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What is Annealing ?

Annealing:-


Annealing Process :-


Annealing is a heat treatment process by which the properties of steel are enhanced to meet machinability requirements.In this process Hypoeutectoid steels (less than 0.83% carbon) are heated above upper critical temp., soaked and cooled slowly and Hypereutecoid (above 0.83%) are heated above lower critical temp., soaked and allowed to cool slowly.


Annealing Process Steps:




1.Heat material into the asutenite region (i.e. above 1600F) – rule of thumb: hold 

 steel for one hour for each one inch of thickness

2.Slowly furnace cool the steel – DO NOT QUENCH

3.Key slow cooling allows the C to precipitate out so resulting structure is coarse 
pearlite with excess ferrite
4.After annealing steel is quite soft and ductile



The purpose of Annealing is :



1)Reduce hardness and brittleness

2)Alter microstructure for desired mechanical properties

3)Soften steels to improve formability

4)Recrystalize cold worked (strain hardened) steel
5)Remove internal stresses induced by some previous processes such as forming, forging, uneven cooling.

Types of Annealing :-

Full Annealing:-



•Heating 25-50°C above upper critical temp. for steel <0.9% C.



• Heating 50°C above lower critical temp. for higher carbon steel.



• Soaking for sufficient time & controlled slow cooling

Process Annealing:- Also called Sub-Critical annealing 


•Heating at 500-650°C.

• Soaking for sufficient time & controlled slow cooling


Stress Relief Annealing:-


•Heat treat between 550 and 650°C-low carbon steel



•Soaking time – One hour per inch of thickness.



•Cooled down slowly in the furnace or in air. A slow cooling speed is important to avoid 


tensions caused by temperature differences in the material, this is especially important 

when stress relieving larger components


Spherodise Annealing:- This can be done by two methods 

  •  Heating and cooling the material alternatively between temperatures just above and below the lower critical temperature.
  • Steel is heated just below the lower critical temperature about 700 Degree centigrade and the temperature is maintained for about 8 hours and allowed to cool down slowly.

Isothermal Annealing:-
  • Steel is heated above the upper critical temperature
  • Forced cooling is done
  • Cooling is done below the lower critical temperature about 600 to 700 Degree Centigrade

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