Rolling Process: Definition, Working, Applications, Defects, Type of Rolling Mills [PDF]


Rolling is the deformation process of a metal that is widely used in the metal forming process. It is done by passing a strip of metal between the rollers. This process is generally used to make flat sheets, bars & other shapes. This process has various advantages and disadvantages which we will discuss in detail.

rolling operation

Definition of Rolling process:

Rolling is defined as a process to form metals where the metal strip is pressed by two or multiple rollers. Due to this a uniform thickness is maintained.

There are two main types of processes.

  • Hot rolled
  • Cold Rolled

If the strip is rolled after heating the strip above the re-crystallization temperature then it is termed as Hot rolled. If this is done at room temperature then it is termed as Cold rolled. Rolling is a process that is widely used and has very high production rate.

Working Principle of Rolling Process:

The rolling process is a metal-forming process, in which a stock of the material is passed between one or more pairs of rollers to reduce and maintain the uniform thickness. This process is mainly focused on the cross-section of the ingot or the metal which is forming. Using this process, we can reduce the thickness of the metal workpiece. The rolling processes are mainly focused on the increasing length and the decreasing thickness without changing the width of the workpiece. There are certain types of the rolling process, whereas, in the hot rolling process, the metal is heated up to its desirable temperature. When the metal is properly heated then the metal should be passed between one or more rolling mills to gain the proper desirable shape. The metal is heated above the recrystallization temperature. In the hot working process, the metal changes its grain structure because of the heat, now there were a new set of strain-free grains in the metal and this process needs less amount of force which correspondingly reduces the quality of the surface finish, of that metal. The cold rolling process is done below the recrystallization temperature of the metal. In this process, the force requirement is much more than the hot working process to pass the metal from the rollers. This process offers a good surface finish.

Types of Rolling Mills:

There are five rolling mills which are commonly used for rolling metals:

  • Two-High Rolling Mills
  • Three-High Rolling Mills
  • Four High Rolling Mills
  • Tandem Rolling Mills
  • Cluster Rolling Mills

1. Two-High Rolling Mills:

They are further categorized into the following types:

  • Reversing Mills
  • Non reversing Mills

Two High Reversing Mill:

In this type of mill, both rollers are adjustable. The rotation of that two rolls is made in two different directions. The metal is passed between two rollers which rotate at the same speed but it is in the opposite direction. It is used in slabbing, plumbing, rail, plate roughing work, and many other areas. As there is a need for a reversible drive, this mill is cheaper as compared to the others.

Two High Non-Reversing Mills:

In this type of mill, two rolls continuously revolve in the same direction and we can’t reverse the direction of the rollers. In this operation, the motive power is less costly.

two high rolling mills

2. Three-High Rolling Mills:

In this mill, the three rolls stand parallel to each other. The rolls are rotating in opposite directions. In this mill, between the first and the second rolls, the material passes. If the second roll rotates in one direction then the bottom roll rotates in another direction. The material is rolled both forward and returns in three high-rolling mills. At first, it passes forward through the last and second roller and then comes back through the first and second roller. The thickness of the material is reduced. Here transition system and a motor are needed which is less powerful.

three high rolling mills

3. Four High Rolling Mills:

In this type of mill, there are four parallel rolls one after another. In this operation, the rotation of the first and the fourth rolls take place in the opposite direction of the second and the third rolls. The second and third rolls are smaller to provide rigidity in necessity. So those are known as backup rolls. It is used in the hot rolling process of the armor and the cold rolling process of sheets, strips, and plates.


4. Tandem Rolling Mills:

In this type of rolling mill, there are two or more sets of rolls in the parallel alignment which make continuous passes and successively decrease the thickness to make it uniform.


5. Cluster Rolling Mills:

In this type of rolling mill, two basic rolls are backed up by two or more rolls that are bigger. These backed-up rolls give more pressure to the basic rolls to heavily press the strip.


Application of Rolling:

The rolling operation used in various industries such as:

  • Rods, seamless hollow tubes are made by rolling.
  • Rolling is used to producing cross-section of large sections.
  • Rolling is used to cutting the gears on the gear blank.
  • The threaded parts, bolts, screws, etc. which have mass production is made by the rolling process.
  • In automotive industries, various parts are manufactured by the rolling process.
  • The rolling process is used to made plates, steel sheets, etc.
  • Bearing, Turbines rings are rolling products.

