Why are TTT diagrams called isothermal temperature diagram?As far as I know the temperature variation is plotted with the variation of time in a TTT diagram. Then why do we call it "isothermal".What is so constant about the temperature?
They are two names for the same procedure. A steel is heated to a temperature in the austenite range then quenched/quickly cooled to some specific temperature. Held at that constant ( isothermal) temperature ( T) for a specific time (T) then quenched to room temperature for examination of the transformation (T) products. You may be thinking of CCT ; continuous cooling diagrams ; where a steel sample is austenitized then continuously cooled at some specific rate. Then examined for transformation products. The results can have similarities depending on alloy content , etc.