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What is the T-T-T diagram?

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).

TTT Diagram for Eutectoid Steel :

TTT Diagram For steel — TTT Diagram For eutectoid steel

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 the 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.

Time-Temperature-Transformation (TTT) Curve For AISI 1080 steel

Isothermal transformation diagrams (also known as time-temperature-transformation (TTT) diagrams) are plots of temperature versus time (usually on a logarithmic scale). They are generated from percentage transformation-vs logarithm of time measurements and are useful for understanding the transformations of an alloy steel that is cooled isothermally. An isothermal transformation diagram is only valid for one specific composition of material, and only if the temperature is held constant during the transformation, and strictly with rapid cooling to that temperature. Though usually used to represent transformation kinetics for steels, they also can be used to describe the kinetics of crystallization in ceramic or other materials. Time-temperature-precipitation diagrams and time-temperature-embrittlement diagrams have also been used to represent kinetic changes in steels.

The TTT diagram for AISI 1080 steel (0.79%C, 0.76%Mn) austenitized at 900°C

Time Temperature Transformation for steel

Isothermal Transformation (IT)

Diagram or the C-curve is associated with mechanical properties, micro constituents/microstructures, and heat treatments in carbon steels. Diffusional transformations like austenite transforming to a cementite and ferrite mixture can be explained using the sigmoidal curve; For example the beginning of pearlitic transformation is represented by the pearlite start (Ps) curve. This transformation
is complete at Pf curve. Nucleation requires an incubation time. The rate of nucleation increases and the rate of micro constituent growth decreases as the temperature decreases from the liquidus temperature reaching a maximum at the bay or nose of the curve. Thereafter, the decrease in diffusion rate due to low temperature offsets the effect of increased driving force due to the greater difference
in free energy. As a result of the transformation, the micro constituents, Pearlite and Bainite, form; Pearlite forms at higher
temperatures and bainite at lower.
Austenite is slightly undercooled when quenched below Eutectoid temperature. When given more time, stable microconstituents can form ferrite and cementite.
Coarse pearlite is produced when atoms diffuse rapidly after phases that form pearlite nucleate. This transformation is complete at the pearlite finish time (Pf).
However, greater undercooling by rapid quenching results in the formation of martensite or bainite instead of pearlite. This is possible provided the cooling rate is such that the cooling curve intersects the martensite start temperature or the bainite start curve before intersecting the Ps curve. The martensite transformation being a diffusionless shear transformation is represented by a straight line to signify the martensite start temperature.

TTT diagram gives

TTT diagram is used to find out the following parameters,

1- Nature and type of transformation.
2- Rate of transformation.
3- Stability of phases under isothermal transformation conditions.
4- Temperature or time required to start or finish the transformation.
5- Qualitative information about the size scale of the product.
6- Hardness of transformed products.

Factors affecting TTT diagram

1- Composition of steel-
(a) carbon wt%,
(b) alloying element wt%
2- Grain size of austenite
3- Heterogeneity of austenite.
4. Effect of Alloying Elements on the TTT Curve:

Alloying elements like Cr, Ni, Mo, and W can have two effects:
i. To shift the nose of Austenite to pearlite transformation diagram to longer times.
ii. Formation of a separate Bainite Nose.

Limitations of TTT Diagram:

i. Only applicable for an iron-carbon alloy of eutectoid composition thus for other compositions, curves will have different configurations.
ii. These plots are accurate only for transformations in which the temperature of allow is held constant throughout the duration.

Applications of TTT diagrams

• Martempering –

The process of quenching an austenitized ferrous alloy is a medium at a temperature in the upper part of the martensite range, or slightly above that range, and holding it in the medium until the temperature throughout the alloy is substantially uniform is known as martempering. The alloy is then allowed to cool in air through the martensite range.

• Austempering –

The heat treatment for ferrous alloys in which a part is quenching from the austenitizing temperature at a rate fast enough to avoid the formation of ferrite or pearlite and held at a temperature just above that of martensite formation until transformation to bainite is complete is called austempering.

• Isothermal Annealing –

Annealing means heating the material to and holding at a suitable temperature and then cooling at a suitable rate. Annealing is for the purpose of
i) Reducing hardness
ii) Improving machinability
iii) Facilitating cold working
iv) Producing the desired microstructure and
v) Obtaining desired mechanical, physical, and other properties.

• Patenting –

A special application of isothermal hardening is called patenting and is used for steel wire. Steel wire with 0.40 – 1.10% carbon is quenched from the hardening temperature in a bath of molten lead to about 400 degrees C to 500 degrees C. A structure results with possesses good ductility in addition to a hardness.

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