Cryogenic Processing
 What first comes to mind when one hears
the words Cryogenics is the Sci-Fi reference to freezing live tissue with the
intention of reviving that tissue in the future. This is actually Cryonics
and not Cryogenic Processing.
The first is not what CPM Fastools does; we do not have frozen heads in the
back room.
So what is Cryogenic Processing?
The Cryogenic Society of America defines Cryogenic Processing as temperatures
below 1200K (-2440F, -1530C). This is different from cold treatments that
are often spelled out in heat treating specifications. Cold Treatments
generally only go down to temperatures of 1780K (-1400F, -960C) Cryogenic
treatments have been found to offer performance over and above that of cold
treatments by studies performed by the Illinois Institute of Technology Research
Institute for the US Army Aviation and Missile Command.
So what does it do and how does it benefit my
application?
Cryogenic processing makes changes to the crystal structure of
materials. The major results of these changes are to enhance the abrasion
resistance and fatigue resistance of the materials. In general, the
process refines the crystal structure of metals and crystalline plastics.
Although there has not been definitive research on the subject, the theory is
that it crystal structures are not perfect. Some research shows that there
are millions of vacancies per cubic inch of metal in the crystal lattice.
Also, some atoms are not ideally located in respect to their nearest
neighbors. The process relieves residual stresses in metals.
This has been borne out by several research projects as well as by practical
use. Our machine manufacturer has worked closely with customers who
cryogenically treat metal before heat treat to reduce the distortion of the
metal during heat treat. NASA is one of them. Gage makers have used
cold temperatures to stabilize metals for years. In Ferrous metals,
it converts retained austenite to martensite and promotes the precipitation of
very fine carbides.
It has been known for many years that cold
will cause retained austenite to change to martensite. (The terms austenite and
martensite refer to the way the carbon atoms relate to the ferrous atoms in the
crystal lattice structure. Note that we refer to a crystal lattice
structure. A lot of people try to talk about the "molecular" structure of
metals. Metals are metals because they are crystalline in nature.
The crystal structure is what gives the metals their ability to conduct heat
and electricity, their ability to plastically deform, and their ability to be
hardened.) This can be verified through publications such as Machinery's
Handbook, ASM's Metals Handbook and more. Even the best heat treating
facility will leave somewhere between ten and twenty percent retained austenite
in ferrous metals. We've seen over 40% on gears and shafts made for
commercial and racing applications. Because austenite and martensite have
different size crystal structures, there will be stresses built in to the
crystal structure where the two co-exist.
Cryogenic processing eliminates these stresses by converting most of the
retained austenite to martensite. This also creates a possible
problem. If there is a lot of retained austenite in a part, the part will
grow due to the transformation. This is because the austenitic crystals
are about 4% smaller than the martensitic crystals due to their different
crystal structure.
For this reason CPM Fastools considers the joining of Laser Sintering and the
use of Cryogenics proprietary to our manufacturing process. Extraordinary
effort has been exerted in finding the correct processing curves and mold
shrinkage factors.
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