T.T.T Diagram

T.T.T Diagram

Introduction

T (Time) T (Temperature) T (Transformation) diagram is a plot of temperature verses the logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin & end for an isothermal (constant temp.) heat treatment of a previously austenitized alloy. When austenite is cooled slowly to a temp. below LCT (Lower Critical Temp.), the structure that is formed is pearlite. As the cooling rate is increases, the pearlite transformation temp. gets lower. The microstructure of the material is significantly altered as the cooling rate increases. By heating & cooling a series of samples, the history of the austenite transformation may be recorded.

The principle source of information on the actual process of austenite decomposition under non-equillibrium condition is the T.T.T diagram, which relates the transformation of austenite to the time & temp. conditions to which it is subjected.

Importance of T.T.T Diagram:-
1. It indicates the phases existing in steel at various temp. verses time.
2. Due to this diagram one can choose proper heating & cooling cycle to obtain different property in the component.
3. It indicates when a specific transformation starts & ends and it also shows what percentage of transformation of austenite at a particular temp. is achieved.
Steps to construct a T.T.T Diagram
1. Obtain a large no. of relatively small specimens & placed them in a molten salt bath held at the proper austenitizing temp. for long period to form complete austenite.
2. When austenilized, the samples are quickly transformed to an other molten salt bath held at the desire reaction below A1.
3. After a given specimen has been allowed to react isothermally for a certain time. It is quenched in cold water or iced brine.
The first specimen may be allowed to react isothermally for 2 secs, second for 4 secs, third for 8 secs, fourth for 15 secs & so on upto say 15 hours.
4. As the specimen is quenched in water this stops the isothermal reaction by causing the remaining austenite to change almost instantly to martensite.
In microstructure both pearlite & martensite can be seen. Pearlite is the result of isothermal heat treatment & its amount depends upon the time permitted for isothermal reaction to continue. Martensite is the result of water quenching of the specimen after the isothermal heat treatment.
5. When large no. of specimens isothermally reacted for varying time periods are metallographically examined.
6. The result obtained from a series of isothermal reaction curves over the whole temp. range of austenite instability for a given composition of steel is summarized, the result is T.T.T diagram for that steel.
T.T.T Diagram for an Eutectoid Steel:-
 Austenite is stable above A1 temp. line & below this lne austenite is unstable i.e. it can transform into peralite, bainite or martensite.
 In addition to the variation in the rate of transformation with temp., there are variations in the structure of the transformation products also.
 Transformations at temperatures between approximately 1300F & 1020F (550C) result in the charecteristics lamellar microstructure of peralite. At a temp. just below A1 line, nucleation of cementite from austenite will be very slow. But diffusion & growth of nuclei will proceed at maxm speed, so that there will be few large lamellar & the peralite will be coarse.
However the transformation temp. is lowered i.e. it is just above the nose of the c-curve, the peralite becomes fine.
 At temp. between 1020F & 465F, transformation becomes more sluggish as the temp. falls, for although austenite becomes increasingly unstable. The slower rate of diffusion of carbon atoms in austenite at lower temperatures outstrip the increased urge of the austenite to transform. In this temp. range the transformation product is bainite.

Bainite consists of a ferrite matrix in which particles of cementite are embedded. The individual particles are much finer than in pearlite.

The appearance of bainite may vary between feathery mass of fine cementite & ferrite for bainite formed around 900F & dark acicular crystals for bainite formed in the region of around 600F.
 At the foot of the T.T.T diagram, there are two lines Ms (240C or 465F) & Mf(-50C).
Ms reperesents the temp. at which the formation of martensite will start & Mf, the temp. at which the formation of martensite will finish during cooling of austenite through this range. Mf is a fairly low temp.
 Martensite is formed by the diffusionless transformation of austenite on rapid cooling to a temp. below 240C designated as Mf temp.
The martensitic transformation differs from the other transformation in that it is independent of holding time & occurs almost instantaneously. The proportion of austenite transformed to martensite depends only on temp. to which it is cooled.

T.T.T Diagram for Hypoeutectoid Steel
In hypoeutectoid steel, proeutectoid phase separates out in upper temp. region. For this type of steels, ferrite starts separating out from the austenite as soon as austenite is cooled below the critical temp. (A3). The amount of proeutectoid ferrite decreases as austenite is undercooled more & more below the critical temp. after a certain degree of undercooling, austenite will transform directly to pearlite. On further cooling, there will be no surplus ferrite.

T.T.T Diagram for Hypereutectoid Steel
Similar to hypereutectoid steel, in hypereutectoid steel proeutectoid phase separates out in upper temp. region. Here cementite is separates out from austenite on cooling below the upper critical temp. (Acm). The amount of cementite decreases with increased degree of supercooling & finally reduces to zero when austenite is cooled below a particular temp.

Effect of alloying elements on T.T.T Diagram
Almost all alloying elements except cobalt, decrease both the tendency & rate of decomposition of austenite due to austenite stabilizing elements. Alloy carbides are more stable than cementite as they retard the diffusion of carbon which inturn decreases the decomposition of austenite. T.T.T diagram alloy steels can broadly be classified into 4 types.

The first type of T.T.T diagram is similar to that of carbon steel. There is practically no difference in the pattern of austenite decomposition in the presence of non-carbide forming elements, supercooled austenite decomposes to a mixture of ferrite & carbides rather than to an aggressive to ferrite & cementite. Generally plain carbon steels exhibit this type of diagram.

The second type of T.T.T diagram differs from the remaining T.T.T diagram as it consists of two minima with respect to the stability of austenite. The upper bay (at higher temp.) corresponds to the transformation of austenite to pearlite, where as the lower bay corresponds to the transformation of austenite to bainite. Very few steels exhibit such a T.T.T diagram. This type of T.T.T diagram generally observed for low alloy steels.

The third type of T.T.T diagram is peculiar that means bainitic region is not present. This implies that bainite can not be formed in these steels. Such a T.T.T diagrams obtained in general, for high alloy steels. Specially those in which start of martensite transformation temp. has been shifted to sub zero region. In such steel, stable austentite structure is obtained at room temp.

The fourth type of T.T.T diagram does not exhibit pearlitic bay. Here, under normal cooling conditions, either bainite or martensite is formed. Such T.T.T diagram is obtained for special alloy steels.

Limitations of T.T.T Diagram
 In practice transformation during heat treatment occurs by continuous cooling and not isothermally.
 For most of the heat treatment processes, these diagrams are useful only qualitatively and not quantitatively.


Conclusion

The T.T.T diagram have gained great importance from the heat treaters point of view. This is due to the simple reason that these diagrams are extremely used as they give information about the hardening response of steels & the nature of transformed products of austenite at varying degree of supercooling.