Continuous cooling transformation

A 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-transformation diagrams because it is more convenient to cool materials at a certain rate (temperature-variable cooling), than to cool quickly and hold at a certain temperature (isothermal cooling).

There are two types of continuous cooling diagrams drawn for practical purposes.

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Continuous cooling transformation diagram of EN S355 (ASTM A-572 Grade 50, warranted yield stress 355 MPa) for welding (fast cooling, linear scale).
TTT diagram for constant cooling rate transformations of steel.