TYPES OF HEAT TREATMENTS
1) NORMALIZING.
2) ANNEALING.
3) STRESS RELIEVING.
4) SOLUTION ANNEALING.
5) AGEING.
6) TEMPERING.
1) NORMALIZING. :
Normalizing is the process in which heating a material at elevated temperature and then allow it to cool back to the room temperature by exposing it to room temperature air after it heated. In normalizing the steel is heated to 40 Degree C above upper critical temperature followed by cooling in the still air in order to achieve,
i) Uniform structure of material
ii) To refine grain structure.
iii) Change in UTS (Ultimate Tensile Strength)
iv) Change in YS (Yield Strength)
v) Impact Properties
vi) Relieving internal stresses
vii) Hardness
The stages involve in Normalizing shown with diagram as shown below
Pic.01
The main aim of normalizing is to increase the toughness of steels. It is the type of heat treatment which applicable to ferrous material only.
Advantages over annealing process:
i) It is faster than annealing process
ii) Normalizing is less expensive than annealing process.
Disadvantages:
i) As increases the cooling rate results in high hardness.
ii) Cooling rate in normalizing is different at different depths so the formed structure is depending on thickness of material.
2) ANNEALING. :
Annealing is one of the heat treatments which change the mechanical and physical properties of material in order to increase the ductility and reduce the hardness. In annealing steel is heated at 10 to 50 degree C above the upper critical temperature and held for the desired length of time followed by very slow cooling in furnace in order to achieve softness of material and better ductility.
Advantages:
i) It increases toughness of material.
ii) It increases the ductility of the material.
iii) It reduces the brittleness of the material.
iv) Helps to increase machinability of the material.
3) STRESS RELIEVING. :
The steel is heated at temperature below or close to the lower critical temperature with specific rate of heating call stress relieving process. In this heat treatment metal is held at temperature for specific length of time and then cooling with specific rate of temperature. One thing is noted that in stress relieving there is no change grain structure of the material. Stress Relieving done in order to soften the material partially, reduce internal stresses also soften HAZ (Heat Affected Zone).
The temperature range in stress relieving is in order to 593 degree C , 600 degree C -640 degree C, 650 degree C – 690 degree C
The time is 1 Hr/ inch Thick.
Heating Rate: Normally 300 C per Hr Per Inch.
Cooling Rate: Normally 300 C per Hr Per Inch.
4) SOLUTION ANNEALING.:
In Solution annealing heat treatment, the austenitic stainless steel is heated above 1050 degree C and held for the desire time followed by cooling to the room temperature within few minutes by quenching process or blowing the air.
Basically solution annealing is done on non-ferrous alloys and stainless steel in order to soften the material , to improve corrosion resistance & in order to remove carbide precipitation formed at grain boundaries during the process of manufacturing.
5) AGEING. :
Aging is the simple heat treatment in which the material heated at a certain temperature and held for desire time followed by the process of quenching or cooling in air. The aging heat treatment done on material susceptible for ageing characteristics like maraging steels. The ageing heat treatment is increasing the strength of the material, also improve toughness of the material.
6) TEMPERING. :
Tempering is the process which improve the characteristics of steel by heating at high temperature below the melting point of material and then cooling it normally in air Tempering process reduces the internal stresses in the material. Tempering process used to increase toughness of iron-base alloys.
7) HARDENING. :
Hardening is the process is used to increase hardness of metal. The hardening process in which heating the material above critical temperature holding at this temperature for one hour per inch of thickness and cooling fast in order to material much harder and string structure.
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