Heat treatment properties for Various Materials

1. HEAT TREATMENT

2. HEAT TREATMENT PROCESS
• Heat treatment may be defined as operation or
combination of operations involving heating and
cooling of metal / alloy in solid steels to obtain
desired conditions ( Relieve stress ) and
properties ( like Machinability, ductility etc. ).

3. Purpose of Heat Treatment:
• To relieve stress created during cold working,
welding, casting etc.
• Improve Machinability.
• Change grain size.
• Improve ductility and
• Homogenous structure.
• Cooling rates also plays a important role. Slow
cooling produces pearlitic structure and rapid
cooling produces a Martensitic ( hard ) structure.

4. Iron
• Iron is a chemical element with the symbol Fe
and atomic number 26. It is a metal in the first
transition series. It is the most common
element forming the planet Earth as a whole,
forming much of Earth's outer and inner core.
• Melting point: 1,538 °C
• Atomic number: 26
• Boiling point: 2,862 °C

5. .
IRON – CARBON
EQUILIBRIUM DIAGRAM
OR
Fe –C –PHASE DIAGRAM

6. .

7. .

8. .

9. .

10. .

11. .
MICRO-STRUCTURE
OF STEEL

12. Austenite
• It’s a FCC -----------structure
• The 2 % carbon is dissolved in 1180
• It has non – magnetic property
• Its not stable in room temperature
• This is the first structure formed in all heat
treatment processes

13. Ferrite
• Its having BCC structure
• Above 723 degree C it having non – magnetic property
• Below 723 degree C it having magnetic property
• The carbon dissolves 0.025% only
• It’s a soft ductile material
• Low carbon and mild steel having this structure.
• We cannot harden these structure by sudden cooling.

14. Cementite
• Its having orthorhombic structure
• It denotes Iron – Carbide
• It’s a hard and brittle material
• When the carbon is more than 0.8 % we get this
structure
• More increase of carbon give increase of
cementite property

15. Martensite
• This is obtained by sudden cooling of Austenite
structure
• It’s a hard brittle structure
• It has magnetic property
• It look like a group of slanting needles

16. Pearlite
• It has 0.8% of carbon content
• The structure look like pearls
• It’s a mix of ferrite and cementite

17. DIFFERENT STRUCTURES
•.

18. .

19. Heat Treatment Processes
Softening Processes -
Annealing
Normalizing
Hardening Processes -
Hardening
Tempering
Surface Hardening

20. TYPES OF HEAT TREATMENT PROCESS

21. .

22. .

23. ANNEALING
Annealing is a process of heating the steel slightly
above the critical temperature of steel (723 degrees
Centigrade) and allowing it to cool down very slowly.
.

24. Annealing of Aluminium

25. PHYSICAL PROPERTIES
• Annealed metals are relatively soft and can
be cut and shaped more easily. They bend
easily when pressure is applied. As a rule they
are heated and allowed to cool slowly.

26. ANNEALING

27. TYPES OF ANNEALING PROCESS
• Full Annealing
• Process Annealing
• Stress Relief Annealing
• Spherodising Annealing
• Recrystallisation (or) Isothermal Annealing

28. FULL ANNEALING
• Full annealing is the process of slowly raising the temperature about 50
ºC (90 ºF) above the Austenitic temperature line A3 or line ACM in the
case of Hypoeutectoid steels (steels with < 0.77% Carbon) and 50 ºC
(90 ºF) into the Austenite-Cementite region in the case of
Hypereutectoid steels (steels with > 0.77% Carbon).
• It is held at this temperature for sufficient time for all the material to
transform into Austenite or Austenite-Cementite as the case may be. It
is then slowly cooled at the rate of about 20 ºC/hr (36 ºF/hr) in a
furnace to about 50 ºC (90 ºF) into the Ferrite-Cementite range. At this
point, it can be cooled in room temperature air with natural
convection.

29. Full Annealing
• The process involves heating the steel to 30 to
50 degrees Centigrade above the critical
temperature of steel and maintaining the
temperature for a specified period of time, then
allowing the material to slowly cool down inside
the furnace itself without any forced means of
cooling.
• Hot Worked sheets, forgings, and castings made
from medium and high carbon steels need full
annealing.

