3 February 2016

What is annealing?

What is the annealing process?

Annealing is a heat treatment process in the field of metallurgy and materials science that alters a physical and sometimes chemical property of a material to increase its ductility and reduce its hardness and make it more workable and this process used to bring a metal closer to its equilibrium state.

Annealing is a process of heat treatment used to bring a metal closer to its state of equilibrium. Annealing is the heating process of metal at a particularly high temperature held there for a period of several hours to several days and then allowing it to cool down.

How annealing can be performed?

Heating the steel in a furnace to a point not exceeding 500 above or upper critical point in the annealing process and then holding it at that temperature for a considerable time likely to convert the entire steel into the austenite form for about 30 to 50 minutes after that a medium of hot sand or hot ashes or hot time dust allows the steel to cool down slowly. The cooling level must be sustained at 150-2000 per hour. Once this process has been finished, the formed metals are called annealed metal which is relatively soft and can be cut and shaped more easily, so when pressure is applied they bend easily.

Purpose of the annealing process

  • To increase ductility and relieve internal stresses that contribute to brittleness.
  • To reduce hardness, improve plasticity.
  • To increases the toughness and homogeneity of metals.
  • For improve machinability.
  • Annealing softens the metal but working on the cold condition, it becomes hard and again works on the metal without any cracks, it should be softened by an annealing process.
  • Eliminate internal stresses and process hardening to prevent cracking. 

Classification of annealing 

According to heating temperature annealing methods is divided into two types. 

Phase change recrystallization annealing above the critical temperature. 

  • Complete annealing 
  • Diffusion annealing 
  • Incomplete annealing 
  • Spherification annealing 
Phase change recrystallization annealing below the critical temperature. 
  • Recrystallization annealing 
  • Stress annealing

Advantages of annealing 

  • It softens the steel.
  • It reduces thermal stresses which occur due to a temperature gradient.
  • It enhances and improves the machinability of steel.
  • It increases the ductility of steel.
  • It enhances the toughness of steel.
  • It improves the homogeneity in steel.
  • The grain size of the steel is refined by annealing.
  • It prepares the steel for further heat treatment.

Disadvantages of annealing 

  • The electrical resistance of the metal is decreased.