Heat treatment Process of Steels : Material Science

Heat Treatment Process Of Steel :

Heat treatment is defined as combination of operations involving heating and cooling of a metal or alloy . 

The Purpose of heat treatment :

1. To relieve internal stress that is developing by casting , welding , forging .

2. To increasing ductility and toughness .

3. To improve machinability , corrosion resistance .

4. To changing grain size .

5. To soften metals

6. To improve mechanical properties .

7. To increase wear resistance .

8. To improve trapped gases like parasity .

Process :

In all of heating process some basic step is followed .

1. Heating :

Small grains are combined into large grain .

2. Holding :

All grain is become into uniform shape and size .

3. Cooling :

(A) Rapid cooling Process :

• Large grains are break into small grains .
• Hardness increases .

(B) Slow cooling Process 

• Small grains are combined into larger grain .
• Hardness increases .

Types of heat treatment process :

1. Annealing

2. Normalizing

3. Hardening

4. Tempering

Special heat treatment process :

1. Surface hardening :

1. Induction Hardening
2. Flame Hardening or touch hardening

2. Case hardening :

1. Carburizing
2. Nitriding
3. Cyaniding
4. Carbonitriding 

Quench or cooling medium :

1. Cold water.                                   : at 16 °

2. Hot water                                      : at 60°

3. Oil Quench.                            : silicon oil

4. Polymer : Ethelene glycol + ethyl alcohol

5. Low temperature.       : liquid nitrogen

6. Salt water.         : NaOH + water at 15 °

Rate of cooling :

 1. Fast or Rapid cooling

 2. Slow Cooling

Annaling :

• Annealing is done to soften the metal 
• The process involves heating the metal slowly above the critical temperature (724 °c) .
• Then holding at that temperature for one hour and finally cooling in furnace at above 30°c to 15°c per hour .

Types of Annealing :

1. Full annealing

2. Process annealing

3. Spheroidization annealing

4. Diffusion annealing

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