30. FULL ANNEALING

31. Process Annealing
• Process Annealing is used to treat work-hardened parts made out of low-
Carbon steels (< 0.25% Carbon). This allows the parts to be soft enough to
undergo further cold working without fracturing. Process annealing is done
by raising the temperature to just below the Ferrite-Austenite region, line
A1on the diagram. This temperature is about 727 ºC (1341 ºF) so heating it to
about 700 ºC (1292 ºF) should suffice. This is held long enough to allow
recrystallization of the ferrite phase, and then cooled in still air. Since the
material stays in the same phase through out the process, the only change
that occurs is the size, shape and distribution of the grain structure. This
process is cheaper than either full annealing or normalizing since the
material is not heated to a very high temperature or cooled in a furnace.

32. Process Annealing
• This process is mainly suited for low carbon
steel. The material is heated up to a
temperature just below the lower critical
temperature of steel.
• Cold worked steel normally tends to posses
increased hardness and decrease ductility
making it difficult to work. Process annealing
tends to improve these characteristics. This is
mainly carried out on cold rolled steel like wire
drawn steel, etc.

33. Stress Relief Anneal
• Stress Relief Anneal is used to reduce residual
stresses in large castings, welded parts and
cold-formed parts. Such parts tend to have
stresses due to thermal cycling or work
hardening. Parts are heated to temperatures
of up to 600 - 650 ºC (1112 - 1202 ºF), and
held for an extended time (about 1 hour or
more) and then slowly cooled in still air.

34. Stress Relief Annealing
• Large castings or welded structures tend to possess
internal stresses caused mainly during their manufacture
and uneven cooling. This internal stress cause brittleness
at isolated locations in the castings or structures, which
can lead to sudden breakage or failure of the material.
This process involves heating the casting or structure to
about 650 Degree centigrade. The temperature is
maintained constantly for a few hours and allowed to
cool down slowly.

35. Spheroidization
• Spheroidization is an annealing process used for high
carbon steels (Carbon > 0.6%) that will be machined or
cold formed subsequently. This is done by one of the
following ways:
• 1.Heat the part to a temperature just below the Ferrite-
Austenite line, line A1 or below the Austenite-Cementite
line, essentially below the 727 ºC (1340 ºF) line. Hold the
temperature for a prolonged time and follow by fairly
slow cooling.
Or
• 2.Cycle multiple times between temperatures slightly
above and slightly below the 727 ºC (1340 ºF) line, say
for example between 700 and 750 ºC (1292 - 1382 ºF),
and slow cool. Or 3.For tool and alloy steels heat to 750
to 800 ºC (1382-1472 ºF) and hold for several hours
followed by slow cooling.

36. Spherodise Annealing
• This is a process for high carbon and alloy steel in order to
improve their machinability. The process tends to improve
the internal structure of the steel. This can be done by two
methods
• a. The material is heated just below the lower critical
temperature about 700 Degree centigrade and the
temperature is maintained for about 8 hours and allowed to
cool down slowly.
• b. Heating and cooling the material alternatively between
temperatures just above and below the lower critical
temperature.

37. Isothermal Annealing
• This is a process where is steel is heated above the upper
critical temperature. This causes the structure of the steel to
be converted rapidly into austenite structure. The steel is then
cooled to a temperature below the lower critical temperature
about 600 to 700 Degree Centigrade. This cooling is done using
a forced cooling means. The temperature is then maintained
constant for a specified amount of time in order to produce a
homogenous structure in the material. This is mainly
applicable for low carbon and alloy steels to improve their
machinability.

38. NORMALISING

39. Normalizing
It is mainly applied to achieve higher hardness and
strength.
In this process, the temperature is raised Upper
critical temperature.
Under this temperature, the structure is converted
into Austenite and removed from the furnace then it
is cooled under natural convection at controlled
room temperature.
This results in a grain structure of fine Pearlite with
excess of Ferrite or Cementite. The degree of
softness of a material in this process is mainly
depends upon the cooling condition.

40. PURPOSE OF NORMALISING
To produce a harder and stronger steel than full
annealing
To improve the machinability
To modify and refine the grain structure
To obtain a relatively good ductility without
reducing the hardness and strength

41. NORMALIZING
• Normalizing can be applied above the UCT for
both hypoeutectoid and hypereutectoid steels.

42. .

43. .

