Why "Cryo" anything?

John,
For martensitic formation to occur, steel needs to be heated well above room temperature first. That temperature will depend on the composition of the steel. (how much carbon and other metal additive and etc) Again, a heating cycle is needed.

For a reaction to occur, a driving force is needed. (physical force, thermal, elctro-magnetic and etc) Reducing the temperature certainly will not provide that driving force.

This brings up another theory for me. If you combine cryo treatment with some kind mechanical machining at the lower temperature, you can potentially change the micro-structure due to internal stress difference and mechanical force; however, this is not what I have understood about today's Audiophile "cryo" technique. From what I read in the brochure, it's dipping the desired object in a cool solution.

Eric

Yes, you are correct in part. In order for the transformation to martensite to occur, austenite is indeed required..and that is obtained by heat.

What is more important, though, is the fact that it is a diffusionless process, one that does not require heat treatment for it to start..it requires the driving force, which in the case of the change from austenite to martensite, is not an increase in temp, but a decrease to below the martensite start temp.

The fact that cryo is indeed used to alter the macro properties of any material means that one is using a diffusionless process..and, it does not necessarily require a pre-treatment to higher temperatures first..The argument that all metal objects require heat to actually form them, before one can cryo them, is just a semantic one. There are processes that do not require the end manu heat them prior to cryo..

But, a diffusionless process does not require a heat precursor, but instead, use the internal lattice forces being created by the cooldown..that is the driver force..not heat..

Some quotes:page 311, same text..

"T3 is so far below the equilibrium transformation temperature Teu, that the driving force for FCC austenite to transform to BCC ferrite is enormous.....

"The diffusionless transformation by which (greek symbol meaning austenite phase, no html codes here) decomposes to martensite takes place by a complicated shearing of the () lattice. Each atom moves only a small distance relative to it's neighbors, less than one atomic distance. Consequently, thermal activation in the sense of vacancy motion or solid state diffusion is not required for the formation of martensite.

Since martensite formation is a diff. transformation, it cannot be supresses by quenching and, irrespective of time, a certain amount of martensite will form at a given temperature...the amount of martensite that forms at a given temperature will increase with increased cooling...at a temp Tf, all the austenite will have transformed."end of quotes..

So, clearly, the diffusionless transformation equilibrates at any temperature between the start and finish temp. If the object is, at a later date, taken down to a lower temperature, more martensite will form..this will continue until all the austenite is gone...

Now, the real question, is...is this type of diffusionless process possible with plastics?..I don't know.

Cheers, John

Is this some kind of set-up? Nobody dips anything into a cool solution. Almost everyone knows cryo treatment is a two-day affair. This is some sort of joke, right?

John,

the point I am making is that the starting phase of the steel must be in austenite. Once quench to to a lower temperature. Martensitic transformation occurs. Certain percentage of the marensite is formed, but the other material do not stay in Austenite phase. All the left over goes into Pearlite or Ferrite depending on compostion.

So any further quenching will not continue the martensitic transformation. The material must be raised back to a higher temperature level and reform Austenite before that's possible.

Put it another way. A piece of Steel can have a dramatic phase change by dropping rapidly from 900 to 20C, but that change is near permanent. Dropping the temperature from 20C to -150C do not continue the phase change. You must heat back up above ~800-900C to reform the inital Austenite phase.

The driving force for the martensitic transformation is the instable crystal structure of Austensite at lower temperature. So without forming austenite again. The driving force is gone.

For anyone who is interested, check out an example of phase diagram: (note that phase diagram changes rapidly depending on level of impurity.)

http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/examples/kimcon.html

For amorphous material like glass, the temperature change will mostly introduce lots of stress on the material. Eventually it formed a solid. The phase change most likely will cause physical breakage. Not sure what would be the audible effect, but I think the end result would most likely lower reliability.

Eric

""The driving force for the martensitic transformation is the instable crystal structure of Austensite at lower temperature. So without forming austenite again. The driving force is gone.""

The point is, at a specific temperature, austenite/martensite ratio will be stable, and lowering the temp changes that ratio to another, stable one..eventually, a temp is reached where all the austenite is gone.

Now, purchase a material, it is delivered, and in your hand..next, assume that what you have in your hand is not fully stable at the atomic level, and if you lower it's temp, a diffusionless process stabilizes the lattice..that is what the cryo process is all about..yes, heat was initially involved, but after you got the material, all you did was cool it..

It's that instability I'm talking about..the austenite to martensite transformation was the easiest example to use to explain diffusionless transformations..

It's the concept of a transformation that occurs as a result of cooling something that is important..

Cheers, John.

PS..be back in a week..on vaca...it's been a pleasure..