Cryo for steel is useless??

The one fact everyone agrees on is that it significantly reduces retained austenite.
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Yes, but the point is not to just lose the austenite, it is also to gain the martensite. The hard thing to predict is how much austenite remains after the initial cool. Two steels with exactly the same chemical makeup can have very different grain makeup. Austenite is not something quality steel manufacturers want in the first place, and they try to avoid. Careful control of the initial cooling will reduce austenite. Also, lower carbon steel will have less austenite. In a nutshell, the better the steel in the first place, and the more care you take cooling it, the less you have to gain by the cryo.
 
I thought that sub-zero quenching for steels like T-1 or other steels with high austentizing temperatures was non-controversial. I also thought it would be pretty irrelevant for, say, 1095.

I thought Kevin had made the point (and please don't beat me up too much if I am incorrectly paraphrasing him) that for steels in-between that you should solve the problem as much as possible by not having retained austenite in the first place by carefully managing heat-treating up to that point.

Anyhoo, from Crucible's website describing the heat treatment of 154 CM:

Recommended Heat Treat Practice
To completely transform any retained austenite, a freezing treatment with dry ice at -100°F (-74°C) is recommended either after the quench or in between the two tempers. The freezing treatment is most effective right out of the quench, however complex parts with sharp corners are more safely frozen between the two tempers. Thin sections can be successfully quenched in forced air and will obtain results to those in the table above.

Here's the PDF file:
http://www.crucibleservice.com/datash/ACF13AF.pdf

The two things that leap out at me are the 9 point HRC gain at the 2000 degree Austenitizing/1000 degree Tempering point between cryo and no cryo, and the secondary hardening of higher tempering temperatures.
 
I did some (admittedly very limited) testing on cryo and edge-retention some years ago as part of a cutlery set-up I had going at the time. I was using vacuum hardening which was completed with a gaseous nitrogen treatment (done for me by a UK company called Torvac.) There was the option of a second cryo treatment, also using gaseous nitrogen, but I wanted to find out if this was worthwhile.

I had test samples of knives made, ones with the 2-stage cryo treatment and ones without (incidentally, prior to deciding on the vacuum hardening I had tested knives using all sorts of hardening techniques.) In terms of strength I could determine no difference between the knives. Attempting to dull the edges was extremely difficult as these were very sharp knives with amazing edge retention (see thumbnails below.) To try and be as scientific as possible, I settled on cutting through blocks of cardboard - repeatedly. It took thousands of cuts before any difference could be detected, and then it was only slight since the knives were still very sharp, but the knife with the second stage cryo treatment definitely held its edge better. Why that was I don't know, and of course it's a limited study, but that's what I found, and I went with the 2 stage cryo treatment.



 
I don't think Cryo has been in the main stream that long that might be part of the reason why we don't know much about it.

Much of the info appears to be from academic institutions and not just business. So something must be happening.


METE ,

You appear to believe that it's nothing more than hype and BS. So do you have some scientific evidence to support you point of view?

I would do a search but that function is not avaliable to those of us in steerage class.

So give us your take I'm interested in hearing your point of view.
 
Larrin says it reduces retained austenite which is true. Let me just fill in some details.

With a higher austenitizing temperature (hardening temp) more carbon and chromium is dissolved into the steel mass. C and Cr (and Mo) breaks lose from carbides and dissolves into the steel mass.

Carbon increases hardness, so more carbon in the mass (matrix) enables higher hardness.

Chromium increases corrosion resistance, so more Cr in the mass enables higher corrosion resistance.

BUT, the more C and Cr dissolved in the mass, the more difficult it is for the transition from austenite (soft) to martensite (hard). So with everything being the same just increasing hardening temperature will not increase hardness, it will just increase the amount of retained austenite.

Now if you use cryo on the sample with higher hardening temp you will get the extra benefit of deep freezing. Which is 1-3 HRC depending on grades and other things.

Cryo enables higher hardness and corrosion resistance.

Larrin, you claim that you get AEB-L (13C26) to 63 HRC, this I believe is only possible with cryo. I'm I right?