44. Hardening
• Its a process of heating the steel above or
below the critical temperature for a particular
period and then allow to cool by oil or water
rapidly

45. PURPOSE OF HARDENING
• Hardness of the metal can be improved to
resist wear
• Cutting ability of the material can be improved
to cut other material

46. Factors for getting good hardness
• Carbon content
when the % carbon is less than 0.3% we cannot do the
process. It should be 0.3 – 0.7%
• Rate of cooling
To get martensite structure we have to cool suddenly
• Work size

47. QUENCHING
• It’s the operation of rapid cooling by dipping
the hot metal piece into a quenching bath.
• The heated steel become much harder and
stronger by a rapid cooling

48. Quenching Medium
• Cold water
• Liquid salt
• Oil
• Air
• NaCl
The rate of cooling determines the level of hardness
and microstructure of steel

49. TEMPERING
• In the hardening process we obtained
martensite structure . In this structure, the
material having brittle property and also it has
internal stresses.
• For minimizing the harness and removing the
internal stresses we heat the metal near to
upper critical temp once again and let it for
some time then cool slowly by using salt liquid
or oil

50. .

51. TYPES OF TEMPERING
• Low temperature tempering (150 – 250 ‘ c)
• Medium temperature tempering (350 – 450 ‘ c)
• High temperature tempering (500 – 650 ‘ c)

52. INTERRUPTED QUENCHING
• The rapid cooling of molten metal gives more problems
like induced stresses
distortions (warping)
crack formation in steel
In order to overcome the disadvantages a modified
quenching is to be followed called interrupted quenching
Two forms of modified Quenching are
MARTEMPERING
AUSTEMPERING

53. MARTEMPERING
(Mar-quenching)
• It’s a interrupted cooling procedure for a steel to
reduce the stresses, distortions and cracking of
steels that may develop during rapid quenching

54. Step-1
Heat the metal to obtain Austenite structure level
Step-2
Quench the austenite steel in hot oil or molten salt at a
temperature just above the martensite start temp.
Step-3
Hold it for some time and stop the treatment before the
transformation of austenite to banite
Step-4
Cool it in a room air.

55. AUSTEMPERING
(isothermal Quenching)
• It’s a isothermal transformation of steel at a temp below
that of pearlite formation and above that of martensite
transformation
• Its usually used to reduce the quenching distortion and
to make a tough and strong steel
• Banite is the structure formed at the end of the process
• ADVANTAGES
Increased ductility , Toughness and reduced distorsion

56. .
Step-1
Heat the metal to austenite temperature
Step-2
Then quench the steel in a molten salt bath at a temp
just above the martensite start temp of the steel.
Step-3
Holding the steel isothermally to allow the austenite to
banite transformation to take place
Step-4
slow cooling to room temperature in air

57. AUSTEMPERING

58. TTT DIAGRAM
UNIT -2

59. WHY – TTT & CCT DIAGRAMS ?
• The phases martensite and bainite are non-equilibrium
phase that do not appear in fe-fe3 c (iron-iron carbon)
phase diagram
• also strengthening treatment like hardening and
tempering are non-equilibrium process.
• in order to show the influence of varying cooling rates,
that is time, on the transformation of austenite other
types of diagrams are necessary.
• The time temperature transformation or ttt diagram and
the continuous cooling transformation or cct diagram
are used to explain the things in the cooling operation

60. .
• Non-equilibrium cooling will result in different
microstructures hence altered properties.
• TTT diagrams are the tools that we can use to take into
account the kinetics of the transformation.
• They show the relationship between time, temperature
and (percent) transformation.
• There are two types of TTT diagrams:
isothermal transformation (IT) TTT diagrams
continuous cooling transformation (CCT) TTT diagrams

61. TTT
DIAGRAM
• The time-temperature
transformation curves
correspond to the start and
finish of transformations which
extend into the range of
temperatures where austenite
transforms to pearlite.
• Above 550 C, austenite
transforms completely to
pearlite.
• Below 550 C, both pearlite and
bainite are formed and below
450 C, only bainite is formed.
• The horizontal line C-D that runs
between the two curves marks
the beginning and end of
isothermal transformations.
• The dashed line that runs
parallel to the solid line curves
represents the time to transform
half the austenite to pearlite.
• .

