What is Hot Working? (Examples, Advantages & Disadvantages)

hot working of metals

What is Hot Working?
Definition: Hot working (also known as hot forming) is defined as a metal forming process in which the plastic deformation of metals takes place above their recrystallization temperature.

Now you might have a question, “what is recrystallization temperature?”, Isn’t it?

Recrystallization temperature is a temperature in metal forming operations at which the old grain structure are destroyed and new grain crystals are formed by applications of mechanical pressure on the metals.

Metals have very high recrystallization temperatures, so they are necessary to be heated at such high temperatures for hot working.

During the hot working of metals, the upper limit of temperature of the metals should be less than its melting point.

The hot working of metals is carried out at a temperature of about 0.6 of its melting point.

Examples of Hot Working

Examples of hot working are listed below.

  1. Hot rolling
  2. Hot forging
  3. Hot spinning
  4. Hot drawing
  5. Hot extrusion
  6. Hot piercing

#1) Hot rolling

hot rolling

Hot rolling is a process in which the metal heated above recrystallization temperature gets plastically deformed when it is allowed to pass in between the two opposing spinning rolls.

Hot metal rolling is a deformation process in which the thickness of the workpiece is reduced by the application of compression force exerted by the two rotating opposed rolls.

#2) Hot forging

hot forging

Hot forging is a metal working process in which the desired shape of metal is achieved through the plastic deformation of the metal which is heated above its recrystallization temperature.

Depending upon the complications, forging is carried out as open die forging and closed die forging.

#3) Hot spinning

hot spinning

Hot spinning operation is a sheet metal forming operation in which the metal is heated above its recrystallization temperature and it is revolved at a high speed. Then it is pressed against the headstock spindle in order to give the desired shape.

#4) Hot drawing

hot drawing

In hot drawing, the metal is heated above its recrystallization temperature and the wires, tubes, rods and other sections are produced by pulling it through a die.

(Note: Drawing is generally a cold working process, but for large diameter tubes or wires, hot drawing is required which helps in reducing the forces.)

#5) Hot extrusion

hot extrusion

Hot extrusion is a metal forming process in which the block or billet of metal (which is heated above its recrystallization temperature) is forced to flow through the die opening to produce a desired cross section shape under a high pressure.

#6) Hot piercing

hot piercing

Hot piercing is a process in which a hole is pierced into the rotating heated metal workpiece.

Having said that, let’s now discuss the advantages and disadvantages of hot working.

Advantages of Hot Working

Advantages of metal hot working are mentioned below.

  • The main advantage of hot working is that, when the metal is heated to its recrystallization temperature, it becomes soft and its yield strength decreases. Because of this, less force is required to change the shape of that heated metal.
  • The formability of the metal increases at the higher temperature as the metals are more ductile at hot working temperature.
  • Casting defects like internal shrinkage cavities, blow holes etc, gets welded during the hot working because of recrystallization.
  • By hot working of metals, large components can be easily formed by its plastic deformation.
  • Recrystallized metal with favourable grain size gives improved mechanical properties.
  • Residual stresses can be minimized by using hot working.
  • Hot forming requires less force as compared to cold forming.
  • Fracture of the metals are avoided due to decrease in yield strength of the metals at high temperature.

Disadvantages of Hot Working

Disadvantages of hot working are mentioned below.

  • In hot working, the oxidation of surface takes place which reduces the surface quality.
  • Hot worked components show less dimensional accuracy.
  • The tools used for hot working have less life because of the extreme heat of hot working operation.
  • Sometimes hot metal is difficult to handle during the hot working operation.
  • For some ferrous metals, cold working is required even after the hot working is done on them. This cold working is done to improve the surface finish.
  • Very high amount of thermal energy is required to heat the metal above its recrystallization temperature.

External links:
Metal Rolling: Image via wikimedia commons, image license CC BY 3.0
Tube piercing: Image by PeterW, CC BY-SA 3.0, via Wikimedia Commons

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