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Thread: TTT vs CCT chart usage?

  1. #1
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    TTT vs CCT chart usage?

    I can get a fair idea of how to heat treat a particular steel from a TTT (Time Temperature Transformation)chart, but I'm pretty foggy on how a CCT (Constant Cooling Transformation) chart is read. Are there any tutorials out there online?

    Thanks, everyone.

  2. #2
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    If the cooling rate of the quench media is know, just follow the appropriate rate-line on the chart (or an extrapolation thereof) to estimate percentage of pearlite, bainite, etc.


    I hope I have the following right:

    If the temperature is decreasing at a constant rate, then there is no "iso" for isothermal stuff to happen. Due to thermodynamic stuff that I don't grock, bainite and pearlite transformations take longer to begin if the steel is cooling than if it is held at constant temperature. Things happen at a lower temperature too. The nose ducks down and to the right, at least for plain-carbon steels.
    What are the facts? Again and again and again - what are the facts? Shun wishful thinking, ignore divine revelation, forget what "the stars foretell," avoid opinion, care not what the neighbors think, never mind the unguessable "verdict of history" — what are the facts, and to how many decimal places? You pilot always into an unknown future; facts are your single clue. Get the facts! - - Heinlein

  3. #3
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    This is actually a very good question, and something I have been wondering if people understood correctly most of the time.

    As Lee said, a TTT diagram is ONLY applicable if the steel is quenched extremely rapidly to a set temp, and then held there until transf. is complete. A CCT is what happens to a TTT under continuous cooling conditions (as with realistic quenching). The graph is not the same, because you move through a range of temperatures gradually, where rates of transformation are different. Therefore, the transformation lines are pretty much what you would get if you devided the continuous cooling curve into little isothermal steps, in stead of a straight line, and averaged the amount of transformation that would happen on each step downt to the temp you want the transformation time of (sorry, but hard to describe in a way that is easy to understand).

    CCTs are a little more difficult to follow, and are either marked in cooling rates (degrees/time) or thicknesses+known cooling rates of those thicknesses in different quenchants. As Lee also said, you have to know the approximate cooling rate of each part you want to know the microstrure resulting from quenching of. This can be hard to know, especially with complex crossections. There are tabes that can be used to get the approximate cooling rates of different quenchants, along with convertions for a few standard crossections that can be used for getting at least an approximation. You will probably be able to find this, and related info if you search for the data needed to do Grossman (sp?) hardenability calculations.

    Therefore, because TTTs are easyer to draw up (and thus to find), it is good to use them to get an approximate idea on how a certain HT will do - but they should not be assumed to be too directly applicable. CCTs will give you a more accurate description of actual conditions.

    The slower the quench to a certain temp, the bigger the potential for a difference between TTT and CCT for that temp.
    Last edited by B. le Roux; 07-21-2005 at 01:51 AM.
    Bertie

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    Whether you find them or lose them, you often do not notice" - verse from a Japanese song.

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    I suppose this means that the difference is more important for steels that have very short transformation times, i.e. low carbon steels. When you have to get from austenite to below Ms in less than one second, a TTT will be much less accurate than a CCT, right? Since on those time scales, an instant transition to a given holding temperature is unrealistic, I mean.

    Phew- I looked up the Grossmann stuff- luckily I recovered from a serious partial differential equation infection years ago... It all looks like greek to me these days..

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    It's important to remember that all of those curves are developed under laboratory conditions with very specific conditions . They should be used as a guide not an absolute. We always used the TTT curves and Jominy hardenability data , rarely the CCT curves.

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    It is funny how it works, there are threads that I wish were in the Cafe instead, and then gems like this are in the Cafe and I wished it were it were in the metallurgical forum. Are the any objections to my making a copy of this thread for the Metallurgical forum as well?

    I will do so and if there are any objections I can always delete the pilferred posts.

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    Talking

    Kevin, now that you have locked a thread you have completed successfully the final test in becoming an official Curmudgeon !!!

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    Hmmm.. that doesn't have the same ring to it as Guy's suggestion of "Kevin the Pilferer". Let's see, my MS thesaurus gives me the synonyms of "killjoy" or "wet blanket" for Curmudgeon. I really don't think that adequately covers my total wickedness as I sit here and rub my evil hands and twist my handlebar mustache.

    Anyhow, I did pilfer this thread because I wanted it here in order to reference it in the Rosetta stone and in future searches of this forum. I had had a problem reading those blasted CCT's so I was learning as well, so I liked it.

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    curmudgeon in the sense of 'cantankerous and full of stubborn ideas' [to keep this a scientific forum].

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