There are two main types
of transformation diagram that are helpful in selecting the
optimum steel and processing route to achieve a given set of
properties. These are time-temperature transformation
(TTT) and continuous cooling transformation (CCT) 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.
- Time-temperature transformation (TTT) diagrams
- measure the rate of transformation at a constant temperature.
In other words a sample is austenitised and then cooled rapidly
to a lower temperature and held at that temperature whilst the
rate of transformation is measured, for example by dilatometry.
Obviously a large number of experiments is required to build up
a complete TTT diagram.
- Continuous cooling transformation (CCT) diagrams
- measure the extent of transformation as a function of time for
a continuously decreasing temperature. In other words a sample
is austenitised and then cooled at a predetermined rate and the
degree of transformation is measured, for example by
dilatometry. Obviously a large number of experiments is required
to build up a complete CCT diagram.
Given below is a typical TTT diagram.
 Click on a temperature below
to show how the microstructure develops relative to the TTT
diagram.
TTT curve for a 1%C, 0.3%Si, 0.4%Mn steel
P - pearlite
Ms - martensite
start temperature.
Given below is a typical CCT diagram.
Click on a cooling rate below
to show how the microstructure develops relative to the CCT
diagram.
CCT diagram for a 0.4% C, 1.5% Mn, 0.5% Mo steel
As you might expect there is a significant influence of composition
on the TTT and CCT diagrams. You can review the influence
of composition (and grain size) on hardenability by reading
through the section on Jominy Quench
Tests. For the Jominy Quench Test samples we saw the effect
by the change in shape of the hardness curves for the transformation
diagrams we see the effect through a shift in the transformation
curves. For example:
- An increase in carbon content shifts the CCT and TTT curves to
the right (this corresponds to an increase in hardenability as
it increases the ease of forming martensite - i.e. the cooling
rate required to attain martensite is less severe).
- An increase in carbon content decreases the martensite start
temperature.
- An increase in Mo content shifts the CCT and TTT curves to the
right and also separates the ferrite + pearlite region from the
bainite region making the attainment of a bainitic structure
more controllable.
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