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Theory of Cryogenics

This was written to help people understand more about cryogenics and cryogenic processing.  It is more textbook than advertisement, so it may be a little boring.  We've tried to keep the superlatives to a minimum. It is a work in progress, so please bear with us. 

Metallurgy for Non MetallurgistsIf you are new to engineering, or not very knowledgeable about metals, it would help you to get some basic knowledge in the field.  We suggest that you go to ASM International website and get a copy of their book Metallurgy for the Non-Metallurgist    This book will give you an excellent overview of general metallurgy without going into mind-numbing detail. 

 

Steel Metallurgy for the Non-MetallurgistA second good book if you are interested in steels is Steel Metallurgy for the Non-Metallurgist.  This is also published by ASM.  Again, it is aimed at simple explanations.

Another good source for basic metallurgy, but without credentials of the ASM is on the Pirate4x4.com website.  They have a section titled The Tech Behind the Talk - Steel and Material Strength.  You should take a look at this article.  By the way, our listing this article here does not imply their endorsement of our process or our endorsement of their products. 

If you want to know more about cryogenics in general, the National Institute of Standards and Technology (NIST) has a Cryogenic Technology Group. This group has a web site with tremendous amounts of information about cryogenics in general. Click here to see their site.

There are more links to cryogenic sites at the Cryogenic Society of America's link page.

What is Cryogenic Processing?

There are no "official" definitions of the term cryogenic processing.  Controlled Thermal Processing, Inc. considers the definition of the process to be:

"The modification of a material or component using cryogenic temperatures."

Cryogenic temperatures are defined by the Cryogenic Society of America as being 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.

The word Cryogenics is derived from the Greek words 'Kryos" (meaning cold) and "Genes" (meaning born).  It was first seen around 1894 by a fellow named Kamerlingh Onnes to describe his Leiden Cryogenic Laboratory.  He was a professor of Physics from 1882 to 1923 at the University of Liden. In 1904 he founded a very large cryogenics laboratory and invited other researchers to the location, which made him highly regarded in the scientific community.  He was the first physicist to liquify helium, doing so in 1908.  By 1937, "cryogenics" was being freely used in relation to gasses and their critical points.  This usage in the 1930's led to the definition of 120oK as the upper limit of the cryogenic temperature range.

Since 1945, the increased use of "cold" in medicine and industry has resulted in higher temperatures being labled as cryogenic.   In our industry this has led to some problems as heat treaters tend to call the -110oF cold treatment to convert retained austenite to martensite a cryogenic treatment. It is not cryogenic, and it does not do the same thing.

Since there is no official definition of the process, the process parameters vary widely from one company to the next. Processes and equipment vary considerably from one company to the next.  Simply specifying "cryogenic treatment" is not sufficient.  One must be very careful when comparing results from one company to those of another.  Controlled Thermal Processing, Inc. has been in business since 1982.  Our equipment is the best available.  Where our cryogenic processing machinery use the latest vacuum insulated dewars and heat exchanger technology, we have competitors who pour raw liquid nitrogen on their customer's expensive tooling.  Our techniques are proven ones.  For instance, Controlled Thermal Processing, Inc. processes parts for other "cryogenic processing" companies because our process and machinery works where theirs fail.

The process is referred to by quite a few different terms.  Terms heard in industry are:

Cryogenic Processing Cryogenic Tempering Cryogenic Stress Relief Cryogenics
Cryogenic Hardening Cryo Cryogenic Treatment Cryogen        
Deep Cryogenic Tempering Cryotempering         Thermal Cycling Deep Cryogenic Treatment (DCT)
Cryoing Cryogenic Stress Relief

Cryogenic Tempering

The term "cryogenic tempering" is used a lot in relationship to cryogenic processing.  It is a nonsense term.  According to Metals Handbook, published by ASM, tempering is,

"....reheating hardened steel to some temperature below the eutectoid temperature to decrease hardness and/or increase toughness."

