just keep some basic in mind, its give u enough information about this topic.

1. HARDENING
Dr. H. K. Khaira
Professor in MSME
MANIT, Bhopal
1

2. Introduction
• Steels can be heat treated to high hardness and
strength levels.
• The reasons for doing this are obvious.
Structural components subjected to high
operating stress need the high strength of a
hardened structure.
• Similarly, tools such as dies, knives, cutting
devices, and forming devices need a hardened
structure to resist wear and deformation.
2

3. Hardening
• In hardening, the steel is heated 30 to 50oC
above Ae3 temperature in case of hypoeutectoid steels and 30 to 50oC above A1
temperature in case of hyper-eutectoid steel,
kept at that temperature for some time,
followed by quenching at a rate faster than
the critical cooling rate of the steel.
3

4. Heat Treatment Temperature for
hardening and other heat treatments
In hardening, the steel
is heated 30 to 50oC
above Ae3
temperature in case
of hypo-eutectoid
steels and 30 to 50oC
above A1 temperature
in case of hypereutectoid steel …......
←Acm
A3 →
4

5. Critical Cooling Rate
…. followed by
quenching at a rate
faster than the critical
cooling rate of the
steel.
The critical cooling rate
is the cooling rate
which is tangential to
the TTT diagram at the
nose of the curve.
Critical cooling rate
Cooling at or faster than
critical cooling rate will
transform the austenite
to martensite and not
to pearlite or bainite.
5

6. Hardening Procedure
1. The steel is heated
30 to 50oC above Ae3
temperature in case
of hypo-eutectoid
steels and 30 to 50oC
above A1 temperature
in case of hypereutectoid steel
←Acm
A3 →
2. Kept at that
temperature for
some time
6

7. Hardening Procedure
3. Followed by
quenching at a
rate faster than
the critical cooling
rate of the steel.
Critical
cooling
rate
7

8. Ms and Mf Temperatures
4. The martensite starts forming at Ms temperature
5. At Mf temperature, the martensite formation gets completed.
6. Therefore, the steel has to be quenched to a temperature below Mf.
Critical
cooling
rate
8

9. Retained Austenite
If the quenching is stopped before Mf temperature, proportionately less
austenite will transform to martensite.
The remaining austenite will also be present in the microstructure.
This remaining austenite is known as retained austenite.
Critical
cooling
rate
9

10. Sub-Zero Treatment
The presence of retained austenite will result in decreased hardness after
quenching.
Thefore, retained austenite is not desirable in the hardened steel.
In some steels, the Mf is below room temperature and, therefore, the steel is
quenched to sub-zero temperatures . It is known as sub-zero treatment.
Critical
cooling
rate
10

11. Comparison of
Cooling Rates for Different heat treatments
Different heat
treatments result in
different phases in
steel with different
properties.
11

12. Martensitic Transformation
• Because Martensite transformation is almost instantaneous, the
Martensite has the identical composition of the parent phase,
unlike ferrite and pearlite which result from a slower chemical
diffusion process, so each have different chemical compositions
than the parent austenite.
• Formation of Martensite involves a transformation from a facecentered cubic (FCC) structure to body - centered tetragonal (BCT)
structure.
• Martensite is super-saturated solid solution of carbon in iron having
BCT structure.
• The large increase in volume that results creates a highly stressed
structure.
• This is why Martensite has a higher hardness than Austenite for the
same composition.
12

13. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Figure (a) The unit cell of BCT martensite is related to the FCC austenite unit
cell. (b) As the percentage of carbon increases, more interstitial sites are
filled by the carbon atoms and the tetragonal structure of the martensite
becomes more pronounced resulting in increase in hardness.
13

14. Hardness of Martensite
• The hardness of martensite depends on its carbon content.
14

15. Effects of Carbon Content on Hardness and
Distortion in Martensite
The hardness of martensite increases with carbon content because
of the increasing supersaturation and distortion of the lattice
The distortion results in brittleness also.
More carbon → More distortion → More internal stresses → More brittleness
15

16. Comparison of
Hardness of Different Phases in steels
Hardening produces
martensite whereas
normalizing results in
fine pearlite.
16

17. Martensite
Some Important Points
• Cooling at a rate faster than the critical cooling rate produces martensite
in the steel.
• Martensite is formed by diffusion less transformation.
• Because Martensite transformation is almost instantaneous, the
Martensite has the composition identical to that of the austenite.
• Martensite is a super saturated solid solution of carbon in alpha iron.
• Martensite has BCT (body centered tetragonal) structure.
• Martensite , therefore, is hard.
• Hardness of martensite increases with increase in its carbon content.
• Formation of Martensite from austenite involves large increase in volume
that results in a highly stressed structure. It increases the hardness of
martensite.
• Martensite makes steel hard and strong.
• But martensite is very brittle also.
• Therefore, hardened steels are hard and strong but brittle.
17

18. Microstructure of Martensite
18

19. Increase in carbon content decreases Ms and Mf temperatures
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a
trademark used herein under license.
Effect of Carbon Content on Ms and Mf
Temperatures
19

20. Effect of Alloying Elements on Ms and
Mf
20

21. Quenching Media
Four commonly used quenching media are:
– 1. Brine – the fastest cooling rate
– 2. Water – moderate cooling rate
– 3. Oil – slowest cooling rate
– 4. Liquid Nitrogen – used in automatic furnaces,
can be very fast cooling.
Too rapid cooling can cause cracking in complex
and heavy sections.
21

22. Quenching Media
• Many quenchants can be used for hardening of
steels
– Each quenchant has its own thermal properties
• Thermal conductivity
• Specific heat
• Heat of vaporization
– These properties cause rate of cooling differences
– Different quenching medium have different cooling or
quenching capacity.
– The severity of quenching obtained from the
quenching medium is indicated by H Value.
22

