What Is Rolling ? Difference Between Hot Rolling & Cold Rolling - SKengineers

 

  ROLLING

·        When metal is required in long length of uniform cross-section , the deformation process applied is known as rolling.

·        Rolling is more economical than forging.

·        It is a rapid process of forming metal into desired shape by plastic deformation.

·        In this method metal is compressed between two rotating roll to increase in length thus reducing its cross-sectional area.

·        Rolling is most widely used mechanical working process used in industry today.

Ø  PRINCIPLE OF ROLLING –

·        Work in the form of bar or billet is passed between two rollers rotating in opposite directions.

·        These rollers exerts high compressive force on the stock due to squeezing actions. These rollers are called as rolls.

·        Figure shows rolling of rectangular slab of thickness t1, which after rolling is reduced to t2. Also frictional forces between the rolls are responsible for drawing out the metal.

·        Rolling can produce a stock having constant cross-section throughout its length.

·        It can produce many shapes such as I, T, L, C and various channel sections.

·        Rolling is done in both i.e. hot rolling and cold rolling. Normally rolling is considered as hot rolling unless specified as cold rolling.

Ø  HOT ROLLING –

1.      In hot rolling process, the metal in a hot plastic state is passed between two rolls revolving at the same speed but in opposite direction.

2.      As the metal passes through the rolls, it is reduced in thickness and increased in length.

3.      The metal provided for hot rolling first cast into ingots of suitable form i.e. rectangular, square or round. These ingots then heated in gas fired furnace to attain uniform working temperature.

4.      Then these ingots are taken to rolling mills for hot rolling processes.

5.      The flow of metal during rolling is continuous and almost in longitudinal direction.

6.      Hot rolling produces components like plates, bars, sheets, wide, variety of bars ad sections, rods and other structural shapes.

Ø  COLD ROLLING –

1.      Cold rolling is a finishing operation generally performed on products of hot rolling to give good surface finish, dimensional accuracy and strength of material.

2.      Bars of different shapes, sheets, rods, strips produced by hot rolling are commonly finished by cold rolling process.

3.      Cold rolling is done below the recrystallisation temperature of the metal.

4.      The operation consists of cleaning the stock in the form of sheet or strip in acid solution to remove scale.

5.      Next step is to wash in water and then dried. This cleaned metal is then passed through set of rolls.

6.      In each pass, there is slight reduction in area. The process is repeated until the required thickness is obtained.

 

DIFFERENCE BETWEEN HOT ROLLING AND COLD ROLLING

Sr. No.	HOT ROLLING	COLD ROLLING
1.	Metal heated above recrystallization temperature is fed to the rollers.	Metal is heated below the recrystallization temperature is fed to the rollers.
2.	Poor dimensional accuracy and finish.	Good dimensional accuracy and finish
3.	Suitable for production of large sections.	Deformation is limited to small reductions.
4.	Very thin sections cannot be obtained.	Thin sections can be obtained.
5.	Roller radius is generally larger in size.	Comparatively roller radius is smaller.
6.	Wide range of shapes like billets, blooms, slabs, sheets, burs etc. can be produced.	Suitable for production of plates, sheets, foils, etc.