1. Advantage of TTT diagram over Iron-Iron carbon diagram - Has time axis 2. Disadvantage of TTT diagram over Iron carbon diagram - TTT diagram for only single composition
Sir, on 21:07, you wrote in athermal transformation, rate of transformation does not depend on temperature; shouldn't it be that the rate does not depend on time?
Well, time is anyway insignificant, because the transformation is expected to be instantaneous. Transformation takes place between Ms and Mf temperatures. At any temperature between these limits, the transformation happens at the same extremely fast rate. So we say that it is independent of temperature.
Consider the C curves for austenite to pearlite transformations. Let us say I come down to 625 Celsius and hold my material at that temperature. In that case after a period of time, my material will start transformation at the start line (ts) and reach the finish line (tf) after some time [draw horizontal line from 625]. This means, at 625 Celsius, transformation rate to 100% pearlite takes (tf - ts) time. This indicates my rate of transformation. This will be different for different temperatures. But for Martensite, even if I draw a horizontal line (temperature) below Ms, I will not encounter any finish line. So my rate of transformation is independent of the temperature.
Thank you Sir for this wonderful lecture. I have one question. What is the range of temperature for Martensitic start and finish temperature (Celcius scale)
Sir you said that we do not find microstructures like martensite, bainite in phase diagram since there is no time graph there. But, in the end of quenching you said that in martensitic transformation the curve is horizontal since the amount transformed is independent of time. Could you please clarify.
Sir in TTT diagram you told that on quenching we obtain a amorphous structure. But in TTT diagram of eutectoid steel you are telling that on quenching a BCT structure will be formed. Also in one of the comment of this video you told that carbon will occupy fraction of edge centres randomly. So if its a BCT structure how will we able to define the motif for random positioning of carbon
sir as martensite is obtained below the eutectoid temperature exmple for .8wt% carbon, will it be wrongto say that it is also a combination of alpha ferrite and and cementite with different property??or is it a different phase altogether kindly do respond to the query sir, i will be very thankful to you.
It is a different phase altogether. Firstly, it is a single phase and not a mixture of two phases. Secondly, it has a very different crystal structure.
Martensite has even finer grains than pearlite due to very fast cooling rate applied to produce martensite. When grains are fine or say smaller in size then it obviously has more grain boundaries. If more grain boundaries then there will be more resistance to deformation. Hence the material will be hard and not ductile but brittle 👍 thus Martensite is brittle.
Sir, I have a doubt if steel quenched to a temperature I.e.,in between the Ms and Mf temperature ,will the structure is stable(maternity retained austenite ) or upon further cooling will it transform to complete martensite !
At a temperature between Ms and Mf austenite+martensite is stable (well metastable, as martensite is inherently not a stable phase). Upon further cooling, the remaining austenite will transform to martensite. However, if you wait long enough at a temperature between Ms and Mf then both martensite and austenite can transform to ferrite+cemntite as this is the stable mixture at such tempeartures.
Sir, martensitic transformation do depends on cooling rate. So, time should be considered ? Also, there is confusion in point number 2 regarding athermal transformation. Pls explain sir.
Yes, we do have to cool faster than a critical cooling rate. But this is to avoid the formation of the equilibrium products. Once the transformation begins, it is extremely rapid and, for all practical purposes completes instantaneously. In this sense, time is not important. That is why we do not have start and finish time curves for martensite; we only have start and finish temperatures. Nucleation and growth transformations occur at a finite rate, so there is a start time and a finish time. These are given by the two C curves. The rate of transformation depends strongly on temperature. This is the reason for the C-shape of the curve. In contrast, the rate of transformation of martensite does not depend upon temperature. Therefore there is no C-curve between Ms and Mf. This is the reason the transformation is called 'athermal'.
@@introductiontomaterialsscience Thank you very much sir for complete explanation 🙏. You are one of the top most professor I ever know who know how to make his student understand in best possible way.
Sir for ALLOY STEEL which medium is used for quenching .( Air , water , oil or brine soultion)..I mean whether all are used or one of them . Please suggest
You can get it only at a very fast cooling (quenching). This is a nonequilibrium processing. It cannot be formed by slow cooling. The equilibrium phases alpha and cementite form even on slow cooling. Also, once formed, alpha and cementite do not transform into other phases. But martensite will transform to equilibrium alpha and cementite if heated (tempering). This is an indication of the metastability of martensite.
Sir... What happens if the steel is cooled through a cooling curve which intercepts the starting C curve (the C curve where the transformation starts) twice..?? Thank you sir...
An important thing to keep in mind is that all the transformation lines affect only the untransformed austenite. A cooling curve that intersects the C-curve will always intersect it twice: once on the upper branch of C and then on the lower branch. There are two possible cases after the first intersection: 1. It intersects the finish curve before hitting the start curve again. The C-curves have relevance only for untransformed austenite. Intersecting the finish curve means that all austenite is transformed to pearlite. After this, the curve hitting the start or finish curve again has no relevance as there is no untransformed austenite in the sample. 2. It never intersects the finish curve and hits the start curve again. Since it never hits the finish curve, it means that that only a part of austenite transformed to pearlite. Hitting the C-curve again has no relevance. When further cooled, it will hit Ms and Mf lines. This will lead to the start and finish of martensitic transformation of the remaining austenite. So you will end up with a mixture of pearlite and martensite.
This is an interesting question. As the solution is interstitial it is obvious that there will e an upper limit to the solubility determined by filling of all interstices. But the actual limit can be far lower than this limit. I have not seen such a value reported anywhere, but I have not done an extensive literature search.
No, edge centres are the possible sites for C atoms. But all sites are not occupied as we have much less carbon. So depending on the carbon concentration, only a fraction of such sites are occupied randomly.
It is reversible in the sense that if you het the nmartensite back to a temperature where austenite is the stable phase it will transform to austenite.
Sir, in some journals it's mentioned as reversible and in some it's mentioned as irreversible due to the immobile dislocations and intersection zones of martensite (In case of Fe- C steels). That's why this ambiguity.
Sir, you said if c/a=1 ,then the structure become BCC and it's called alfa ferrite, but alfa ferrite have complete different compositions of carbon than martensite , how can it be called so ?(because even if structure is same there composition is different sir)
In fact c/a itself depends on the carbon concentration. The value of c/a=1 is achieved as the carbon concentration tends to zero, i.e towards the pure Fe.
