Purpose of Heat treatment processes with basic detailed description and comparative study of different process such as bulk & surface process .

1. GUJARAT
Technological
University
Dr . S & S.S Gandhy Government Engineering
College , Surat
Branch :-Mechanical Engineering
Subject :- Material science andmetallurgy

2. Heat treatment Process
Purpose :-
• Improve metals properties ( Ex. Mechanical ,
Electrical etc..)
• Change in Grain structure
Process :-
It consists of 1. Heating
2. Soaking
( Holding )
3. Cooling

3. Types of Heat treatment Process
• Annealing
• Normalizing
• Hardening
• Tempering
• Carburizing
• Nitriding

4. Annealing process
Types of annealing
process :-
• Stress relief
annealing
• Recrystallization
annealing
• Spheroidizing
annealing
• Full annealing

5. Types of annealing process
Parameters
Steps
1.
Stress relief
annealing
2.
Recrystallization
annealing
3.
Spheroidizing
annealing
4.
Full annealing
Purpose
• Relieve
Stresses that
have been
absorbed by
metal from
different
process
• Remove strain
Harding and
Internal
Stresses
• Improve
machinibility
• Remove all
structural
imperfections
Process Step 1
1. Heating the
cold work
steel at temp.
between 500-
550°c
1. Heating the
steel
components at
temp. between
625-675°c
1. Heating the
steel
components at
temp. between
650-700°c
1. Heating the
steel
components at
a slowly
raising temp.
about 30-50°c
above upper
critical
temperature.

6. Parameters Steps
1.
Stress relief
annealing
2.
Recrystallization
annealing
3.
Spheroidizing
annealing
4.
Full annealing
Step 2
2. Holding the
steel components
at this temp. for
1-2 hours.
2. Holding the
steel components
at this temp. for
sufficient time .
2. Heating and
cooling alternate
for a 8hour period
of time.
2. Holding the
steel components
at this temp. for at
least 20min per
cm of the thick
section .
Step 3
3. To cool the
steel components
at room
temperature in air
3. To cool the
steel components
at room
temperature in air
3. Heating at
temp. above
lower critical
temp. 730-770°c
with very slow
cooling in furnace
3. To cool the hot
steel components
to room temp.
slowly in the
furnace
Benefits
• Reduce risk of
distortion while
machining
• Increase
corrosion
resistance
• Allows
recovery
process by
reduction of
work hardening
effect.
• Steel have
carbide and it
shape is
sphere .
Sphere shape
are soft so we
can improve
machinibility.
• Steel become
soft & ductile
• A
microstructure
having small
grains and
uniform grain
structure

7. Parameters Steps
1.
Stress relief
annealing
2.
Recrystallization
annealing
3.
Spheroidizing
annealing
4.
Full annealing
• No loss in
strength &
Hardness
• No change in
microstructure
• Increase
equiaxed
ferrite grains
• Decease
strength &
Hardness
• Increase
ductility
• Increase
Ductility &
toughness
• Decrease
Hardness &
Strength
--
Applicable • Low carbon
steel
• Hypo Eutectoid
Steel
• Low carbon
steel
• Hypo Eutectoid
Steel
• Medium & High
carbon steel
• Low & Medium
carbon steel

8. Normalizing process
Purpose :- Inhance Mechanical properties of a material by
refining microstructure
• Normalizing involves
1. Heating
2. Soaking
3. Cooling

9. The Normalizing Process
Heating :-
The metal is heated in a furnace for normalizing heat treatment process. The
temperature of the furnace is kept between 750-980 °C (1320-1796 °F), depending upon the
carbon content in the material.
Soaking:-
The material is kept at the temperature above austenite temperature for 1-2 hours, until
all the ferrite converts into austenite.
Cooling :-
The ferrite converts into austenite, and then cooled to room temperature in still air or
Nitrogen.

10. Applicable Metals :-
• Iron based alloys (tool steel, carbon steel, stainless steel, and
cast iron)
• Nickel-based alloys
• Copper
• Brass
• Aluminum

11. Common Applications of Normalizing
• Ferritic stainless steel stampings in the automotive industry may be
normalized following the work hardening that occurs during their
forming process.
• Nickel-based alloys in the nuclear industry may be normalized
following the thermal microstructure alteration that occurs following
welding.
• Carbon steel may be normalized after it is cold-rolled to reduce the
brittleness caused by work hardening.

