Why is a ttt diagram c shaped

Siddhesh Deshmukh. Show More. Views Total views. Actions Shares. No notes for slide. TTT diagram 1. These are 1. Time-temperature transformation TTT diagrams 2. Continuous cooling transformation CCT diagrams 8. Indicates the amount of transformation at a constant temperature. Samples are austenitised and then cooled rapidly to a lower temperature and held at that temperature whilst the amount of transformation is measured, for example by dilatometry.

Obviously a large number of experiments are required to build up a complete TTT diagram. Indicates the extent of transformation as a function of time for a continuously decreasing temperature. Samples are austenitised and then cooled at a predetermined rate and the degree of transformation is measured, for example by dilatometry. In this case also a large number of experiments are required to build up a complete CCT diagram also TTT Diagram TTT diagram for an eutectoid carbon steel.

Eutectoid Steel Spring Dr. How Transformation Ocures? Growth of nuclei — Growth of nuclei is a diffusion controlled process. Transformation Rate. Time required for transformation fist decreases, reaches a minimum and then starts increasing with decrease in temperature.

Let us do some experiment Isothermal transformation of eutectoid steel Let us take a eutectoid steel and do the following experiment — — — — — Step 1 — Heat the sample above A1 temperature for austenitisation Step 2 — Transfer the sample to a salt bath kept below A1 Temp.

Step 3 - Keep it at the bath temperature for a specified time Step 4 - Quench to room temperature Step 5 — Find out the amount of phases present Fraction of transformation Vs the logarithm of time at constant temperature The S Curve. Plot the result of the experiment This is known as S curve The transformation starts but it takes some time before we can see a precipitate.

The time required for transformation required to initiate the transformation is known as Incubation Period. The rate of transformation first increases and then starts decreasing Let us consider eutectoid reaction as an example The S-shaped curves are shifted to longer times at higher T showing that the transformation is dominated by nucleation nucleation rate increases with supercooling and not by diffusion which occurs faster at higher T But at lower temperatures ….

Transformations of austenite to Martensite 4. When the austenite is quenched to temp. Diffusion rate is extremely slow. Complete TTT isothermal transformation diagram for eutectoid steel. In those diagrams, continuous cooling takes place while this diagram explains constant temperature transformations. TTT diagram of steel has a very important application like austempering, martempering Patenting, and isothermal annealing which is commonly employed in industry for achieving specific properties in steel.

TTT diagram of steel is called isothermal transformation diagram or time-temperature-transformation diagram. It presents a logarithmic plot between temperature and time. The common application of the TTT diagram of steel is to understand elevated transformation is alloy steel which follows non-equilibrium cooling and requires important microstructural features.

For history, it should be noted that Pearlitic and Bainitic bay in the TTT diagram of steel are identified and explained by Davenport and Bay. Martensitic start and end transformation lines were later added by Cohen.

Follow isothermal transformation in steel on Wikipedia for complete history. Presented above is the TTT-diagram for eutectoid steel. For each composition, there will be a slight variation in diagram curves. TTT diagram of steel indicates the time-temperature and transformation curve.

This means transformation is dependent upon time, temperature, and cooling mechanism. For a clear understanding of this diagram, it is better to understand the phase transformation of the Fe-Fe3C curve.

The phase transformation diagram and TTT diagram has a major difference that you must understand. Phase transformation just describe continuous and equilibrium cooling product. All remaining products or transformation which are the result of isothermal cooling, fast cooling like bainite and martensite are only understood by the TTT diagram. Follow the quenching procedure and martensitic formation for understanding the Phase transformation diagram.

We have mentioned before, TTT diagram is time and temperature transformation diagram. Pearlitic transformation is a diffusion-based transformation which involves solid-state conversion into alpha iron and iron carbide colonies.

Without the required temperature and time, diffusion is not possible, and structure will, then, have martensite or bainite transformations. The edge point on the nose is called a critical point. If the cooling or temperature drop is high enough that the line does not touch the nose, then it generates diffusionless transformations.

This critical point in the nose also indicates the point above which diffusion is possible. Picture of nose for clear understanding is shown below;. In this picture, four important concepts are being explained. In this figure, we have shown that with a decrease in transformation temperature up to the nose of the TTT diagram, the size of critical radius decreases i.

High critical radius, more time is required for stabilization. This is the very reason, nose shifts towards the right as we move towards high temperature. At high temperature, critical size is very large which requires more time for stabilization. But, as soon as nuclei forms, the end of transformation takes very little time due to a higher diffusion rate about temperature.

At lower temperatures closer to the nose, the temperature is low and the diffusion rate is, also, low. This low diffusion rate causes slow growth and requires more time for complete transformation. If we summarize this concept, we can say, large nuclei are formed at high temperatures, and transformation takes less time for completion but, closer to the nose, small nuclei are formed and transformation takes more time for completion.

This is clear in the TTT diagram, End transformation line is closer to start the transformation line at high temperature. 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.

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.

A special application of isothermal hardening is called patenting and is used for steel wire. Steel wire with 0. A structure results with possesses good ductility in addition to a hardness. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

Your email address will not be published. Save my name and email in this browser for the next time I comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. Knuckle Joint A knuckle joint is used to connect two rods which are under the action of tensile loads.

However, if the joint is guided, the rods may support a compressive load. A knuckle joint Plastic Material Definition Of Plastics : i Plastics: Plastics are basically, the synthetic organic materials of high molecular weight, which can be moulded into any desired shape by the Skip to content Home - Blog - Material Science.