BeginnerTopic 20 of 100Materials & Chemical
TTT and CCT diagrams
Show transformation of austenite with time and cooling rate.
In depth
TTT (isothermal) and CCT (continuous cooling) diagrams show how austenite transforms to pearlite, bainite or martensite with time and temperature; the nose defines the critical cooling rate to get martensite. Ms and Mf mark martensite start and finish.
Key points
- C-curve nose
- Bainite formed isothermally
- Alloying shifts curves right
ExampleQuenching faster than the critical rate avoids pearlite and gives full martensite.
What is TTT and CCT diagrams?
Show transformation of austenite with time and cooling rate.
Why is it important to understand TTT and CCT diagrams?
Transformation diagrams show what structure forms at each cooling rate, which predicts the hardness and microstructure of heat-treated parts.
What does a TTT diagram show?
The transformation of austenite at a fixed temperature against time, with curves for the start and end of each transformation product.
What is the difference between a TTT and a CCT diagram?
A TTT diagram is for isothermal holds, while a CCT diagram shows the transformation during continuous cooling at different rates, which is closer to real quenching.
How is TTT and CCT diagrams used in practice?
Typical applications include predicting whether a cooling rate in a forging will produce martensite, designing a quench for a gear to reach a required hardness and reading the time to start the bainite transformation at a fixed temperature.
What should I learn before TTT and CCT diagrams?
Study "Heat treatment of steel" first (topic 19 of 100 in Materials & Chemical). It covers the groundwork this topic builds on.
What are the key points of TTT and CCT diagrams?
C-curve nose. Bainite formed isothermally. Alloying shifts curves right.
Can you give an example of TTT and CCT diagrams?
Quenching faster than the critical rate avoids pearlite and gives full martensite.
What comes after TTT and CCT diagrams?
The next topic is "Hardenability", which builds directly on this one.
How difficult is TTT and CCT diagrams?
It is classed as Beginner in Materials & Chemical (topic 20 of 100). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.