Mumbai University > Mechanical Engineering > Sem 4 > Material Technology

Marks: 10M

Year: May 2014

T (Time) T(Temperature) T(Transformation) diagram 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 temperature) heat treatment of a previously austenitized alloy. When austenite is cooled slowly to a temperature below LCT (Lower Critical Temperature), the structure that is formed is Pearlite. As the cooling rate increases, the pearlite transformation temperature gets lower. The microstructure of the material is significantly altered as the cooling rate increases. By heating and cooling a series of samples, the history of the austenite transformation may be recorded. TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved.

Cooling rates in the order of increasing severity are achieved by quenching from elevated temperatures as follows: furnace cooling, air cooling, oil quenching, liquid salts, water quenching, and brine. If these cooling curves are superimposed on the TTT diagram, the end product structure and the time required to complete the transformation may be found.

In Figure the area on the left of the transformation curve represents the austenite region. Austenite is stable at temperatures above LCT but unstable below LCT. Left curve indicates the start of a transformation and right curve represents the finish of a transformation. The area between the two curves indicates the transformation of austenite to different types of crystal structures. (Austenite to pearlite, austenite to martensite, austenite to bainite transformation)

Transformation of austenite to pearlite is not linear as shown at constant temperature i.e600°C. Initially the rate of transformation is slow, then it increases rapidly, and finally it slows down towards the end.

Difficulties in plotting TTT Diagram

• During transformation of Austenite to Martensite or Pearlite it is impossible to change the temperature from 727°C to 500°C in zero time and from 500°C to 0°C.
• Specimen should be small in nature and care should be taken and special care should be taken during their rapid transfer between baths.
• Austenite to Martensie transformation does not occur at the room temperature and the material should be cooled below room temperature after transformation

CCT Transformation Curve

• In practice, structural steels are strengthened through microstructural transformations which take place under continuous cooling conditions rather than isothermal conditions. In continuous cooling heat treatments, the material is cooled from above the A3 temperature to room temperature continuously; the transformation from austenite to pearlite and/or martensite therefore occurs over a range of temperatures instead of at a single temperature, resulting in a more complex microstructure.

• The products formed by continuous cooling procedures can be predicted using a continuous cooling diagram which differs from the isothermal transformation diagram in that the beginning and the end of the transformation are generally shifted to lower temperatures and longer times. Different rates of cooling, and hence different microstructures are achieved through the use of different cooling media, as shown in Fig. Essentially as the cooling rate is increased the hardness and the strength of the steel increases.

• Rapidly cooling or quenching, the austenite at a rate equal to or greater than the critical cooling rate will bypass the knee of the TTT curve and result in the formation of martensite. However, martensite is too hard and brittle for most purposes and it are usually transformed to the equilibrium phases ( and Fe3C) using a low-temperature heat treatment called tempering. The resulting microstructure is not lamellar like that of pearlite but contains many dispersed carbide particles. Tempering causes the strength and hardness of the martensite to decrease, while the ductility and impact properties are improved. By selecting the appropriate tempering temperature, a wide range of properties can be obtained.