CCT and TTT Diagrams: Understanding and Reading Transformation Diagrams

CCT and TTT diagrams show which microstructures form in a steel when it is cooled in a particular way after austenitizing. CCT stands for continuous cooling transformation, TTT for time-temperature-transformation, which usually refers to the isothermal diagram. In German, both are called ZTU diagrams. They are a central tool for heat treatment, for example hardening and quenching and tempering, and an important input for simulating heat treatment processes.

What a transformation diagram shows

A transformation diagram describes the microstructural evolution of a single material for different time-temperature paths. Temperature is plotted vertically and time horizontally, usually on a logarithmic scale. The diagram shows the start and end of transformations, the resulting microstructures and often the hardness reached.

A complete diagram also states the material including the chemical composition of the tested heat, the initial microstructure and the austenitizing conditions. Without this information, a diagram cannot be interpreted reliably.

Transformation diagrams are usually determined with a quenching dilatometer: the change in length of a specimen shows when a transformation starts and ends. The microstructure that has formed, and its fraction, is then determined metallographically.

Continuous and isothermal diagrams

There are two fundamentally different types, which must be kept strictly apart:

• CCT diagram (continuous cooling): after austenitizing, specimens are cooled to room temperature along different cooling paths. The diagram contains these cooling curves. It fits processes in which a part cools continuously, for example in air, oil or water.
• TTT diagram (isothermal): after austenitizing, the specimen is cooled very quickly to a temperature and held there until the transformation is complete. It fits processes with a holding step, for example isothermal transformation in the bainite range.

An isothermal diagram cannot simply be applied to continuous cooling, and vice versa.

Microstructures in the diagram

Depending on the cooling, austenite transforms into different microstructures:

• Ferrite and pearlite form with slow cooling or at high transformation temperatures through diffusion-controlled transformation. Pearlite consists of ferrite and cementite.
• Bainite forms at temperatures and cooling rates between those for pearlite and martensite. Diffusion and shear processes in the crystal lattice act together.
• Martensite forms with rapid cooling below the martensite start temperature Ms. The transformation is diffusionless, by a shear movement of the lattice. Martensite is metastable and, with sufficient carbon content, very hard.

Depending on the steel, further constituents may occur, for example retained austenite or carbides.

How to read a CCT diagram

1. Check the conditions: compare material, composition of the tested heat and austenitizing conditions with your own case.
2. Find the characteristic lines: the transformation temperatures Ac1 and Ac3 determined on heating are usually drawn as horizontal lines at the top, the martensite start temperature Ms at the bottom. The regions for ferrite, pearlite and bainite lie in between.
3. Read only along a cooling curve: a CCT diagram may only be read along the cooling curves drawn in it. Where a curve enters or leaves a region, the corresponding transformation starts or ends.
4. Read microstructure fractions and hardness: the numbers along the curve usually give the percentage of each microstructure. The hardness reached is often given at the end of the curve.
5. Match your cooling path: compare the cooling of your own part with the curves in the diagram. The result is an estimate, not an exact prediction.

A TTT diagram, by contrast, is read horizontally: at the level of the holding temperature, the lines show after which holding time the transformation starts and ends.

Limits of transformation diagrams

• Valid for one material only: a diagram is strictly valid only for the tested composition and the respective austenitizing condition. Differences within the permitted composition range of a steel grade or a different austenitizing temperature can already change the transformation behavior.
• A specimen is not a part: the cooling curves apply to small specimens. In a real part, surface and core cool at different rates, so different microstructures can form at different locations.
• History matters: forming before cooling affects the transformation behavior; there are separate deformation CCT diagrams for this. For welding there are welding CCT diagrams, and for heating there are time-temperature-austenitization (TTA) diagrams.

Finding and calculating transformation data

Measured transformation diagrams are published in collections. The phase transformation data in StahlDat SX comprise more than 400 diagrams based on the atlas on heat treatment of the VDEh. They can be searched by phase composition at different austenitizing temperatures and compared in tables. Steel grades can be found there by material number and steel name.

If no measured diagram is available for a composition, materials simulation helps: JMatPro® calculates continuous and isothermal transformation diagrams as well as time-temperature-austenitization diagrams, and thus also material properties as a function of cooling and heating rate. EDA MM visualizes multidimensional data such as transformation diagrams, and materials data management keeps them together with test and process data.

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Frequently asked questions

What is the difference between a CCT/TTT diagram and the iron-carbon phase diagram?

The iron-carbon phase diagram describes equilibrium states that are reached with very slow temperature changes. CCT and TTT diagrams show which microstructures form after which time for a given cooling path or holding temperature, including non-equilibrium microstructures such as bainite and martensite. They also apply to one specific steel only.

What does Ms mean in a CCT or TTT diagram?

Ms is the martensite start temperature. Below this temperature, the diffusionless transformation of austenite into martensite begins. In the diagram, Ms is usually drawn as a horizontal line.

What do the numbers on the cooling curves mean?

Numbers where a cooling curve crosses the transformation regions usually give the percentage of the microstructure formed in that region. The hardness reached, for example in HV, is often given at the end of the cooling curve.

Can CCT and TTT diagrams be calculated?

Yes. Simulation software such as JMatPro calculates continuous and isothermal transformation diagrams for given alloy compositions. Calculated diagrams complement measured diagrams; where exact values for a specific heat matter, testing is still needed.