cryo treatment

curtisk

Well-Known Member
What exactly is cryo treatment? Does anyone have information on the process?

thanks, curtisk
 
Essentially "cryo treatment" refers to a portion of the "heat treat" process. As it applies to knife blades, once the steel is hardened, and then given an initial tempering, the blade(s) are placed in an extremely cold environment. This most common is liquid nitrogen (-300F). This process converts retained austenite to martensite. By transforming retained austenite, a couple points of Rc hardness can be realized, which is addressed in further heat treating steps.

Some folks will use dry ice and acetone in an attempt to cryo, but the max you can achieve with that combo is approx -108F. This MAY have some benefits on simpler steels, but is not cold enough for more complex/high alloy steels.

Personally, I cryo treat my blades in liquid nitrogen. On some of the stainless steels such as 13C26, ATS-34, and 154CM it make a night and day difference in the cutting performance of the blade(s). My setup consists of a "dewar" tank (used to preserve semen for artificial insemination by Ranchers/Veterinarians) that holds up to 56lbs of liquid nitrogen (28 liters). After the blades go through their first tempering cycle, I place them on a stainless wire, and lower them onto the tank, where they remain for 8-12 hours. I then take the blades out, and place them between two layers of Kawool, which allows them to warm slowly to room temp. Once they are at room temp, they go back into the tempering oven for 2 hours, at 25F higher than the previous temper(s).

I've had great results "cry treating" 52100, 5160, CruForge V, as well as the steels I mentioned above. Once you get over the sticker shock of a dewar tank, and the fact that you'll have to fill the tank nearly every time you use it, it's just another step we can take to produce a better blade.

NOTE: Something I did, that makes my tank of liquid nitrogen last: I built a plywood box, (including a hinged lid) lined with 2" thick rigid foam insulation. As soon as I get the tank home from a fill-up, I place it in the box, and fill all the surrounding air space with vermiculite, then place a layer of fiberglass insulation over the top of the tank...then shut the lid. With the dewar full, and in the box, it will routinely last 2 months, give or take for weather conditions.
 
Cryo also improves toughness by converting retained austenite to martensite. Without the cryo the retained austenite will gradually transform itself to untempered martensite, which is quite brittle. When you force the conversion with cryo then temper you get tempered martensite instead.
 
That is interesting, very interesting. Thank you Ed and Dan. Ed, I really appreciate you “breaking down” the process. I have some 5160 and some 01 that I could “play” with. As luck would have it, my neighbors own their own AI business and have some of those Dewar tanks. That being said, I reckon I am a bit ahead of the game. My neighbor also knows my obsession with making the best knife possible.
I did some more reading on the subject yesterday and it seems there are a plethora of ways to perform this process, but they all come down to two steps: controlled cooling, controlled warming. The controlled cooling I could do (just as you described Ed) by suspending the blade in the chamber so just the vapor came into contact with the blade, it was the controlled warming that stumped me. Ed, great idea with the Kawool! Controlled warming (even in Texas weather) is now possible.
Ed, I am going to pass on your insulating the dewars idea to my neighbors. They have a few of the small tanks that this would work on.
Thanks again!
 
Since we are on the topic, I was reading somewhere on the web a while back on a tool & die maker's site and he listed the effects of cryo for 01 - 400% increase in toughness I believe was his number:what!::les: Has anybody out there tested this for knife use? I have been operating under the impression that cryo had limited benefit at best for anything besides SS and maybe D2. What's the beef on this?
 
It's got limited benefit at best even on SS and D2.

I am told Scott Devanna and some other PHD's in metallurgy and heat treating said at the Strider/Crucible talk at Blade that there was no benefit to cryo in a knife blade and actually a small amount of retained austenite is not a bad thing. I believe the "Bos" man himself even agreed that a little retained austenite is OK.
 
I'm in a picky mood tonight, please forgive me.

"No benefit"? I'm no PhD but even with my very limited testing I have seen a difference. 300-500% difference? Heck no.

How much is "a little RA"? How much RA is a bad thing?

Why would so many firms that cryo blades (including Bos) do it if it had little or no benefit?

I'm not trying to be a pain, I just honestly want to understand the subject better. The cryo discussion includes wild claims of HUGE increases in toughness, and claims that it's pointless. I suspect the truth is somewhere in the middle.
 
Why would so many firms that cryo blades (including Bos) do it if it had little or no benefit?

I can't answer that for sure but now that they know what they know possibly because the customer want's it. I do a lot of things in my business that I know makes no difference but the customers insist on paying to have it done since it makes them feel better.

Scott Devanna has said for a long time that he saw no benefit to it but said if your doing it and you have good results go ahead with it.

They have all said it's all in having a good heat treatment. I'm no PHD in steel so don't know how much RA is left in a properly heat treated blade but however much is apparently exceptable and some even have said it's a good thing. Cryo isn't going to fix a bad heat treat job to begin with.

All I know is they are the experts and I tend to believe them since my testing has shown absolutely "NO" difference in the way a blade performs I see no reason to shock it by dunking it in a pot of LN.

The bottom line is as more and more comes out on cryo studies it is apparent it's more useful as a marketing tool than to make a better blade.
 
Certainly, cryo won't fix anything that was really wrong with the hardening to begin with.

