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Underlying metallurgy

Transformation diagrams (CCT & TTT)    
 
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.

Have a go!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.

Have a go!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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