Heat treatment is a process that involves heating and cooling metals and alloys to modify their properties. There are two main types: softening treatments like annealing, and hardening treatments. Annealing involves heating steel above its critical temperature, holding, and slow cooling to develop an equilibrium grain structure and increase ductility. Common annealing processes are full annealing, diffusion annealing, process annealing, and spheroidizing annealing, which converts cementite into a spherical shape to improve machinability.

1. MODULE-3
3.1. HEAT TREATMENT
Presented by-Prof.F.J.Khan

2. WHAT IS HEAT TREATMENT ???
Heat Treatment is a very broad term which includes any heating and
cooling operation or a sequence of such operations applied to any
material in order to modify its internal structure or to alter its
physical mechanical or chemical properties.

3. CONTD….
The heat treatment processes are carried out in 3 stages such as
heating the steel at some temperature above critical range in order
to from austenite , hold at constant temperature and cool at room
temperature .

4. HEAT TREATMENT OF STEEL:-
•The heat treatment is a very broad term and includes any heating
and cooling operation or any sequence of two or more such
operation.

5. OBJECTIVE OF HT OF STEEL:-
1.To increase hardness, wear and abrasion resistance.
2.To remove internal stresses.
3.To increase machinability.
4.To refine grain structure.
5.To change the composition of steel.
6.To increase ductility and softness (tempering, recrystallization
annealing)
7.To improve surface properties
8.To improve electrical properties

6. TYPES OF HEAT TREATMENT
➢ The heat treatment processes is classified are as follows:-
A . Softening heat treatment
• Annealing
• Normalizing
B . Hardening heat treatment
• Conventional hardening • Maraging
• Ausforming • Austempering
• Martempering

7. SOFTENING HEAT TREATMENT
➢ ANNEALING PROCESS
•Annealing is a heat treatment in which the metal is heated to a
temperature above its recrystallisation temperature, kept at that
temperature for some time for homogenization of temperature
followed by very slow cooling to develop equilibrium structure in
the metal or alloy.
•Steel is heated 30 to 50oC above its critical temperature.
•The cooling is done in the furnace itself.

8. TYPES OF ANNEALING PROCESS
1.Full annealing
2.Diffusion annealing
3.Process annealing
4.Spheroidising annealing (Cyclic Annealing)

9. 1.FULL ANNEALING
•Full annealing is the process of slowly raising the temperature
about 50 ºC above the Austenitic temperature in the case of
Hypoeutectoid steels=A3+50 (steels with < 0.8% Carbon)
Hypereutectoid steels=A1+50 (steels with > 0.8% Carbon).
•After heating steel in above temp. range it is held at this constant
temp. for min. period of 1hr for each 25 mm dia. or thickness.
•It is then slowly cooled in a furnace.
•The grain structure has coarse Pearlite with ferrite or Cementite
(depending on whether hypo or hyper eutectoid).

10. FULL ANNEALING
● The steel becomes soft and ductile.

11. 2.DIFFUSION ANNEALING
•Diffusion Annealing is also known as homogenization.
•Process Involved:
(a)Heating:-
Steel is heated to temperature 1100 to 1200OC .
(b)Holding:-
Holding time ranges from 8 to 20 hrs.
(c)Cooling:-
Total time required for diffusion annealing is 50 to 100 hrs.,
depending upon the composition of steel.

12. DIFFUSION ANNEALING

13. 3.PROCESS ANNEALING
● In process annealing, the cold worked metal is heated above its
recrystallisation temperature (usually bet. 550-650ᵒC), kept for some
time (at constant temp. for 1hr for each 25mm dia.)followed by slow
cooling.
● Cold worked grains are quite unstable due to the strain energy. By
heating the cold worked material to high temperatures where sufficient
atomic mobility is available, the material can be softened and a new
microstructure can emerge.
● This heat treatment is called process annealing where recovery and
recrystallization take place.

14. 4.SPHEROIDIZING ANNEALING (CYCLIC
ANNEALING)
● In spheroidizing annealing, the steel is heated to a temperature
below A1 temperature, kept at that temperature for some time
followed by slow cooling.
● The aim of spheroidizing annealing is to improve the
machinability of steel.
● In this process the cementite is converted into spheroidal form.
● The holding time varies from 15 – 25 hours.

15. PROCESSES OF CYCLIC ANNEALING
1. Heating: Steel is heated to 727˚C
Above 727˚C - Austenite structure is formed
Below 727˚C – Austenite tries to transform into pearlite.
1. Thermal Cycling: The process of heating below and above 727OC is
repeated for long time. Cementite layers from pearlite breaks and
converted into round or spheroidal shape. Spheroidised structure is
softest and gives excellent machinability.
3. Cooling: Cooling of spheroidised cementite structure to room
temperature in air. No change in microstructure during cooling.

16. CONTD...
Fig .The Spheroidisation of Pearlitic Cementite

17. APPLICATIONS