Isothermal transformation diagram

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Dividends Paid, FY —. From the creators of MultiCharts. This little scam has already been done. If you keep your sample just a few degrees below the transformation temperature or at very low temperatures, it will take a long time before the process is over. With decreasing temperature we expect that the interface velocity goes up because of an increasing driving force, and that the interface velocity goes down because of decreasing jump rates.

1. The Basic Idea

Time Temperature Transformation (TTT) Diagrams R. Manna Assistant Professor Centre of Advanced Study Department of Metallurgical Engineering Institute of Technology, Banaras Hindu University.

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Sharing a chart for a friend. Price prediction for TGT. Enterprise Value, FQ —. Market Cap - Basic —. Number of Employees —. The rate of nucleation increases and the rate of microconstituent 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 greater difference in free energy. As a result of the transformation, the microconstituents, 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: Coarse pearlite is produced when atoms diffuse rapidly after phases that form pearlite nucleate.

This transformation is complete at the pearlite finish time P f. However, greater undercooling by rapid quenching results in 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 P s curve. Well, it's good for you. If all of this is clear, we can now look at a real TTT diagram without all the frills and explanations given above, and progress a bit about how to use it.

Here is the "official" isothermal TTT diagram for eutectoid carbon steel. Look up the phase diagram if you wonder about this. As before, we have an austenite region, stable above the transformation temperature A 1 at o C. Instead of only a ferrite, we now have ferrite and cementite Fe 3 C inside the "nose". In addition, we have the martensite region at low temperatures. This is the "official" TTT diagram for this steel. If we follow isothermal lines like the green ones, nothing new emerges.

However, when we make or process steel, we usually do not keep it a some constant temperature for a very long time but let it cool down "naturally" or, as we call it, continuously. Naturally occurring continuous cooling would follow the yellow lines.

Now you should have two questions: Those yellow lines do not look like the diagrams for natural cooling we had before. I did pay close attention to the beer module , after all.

If those yellow lines signify anything - what is it? How can I use a TTT diagram that was made for predicting what happens if you keep the temperature constant "isothermal" to say anything about what happens if you cool continuously? The first one is easy: That distorts a typical exponential decay curve like this:. Continuous cooling temperature vs. The answer to the second question is a bit more difficult. Isothermal TTT diagrams indeed do not really show what will happen if you run some temperature profile "through" them.

What really happens is shown below:. Continuous cooling and TTT diagrams. Enlargement of the diagram above. This is just an enlargement of the figure above.

To see what happens, let's assume that the left cooling curve, when it hits the nose of the TTT diagram upper green point , would stay at constant temperature for the time it takes it to leave the nose lower green point. It would follow the green line up to the white point. There it would end on a curve that represents only a small amount of transformed material. What will be really transformed must be even less since the temperature during that time interval goes down, after all.

Far less material thus will be transformed then indicated by the positions of the yellow lines inside the nose. Does that mean that we cannot use isothermal TTT diagrams to predict what will happen during continuous cooling? Yes, that is exactly what that means. Now I have good news: Push the upper part of the "nose" a bit down and to the right - and you are done! That's what it looks like:.

The difference between the isothermal TTT diagram and a CCT diagram, that is valid for continuous cooling that follows approximately some exponential decay law, seems to be minor.

I need to made two points to this: It looks like a minor difference but, since we have a logarithmic time scale, this can be deceiving. So if you look for quantitative data, wanting to extract numbers from these diagrams: If you use these diagrams only for qualitative reasoning, you don't need to bother distinguishing between TTT and CCT.

They all looks roughly the same anyway. People use these kinds of diagrams for both purposes; they are absolutely essential to metal technology. On occasion, however, some confusion is produced. The pictures above, for example, can easily lead you astray. In the pictures above the thermal history of the sample was given by temperature-time "pathways" in the form of green or yellow lines with an arrowhead.

In the case of continuous cooling yellow lines , the pathways always end around room temperature because that it what your sample does. The uninitiated then tend to assume that whatever is found a the end of the pathway arrow is what you will find in your sample. On occasion that might take a long time - but that is of no consequence.

That is all important and needs to be considered - but the phase or phase mixture doesn't change anymore. That two out of three yellow pathway arrows hit the martensite region in the figure above thus doesn't mean anything. The remaining austenite then is going to transform to martensite. So why do I extend the martensite field all the way to the right of the nose?

Simply because there will be pathways not yet considered, where this is not the case. It is just easier to keep the TTT diagrams a bit "open", hoping that the user knows how to deal with it.

We will see a bit better how this works in the next examples. Shown are actual and somewhat more involved diagrams for 0. After "Einführung in die Werkstoffwissenschaften"; ed. Schatt, 7th edition Some new features and additional information appear in both diagrams. First, we now have ferrite region in front and above the pearlite nose. Well - why not? We have hypo-eutectoid steel here, so between the temperatures A 3 and A 1 you need to form some primary ferrite before whatever is left transforms to secondary ferrite and pearlite look it up yourself!

Second the approximate Vickers Hardness is given that you end up with following various pathways. You can do that because you will obtain a defined structure with a defined hardness if you follow a given cooling pathway. Of course, you know that for isothermal annealing you will eventually go down to room temperature, essentially "freezing in" the high-temperature structure since nothing much changes anymore.

Third , Bainite is drawn in - even so it is not a proper phase. There is no transformation from pearlite into bainite and means that you can't give a transformation percentage boundary.

That's why I made it fuzzy in some parts. In old times, making a draw ing was far more difficult than it is today. You actually had to draw it with ink on a piece of paper, so people tended to draw a clean line, hoping that everybody would know that this is overdoing it a bit.

Fourth , the CCT diagram contains some numbers at the points where the cooling curve crosses phase lines. These numbers give the degree of transformation in percent. It's just not possible to have a CCT diagram where one line gives the same transformation percentage for all cooling lines crossing it somewhere.

For example, if you look at the second green cooling curve from the left, you see a "1" at the point where it leaves the ferrite region, diving into the pearlite part, a "10" where it leaves the pearlite part, and a "20" where it leaves the bainite region.

Of course, in those rather busy diagrams there is just not enough room to write in all the numbers, so only a few or none are given, hoping that the reader knows how to interpolate or to guess what it should be.

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