TTT DIAGRAM | Time Temperature Transformation

TTT (time-temperature-transformation)


A diagram which can include all the three parameters of time, temperature and transformation will be of great importance, specially to the heat treaters. This diagram is popularly known as isothermal or time temperature transformation. The TTT curve is an extension of isothermal transformation of austenite diagram.

● The kinetics of austenitic transformation can be studies best at a constant temperature rather than by continuous cooling. The constant temperature transformation is also referred to as isothermal transformation. Here samples are heated to predetermined austenitizing temperature and are held at this temperature for a sufficiently long period so as to obtain a homogeneous austenite.

● These samples are taken out one by one from the subcritical temperature bath after different time interval and are quenched immediately. The quenching of sample results in the formation of martensite form the untransformed austenite. By this technique, the amount of transformed austenite can be determined as a function of time at constant temperature.

● After a particular time all the austenite will transform to an aggregate to ferrite and cementite at a given temperature. Transformation of austenite to ferrite - cementite mixture occurs after a definite time. This time during which transformation does not proceed is known as incubation period.

● The magnitude of incubation period provides a qualitative idea about the relative stability of super cooled austenite. Smaller incubation period corresponds to lesser stability of austenite.

Limitations of TTT Diagram:

1. If only related the amount of transform austenite with different transformation time for a constant temperature. Which shows " when a sample is heated at T4 temp and resulted with a minimum incubation period. This temperature known as Critical transformation temperature.
TTT diagram for eutectoid steel
TTT Diagram for eutectoid steel

2. It can be understand that the higher the transformation temperature the more is the incubation period and more time will be required for completion of this transformation.

3. In Cubation Period and transformation time decreased with lowering of transformation temperature. However after a particular temperature the decrease is reversed and both incubation period and transformation time increase again with further lowering the transformation temperature.

4. In above fig shows TTT diagram of hypoeutectiod and hypereutectoid steel. A common feature of these TTT dia is that proeutetoid phase separate out in upper temperature region.
TTT diagram for hypoeutectoid steel - TTT diagram for hypereutectoid steel
TTT diagram for hypoeutetoid and hypereutectic steel

5. For hypoeutectiod steel, ferrite starts separating out from the austenite as soon as austenite cooled below the upper critical temperature (A3). The amount of protection ferrite decrease as austenite is undercooled more and more below the lower critical temperature. Then after a certain degree austenite will transform directly to the pearlite. Similarly cementite is separate out in hypereutectoid steel from the austenite on cooling below the upper critical temperature (Acm). The amount of cementite decrease with increased degree of supercooling and finally reduces to zero when austenite is cooled below a particular temperature.

Important of TTT diagram :

1. It indicate the phase existing in Steel at various temperature and various time. It gives a useful information in heat treatment of steel.

2. One can choose proper heating and cooling cycle to obtained different transformation product as well as about required property in this component.

3. Where as iron-carbon equilibrium dia given a limited used in the study of steel cooled under non equilibrium condition. But this dia gives a principle source of information in the actual process if austenite decomposition under non equilibrium condition.

Effect of Alloying Elements on TTT Diagram :

● In an alloy steels except cobalt, other alloys decrease both the tendency for the rate of decomposition of austenite because of austenite stabilizing elements. Ferrite stabilize by forming carbide, alloy carbides are more stable then cementite and hence they related the diffusion of carbon which decrease the rate of decomposition of austenite.

● Since pearlite transformation involve diffusion of both carbon and metallic atoms, the effects of alloying elements is much more pronounced in pearlite region. The effect is less in bainitic region as a bainitic transformation involved diefision of carbon atoms only.
TTT diagram of alloy steels
TTT diagram for alloy steel
(a) with plain carbon steel
(b) with both pearlitic and bainitic bays
(c) without bainitic bay
(d) without pearlite bay

● The TTT diagram for alloys steel can be broadly classified as
1. TTT dia with resemblance of plain carbon steel in which the presence of carbide forming elements, supercooled austenite decompose to a mixture of ferrite and carbide rather then to an aggregate of ferrite and cementite.

2. With both pearlite and bainite byes : which consists of two minus with respect to the stability of austenite. The upper bay corresponds to the transform of austenite to pearlite and the lower bay transformed if austenite to bainite.

3. Without  bainite bays : This implies that bainite can not be formed in such steel. For high alloys steel specially those in which the start of martensitic transformed shifted to subzero region.

4. Without pearlite bay : Here under cooling conditions either bainite or martensite is formed.








References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharks,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.
Metallurgist.


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