Curie temperature VS fast heating

Discussion in 'General Science' started by thingmaker3, May 30, 2009.

  1. thingmaker3

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    May 16, 2005
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    The curie temperature of hypoeutectoid steel approximately 50°F (30°C) above the isothermal austenitizing temperature.

    The austenitizing temperature increases as Δtemp/Δtime increases.

    Does the curie temperature increase with heating rate increase as well? Or is it based strictly on temperature?
     
  2. studiot

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    No the Curie temp is a fixed point.

    I'm not quite sure what you mean by this as the final composition at any temperature is also fixed. Since we are talking solid solutions it may take a very long time to achieve though.
     
  3. thingmaker3

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    Thank you. No matter where I dug, I couldn't turn up data on that one.

    Isothermal conditions do have the Ar and Ac temperatures at the same point, and fixed. But increasing the heating or cooling rate will increase or decrease (respectively) Ac and Ar.

    Time required depends on diffusion rates of carbon in any given alloy, on microstructure prior to heating (if heating), degree of superheaing/supercooling relative to transition temperature, and other variables.
     
  4. studiot

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    Are you referring to austempering and TTT diagrams?

    Austenite is not stable below 723°C but any change from Body Centered Cubic (stable below 910°C) to Face Centered Cubic (stable above 910°C) obviously takes time.
     
  5. thingmaker3

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    No, my friend. I am talking about Ar = "Arret Refroidissement" (Cooling Arrest) and Ac = "Arret Chauffage" (Heating Arrest) - the transitions from BCC to FCC and vice-versa.

    As a sidenote, you might want to search "retained austenite." Austenite can and does exist at surprisingly low temperatures.

    "Austempering" is a bit of a misnomer; it is a technique for creating bainite. The TTT curves are isothermal cooling curves. CCT curves are of more interest to me, since I do nothing isothermally.:cool:

    My interest in starting this thread was to determine if a Hall-effect sensor could ballpark austenitization during rapid heating of hypoeutectoid steel. Appearantly it won't. (Works great for slow heating, though.)
     
  6. studiot

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    I'm sorry I didn't explain my point very clearly.

    I was trying to say that the the phase diagram for the iron carbon system is an equilibrium diagram and determined by action at the molecular level. As such there is a theoretical composition at any temperature, once equilibrium has been reached. But since these are solid state transitions we can be talking about a mighty long time to reach that equilibrium. So rapid heating or cooling can temporarily bring about varying non-equilibrium compositions or appear to change the temperatures needed to effect the transitions. Crystal structure is a molecular phenomenon; we can input/extract energy faster than it can change.

    If we waited long enough or cooled/heated slowly enough we would always get the same result.

    On the other hand magnetism is an effect generated at a sub atomic level by electrons. As such it is insensitive to the rate of input of themal energy and thus the Curie temp is also.

    And thanks for the French, I'll look them up. I have one French materials textbook. They certainly offer a different worms eye view of the subject.
     
  7. thingmaker3

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    When you say "compostition," do you mean "phase?" :confused: My texts use "composition" to indicate quantities of alloying elements.

    I concur re: time required for transition to austenite, pearlite, or bainite. Highly alloyed steels might have to "soak" for half an hour or more once at temperature in order to fully austenitize. The bainite phase can take hours or days to form.

    But transition to martensite takes place at the speed of sound. Amazing stuff, steel is. :)
     
  8. studiot

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    I mean the bulk percentages of austenite, pearlite, ferrite etc.

    The following extract may interest you.
     
  9. thingmaker3

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    I have a copy of the Bain & Paxton text the chart was borrowed from. ;) It is much easier to read than Bain's first edition. (Bain's writing style was an older style. It reminds me of Byron.)
     
  10. studiot

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    I'm glad not everyone has thrown out paper records!
     
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