Isothermal Transformation Temperature: The isothermal transformation temperature is commonly known as the Triple T Diagram (TTT).

It is a diagram that is used to assist the heat treater in knowing how to select the critical cooling curve of the steel being quenched or cooled to achieve a particular metallurgical structure. It also is known as the isothermal transformation diagram (IT diagram). This diagram is consulted for a process known as isothermal annealing.