TTT DIAGRAM / ISOTHERMAL TRANSFORMATION DIAGRAM
Time-Temperature-Transformation (TTT) diagram or S-curve refers to only one steel of a particular composition at a time, which is applicable to all carbon steels. This diagram is also called as C-curve isothermal (decomposition of austenite) diagram and Bain’s curve. The effect of time-temperature on the microstructure changes of a steel can be shown by TTT diagram. These diagrams are extensively used in the assessment of the decomposition of austenite in heat-treatable steels. We have seen that the iron-carbon phase diagram does not show time as a variable and hence the effects of different cooling rates on the structures of steels are not revealed. Moreover, equilibrium conditions are not maintained in heat treatment. Although, the iron-carbon equilibrium diagram reveals on the phases and corresponding microstructures under equilibrium conditions but several useful properties of the steels can be obtained under non-equilibrium conditions, e.g. variable rates of cooling as produced during quenching and better transformation of austenite into pearlite and martensite.
For each steel composition, different IT diagram is obtained. Fig shows TTT diagram of eutectoid steel (i.e. steel containing 0.8% C).
Austenite is stable above eutectoid temperature 727 °C. When steel is cooled to temperature below this eutectoid temperature, austenite is transformed into its transformation product. TTT diagram relates transformation of austenite to time and temperature conditions. Thus, TTT diagram indicates
transformation product according to temperature and also time required for complete transformation.
Curve 1 is transformation begin curve while curve 2 is transformation end curve. The region to the left of curve 1 corresponds to austenite (A’). The region to the right of curve 2 represents complete transformation of austenite (F+C). The interval between these two curves indicates partial decomposition of austenite into ferrite and Cementite (A’+F+C).
At temperatures just below eutectoid temperature, austenite decomposes into pearlite; at lower temperatures (600 deg C) sorbite is formed and at 500 – 550 degree C troostites is formed.
If temperature is lowered from 500 deg C to 220 deg C acicular troostite or bainite is formed. In eutectoid steels, the martensite transformation begins at MS (240 deg C) and ends at MF (50 deg C).
The change in the hardness of the structures is shown in Rockwell units (RC) at the right hand side of the diagram.