The time–temperature–transformation curves correspond to the start and finish of transformations which extend into the range of temperatures where austenite transforms to pearlite (Figure 24.2). Above 550°C, austenite transforms completely to pearlite. Below 550°C, both pearlite and bainite are formed and below 450°C, only bainite is formed. The horizontal dotted line that runs between the two curves marks the beginning and end of isothermal transformations. The dashed line that runs parallel to the solid line curves represents the time to transform half the austenite to pearlite.
Figure 24.2 TTT Diagram
The transformations through various paths are described as follows:
- Path 1: The specimen is cooled rapidly to 160°C and left for 20 min. The cooling rate is too rapid for pearlite to form at higher temperatures; therefore, the steel remains in the austenitic phase until the Ms temperature is passed, where martensite begins to form. Since 160°C is the temperature at which half of the austenite transforms to martensite, the direct quench converts 50% of the structure to martensite. Holding at 160°C forms only a small quantity of additional martensite, so the structure can be assumed to be half martensite and half retained austenite.
- Path 2: The specimen is held at 250°C for 100 s, which is not long enough to form bainite. Therefore, the second quench from 250°C to room temperature develops a martensitic structure.
- Path 3: An isothermal hold at 300°C for 500 s produces a half bainite and half austenite structure. Cooling quickly would result in a final structure of martensite and bainite.
- Path 4: Austenite converts completely to fine pearlite after 8 s at 600°C. This phase is stable and will not be changed on holding for 100,000 s at 873 K. The final structure, when cooled, is fine pearlite.
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