TTT diagram, CCT diagram and Heat treatment processes

1. TTT diagram and heat treatment
processes
By:
Saumy Agarwal
Asst. Professor (MED)
BTKIT Dwarahat

2. Time-Temperature-Transformation
(T.T.T) Diagram
• Iron has having different crystal structures at different
temperatures.
• It changes from FCC to BCC at 910oC.
• At eutectoid temperature (727oC), austenite transforms
to pearlite
• This transformation of austenite is time dependent.

3. Limitations of Iron-carbon Diagram:
• Iron carbon diagram does not provide information about
the transformation of austenite to any structure other
than equilibrium structures,
• It also does not provide any details about the influence of
cooling rates on the formation of different structures.
• It provides no information on time taken to form any
phase.

5. • So to overcome this problem TTT diagram is required.
• TTT diagram shows what structures can be expected
after various rates of cooling.
• TTT diagram graphically describes the cooling rate
required for the transformation of austenite to pearlite,
bainite or martensite.
• It also gives the temperature at which such
transformations take place.

6. Bainite:
It is a fine pearlite and contain very fine distribution of ferrite
and cementite. It involves diffusional transformation (carbon
diffusion) where phase composition is changed. (Austenite to
ferrite and cementite)
Martensite:
It is a solid solution of Carbon in BCC iron having Body centered
Tetragonal (BCT) structure. It involves diffusion-less
transformation where change in crystal lattice takes place (FCC
to BCT)

8. Fig: Isothermal transformation diagram including austenite-to pearlite
(A–P) and austenite-to-bainite (A–B) transformations.

9. Fig: Complete TTT diagram for the Iron carbon alloy of eutectoid composition

10. Continuous Cooling Transformation
(CCT) Diagram
• Instead of isothermal cooling, there is mostly continuous
cooling for practical applications.
• Due to continuous cooling, TTT curves are to be modified to
show the transformations occurring at constantly varying
temperature.
• For continuous cooling, the time required to start and end the
reaction is delayed.
• Thus, isothermal curves are shifted to longer time and lower
temperature.

12. • It is possible to form 100% pearlite or 100% martensite by
continuous cooling, but it is not possible to form 100%
Bainite.
• To obtain a bainitic structure, cool rapidly enough to miss the
nose of curve and then holding in the temperature range at
which bainite is formed.
• Cooling curve tangent to the nose of TTT curve is known as
Critical cooling rate (CCR).
• CCR is the minimum rate of cooling required to convert the
austenite into 100% martensite.

16. Heat Treatment Processes
• Heat Treatment process is a series of operations involving the
Heating and Cooling of metals in the solid state.
• Its purpose is to change a mechanical property or
combination of mechanical properties to make the metals
suitable for required operation.
• These processes refine the grain structures, remove gases
from castings and improve machinability

17. • By heat treating, a metal can be made harder, stronger,
and more resistant to impact, heat treatment can also
make a metal softer and more ductile.

18. • The basic steps of heat treatment are:
Parameters to choose:
1. Temperature
2. Time of soaking
3. Medium of cooling
4. Rate of cooling

19. Types of Heat Treatment Processes
• Annealing
• Normalizing
• Quenching
• Tempering
• Surface Hardening

20. Annealing:
The various purpose of these heat treatments is to:
1) Relieve Internal stresses developed during solidification,
machining, forging, rolling or welding,
2) Improve or restore ductility and toughness,
3) Enhance Machinability,
4) Refine grain size, &
5) Reduce the gaseous contents in steel.

21. Annealing is a heat treatment which involves
1. Heating of metal to a temperature above its
recrystallization temperature
2. Then the heated metal is kept at that temperature for
some time for homogenization of temperature
3. Then after soaking, it was slowly cooled.
• The cooling is done in the furnace itself.

22. Stages of Annealing :
There are three stages of annealing
1. Recovery : The relief of some of the internal strain
energy of a previously cold-worked material.
2. Recrystallization: The formation of a new set of strain-
free grains within a previously cold-worked material.
3. Grain Growth: The increase in average grain size of a
polycrystalline material. Reduction in concentration of
grain boundaries

23. Types of Annealing:
1. Full Annealing
2. Process Annealing
3. Spheroidizing Annealing

24. Full Annealing: (Austenite to coarse Pearlite)
It is heating the steel at
1. 30 to 50oC above A3 temperature (Upper critical
temperature) in case of hypo-eutectoid steels, and
2. 30 to 50oC above A1 temperature (Lower critical
temperature) in case of hyper-eutectoid steel
• The steel is then kept at that temperature for some time (4
min per mm thickness) for homogenization of temperature
followed by cooling at a very slow rate (furnace cooling).
• The cooling rate may be about 30oC per hour.

25. Process annealing
It is often referred to as a stress relief annealing
• This treatment, which is usually applied to hypoeutectoid
steels with less than 0.3% C, is carried out at a temperature
below the lower critical temperature, usually between 550°C
and 650°C.

