Mumbai University. Mechanical Engineering SEM III Material Technology MOdule 3 TTT diagram, CCT diagram Hardenability concepts and tests, Graphitization of Iron- Grey iron, white iron, Nodular and malleable irons, their microstructures, properties and applications

1. Chapter 3.
Theory of Alloys and Alloys
Diagrams
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2. Phase Transformations in Steels
• Iron has having different crystal structures at different
temperatures.
• It changes from FCC to BCC at 910o C.
• This transformation results in austenite
transforming to pearlite at eutectoid
temperature.
• This transformation of austenite is time dependant.
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3. Fe-C Equilibrium Diagram
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4. Fe-C Equilibrium Diagram
 Though the Fe-C equilibrium diagram is very useful, it does
not provide information about the transformation of austenite
to any structure other than equilibrium structures, nor does it
provide any details about the influence of cooling rates on the
formation of different structures.
 In other words, Fe-C diagram does not explain the
decomposition of austenite under non-equilibrium
conditions or conditions involving faster rates of cooling
than equilibrium cooling.
 Several structures (e.g. martensite) not appearing on the
equilibrium diagram may be found in the microstructures in
steels.
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5. TTT Diagram
 On the other hand, TTT diagram is a more
practical diagram.
 It shows what structures can be expected after
various rates of cooling.
 It graphically describes the cooling rate
required for the transformation of austenite to
pearlite, bainite or martensite.
 TTT diagram also gives the temperature at
which such transformations take place.
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6. Phase diagram :
– Describes equilibrium
microstructural development
that is obtained at extremely
slow cooling or heating
conditions.
– Provides no information on
time taken to form phase
TTT diagram
For a given alloy composition, the
percentage completion of a given
phase transformation on
temperature-time axes is described.
Phase diagram and TTT diagram
Which information are obtained from phase diagram or TTT
diagram?
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7. Transformation Diagrams
 There are two main types of transformation
diagrams that are helpful in selecting the
optimum steel and processing route to achieve
a given set of properties. These are
 Time-temperature transformation (TTT) diagrams
 Continuous cooling transformation (CCT)
diagrams
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8. Transformation Diagrams
 Time-temperature transformation (TTT) diagrams
 Indicates the amount of transformation at a
constant temperature.
 In TTT diagrams, after cooling to a transformation
temperature, you keep the temperature constant
until the transformation of austenite to the required
transformation product (usually pearlite or bainite)
is complete and then cool to the room temperature.
 One such process is austempering in which
austenite is transformed to bainite isothermally
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9. TTT Diagram
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10. Transformation Diagrams
 Continuous cooling transformation (CCT) diagrams
 Indicates the extent of transformation as a function of time
for a continuously decreasing temperature.
 In CCT diagrams, there is continuous cooling i.e. there is
no holding of temperature.
 The components are cooled at a constant or varying rates.
 The end products are usually martensite or pearlite
depending on the cooling media as well as the material of
components.
 Fully bainitic structure cannot be obtained using continuous
cooling.
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11. Transformation Diagrams
 CCT diagrams are generally more appropriate
for engineering applications as components
are cooled (air cooled, furnace cooled,
quenched etc.) from a processing temperature
as this is more economic than transferring to a
separate furnace for an isothermal treatment.
 CCR- Slowest cooling rate at which unstable
austenite can be transformed into martensite
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12. CCT diagram
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15. Thanks
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