If cryo is hype or BS, then +60 hardness is also hype and BS. I personally prefer my knives in the 58-59 range where cryo is only needed for low carbon grades like 420. But I believe 60+ hardness is desired by alot of people for the extra wear resistance (with a small sacrifice of toughness).
 
Razorsharp244 said:
Larrin, you claim that you get AEB-L (13C26) to 63 HRC, this I believe is only possible with cryo. I'm I right?
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The retained austenite is so low with AEB-L using a low austenitizing temperature (1925F) that you could probably get 63 Rc without it, but for maximum hardness, even when the rockwell tester isn't showing a difference, you need cryo.
 
Something that occurs to me is that knives are seldom tested for abrasion resistance alone, edge retentions is a function of many things, abrasion resistance is just one of those things.
 
There may be some benefit to cryogenic processing beyond conversion of retained austenite to martensite, but at least one guy who claims to have proof also states that trade secrets prevent him from providing said proof.

I'd be quite surprized if gains in wear-resistance weren't from getting additional martensite.
 
Mike Stewart at Bark River uses cryo on his A2 blades and that's enough of an endorsement for me. All of my A2 Barkies have been nothing short of remarkable.

FWIW, his website says it is done for "stress relief," whatever that means in context.
 
Don't Frosts of Mora use cryo throughout their range (and have done for a long time?)
 
thombrogan said:
I'd be quite surprized if gains in wear-resistance weren't from getting additional martensite.
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I agree. Additional martensite means extra hardness, this gives extra wear resistance. But since retained austenite is soft and ductile it also loses some toughness. Probably not noticeble for a small folder but it could be significant for like a big chopper.

Larrin: Ok I see. With 0 retained austenite the hardness reaches high levels. Is the toughness still good enough? Do you know roughly what retained austenite levels you normally get for AEB-L at 62-63 HRC?

Btw Larrin, if you didn't know, I'm the guy who mailed you the microphotographs during the summer.
 
Razorsharp244 said:
I agree. Additional martensite means extra hardness, this gives extra wear resistance. But since retained austenite is soft and ductile it also loses some toughness. Probably not noticeble for a small folder but it could be significant for like a big chopper.

Larrin: Ok I see. With 0 retained austenite the hardness reaches high levels. Is the toughness still good enough? Do you know roughly what retained austenite levels you normally get for AEB-L at 62-63 HRC?

Btw Larrin, if you didn't know, I'm the guy who mailed you the microphotographs during the summer.
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According to Verhoeven, the retained austenite is about 4% after austenitizing from 1925F. For fine cutting knives I'm more concerned about the edge being fully hard rather than the blade having a higher overall toughness. As you know, it has been recommended by Sandvik to shoot for approximately 15% retained austenite. I would rather have full martensite and heat treat to a lower hardness (through lower austenitizing temperature, higher temper, or both). That's just me, though.

Oh, and I still love the micrographs, they've been very useful.
 
Here is another proponent, and the parts that seem to be common to all cryo companies is what I highlighted in bold and red. I was just going to post the link but I could not as I could only reach the article by cache.


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."

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.

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 Cryogenating
Deep Cryogenic Tempering Cryotempering Thermal Cycling Deep Cryogenic Treatment (DCT)
Cryoing

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, the term "Cryogenic Hardening" is also confusing, inaccurate, and does not convey the full impact of what cryogenic processing can do. Also, 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. 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.

What Does Cryogenic Processing Do?

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 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.

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.


The process also promotes the precipitation of small carbide particles in tool steels and steels with proper alloying metals. A study in Rumania 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 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.

Stabilizing metal to prevent distortion

Cold processes have been used for years to stabilize fixtures and tooling. The process will relieve stresses and that will help to machine parts to the proper size and shape. Cryogenic processing establishes a very stable piece of metal that remains distortion free. The process will also stabilize some plastics.

The process will eliminate the retained austenite in ferrous metals. Retained austenite can be transformed to martensite through the heat, pressure, or vibration of use. This can cause the part to go off specification due to the difference in crystal size between austenite and martensite. We have customers who will cryogenically treat metals before heat treat also. This reduces the distortion due to heat treat. Note that treating the metal before heat related processes such as the Toyoda process also reduces size and shape change due to that process. Some companies will cryogenically treat any precision part before and after heat treating to stabilize the part and reduce warping.