Following are the two types of Rolling Product:

Hot Rolled Products:

Hot-rolled products are made by a hot-rolling process. The process is done at a very high temperature of (above 1,700 degrees Fahrenheit). For most of the metals, this temperature is the re-crystallization temperature. Such Hot Rolled products are as follows:

  • Rods
  • Rails
  • Plates
  • Sheets and strip
  • Structural
  • Bars

Hot Rolled products also used to make body panel, piping and tubes, and construction materials.

The following are the characteristics of a hot rolled product:

  • As the product shrinks from very high temperature, the edges of the product and the corners of the products such as plate or bars becomes rounded slightly.
  • As the product is cooled from very high temperature there might be a chance of distortion. In many cases, the product becomes slightly trapezoidal in shape instead of the perfect square.
Cold Rolled Product: 

Cold-rolled product is made from hot-rolled product. When the hot-rolled product is re-rolled at room temperature it is called as Cold rolled Product. It is used to get exact dimensions and to make the surface quality better. Following are some examples of Cold Rolled Process:

  • wire sheet etc.
  • Cold-rolled products, such as a wire is used to make screws and bolts, cold-rolled sheets are used to make strip and sheets for external applications of the automotive industries like doors, electrical motors, etc.

The following are the characteristics of a cold rolled product:

  • Glossy and smooth oily surface.
  • The surface is more finished and better with closer tolerances.
  • Tubes have straightness and the concentric uniformity is better.
  • The edges or the corners of the bars are well defined and the shape of the bars are more perfect than hot rolled products.

Defects of Rolling:

Following are the types of defects in rolling:

  • Surface rolling defects
  • Internal structural rolling defects

Surface Defects:

Surface defects can occur during the rolling process due to a number of factors such as uneven deformation, cracks, scratches, etc. The following are some common surface defects in rolling process:

  • Scale Pits – They are cavities or depressions that appear on the material surface.
  • Edge Cracks – They appear on the edges on metal stock due to excessive deformation.
  • Scratches – They are caused due to abrasive particles or other debris on the surface.

Internal Structural Defects:

The following are some common Internal Structural Defects in rolling process:

Wavy Edges Crack: 

The result is thicker as the middle portion of the rolling part is bent or deflected by the compressive load. There are some different cases. Those are as follows.

  • For the imperfection of the roll gaps, variation occurs on the rolling sheets.
  • If the thickness varies and along with that volume and width are constant then the center is shortened than the edges. But the body is continuous.
  • The edges portions are in the compression and the center portion is the tension.
  • The result of the edge is wavy.

Zipper Cracks in Centre of Strip: 

Zipper Crack is a type of Wavy Crack. If there is an uneven stress distribution on the strip, then the crack occurs in the centerline of the strip. It is called Zipper Cracks in the Center of the Strip. This crack looks like a zip so it is called Zipper Cracks.

Edge Crack: 

Edge cracks occur when the hot rolls are cooled. It happens as excessive quenching effects on the strip. Excess water is used to cool the edges. The use of excess water might give result into unflatten strips. The edges of the metal gets rounded off as the friction force prevents the corners and increases the length of the center portion.

Alligator Crack:

Alligator Crack is a type of cracking where the metal has any inclusions or weakness of metallurgy. As this crack separates the layers and increases the slab openings, it looks like the mouth of an alligator. Hence this crack is known as Alligator Crack.


Which material is used for rolling?

The selection of material depends on the desired result. Some commonly used materials are Steel, Copper, Aluminum, etc. Apart from these other materials that can be rolled are paper, plastic, textiles etc. The material thickness, width, surface properties plays an important role in selecting the material for rolling.

What are the advantages of Rolling process?

The following are the advantages of a rolling process:

1. Improved surface finish
2. Accurate Dimensions
3. More strength
4. Versatility
5. Low cost

Due to all these advantages, the rolling process has become a popular choice for production in various industries.

What are the types of rolling mills?

Rolling mills are used to reduce the thickness of a metal sheet or bar.
The following are the types of rolling mills:
1. Two High Rolling Mills
2. Three High Rolling Mills
3. Four High Rolling Mills
4. Cluster Rolling Mills
5. Tandem Rolling Mills


In terms of manufacturing engineering, every engineer should consider the rolling process as a core metal forming process. This process is mainly used when a cross-sectional area of a material is required to be changed by an engineer.


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