62. TIME TEMPERATURE PATH ON ISOTHERMAL TRANSFORMATION DIAGRAM

63. PATHS
what transformations happen in:
– a. Path 1 (Red line)
– b. Path 2 (Green line)
– c. Path 3 (Blue line)
– d. Path 4 (Orange line)

64. PATH I -
(RED)
• a. (Red) The specimen is cooled rapidly to 150
Degree Celcius and let for 20 minutes. The cooling
rate is too rapid for pearlite to form at higher
temperatures; therefore, the steel remains in the
austenitic phase until the Ms temperature is
passed, where martensite begins to form.
• Since 150 Degree Celcius is the temperature at
which half of the austenite transforms to
martensite, the direct quench converts 50% of the
structure to martensite.
• Holding at 150 Degree Celcius forms only a small
quantity of additional martensite, so the structure
can be assumed to be half martensite and half
retained austenite.

65. PATH 2 -
(GREEN LINE)
• b. (Green) The specimen
is held at 250 Degree
Celcius for 100 seconds,
which is not long
enough to form bainite.
• Therefore, the second
quench from 250
Degree Celcius to room
temperature develops a
martensitic structure.

66. Path 3 –
(Blue line)
• c. (Blue) An
isothermal hold at
300 Degree Celcius
for 500 seconds
produces a half-
bainite and half-
austenite structure.
• Cooling quickly
would result in a
final structure of
martensite and
bainite.

67. PATH 4 –
(Orange Line)
• d. (Orange) Austenite
converts completely to
fine pearlite after
eight seconds at 600
Degree Celcius This
phase is stable and
will not be changed on
holding for 100,000
seconds at 600 Degree
Celcius .
• The final structure
obtained while cooling
is fine pearlite.

68. TTT – MICRO STRUCTURES

69. AUSTENITE
• Slowly cooling from the high temperature
crystal structure (austenite) in carbon steels
will develop microstructures as Shown in this
figure. • .

70. BAINITE
• This microstructure generally
forms as an aggregate of ferrite
(the stable crystal structure of
pure iron at room temperature)
and cementite/ carbides
(stoichmetric combinations of
iron, other metallic elements and
carbon).
• Bainite is an acicular
microstructure (not a phase) that
forms in steels at temperatures
from approximately 250-550°C
(depending on alloy content
• The temperature range for
transformation to bainite (250-
550°C) is between those for
pearlite and martensite. When
formed during continuous
cooling, the cooling rate to form
bainite is more rapid than that
required to form pearlite, but
less rapid than is required to
form martensite (in steels of the
same composition).

71. COOLING RATE –
Depend structures
• Faster cooling will produce structures further
away from the equilibrium structure.
• Fast cooling can produce ferrite
supersaturated with carbon, which has a
tetragonal closed packed structure called
martensite.
• Intermediate cooling rates or isothermal
transformation can form bainite.

72. CCT DIAGRAM
• A continuous cooling transformation (CCT) phase
diagram is often used when heat treating steel.
These diagrams are used to represent which types of
phase changes will occur if a material at it is cooled
at different rates. These diagrams are often more
useful than time-temperature-transformation
diagrams because it is more convenient to cool
materials at a certain rate than to cool quickly and
hold at a certain temperature.

74. CCT DIAGRAM

75. Strength
Ductility
Martensite
T Martensite
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends
Possible Transformations

77. HARDENABILITY

78. HARDENABILITY
• Hardenability of steel is defines as that
property which determines the depth and
distribution of hardness induced by quenching
by austenite condition.
• METHOD OF DETERMINING HARDENABILITY :
– Jominy end quench test

79. JOMINY END QUENCH TEST
• The Jominy end quench test is used to measure
the hardenability of a steel.
• This describes the ability of the steel to be
hardened in depth by quenching.
• steel to partially or completely transform from
austenite to some fraction of martensite at a
given depth below the surface

80. • High hardenability allows slower quenches to be used
(e.g. oil quench), which reduces the distortion and
residual stress.
• The test sample is a cylinder with a length of 102 mm
(4 inches) and a diameter of 25.4 mm (1 inch).
Jominy test specimen

81. • This is usually at a temperature of 800 to
900°C.
• The test sample is quickly transferred to the
test machine,
• where it is held vertically and sprayed with
a controlled flow of water onto one end of
the sample.
• This cools the specimen from one end,

85. • The hardness is measured at intervals from the
quenched end. The interval is typically 1.5 mm
for alloy steels and 0.75 mm for carbon steels.
• High hardness occurs where high volume
fractions of martensite
• Lower hardness indicates transformation to
bainite or ferrite/pearlite microstructures