Sir, on Jominy quench test ,the profile of hardness is like cosx function. But how we will say that hardenablity of that steel is more or less??? Whether we have to perform the test on different sample of different composition or we can predict by 1 sample after Jominy test.??
@@rajeshprasad101 What if we perform Jominy end quench test on one sample. What can we predict about hardenablity ( it shows Cosx like harnesses profile)? . I am very happy that you replied me sir🙂
sir the explanation is somewhat contradictory. alloying shifts the curve to the right so that you have time to reach martensite by slow quenching but accordingly we cannot generalize that alloying always increases hardenability without specifying the rate of quenching
Yes, quenching is fast cooling. But you can still compare two quenching rates. For example, water quenching is faster than oil quenching. Now faster the critical cooling rate, more difficult it is to achieve this cooling rate in the interior of a thick component. So for faster critical cooling rate thick components will not form martensite inside and we will say that the hardenability is low. Oppositely, slower the critical cooling rate, higher the hardenability. Please note that in the above sentence I am using CRITICAL cooling rate (CCR), minimum cooling rate to produce 100% martensite. This is a material property and different from ACTUAL cooling rate (ACR) which depends upon quenching medium. 100% Martenstie forms when ACR>CCR.
Sir you said alloying elements enhances hardenability but adding Ni which is a austenite stabilizer stabilizes austenite at lower temp.Then how the hardenability increases?
Well, this is simply the definition of hardenability. If martensite can be produced by slower cooling the steel has, by definition, higher hardenability. Thus if C-curve is shifted to the right (i.e., higher time) then even a slower cooling rate can avoid the formation of ferrite and cementite, and so martensite can be produced at a slower cooling rate. We thus have higher hardenability.
Below the critical cooling rate, if the rate of quenching is varied you may get a variable amount of martensite in the final product. Then the hardness will certainly vary with the amount of martensite. But if you are cooling faster than the critical quenching rate then 100% martensite should be produced. The hardness should then not be a very strong function of the quenching rate. However, it may still depend upon the quenching rate due to differing amounts of residual stresses and defects.
Sir, I have still a doubt that why faster cooling increases hardness?? Sir,it is true that due to change in microstructure hardness change,but the question is why?? Sir, is there a mathematical relation between hardness and cooling rate??
@@introductiontomaterialsscience sir, one more difficulty I am getting that why an increase in carbon content increases hardness?? What exactly happens at atomic level??
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but sir, what is reason behind getting 2 noses in alloy steel??
very nice lecture, thanks sir
Sir, on Jominy quench test ,the profile of hardness is like cosx function. But how we will say that hardenablity of that steel is more or less???
Whether we have to perform the test on different sample of different composition or we can predict by 1 sample after Jominy test.??
If you want to compare the hardenability of two or more steels then you will certainly have to test all of them.
@@rajeshprasad101 What if we perform Jominy end quench test on one sample.
What can we predict about hardenablity ( it shows Cosx like harnesses profile)?
.
I am very happy that you replied me sir🙂
Then you will get hardenability of that particular steel.
sir the explanation is somewhat contradictory. alloying shifts the curve to the right so that you have time to reach martensite by slow quenching but accordingly we cannot generalize that alloying always increases hardenability without specifying the rate of quenching
sir there is some other elements like Al and Si that inhance shift towards left means nucleation and growth is faster by alloying with this material
Why QUENCHING RATE should be low to obtain high hardenability though we get the same martensite irrespective of quench rate??
Its the other way round. If the hardenability is high then one can form martensite even on a slower quenching rate.
Sir how does the slower quench rate gives higher hardenability when high quench rate is forming martensite which is hard and brittle?
Sir have u got answer?
Sir, quenching itself is fast cooling, what is meant slow quenching rate? why slow quenching rate greater the hardenability?
Yes, quenching is fast cooling. But you can still compare two quenching rates. For example, water quenching is faster than oil quenching.
Now faster the critical cooling rate, more difficult it is to achieve this cooling rate in the interior of a thick component. So for faster critical cooling rate thick components will not form martensite inside and we will say that the hardenability is low. Oppositely, slower the critical cooling rate, higher the hardenability.
Please note that in the above sentence I am using CRITICAL cooling rate (CCR), minimum cooling rate to produce 100% martensite. This is a material property and different from ACTUAL cooling rate (ACR) which depends upon quenching medium. 100% Martenstie forms when ACR>CCR.
Excellent
Sir you said alloying elements enhances hardenability but adding Ni which is a austenite stabilizer stabilizes austenite at lower temp.Then how the hardenability increases?
That is in continuous cooling not in slower cooling rate, what we are discussing here🙂
Thanks from south korea
great lecture. thanks sir!
sir why by shifting the of C curve increases the hardenability of steel
Well, this is simply the definition of hardenability. If martensite can be produced by slower cooling the steel has, by definition, higher hardenability. Thus if C-curve is shifted to the right (i.e., higher time) then even a slower cooling rate can avoid the formation of ferrite and cementite, and so martensite can be produced at a slower cooling rate. We thus have higher hardenability.
What it means slower quenching rate ..? It's confusing me .
If hardness is achieved even by slow cooling that means material hardenability has increase
Thank you Sir...
Sir please explain that why cobalt do not shift the curve toward right?
I have not found answer to this interesting question.
sir could u explain in other video how curves come when I alloying material to that
Sir does the rate of quenching quenching affect hardness?
Below the critical cooling rate, if the rate of quenching is varied you may get a variable amount of martensite in the final product. Then the hardness will certainly vary with the amount of martensite. But if you are cooling faster than the critical quenching rate then 100% martensite should be produced. The hardness should then not be a very strong function of the quenching rate. However, it may still depend upon the quenching rate due to differing amounts of residual stresses and defects.
Sir, I have still a doubt that why faster cooling increases hardness?? Sir,it is true that due to change in microstructure hardness change,but the question is why?? Sir, is there a mathematical relation between hardness and cooling rate??
You have a valid question and one which is difficult to answer.
In case of coarse pearlite
@@introductiontomaterialsscience Thank you sir...
@@introductiontomaterialsscience sir, one more difficulty I am getting that why an increase in carbon content increases hardness?? What exactly happens at atomic level??
Why Cobalt decreases Hardenability?
It doesn't increase hardenabilty. Because it doesn't helps in shifting the c curve to the right.
Sorry, I missed seeing your question for so long. And as I have replied to Hassan, I have no answer to this question.
@@bijoydeori4469 not satisfied with this answer..what is the reason behind to shift c curve right By adding alloying elements?