increased strength and better surface finish. The main purpose of cold rolling is to achieve accurate dimensions, smooth surface, and high mechanical strength.

Characteristics of Cold Rolling:

• Performed at room temperature.
• Produces excellent surface finish and dimensional accuracy.
• Increases strength and hardness due to work hardening.
• Requires more force and power compared to hot rolling.
• May need annealing after rolling to reduce internal stress.

Applications:
Used to produce thin sheets, strips, and rods for applications in the automobile and electrical industries.

Types Based on Roll Arrangement

Rolling mills can also be classified according to the number and arrangement of rolls used in the process:

1. Two-High Rolling Mill:
   Contains two rolls which rotate in opposite directions. It is simple in design and used for rough rolling operations.
2. Three-High Rolling Mill:
   Consists of three rolls placed one above the other. The direction of rolling can be reversed without changing the direction of roll rotation, making it more efficient.
3. Four-High Rolling Mill:
   Has two working rolls and two backing rolls. The backing rolls support the smaller working rolls and help in achieving greater thickness reduction.
4. Cluster and Planetary Mills:
   Used for very thin rolling operations where precise control of thickness and surface finish is required.

Working Principle of Rolling

The rolling process works on the principle of compressive plastic deformation. The metal piece is passed between the rolls, which rotate in opposite directions. The friction between the rolls and the metal pulls the material into the roll gap, where it is squeezed and elongated. The reduction in thickness is achieved gradually in several passes until the desired dimension is obtained.

The deformation is mainly compressive, but a small amount of shear stress is also present due to friction. This process helps in improving the internal structure of the metal and enhances its mechanical strength.

Advantages of Rolling

• Produces uniform cross-section and smooth surface finish.
• High production rate and suitable for mass production.
• Improves mechanical properties by refining grain structure.
• Economical process with low material wastage.
• Can be used for both ferrous and non-ferrous metals.

Disadvantages of Rolling

• High initial cost for rolling mill setup.
• Not suitable for small-scale production.
• Requires large power and heavy machinery.
• Dimensional accuracy is lower in hot rolling due to thermal expansion.
• Internal defects like scale formation and non-uniform thickness may occur.

Applications of Rolling

Rolling is used in a wide range of industries for producing different products such as:

• Steel sheets and strips for automotive and construction industries.
• Railway tracks, beams, and channels.
• Aluminum foils and copper sheets.
• Rods, wires, and pipes.
• Structural parts used in heavy machinery and shipbuilding.

Conclusion:

Rolling is a vital metal forming process used to produce a wide variety of metal products with desired thickness and shape. By using compressive forces between rotating rolls, it refines the structure, improves mechanical properties, and ensures material uniformity. Both hot and cold rolling processes are widely used depending on the product requirements. Due to its efficiency, rolling remains one of the most important and widely applied methods in modern metalworking industries.