Isothermal transformation curves or TTT curves (Temperature – Time – Transformation)

It is to Edgar Bain that we owe the systematic study of the kinetics of constant temperature transformations (isothermal transformations) of ferrous alloys from the austenitic structure. The ferrous alloy (steel or cast iron) is

  • Heated and brought into the austenitic range;
  • Transferred almost instantaneously into a thermostatic bath held at a constant temperature below the austenization temperature;
  • Held at a temperature T2  for a preset timet
  • Taken, rapidly brought to ambient temperature and subjected to microstructural investigation.

By repeating this series of operations for different times at the same temperature T2 and at gradually decreasing temperatures T2, it is possible to identify, for each T2 value, the incubation time ti of the transformation of the austenite (i.e., the time at which transformation begins) and the time Tf at which transformation is completed.

Depending on the isothermal treatment temperature and the specific composition, the ferrous alloy will have a microstructural evolution with the formation of ferrite, pearlite, bainite, martensite (or mixtures thereof).

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In a temperature-time diagram with the time expressed in a logarithmic scale, by plotting the times ti and tf indicating the start and end time of austenite transformation at each temperature T2, two transformation start and end curves are obtained.

The TTT diagram typically identifies three regions:

  • The first, on the left, in which the austenitic structure is unstable and destined to transform;
  • The second, between the start and end transformation curves, in which the austenitic structure still coexists with the microstructures derived from its transformation;
  • The third, on the right of the end transformation curve, consisting exclusively of the structures generated by the austenite transformation: as the isothermal treatment temperature changes, these may be ferrite, pearlite or bainite.

The two start and end transformation curves are limited below, at the level where the two horizontal lines appear, corresponding to the no-longer-isothermal transformation of the austenite in martensite. These lines are identified as Ms (martensite start) and Ms (martensite finish).

Non-isothermal transformation curves or CCT curves (Continuous Cooling Transformation)

In the most common heat treatments, austenite transformation does not occur at a constant temperature but rather in the course of continuous cooling. Therefore, it is important to refer to the austenite non-isothermal transformation curves, that is, the CCT diagrams. These curves are obtained by marking, on each cooling trajectory, the start and end austenite transformation points and identifying the regions corresponding to the various microstructures obtained.

The position and form of the non-isothermal curves depend on the carbon content and presence of alloying elements in the cast iron.

For example, the following figure shows the TTT diagram of cast irons with different Molybdenum (Mo) content. The numbers shown at the cooling trajectories correspond to the hardness characteristics of each obtained microstructure.

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