Another problem occurs due to the transformation of retained austenite to martensite. This causes the part to have residual stresses due to the change in size locally. This residual stress and the fact that the martensite is primary martensite can cause the part to be brittle and to crack. Cryogenic processing prevents these problems by converting retained austenite to martenstite.

Cryogenic treatment has a unique effect on metals. The hardness of the metal becomes less variant after treatment. Typically, the standard deviation of the hardness of treated metals will one third that of a similar untreated metal. Cryogenically treated metals are easier to polish and resist orange peeling more. The net result is that less labor is required to polish the metal and the polish holds up better.

A typical cryogenic cycle consists of:


RAMP DOWN
Lowering the temperature of the object

SOAK
Holding the temperature low

RAMP UP
Bringing the temperature back up to room temperature

TEMPER RAMP UP
Elevating the temperature to above ambient.

TEMPER HOLD
Holding the elevated temperature for a specific time


RAMP DOWN

A typical cryogenic cycle will bring the temperature of the part down to -300F over a period of six to ten hours. This avoids thermally shocking the part.


There is ample reason for the slow ramp down. Think in terms of dropping a cannon ball into a vat of liquid nitrogen. The outside of the cannon ball wants to become the same temperature as the liquid nitrogen, which is near -323F. The inside wants to remain at room temperature. This sets up a temperature gradient that is very steep in the first moments of the parts exposure to the liquid nitrogen. The area that is cold wants to contract to the size it would be if it were as cold as the liquid nitrogen. The inside wants to stay the same size it was when it was room temperature. This can set up enormous stresses in the surface of the part, which can lead to cracking at the surface. Some metals can take the sudden temperature change, but most tooling steels and steels used for critical parts cannot.



A typical soak segment will hold the temperature at -300F for some period of time, typically eight to forty hours.


During the soak segment of the process the temperature is maintained at the low temperature. Although things are changing within the crystal structure of the metal at this temperature, these changes are relatively slow and need time to occur. One of the changes is the precipitation of fine carbides.


We believe that this time in the process also provides time for the crystal structure to react to the low temperature and for energy to leave the crystal structure. In theory a perfect crystal lattice structure is in a lowest energy state. If atoms are too near other atoms or too far from other atoms, or if there are vacancies in the structure or dislocations, the total energy in the structure is higher. By keeping the part at a low temperature for a long period of time, we believe we are getting some of the energy out of the lattice and making a more perfect and therefore stronger crystal structure,




Tempering is important with ferrous metals. The cryogenic temperature will convert almost all retained austenite in a part to martensite. This martensite will be primary martensite, which will be brittle. It must be tempered back to reduce this brittleness. This is done by using the same type of tempering process as is used in a quench and temper cycle in heat treat. We ramp up in temperature to assure the temperature gradients within the part are kept low. Typically, tempering temperatures are from 300F on up to 1100F, depending on the metal and the hardness of the metal

A typical temper hold time is about 3 hours. This time depends on the thickness and mass of the part. There may be more than one temper sequence for a given part or metal. We have found that certain metals perform better if tempered several times.

Why Isn't Cryogenic Processing Well Known?


Cryogenic Processing is relatively new.
Cryogenic processing has been around only for the last sixty years.

It is empirically developed.
Nobody expected to see anything happen at cryogenic temperatures and there are still those who will tell you nothing does. (This causes endless amusement at the Cryogenic Society of America meetings. One only has to look at super conductivity to see that many things can happen at cryogenic temperatures.) The people and companies that developed the process did so because they noticed things happening to materials that were exposed to extreme cold, and took things to the next logical step. The first results were often dreadfully cracked or warped pieces. But eventually recipes were developed which worked. Heat treating was developed much the same way. Look at the recipes that used the urine from a red haired boy as a quenchant. Until this process is well known and attracts more research, we will not really know what we should know about it.