Tempering used to mean hardening in archaic English, hence the persistence of phrases like "fine tempered steel" in advertising.  Such a phrase is virtually meaningless, as any hardened steel would be tempered anyway.  We feel it is confusing and inaccurate to use the word "Tempering" when referring to cryogenic processing.  

Since the process rarely makes materials significantly harder (if it does, there was something very wrong with the heat treat), the term "Cryogenic Hardening" is also confusing, inaccurate, and does not convey the full impact of what cryogenic processing can do.  

In the same light, the term "stress relieving" also indicates a specific process, and the use of cryogenics covers much more than stress relieving.  As far as calling it "Deep Cryogenic Treatment", anyone who is familiar with cryogenic science will know that -300oF is not very deep into the cryogenic range, which starts at -244oF according to the Cryogenic Society of America.  Besides that, we are currently expeimenting with temperatures down to -450oF, which is deep cryogenics, as it is very close to absolute zero.

Is it nit picking to criticize what people call the process?  Maybe it is, but fancy sounding names often confuse the issue as to what the process does. Our preference is to call the process "Cryogenic Treatment" or "Cryogenic Processing".  Both "Cryogenic Processing" and "cryogenic Treatment" have been approved by the Cryogenic Processing Sub-Committee of the ASM.  That's good enough for us.

One term that does not apply to what we do is Cryonics.  The word Cryonics refers to the process of freezing living tissue with the intention of bringing it back to life in the future. .  We do not have Walt Disney  or Ted Williams in the back room.

What Does Cryogenic Processing Do?

Cryogenic processing is not:

A Coating
A Substitute for Heat Treating
A Fix for Bad Heat Treating

Cryogenic processing is not a coating. It affects the entire volume of the part. It does not go away when the part is machined or sharpened. It affects every atom of the structure. It works to enhance plating and many coatings

Cryogenic processing is not a substitute for heat treating. It will not harden parts substantially. Used in conjunction with proper heat treating, it produces a better, more stable part.

Cryogenic processing is not a fix for bad heat treating. Although it will cause retained austenite to transform to martensite, so will cold treating at -140oF. It will not cure the other ills of bad heat treat.

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. The changes we know about are:

Change of Retained Austenite to Martensite in Hardened Steels

Reduction of Residual Stress

Precipitation of Fine Eta Carbides in Steel
Reduction in point defects
Re distribution of alloying elements

The change of retained austenite to martensite happens in steel and cast iron only.  The same with the precipitation of fine carbides.  Many metallurgists will tell you outright that changing austenite to martensite is the only thing that cryogenic processing does. They are very wrong.

We know that there must be more than this. Why? Because cryogenic processing has been shown to work on materials other than steel.  Brake rotors are made of cast iron that is pearlitic in structure but scientific tests show an increase in life of two to four times.  Most metals will respond to cryogenic processing. Some plastics do. There is a lot of evidence that crystals such as diamonds, cubic boron nitride, and aluminum oxide also respond. Basic Crystal Structure

Some Basic Metallurgy

Metals are crystalline in structure.  That means that the atoms of the metal line up in an orderly fashion. A theoretical crysytal structure is pictured at the right.

Notice how perfect it is.  This rarely happens in the real world. A real world crystal structure is pictured below.  The yellow dots are the atoms.  Note that the alignment of all the atoms is not perfect.  There are discontinuities and changes in spacing.

Theory proposed by Dr. Mark Eberhardt at the Colorado School of Mines states that there is a discreet distance between atoms where the energy in the metallic bond is minimum.

 

A Picture of an Actual Crystal Structure

Actual Crystal Structure

Copyright, Nanoscale Physics Research Laboratory, 
University of Birmingham 

Picture courtesy of and used with the permission of Birmingham University's Nonascale Physics Research Lab. We wish to thank Professor Richard Palmer for allowing us the use of the picture. 