26. Spheroidizing Annealing (Cyclic annealing)
It is applied to high carbon steels and hardened alloy steel to soften
them and increase machinability.
1. Heat the steel above A1 temperature (730- 770oC).
2. Thermal cycling: Cooled slowly below A1 temperature (600oC).
Then again heated above A1 temperature and then again
cooled below A1 temperature.
3. This breaks down Fe3C in the Pearlite into a spheroidal shape

28. Effects of Microstructures

29. Normalizing
• It differs from annealing in that the metal is heated to a higher
temperature and then removed from the furnace for air
cooling.
• The purpose of normalizing is
1. To remove the internal stresses induced by heat treating,
welding, casting, forging, forming, or machining.
2. To increase strength of steel compared to annealing
3. Improve grain structure (more uniform)

31. Quenching: (Austenite to Martensite)
It is done to increase the hardness, strength and wear resistance
properties.
• One of the pre-requisites for hardening is sufficient carbon
and alloy content.
• To harden by quenching, a metal (usually steel or cast iron)
must be heated into the austenitic phase and then quickly
cooled to ambient.
• Cooling rate is more than CCR of steel
• Cooling medium are oils, water, brine solution etc.

32. Tempering
• Tempering is carried out by preheating previously quenched
or normalized steel to a temperature below the lower critical
temperature, holding for 1-2 hours, and then cooling in the
air.
• Tempering is used to reduce the brittleness of quenched steel.
• It improves toughness but reduces strength of the steel

33. • Depending on temperatures, tempering processes can be
classified as:
1. Low- temperature tempering (150 – 250 oC),
2. Medium – temperature tempering (300 – 450oC),
3. High – temperature tempering (500 – 650 oC).

34. Austempering and Martempering
• Austempering is an isothermal heat treatment that
produces a bainite structure in plain-carbon steels.
• Here, the steel is first heated to austenitic temperature, then
quenched in a molten salt bath at a temperature above the Ms
temperature of the steel (500oC),
• Hold the steel at this temperature to allow the austenite-to-
bainite transformation to take place, and then cooled to room
temperature in air.

37. • The martempering process consists of
1. Heating the steel to austenitic temperature,
2. quenching it in molten salt at a temperature just slightly
above the Ms temperature (350oC),
3. holding the steel in the quenching medium until the
temperature is uniform throughout and stopping just before
the austenite-to-bainite transformation begins, and
4. cooling at a moderate rate to room temperature to prevent
large temperature differences.
5. The steel is subsequently tempered by the conventional
treatment.

39. Surface Hardening
• Surface Hardening is a process by which a steel is
given a hard, wear resistant surface, while retaining
a ductile interior
• Surface hardening is usually done for the following
reasons: -
1. To improve wear resistance
2. To improve fracture toughness
3. To improve fatigue resistance

40. • Surface hardening techniques can be classified
into two major categories:
1. Processes that change the surface chemical
composition (case hardening)
2. Processes that do not change the surface
chemical composition (selective surface
hardening)

41. Case Hardening
Case hardening methods include:
1. Carburizing
2. Nitriding
3. Cyaniding

42. Carburizing:
• Carburizing is a hardening process in which carbon
is introduced into the surface layer of the steel.
• If a piece of low-carbon steel is placed in a carbon-
saturated atmosphere at an elevated temperature
(above austenite temperature), the carbon atoms
will diffuse (penetrate) into the steel.
• The steel is then tempered to the desired hardness

43. Types of Carburizing:
• Pack Carburizing: Charcoal and Barium
Carbonate
• Gas Carburizing: Methane or ethane
• Liquid Carburizing: Carbonaceous molten salt
bath (NaCN+ BaCO3)

44. Nitriding:
• In nitriding, the steel piece is heated in a furnace
between 500 – 575 oC and at the same time is
exposed to ammonia gas (NH3 ).
• The heat from the furnace causes the ammonia
to decompose into H2 and N.
• Nitrogen reacts with elements in the steel to
form nitrides in the outer layer of the steel
providing high hardness and wear resistance.

45. Cyaniding:
• This process also involves both the diffusion of C
and N into the surface layers of the steel.
• In cyaniding, the steel is heated in a liquid
cyanide bath (NaCN, Na2CO3 and NaCl) and
then quenched in brine, water or oil

46. Selective Surface Hardening Hardening
• Parts to be heat-treated are so large as to make
conventional furnace heating and quenching
impractical and uneconomical.
• Only a small segment, section, or area of the part
needs to be heat treated.

47. 1. Flame Hardening
A combustible gas flame (oxyacetylene torch) is the
source of heat for austenizing the steel, and the
transformation to martensitic structure is obtained by
rapid water quenching.
2. Induction hardening
• In induction hardening, an electric current is induced
in the work piece to produce heating effect.
• If a steel round piece is placed inside the induction
coil, induced current will heat the steel piece and can
be heated to its austenizing temperature and if after
heating, the steel piece is dropped in water bath,
quenching will occur.

48. Thank You