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Hot Rolling vs Cold Rolling – Difference Between Hot rolling and cold rolling

Introduction To Rolling :

Rolling is a fabricating process in which metal is passed through a pair of rolls. Rolling has 2 main classifications. Flat rolling, in which the product is typically a sheet, or profile rolling, in which the product is typically a rod or bar. Rolling is also classified according to the recrystallization temperature of the metal.

Hot Rolling

Hot rolling uses large pieces of metal, such as slabs or steel billets, and heats them above their recrystallization temperature. The metal pieces are then deformed between rollers creating thin cross sections. These cross sections are thinner than those formed by cold rolling processes with the same number of stages. Hot rolling also reduces the average grain size of metal but maintains an equiaxed microstructure.

Cold Rolling

Cold rolling is a process which passes metal through rollers at temperatures below its recrystallization temperatures. This increases the yield strength and hardness of the metal. This is done by introducing defects into the crystal structure of the metal creating a hardened microstructure which prevents further slip. Because the metal is at room temperature, it is less malleable than metal above its recrystallization temperature. This makes cold rolling a more labor intensive and expensive process than hot rolling. Cold rolling can also reduce the grain size of the metal resulting in Hall-Petch Hardening.

Both hot rolling and cold rolling are used to create sheet metal. However, cold rolling produces thinner sheets. Hot rolling is also commonly used to create railroad rails, and cold rolling is often used to make beverage cans.

Difference Between Hot rolling and cold rolling

Hot Rolling	Cold Rolling
The material to be rolled is above its re-crystallization temperature.	The material to be rolled is below its recrystallization temperature.
The roll radius is larger than used in cold rolling.	The roll radius is smaller.
Heavy reduction in area of the work-piece can be obtained.	The heavy reduction area is not possible.
Very thin sections (less than 1.25 mm) are not obtained by hot rolling or even though it is uneconomical.	Very thin sections (0.02 mm) can be made as in case of aluminium foils.
Hot rolling results in; refined grain size, removed blow holes, toughness and strength increases. A fiber-like structure is obtained.	Cold rolling results in; increased hardness generates cracks, reduced ductility, increased tensile strength and yield strength of steel.
Generally hot rolled metal does not shows work hardening effect.	Cold rolled metal shows work hardening effect.
Surface finish is not good due to metal oxides and scale formed.	Smooth and oxide-free surface finish is obtained.
Close dimensional tolerance cannot be achieved.	Close tolerance (0.002 mm) can be achieved.
Coefficient of friction between the rolls and workpiece is higher.	Coefficient of friction between the rolls and workpiece is comparatively lower
Theoretical analysis cannot be perfectly carried out as yield stress varies with the temperature and with each pass.	Theoretical analysis can be easily carried out as extensively developed theory is available.
In hot rolling, experimental measurements are difficult.	In cold rolling, experimental measurements can be easily carried out.