Citical temp or Curie Point?

B

B Finnigan

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Jan 16, 2006
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I was doing some research on iron and ran across the term Curie Point. This is the temp that iron is no longer magnetic and was discovered by Pierre Curie (Madam Curies husband). My question is why do knifemakers refer to it as critical temp when the scientific term, used by physicists, metalurgists etc is the Curie point?

I know that different professions have a habit of bastardizing terminology but shouldn't we be using the proper termonology for it? Who coined the term "critical temp" as it relates to ferromagnetics and why is that term used vs Curie Point?

The term "Critical temperature" has nothing to do with ferromagnetics.

http://en.wikipedia.org/wiki/Curie_point

http://en.wikipedia.org/wiki/Critical_temperature

"The critical temperature, Tc, of a material is the temperature above which distinct liquid and gas phases do not exist. As the critical temperature is approached, the properties of the gas and liquid phases become the same. Above the critical temperature, there is only one phase: that of supercritical fluid."

The above definition has nothing to do with ferromagnetics. (???????)

IMHO this only makes learning that much more challenging when different terms are applied to the same process. A phrase gets misused and then before you know it becomes part of a profession's dialect.
 
First - The Curie point is a ferromagnetic point of change, as you stated.It is coincidental with the approximate area of the critical point of a steel alloy.The Ac point (Austentitic critical temperature)
is the point at which the alloy changes into austenite.This will vary from the eutectoid ,at 1333F,to much higher for complex alloys.For makers without good tables and accurate temperature control,the curie point plus 50-100F degrees is a good rule of thumb for simple steels.A magnet is a simple way to determine the curie point.A magnet will not tell you when the critical point has been reached.
Second - The critical point you listed is a state point in physics and not applicable to metallurgy.IOW it is a different critical point.(there are many other critical points in other fields)
Stacy
 
Much of our reference point for this topic comes from the Fe/Fe3C Equilibrium diagram. This is good for ball park understanding of temperatures but unfortunately bladesmiths don't typically work steel under equilibrium conditions, we heat and cool at all kinds of differing rates, so ball park will have to do. The term "critical temp” is more a generic lay term in bladesmithing for any number of temperatures that could give any particular desired effect. Most often it used in a way that somewhat resembles the mismatched definition provided by Mr. Finnigan, the allotropic shifts responsible for the differing phases of steel.

On the Fe/Fe3c phase diagram there are several points that could be referred to as "critical temperatures", but if one wanted to be accurate and true to the diagram they are called "arrests" (well technically it is the French term for arrest) and that is why the "A" designations. A1 is the point at which the shift from alpha iron to gamma iron begins and denotes the boundary between pure pearlite (or whatever segregated Fe+fe3C condition you heating from) and varying degrees of austenite solution. The Currie point is often designated as A2, at around 1414F and corresponds with the loss of ferro-magnetism, and while it is not directly the result of the BCC to FCC shift, this allotropic shift seems to affect the spin of the electrons which is responsible for the change. A3 would be the point at which all the proeutectoid ferrite would finally be used up and a complete austenite solution would be achieved. On the opposite side of the chart is Acm where all of the excess cementite is in solution and you have pure austenite.

As things get more complicated more "critical temperatures" are added. On heating they are designated Ac1, Ac3, Accm, and on cooling the "c" is replaced with an "r" and you have a whole new set of "critical temperatures" and the magnetic point almost becomes irrelevant since ferromagnetism will not be regained until an allotropic shift occurs- pearlite, bainite, martensite, etc... Give it a try; quench a piece of steel from nonmagnetic to around 450F. and the magnet will not stick. As it cools beyond Ms there will be increasing degrees of magnetic attraction as the shift occurs in the martensite.

From all of this we can gather- the magnet is not fool proof, the farther we are from .8% either way, and the more alloying in the mix, the greater the separation between full solution and Ac2 (the Currie point). So the reliability will vary greatly depending on the alloy. Of even more significance however is the fact that the magnet is all but worthless on a descending heat for determining quenching temperature.

It is indeed incorrect to refer to the Currie point as “the” critical temperature but to bladesmiths overall it is as much a critical temp as any of the others.:confused:
 
Sorry, I just realized that the last post was so much techno-babble to those without a Fe/Fe3c diagram in front of them:

fefe3c.jpg
 
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