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.

Given below is a typical TTT diagram.

TTT diagram
Time-temperature isothermal transformation (TTT) diagram for
AISI type 6150 carbon steel, austenitized at 850°C.

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 CCT diagram.

CCT diagram
Continuous cooling transformation (CCT) diagram for
AISI type M3/2 steel.

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