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

4. Fe – Fe3C
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
 Fe3C (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
 Peritectic reaction 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 + Fe3C (cementite) [ledeburite]
 Eutectoid reaction at 727˚C and 0.77%C,
 g-iron ↔ a–ferrite+Fe3C (cementite) [pearlite]

7. Peritectic Reaction

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

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 steel
Eutectoid Reaction
727o C
g ¾ cool¾® a + Fe3C
¾
0.022
6.67
0.77
Pearlite

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
Hypoeutectoid
steel
Hypereutectoid
steel
a+Fe3C
a+Fe3C
a+Fe3C
Pearlite
Pearlite +
proeutectoid ferrite
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 Fe3C.

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 d ferrite, but
the two forms are identical.
 Pure ferritic steels are rare
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20. Austenite
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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 1837 C , 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
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22. Pearlite
 A laminated structure formed of alternate
layers of ferrite and cementite with average
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 strength of
cementite with the ductility of 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. Phase Transformations
 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

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

27. Time-Temperature
Transformation Diagram

28. Time-Temperature
Transformation Diagram

29. Time-Temperature
Transformation Diagram
Complete

30. Time-Temperature
Transformation Diagram

31. TTT
Diagram

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

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