What kinds of alloys are we talking about? I think it depends a lot on the steel type, and how much chrome and other fun things are in it. I'll refrain from explaining further, for fear of goofing up a detail and making a fool of myself.
 
James is right.... cryo is not a "band-aid" to mitigate bad heat treating. Cryo is an enhancement to proper heat treatment.

MUCH depends on the alloys your dealing with. Plain carbon steels benefit the least from cryo, and as you get into the higher alloy steels, particular some of the stainless steels, cyro becomes almost a must. One steel that I have experience with along those lines is Sandvik 13C26....it's nearly worthless without cryo....but once it's been cyro treated it's a night and day difference in how it cuts, sharpens, and holds up.
 
I don't know what steels if any particular were mentioned at the talk because I wasn't there. Just had some say that Scott and others agreed cryo wasn't needed.

I have noticed cryo blades would test about 1 point harder than ones that weren't when 440C and ATS34 are air quenched. But recently I started plate quenching in large aluminum plates and my blades are testing even harder. From my experience the quicker quench makes a larger difference in hardness than the cryo adds. I'd like to try cryo with the plate quench method and see if they even get harder. There is no question something is going on in the steel but I can't honestly say I can see a performance difference. I think it gets to the point your splitting hairs and only a controlled laboratory test would show any difference.

I've made some test blades from Carpenter's new powdered steel and their metallurgist advised me on the heat treatment. His instructions are very specific and they did not involve cryo. When I get time I'll contact him and get his opinion on it in this application for their stainless steels. I'd be curious because this steel acts very different and really responds differently to plate quench verses air.
 
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I agree on the quenching Cliff. It's amazing the difference that plate quenching versus straight air quench makes on some steels.....ATS34, 13C26, 19C27, and 154 are ones that I have personally experimented with.

On a different note, I had a experience recently that still has me scratching my head. I forged three blades from CruForge (just trying to use it up)...and decided to cyro two of the blades to see what happened. After cyro and another temper, the blades were so tough/hard that a new Norax X30 would not touch them! In order to finish those blades out, I had to re-temper them at 550F! Even then it wasn't easy. If I had any intent to use more of this steel I would send it off for a spectrograph...but I'm not gona waste the money on it. Anyway, once the blades were finished down and sharpened, it was like sharpening 154, and it cut very similar. They were very difficult to bend/break, but once broken I was very impressed with the grain. Not sure exactly what the cryo did to the CruForge, but whatever is was, there was a profound impact on the steel versus blades that were not cryo treated.

In the end it's always gona be up to each maker whether he/she wants to take the time, effort, and money to cryo their blades. As for me, I am convinced that with specific steels it makes a profound difference for the good, and in some cases is a must to obtain the best attributes for a knife blade.
 
Cliff, I have made a few with CTS 40CP and really like it. I cryo'd some, some not. Anecdotally -cryo did seem to help with edge retention in my limited testing. I plate quench and tempered according to the sheet that was sent -1850 quench and 2X2 @ 350. I would say it is at least equal to ATS-34 and superior to simple 440C. I am now about to buy some XHP to try.
Which variety have you used and how did you treat it?
 
knife makers cryo is different than standard cryo of other metal objects ( brake rotors, musical instruments , golf clubs etc.) in that the makers quicly chill and moderately retard the return to room temp. When other objects are cryoed they are done in a very controlled environment the are taken down to -300+ very slow depending on density and material it may be only a few deg an hour, then after a long hold they will be brought back at the same rate. What his doe is align the structure on a molecular level releasing the internal stress of the metal itself . this all happens naturally with age of metal it can take decades for it to happen ( one reason older brass and silver instruments are regarded for ) with a normal cryo treatment this happens almost instantly.
 
Cliff, I have made a few with CTS 40CP and really like it. I cryo'd some, some not. Anecdotally -cryo did seem to help with edge retention in my limited testing. I plate quench and tempered according to the sheet that was sent -1850 quench and 2X2 @ 350. I would say it is at least equal to ATS-34 and superior to simple 440C. I am now about to buy some XHP to try.
Which variety have you used and how did you treat it?

40CP and XHP. Was told to go hotter on the XHP but I can't remember without looking. The XHP performs better yet finishes great. It may quite possibly the best thing that's come along since sliced bread.
 
knife makers cryo is different than standard cryo of other metal objects ( brake rotors, musical instruments , golf clubs etc.) in that the makers quicly chill and moderately retard the return to room temp. When other objects are cryoed they are done in a very controlled environment the are taken down to -300+ very slow depending on density and material it may be only a few deg an hour, then after a long hold they will be brought back at the same rate. What his doe is align the structure on a molecular level releasing the internal stress of the metal itself . this all happens naturally with age of metal it can take decades for it to happen ( one reason older brass and silver instruments are regarded for ) with a normal cryo treatment this happens almost instantly.

I think it would take substantially more aligning than just the molecularly bonded carbides since the vast majority of steel, and certainly other metals not in an Fe/Fe3C system, are not molecular but metallic bonded crystals.

Although I have heard the realignment or super-lattice theory, and I am open to its possibility, since it is one of the better explanations for any verifiable effects and claims of cryo, beyond treating retained austenite.

However it is the eta carbide aspects that I have heard attached to the accelerated aging theories. I am also inclined to lean with less skepticism in this direction as well since carbide precipitation is diffusional and thus more susceptible to time than reordering of vacancies or dislocations in the lattice.
 
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