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
