Iron Carbon Phase Diagram

Iron Carbon Phase Diagram, TTT Diagram, CCT Diagram

1. Metallurgy &
Material Science
Dr.S.Jose
Professor, Dept of Mechanical Engg.,
TKM College of Engineering, Kollam

2. 2
Diffusion in crystals
Theory of Alloys
Equilibrium Diagrams
Iron Carbon Phase diagram
TTT Diagram
Heat Treatment
Recovery, Recrystallisation& Grain
Growth
Module II

3. Allotropes of Iron

4. Fe –Fe
3C
Phase Diagram

5. Five individual phases
a–ferrite (BCC) Fe-C solid solution
g-austenite (FCC) Fe-C solid solution
d-ferrite (BCC) Fe-C solid solution
Fe
3C (Iron Carbide) or cementite –
an inter-metallic compound
Liquid Fe-C solution

6. Three invariant reactions
A horizontal line always indicates an invariant
reaction in binary phase diagrams
Peritecticreaction at 1495˚C and 0.18%C,
d-ferrite + L↔g-iron (austenite)
Eutectic reaction at 1147˚C and 4.3 %C,
L↔g-iron + Fe
3C (cementite) [ledeburite]
Eutectoid reaction at 727˚C and 0.77%C,
g-iron ↔a–ferrite+Fe
3C (cementite) [pearlite]

7. Peritectic Reaction

9. Metals
Ferrous metals Non-ferrous metals
Steels Cast Irons
Plain carbon steels
Low alloy steels
High alloy steels
Stainless & Tool steels
Grey Iron
White Iron
Malleable & Ductile Irons
Low carbon steels
Medium carbon steels
High carbon steels
Fe-C alloy classification

10. Fe-C alloy classification
Fe-C alloys are classified according to wt.% C
present in the alloys
Commercial pure irons% C < 0.008
Low-carbon steels 0.008 -%C -0.3
Medium carbon steels 0.3 -%C -0.8
High-carbon steels 0.8-%C -2.14
Cast irons 2.14 < %C

11. Cast irons
Cast irons that were slowly cooled to room
temperature consists of cementite, look whitish
–white cast iron.
If it contains graphite, look grayish –gray cast
iron.
It is heat treated to have graphite in form of
nodules –malleable cast iron.
If inoculants are used in liquid state to have
graphite nodules –spheroidal graphite (SG)
cast iron.

14. Eutectoid Reaction727
3
o
C
Fe Cga¾ ¾ ¾® +
0.77 0.022 6.67
cool
Pearlite
Eutectoid steel

15. Hypoeutectoid steel
Proeutectoid
Ferrite
Pearlite
Microstructure of 0.38 wt% C
hypoeutectoid steel

16. Hypereutectoid steel
Pearlite
Proeutectoid
cementite
Microstructure of 1.4 wt% C
hypereutectoid steel

17. Eutectoid
steel
a+Fe
3C
Pearlite
Hypoeutectoid
steel
a+Fe
3C
Pearlite +
proeutectoid ferrite
Hypereutectoid
steel
a+Fe
3C
Pearlite +
proeutectoid
cementite

18. Phase vs. Microconstituents
A phase or a mixture of phases which has a
distinct identity in a microstructure is called a
microconstituent
Pearlite is not a phase.
It is a microconstituent and is a mixture of two
phases a-Ferrite and Fe
3C.

19. a-Ferrite
Known as a-iron
Pure iron at room temperature
Body-centered cubic structure
Soft & ductile and imparts these
properties to the steel.
Less than 0.01% carbon will dissolve in
ferrite at room temperature
High temperature form is dferrite, but
the two forms are identical.
Pure ferritic steels are rare

20. Austenite
Known as g-iron
Face-centered cubic
Much softer than ferrite
Not present at room temperatures.
More easily hot worked

21. Cementite
Iron Carbide -an intermetallic compound
Hard, brittle, white
melts at 1837C , density of 7.4 g/cc
On the phase diagram, cementite corresponds
to a vertical line at 6.7% C
Engineers care only about compounds with less
carbon
Its presence in steels causes an increase in
hardness and a reduction in ductility and
toughness

22. Pearlite
A laminated structure formed of alternate
layers of ferrite and cementite withaverage
composition 0.83% carbon
Pearly lustre in the microscope
Interference of light in its regular layers
Most common constituent of steel
It combines the hardness and strengthof
cementite with the ductilityof ferrite and is the
key to the wide range of the properties of
steels.
The laminar structure also acts as a barrier to
crack movement as in composites. This gives
it toughness

24. Phase Transformations
Involve some alteration of microstructure
1.No change in number or composition of the
phases present, diffusion-dependent.
Solidification of pure metals, allotropic
transformation.
2.Some alteration in composition and no of
phases, diffusion –dependent. Eutectoid
reaction
3.A metastable phase is produced,
diffusionless. Martensitic transformation.

25. At least one new phase is formed
Do not occur instantaneously
Begin by the formation of small particles
of new phase –nucleation
Homogenous –occurs uniformly throughout
the parent phase.
Hetrogenous –preferentially at grain
boundaries, impurities, dislocations
Size of these particles increase in size
until completion -growth
Phase Transformations

26. Dependent on
Temperature
Time
Composition
Require some finite time for completion
Equilibrium is rarely achieved in solids
Metastable –intermediate between
initial and equilibrium states.
Phase Transformations

27. Time-Temperature
Transformation Diagram

28. Time-Temperature
Transformation Diagram

29. Complete
Time-Temperature
Transformation Diagram

30. Time-Temperature
Transformation Diagram

31. T T T
Diagram

32. Pearlite 727 -540C
Bainite 540 -210C
Martensite below 210C
Transformation of Austenite in
Eutectoid steel

33. Transformations involving
austenite

34. CCT diagram
Usually materials are cooled continuously, thus
Continuous Cooling Transformation diagrams
are appropriate than TTT diagrams
For continuous cooling, the time required for a
reaction to begin and end is delayed, thus the
isothermal curves are shifted to longer times
and lower temperatures.
Main difference between TTT and CCT
diagrams: no space for bainite in CCT diagram
as continuous cooling always results in
formation of pearlite.

35. CCT diagram

36. CCT diagram