Their explanation for the picture is:

Atomic Love, in Three Dimensions
It's Valentine's day - and they say that the best presents come in small packages. Scientists at Birmingham University's Nanoscale Research Lab have taken this message to heart - their Valentine card, made of pure palladium, is only 8 nanometers in size; you can even see the atoms.  Making the card was also a work of love; clusters of palladium bonded together on the surface of carbon and spontaneously arranged themselves into the world's smallest heart.

 

We know that when we reduce the temperature of an object the atoms in the object come closer together and we take energy out of the object.  We have a theory that when we warm the object back up that some of the mispaced atoms in the crystal structure relocate to the proper spacing.  This produces a more perfect crystal  We've discussed this theory with metallurgists and scientists. Nobody has said we are crazy yet.

Atoms in a crystal structure can line up in different formations.  Most metals use one of three different formations, although other formations occasionally appear.  The three basic formations are:

Body Centered Cubic

Face Centered Cubic

Hexagonal Close Packed

 

Austenite to Martensite Transformation.

What are austenite and martensite?  Let's try to explain. 

What are the other things that happen? We think that they are :

Reduction of Vacancies in the Crystal Lattice
Refinement of the Atom to Atom Spacing
Re-Ordering of the Alloying Elements

.

In general, the process seems to refine 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.  We believe that cryogenic processing makes the crystal more perfect and therefore stronger.  

We have some evidence from work that we did with Honeywell on thin film magnetic memory chips.  They found that our process relieved stresses between the layers, increased conductivity, and also there was some evidence holes in these very thin films (on the order of several atoms thick) were gone after processing.  This indicated a shifting of atoms in the crystal lattice.  Another indication is that metal objects transmit sound or vibration differently after processing.  We repeat that this is just theory and requires verification.  

Change Retained Austenite to Martensite in Hardened Steels

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.

Austenite

The process also promotes the precipitation of small carbide particles in tool steels and steels with proper alloying metals.  A study in Romania found the process increased the countable small carbides from 33,000 per mm3 to 80,000 per mm3.  The fine carbides act as hard areas with a low coefficient of friction in the metal that greatly adds to the wear resistance of the metals.  A Japanese study (Role of Eta-carbide Precipitations in the Wear Resistance Improvements of Fe-12Cr-MO-V-1.4C Tool Steel  by Cryogenic Treatment; Meng, Tagashira, et al, 1993) concludes the precipitation of fine carbides has more influence on the wear resistance increase than does the removal of the retained austenite.

Note that the hardness of a piece of metal becomes more even during the process.  When multiple hardness readings are taken before and after the process, the standard deviation of those readings will drop a significant amount.   

The process relieves residual stresses in metals and plastics

This has been borne out by several research projects as well as by practical use.  We have customers who cryogenically treat metal before heat treat to reduce the distortion of the metal during heat treat.  NASA is one of them.  We processed components for the  space shuttle's robotic arm for this reason.  Gage makers have used cold temperatures to stabilize metals for years.  Our work with Honeywell also showed the process relives stresses as does a recent NASA study on welded aluminum.

 

Cryogenic Processing is not a substitute for heat-treating.   

Cryogenic processing will not in itself harden metal like quenching and tempering.  It is not a substitute for heat-treating.  It is an addition to heat-treating.  Most alloys will not show much of a change in hardness due to cryogenic processing.  The abrasion resistance of the metal and the fatigue resistance will be increased substantially.  

 

Cryogenic Processing is not a coating.  It affects the entire volume of the material.  It works synergistically with coatings. 

You cannot wear cryogenic processing off a part.  It is there for the life of the part unless that part is subjected to such temperatures that it is brought up to the austenization temperatures.  Unlike coated tools, a cryogenically treated tool can be sharpened, dressed, or modified.  The change brought about by cryogenic processing is permanent.  

 

The process works synergistically with most coatings.  This is because coatings generally work by decreasing the coefficient of friction and by preventing metals from galling.  Coatings start to fail when the metal underneath them fails.  It is not unusual to find wear particles with coating on one side and base metal on the other.  The coating did not fail; the base metal under it failed.  Cryogenic processing keeps the metal under the coating from failing while the coating protects the metal.

 

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