12. Difference between Annealing & Normalizing Process

13.  Hardening Process :-
- Material heated to austenitic range and followed
by sudden Quenching.
- Mainly used for increasing hardness of material.
 Purpose of Hardening :-
- To develop High Hardness in material.
- To improve strength.
- To improve wear resistance.

14. Step-2 :- Holding
- At heating temperature for some time.
Step-3 :- Quenching or Rapid cooling
- Cooling at a faster rate than the critical cooling rate.
- Different Quenching mediums are :-
1.Water 2.Brine 3.Oil 4.Salt solution

15.  Result :-
- Due to rapid cooling Austenite will transform to Martensite.
- Internal stresses will be created in the material because
carbon gets trapped and they do not get enough time to
diffuse out of the structure.

16.  TTT – diagram :-
M : Martensite
A : Austenite
P : Pearlite
B : Bainite
N : Nose of curve
Ms : Martensite start
temperature
Mf : Martensite finish
temperature

17.  Result of different cooling mediums :-
1. Furnace cooling
- Slowest cooling
- Coarse or fine pearlite
2. Air cooling
- Moderate cooling
- Pearlite or Bainite
3. Oil quench
- Faster cooling
- Martensite
4. Water quench
- Fastest cooling
- Martensite

18.  Important points about Martensite :-
- It forms during Quenching (rapid cooling).
- Austenite to martensite transformation is a diffusion
less transformation because of very quick arrangement
of atoms, there is not enough time for atoms to diffuse.
- Body-centered Tetragonal (BCT) structure.
- Very high hardness and brittleness.

19.  Tempering Process :-
- Tempering is always followed after Hardnening process.
- Sub critical heat treatment process.
- Martensite obtained is very much brittle that a small
impact can break it, so tempering process is carried
out to improve its toughness.
- This process also reduces the internal stresses which
are produced during rapid cooling.

20.  Purpose of Tempering :-
- To relieve internal
stresses.
- To improve ductility.
- To improve toughness.
- To reduce brittleness.
- To reduce hardness to
some extent.

21.  Procedure :-
Step-1 :- Reheating hardened steel
- Heat below critical temperature (below 700⁰C).
Step-2 :- Holding
- Holding at that temperature for period of time.
Step-3 :- Cooling
- Slow cooling in air.
- Cooled to the room temperature.

22.  Depending on temperature, Tempering process
can be classified as :-
1. Low temperature Tempering (150⁰ to 250⁰C)
2. Medium temperature Tempering (350⁰ to 500⁰C)
3. High temperature Tempering (500⁰ to 650⁰C)
 Result :-
- At tempering temperature, carbon atoms diffuses out and
martensite transforms to fine cementite and softer ferrite
structure so internal stresses gets relieved.
- Good combination of toughness, strength & hardness in
material is obtained.

23. Difference between Hardening and
Tempering :-
Hardening Tempering
1. Heating occurs above critical temperature
( Above 723⁰C).
1. Heating occurs below critical temperature
( Below 723⁰C).
2. Cooling rate is very fast as it is quenched
in water or oil.
2. Cooling rate is slow as compared to hard-
-ening process as it is cooled in air.
3. Austenite transforms to Martensite. 3. Martensite transforms to fine cementite
and softer ferrite.
4. Internal stresses are produced. 4. Internal stresses are relieved.
5. Improves hardness and brittleness. 5. Improves toughness and reduces hardness
and brittleness to some extent.

24. Carburizing Process :-
 Definition :-
A thermo-chemical process in which carbon is diffused on
the surface of steel is called Carburizing.
 Purpose of Carburizing process :-
- To increase surface hardness so that it will respond well
to heat treatment.
- To increase strength.
- To increase wear resistance.

25.  Important points about Carburizing :-
- Thermo-chemical process.
- Mainly done on low carbon steel (0.02 - 0.3%C).
- Carried out on above critical temperature (>750⁰C).
- Fully austenite phase is required for carburizing because
austenite is having very good carbon solubility.
- Carbon content increases upto 0.7-0.9% after carburizing.

26. Types of Carburizing :-
1. Solid or Pack carburizing :-
- Low carbon steel is packed with
carbonaceous solid mixture of
80% charcoal powder and 20%
barium chloride.
- Specimen and this mixture is
kept in a sealed container.
- Then heat upto 790⁰ to 845⁰C for about 6 to 8 hours, then
cooled to room temperature.

