CCT DIAGRAM | Continuous Cooling Transformation

Continuous Cooling Transformation (CCT Diagram)

The CCT diagram gives information about the hardening response of steels and the nature of transformed product of austenite at varying degrees of supercooling. These diagram have been great practical importance to some special heat treatment process such as austempering and isothermal annealing.

● In practice However transformation during heat treatment occurs by continuous cooling and not isothermally. Thus TTT diagram have limited application. For most of the heat treatment processes these diagram are useful only qualitative and not quantitative. A diagram which can correctly transformation, temperature and time during continuous cooling will be of real value to heat treater.

CCT diagram of eutectoid steel
CCT diagram of eutectoid steel

● Continuous cooling transformation - CCT diagram can be obtained by a technique which is similar to that for TTT diagram except that, in the case of CCT diagram, points of start and end of austenitic transformation are recorded on continuous cooling.

● For the construction of CCT diagram for a eutectoid steel a large number of small sample are heated above the lower critical temperature (A1) to get a completely austenite structure.

● From this temperature specimen are cooled at a constant cooling rate, and points corresponding to start and Finish of pearlite are determine. By repeating the same process at various cooling rates, different sets of start and end points for pearlitic transformation are obtained.

● On joining start and end points two curves similar to those in TTT diagram corresponding to start and end of transformation are obtained.

● continuous cooling transformation (CCT) phase diagram is often used when heat treating steel.[1]These diagrams are used to represent which types of phase changes will occur in a material as it is cooled at different rates. These diagrams are often more useful than time-temperature-transformationdiagrams because it is more convenient to cool materials at a certain rate than to cool quickly and hold at a certain temperature.

● In CCT diagrams, there is continuous cooling i.e. there is no holding of temperature. The components are cooled at a constant or varying rates. The end products are usually martensite or pearlite depending on the cooling media as well as the material of components. Fully bainitic structure cannot be obtained using continuous cooling. Below is a CCT diagram.

● So a CCT diagrams simply gives the various transformation products which will be obtained at different cooling rates. It can be seen from the diagram that at cooling rates of more than 100 degrees celsius, ferrite and martensite will be obtained; for cooling rates between 20 and 100 degrees celsius, ferrite, bainite and martensite will be obtained and so on. These cooling rates are dependent on the cooling media. 

● CCT diagrams are more practical than TTT diagrams as most of the processes employ continuous cooling rather than isothermal transformation. Also it is more difficult to hold the temperature constant.

Types of CCT diagram :

There are two types of continuous cooling diagrams drawn for practical purposes
● Type 1: This is the plot of transformation start, a specific transformation fraction and transformation finish temperature for all products against transformation time on each cooling curve.
● Type 2: This is the plot of transformation start, a specific transformation fraction and transformation finish temperature for all products against cooling rate or bar diameter of the specimen for each type of cooling medium.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharks,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Special thanks to quora and Wikipedia for given more information about this topic.

Author :
Subir Kumar Sahu.

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