There has been very little research into the theories of why it works.
Since in the minds of certain learned individuals the process can't possibly work, very little has been spent on finding out why it does work. We have had people and companies look at consistently fabulous results and tell us they can't possibly use the process because they cannot see why it works.

The process does not show up as an easily demonstrated change in microstructure.
Before some companies will even try our process, they ask us to run a sample piece, which they then cross section and inspect for changes in the microstructure. They don't see the changes, because they do not know what they are looking for. Since they don't see changes through an optical microscope, they claim nothing has been done and the process does nothing. They rarely see that the process has doubled or tripled tool life because they must "see" a change. We get much better results when we are allowed to work with an engineer who is open minded and will try the process on a tool or component that both he and we agree on. We did this several months ago with an engineer for an Aeroquip division and handed him a die that lasted over ten times as long as the best die he had used before. We did some more dies for him and repeated the success.


Resources

ASM International
Materials Park, Ohio 44073-0002

Cryogenics Society of America, Inc
www.cryogenicsociety.org
Controlled Thermal Processing, Inc.

www.metal-wear.com
Applied Cryogenics, Inc.

25 Adams Street

Burlington, MA. 01803
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Isn't it the "sub-zero quenching" of the blade in my Cold Steel 420 Peacemaker that makes it cut like a chainsaw? (I got one fairly recently but can't figure out where to plug it in.:D) Anyway, someone said in an old post that cryo only works with high carbon stainless steels, yet some say it's the cryo that makes Buck knives (420HC) so hard and resiliant. If Paul Bos uses it, not to mention the many others, wouldn't that be a good indicator that it works? Also, isn't the process somewhat expensive?
 
Larrin said:
According to Verhoeven, the retained austenite is about 4% after austenitizing from 1925F. For fine cutting knives I'm more concerned about the edge being fully hard rather than the blade having a higher overall toughness. As you know, it has been recommended by Sandvik to shoot for approximately 15% retained austenite. I would rather have full martensite and heat treat to a lower hardness (through lower austenitizing temperature, higher temper, or both). That's just me, though.

Oh, and I still love the micrographs, they've been very useful.
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Yes, I think Sandvik recommends 10-15% retained austenite. I believe it's a bit conservative but also quite logical. It's rare that people complain over to much toughness but quite common that they complain if the knife is brittle and breaks easily. I think that most H/T recommendations made by steel companies is on the conservative side for this reason. Also in Europe, where Sandvik is more active, knives in the HRC 60+ really is not that common. Puukkos, Lagioles, proffessional knives, Moras and Italian knives are usually in the 57-59 HRC range. Probably since high toughness is valued more in Europe than in the US.

America is slightly different with many knives in the HRC 60+, which works since most US designs are more "stout" in their design so the demand for toughness is lower.

Larrin, is it your opinion that a 5% RA blade would be "tough enough" for normal use in for instance AEB-L or 13C26? Is lower possible?
 
Razorsharp244 said:
Larrin, is it your opinion that a 5% RA blade would be "tough enough" for normal use in for instance AEB-L or 13C26? Is lower possible?
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It's hard to say, since I've never made a knife where I didn't attempt to get the RA to 0%. It is of course very difficult to measure the retained austenite once you get below a certain percentage, and I don't have the equipment to measure it. I would like to think that I am getting as close to 0% as possible. In my limited testing, it is "tough enough."
 
Larrin said:
It's hard to say, since I've never made a knife where I didn't attempt to get the RA to 0%. It is of course very difficult to measure the retained austenite once you get below a certain percentage, and I don't have the equipment to measure it. I would like to think that I am getting as close to 0% as possible. In my limited testing, it is "tough enough."
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Thanks for your feedback Larrin.

Here is some background for you about the RA levels. 10-15% basically came from the proffessional industry (butcher and fishing industry). The one thing that is not allowed to happen is blade breakage. It is quite common that this causes injuries so a high toughness level has always been a high priority. So in order to be "better safe than sorry" the general recommendations are 10-15%. For sporting, hunting and cooking knives maybe these recommendations should be re-considered some.

A stainless Mora (I'm not sure if you are familiar with those) is usually about 10-12% RA and they are really tough blades. But they are used almost as pry-bars as cutting tools.
 
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