[TheForge] cryogenics

Mon Apr 1 13:21:00 2002

Cryogenic treatment of steel is based on the same principles and mechanisms
as is conventional heat treatment. It's a recognition of the fact, that in
many cases, the transformations that began when the piece was cooled from
the austenitizing temperature, fails to go to completion. Basically, the
problem is retained austenite at room temperature. Retained austenite is
much softer than the martensite anticipated. Metallographic transformations
require both driving force and temperature. As you cool below the
transformation temperature, the driving force increases (the structure wants
to change) but transformation itself becomes more difficult (temperature is
basically a measure of crystallographic vibration, and the facilitator of
change). You sometimes end up with a piece that is partially austenitic, and
stably so, at room temperature. The austenitic portion wants very much to
transform to martensite but the structure is too quiet to transform. The
idea of cryogenic treatment is simply that by cooling the structure low
enough, the driving force will become sufficient to force the
transformation, in spite of the lack of temperature. It turns out that if
you cool retained austenite to -120 degrees Fahrenheit, transformation will
occur. After reaching the critical temperature, transformation is complete,
and there is no advantage to further cooling or to holding longer. On an
atomic level, the transformation to martensite is essentially the same as if
it had taken place at a higher temperature, a diffusionless shear
(displacive) transformation of austenite. The metallographic appearance
should be similar.

Cryogenic treatment works for some items and is a complete waste of time for
others. You have to know exactly what you are starting with. If you have a
piece that, for one reason or the other, has retained austenite, cryogenic
treatment is a consideration. If you don't have retained austenite, there's
nothing to gain. You really have to know what you are starting with to know
what you'll end up with. Cryogenic treatment is not a generally applicable
process. It has limited applicability, and is usually of limited value even
where it applies. On the other hand, given the right set of circumstances,
the results can be dramatic.

Daniel T. Hayes, P.E.

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of northwoods
Sent: Monday, April 01, 2002 9:26 AM
To: [email protected]
Subject: [TheForge] cryogenics

I've heard a lot over the years on the subject of freezing knife blades, gun
barrels, and other assorted parts in an attempt to make them...better in
some way I guess. What is exactly going on at the molecular level in a piece
of metal when this treatment is used? People have told me that they have
done this to brake drums on vehicles and it has increased the wear
resistance.
Does freezing a piece relieve stress, also what else does it do and has
anyone ever had any experience with the process?

T. Clark

_______________________________________________
http://mailman.qth.net/mailman/listinfo/theforge
theforge mail list group photo site is
http://www.photoaccess.com
Login:  [email protected]
password:  anvil
___________