Need for TTT Diagram:-

It shows what structures can be expected after various rates of cooling. It graphically describes the cooling rate required for the transformation of austenite to pearlite, bainite or martensite. TTT diagram also gives the temperature at which such transformations take place

☆☆ TTT Diagram( Deep Explanation):-

T-T-T diagram is also called isothermal transformation diagram [Temperature-Time –Transformation]. It is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin and end for an isothermal [constant thermal] heat treatment of a previously austenitized alloy.

TTT Diagram For Eutectoid Steel ( Isothermal Transformation Diagram )

Time-Temperature-Transformation (TTT) diagram or S-curve refers to only one steel of a particular composition at a time, which applies 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 steel can be shown by the 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 the 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 a temperature below this eutectoid temperature, austenite is transformed into its transformation product. TTT diagram relates the transformation of austenite to time and temperature conditions. Thus, the TTT diagram indicates transformation products according to temperature and also the time required for complete transformation.

Curve 1 is transformation begin curve while curve 2 is the transformation end curve. The region to the left of curve 1 corresponds to austenite (A’). The region to the right of curve 2 represents the complete transformation of austenite (F+C). The interval between these two curves indicates partial decomposition of austenite into ferrite and Cementite (A’+F+C)

Need for TTT Diagram:-

It shows what structures can be expected after various rates of cooling. It graphically describes the cooling rate required for the transformation of austenite to pearlite, bainite or martensite. TTT diagram also gives the temperature at which such transformations take place

☆☆ TTT Diagram( Deep Explanation):-

T-T-T diagram is also called isothermal transformation diagram [Temperature-Time –Transformation]. It is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin and end for an isothermal [constant thermal] heat treatment of a previously austenitized alloy.

TTT Diagram For Eutectoid Steel ( Isothermal Transformation Diagram )

Time-Temperature-Transformation (TTT) diagram or S-curve refers to only one steel of a particular composition at a time, which applies 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 steel can be shown by the 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 the 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 a temperature below this eutectoid temperature, austenite is transformed into its transformation product. TTT diagram relates the transformation of austenite to time and temperature conditions. Thus, the TTT diagram indicates transformation products according to temperature and also the time required for complete transformation.

Curve 1 is transformation begin curve while curve 2 is the transformation end curve. The region to the left of curve 1 corresponds to austenite (A’). The region to the right of curve 2 represents the complete transformation of austenite (F+C). The interval between these two curves indicates partial decomposition of austenite into ferrite and Cementite (A’+F+C)