23. Severity of Quenching
•
•
•
Cooling capacities (Severity of quench) of quenching medium is known as H value.
H values of some of the quenchants are given below.
Cooling rates are at the center of a 2.5 cm bar.
H Value
–
–
–
–
–
–
–
–
–
Ideal Quench
Agitated brine
Brine (No agitation)
Agitated Water
Still water
Agitated Oil
Still oil
Cold gas
Still air
∞
5
2
4
1
1
0.25
0.1
0.02
Cooling Rate (0C/s)
∞
230
90
190
45
45
18
-
23

24. Ideal Quenchant
• Ideal quenchant is one which brings down the
steel temperature to room temperature
instantaneously and keeps it at that
temperature thereafter.
24

25. Influence of quench medium and sample size on the
cooling rates at different locations.
• Effect of quenching medium
– Severity of quench:
• Water > Oil > Air, e.g. for a 50 mm diameter bar, the
cooling rate at center is about 27°C/s in water, but, 13.5
°C/s in oil.
– Effect of size
• For a particular medium, the cooling rate at center is
lower when the diameter is larger. For example, 75mm
vs. 50mm.
25

26. The Grossman chart used to determine the
hardenability of a steel bar for different quenchants.
This diagram can be used to determine bar diameters having equal
cooling rates when quenched in different quenching media.
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein
under license.
26

27. Quenching
• Water is one of the most efficient quenching media where maximum
hardness is required, but it may cause distortion and cracking of the
article.
• Where hardness can be sacrificed, whale, cotton seed and mineral oils are
used. These tend to oxidise and form sludge with consequent lowering of
efficiency.
• The quenching velocity of oil is much less than water. Ferrite and bainite
are formed even in small sections.
• Intermediate rates between water and oil can be obtained with water
containing 10-30 % Ucon, a substance with an inverse solubility which
therefore deposits on the object to slow rate of cooling.
• To minimise distortion, long cylindrical objects should be quenched
vertically, flat sections edgeways and thick sections should enter the bath
first.
• To prevent steam bubbles forming soft spots, a water quenching bath
should be agitated.
27

28. Important Points
• Hardening of steel is obtained by a suitable quench from within or
above the critical range. The temperatures are the same as those
given for full annealing.
• The soaking time in air furnaces should be 1.2 min for each mm of
cross-section or 0.6 min in salt or lead baths.
• Uneven heating, overheating and excessive scaling should be
avoided.
• The quenching is necessary to suppress the normal breakdown of
austenite into ferrite and cementite, and to cause a decomposition
at a low temperature to produce martensite.
• To obtain this, steel requires cooling faster than the critical cooling
rate.
• Critical cooling rate is greatly reduced by the presence of alloying
elements, which therefore cause hardening with mild quenching
(e.g. oil and air hardening steels).
28

29. Effect of Alloying Elements on Critical
Cooling Rate
Alloying elements shift TTT diagram towards right thereby decreasing
the critical cooling rate.

30. Quench cracks
• The volume changes, which occur when austenite is
quenched, are:
– a) expansion when austenite transforms to martensite
– b) normal thermal contraction.
• When steel is quenched these volume changes occur
very rapidly and unevenly throughout the specimen.
• The outside cools most quickly to martensite.
• Stresses are set up which may cause the metal either
to distort or to crack if the ductility is insufficient for
plastic flow to occur.
• Such cracks may also occur some time after the
quenching or in the early stages of tempering.
30

31. - The outside cools most quickly to martensite.
- Stresses are set up which may cause the metal either to distort
or to crack if the ductility is insufficient for plastic flow to occur.
31

32. Quench cracks
• Quench cracks are liable to occur:
1) due to presence of non-metallic
inclusions, ;
2) when austenite is coarse grained due to
high quenching temperature;
3) owing to uneven quenching;
4) in pieces of irregular section and when
sharp re-entrant angles are present in the
design.
32

33. Precautions against Quench Cracks
• The relation of design to heat-treatment is very
important. Articles of irregular section need
special care.
• When steel has been chosen which needs a
water-quench, then the designer must use
generous fillets in the corners and a uniform
section should be aimed at.
• This can sometimes be obtained by boring out
metal from bulky parts without materially
affecting the design.
33

34. The CCT diagram for a low-alloy, 0.2%
C Steel
Mild steels can not
be hardened
because the TTT
diagram touches
the Y axis
34

35. Protective Atmospheres
• Steel is heated to high temperatures in hardening.
• At such high temperatures, decarburisation at the
surface may take place in the presence of oxygen
• Decarburisation will result in decrease in carbon
content in the martensite formed at the surface
• It will give rise to low hardness in the surface after
hardening
• Hence protective atmospheres may be used in heat
treatment to avoid oxidation or decarburisation
• Innert gases may be used as protective atmosphere
• Heat treatments may also be carried out in vacuum
35

36. Summery
• 1) Martensite is the hardest and most brittle
microstructure obtainable in a given steel.
• 2) Martensite hardness of the steel is a function of the
carbon content in that steel.
• 3) Martensite results from cooling from austenitic
temperatures faster than the critical cooling rate by
pulling the heat out using a quenchant before pearlite
can form.
• 4) As quenched Martensitic structures are too brittle
for economic use. They must be tempered.
• 5) Tempering a hardened steel results in the best
combination of strength and toughness.
36

37. Need of Tempering
• As-quenched hardened steels are so brittle
that even slight impacts may cause fracture.
• Tempering is a heat treatment that reduces
the brittleness of a steel without significantly
lowering its hardness and strength.
• All hardened steels must be tempered before
use.
37

38. Hardened and Tempered Steels
• Fully hardened and tempered steels develop
the best combination of strength and notchductility.
38

39. THANKS
39