27. - Reactions :-
BaCo3 = Bao + Co2 (This Co2 reacts with charcoal)
Co2 + C = 2Co
2Co + Fe = FeC + Co2
- Advantages :-
1. It is simple method and economical.
2. No atmosphere furnace is required.
- Disadvantages :-
1. Carburizing time is very long.
2. Difficult to control surface carbon and case depth.

28. 2. Liquid carburizing :-
- Specimen is kept in molten salt baths or various salt
mixtures : Sodium cyanide (20-50%),
Sodium carbonate (40%).
- The salt is usually a cyanide – chloride – carbonate
mixture and is highly toxic.
- Then it is heated on about 845⁰ to 950⁰C temperature.
- Time required is about 55 min to 1 hour.

29. - Reactions :-
BaCl2 + 2NaCN = Ba(CN)2 + 2NaCl
Ba(CN)2 + Fe = FeC + BaCN2
- Advantages :-
1. Rapid rate of penetration.
2. Free from oxidation.
- Disadvantages :-
1. Sodium cyanide is highly poisonous; hence care should
be taken while storage.
2. Salt sticks to the components.

30. 3. Gas carburizing :-
- Specimen is kept in a gaseous
atmosphere of methane or
propane.
- Methane and propane diluted
with carrier gas which is a mix-
ture of N2, Co, Co2, H2, CH4.
- Heating is carried out between
900⁰ to 950⁰C temperature for
about 3 to 4 hours.

31. - Reactions :-
C3H8 = 2CH4 + C
CH4 + Fe = FeC + 2H2
CH4 + Co2 = 2Co + 2H
2Co + Fe = FeC + Co2
- Advantages :-
1. It gives more uniform case depth.
2. Total time of carburization much less than the pack
carburization.
- Disadvantages :-
1. Furnace and gas generators are expensive.
2. Handling of fire hazards and toxic gases are difficult.

32.  Nitriding Process :-
 Definition :-
A thermo-chemical process in which nitrogen is diffused on
the surface of steel is called Nitriding process.
 Purpose of Nitriding Process :-
- To increase surface hardness.
- To improve wear resistance.
- To improve corrosion resistance.
- To improve fatigue life.

33.  Important points about Nitriding :-
- Thermo-chemical process.
- Most commonly used on high carbon, low alloy steels.
It can also used for medium carbon steels, titanium,
aluminium and molybdenum.
- Carried out on below critical temperature (<723⁰C).
- Better modification than Carburizing.

34.  Procedure :-
- Nitrogen in monoatomic
form is diffused on the
surface of steel and very
hard nitrides of iron are
formed.
- Resulting nitride case is
much harder than carbu-
rized case.
- Subcritical temperatures are used and hardness is gained
without quenching.

35. - Source of nitrogen used in diffusion process is ammonia
and the nitriding temperature is 500⁰ to 575⁰C.
- Reaction :-
2NH3 = 2N + 3H2
- Nitrogen is diffused on steel and hydrogen is exhausted.
- Nitrided case is categorized into three zones :-
1. White layer, which is brittle.
2. A hard nitride layer.
3. A diffusion zone of decreasing hardness.

36.  Advantages :-
- Process requires less time.
- Improves hardness, fatigue properties, resistance to wear
and corrosion.
- No cracks produced during process.
 Disadvantages :-
- High furnace cost required.
- Medium used is expensive.
- Necessity of using special alloy steels.

37. Difference between Carburizing and
Nitriding :-
Carburizing Nitriding
1. Carburizing is done in presence of
carbonaceous environment.
1. Nitriding is done in presence of
nitrogenous environment.
2. Carbon gets diffused on surface of
steel.
2. Nitrogen gets diffused on surface of
steel.
3. Heating is carried out above critical
temperature.
3. Heating is carrired out below critical
temperature.
4. After heating, Quenching is required. 4. After heating, Quenching is not
required.

38. Reference material
• Material Science and Engineering ( An introduction) William D. Callister
• Design For Heat treatment Lecture
• IIT Bombay
• Nptel
• Online resources
http://www.metalsupermarkets.com
http://www.inspection-for-industry.com
Figures from Google sources

39. Any
Questions