Is Cryo Really Necessary?

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SLC_Blades

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Hello everyone! I just finished constructing an electric heat treat oven so that I can heat treat ovens at my new house without worrying about scaring neighbors with large propane burners and forgers. Now that I have this equipment, I’ve been considering getting into stainless steel for kitchen knives and that kind of thing. My oven is perfectly capable of getting up to the temperatures required for stainless but I’m concerned about doing cryo treatments at home.

I’m not thrilled about the thought of paying a bunch of money for a dewar and then constantly filling it with liquid nitrogen just to do cryo treatments. Are these treatments REALLY necessary to achieve an acceptable hardness on stainless knives? I find it hard to believe that after soaking at temperatures above 1900F and then plate quenching with compressed air followed by a standard temper cycle that the steel wouldn’t be at an “acceptable” hardness. Then again, I’m not a metallurgist.

Thanks!
 
I too need more info. Some good links to past discussions or documents by Larrin would be swell. A young co worker noticed on one of my German bladed knives the words "Deep Cryo" and ask about it. I would like to just print something out to give to him.
I told him that personally I felt it wasn't all that big of a deal for pocket knives.

Somebody tell me : Was my Spyderco M4 steel Deep Cryo treated ?
Since that steel is my be all end all that will tell me all I need to know.
 
Willie71 said:
If you want the proper performance then do dry ice/acetone at minimum. I did that for a few years prior to getting my dewar. Now that I have the dewar, I wouldn’t go back. I’m quite obsessive about heat treat though.
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I agree with Warren, subzero at a minimum,cryo for optimum. Also I ony fill my dewar 1/2 full and have blades and springs ready for about 6 knives, do them all at once and then build out the knives. Then refill as necessary. My dewar will hold a half fill for about 5 weeks.
 
Lots of great info available regarding this subject if you search cryo and Larrin. Or go to knifesteelnerds.com for more info.
Cryo is an important step in the proper heat treatment of most high alloy steels. When I decided to do in-house heat treating I didn't want to skimp on any step that would allow a blade to reach its maximum potential. Otherwise it makes sense to have them professionally heat treated.
 
Cryo seems absolutely necessary for many steels. It is a function of, or a continuation of the quench, as I understand it.

If a steel calls for cryo to -285 F. then why have the mindset that -95 F. is 'close enough'?

To my mind that's no different than saying that a certain carbon steel needs to reach at least 1450 F. to get sufficient carbon into solution...........but if you can't hit 1450F. at least hit 1375 F. and that will be close enough.

Either one of those scenarios doesn't make sense and leads to inferior/subpar products.
 
Dry ice is cheap. Get a 2" thick foam cooler, place the dry ice, blade on top, with a second piece of dry ice on top of that making a knife/dry ice sandwich. I never understood the need for acetone. The blade contacting the dry ice will transfer the heat just fine and the blade will quickly sublimate a blade-shaped form in the dry ice. Also the acetone will be warmer than the dry ice on it's own.
 
Proper Cryo gives you eta carbides and maybe 1-2 higher HRc. There are still some things not understood about it . You can get that small hardness increase without significantly lower RA. There are steels and applications that should have some RA for toughness.
If I was to have a choice I'd take a CPM grade rather than Cryo.
Be careful with containers as some plastics may me damaged by direct contact at sub-zero or below or contact with .
 
There are almost no steel really "require" cryogenic or subzero process (properly some nitrogen steel?). We just do it for higher finished martensite thus some increase in HRC. Seems like it has been proven that the increasing in performance is not that significantly in knife application.

You will be surprise how many commercial HT industry that overlook these freeze treatment process.
 
"Necessary", no. "Good to do", yes.

Sub-zero at around -95°F will complete the cooling curve for any stainless steel I know of. Cryo may offer some "extras" but your blade will be fully hardened with just Dry ice.

I have a 20L dewar that I haven't filled in a good while. I send off my larger batches of blades to Peter's, but when doing a small run of blades ... or a single blade … I prefer the DI bath with alcohol. The solid block method Atlas Knife mentioned works, but takes much larger blocks of DI that a bath will. It may be hard to do a 10" -15" knife that way.

I use a long thin cooking pan called a "Whole Fish Poacher/Pan" for the bath container. Any metal pan that is water tight will work. I would not use foam or plastic.
I put in about 1/2 gallon of denatured alcohol form the hardware store ( $14 a gallon) and 2-3# of dry ice from the local grocery store ( $1.29/lb at Harris Teeter). I break up the DI into chunks and small bits before adding it to the liquid. Then I put in the blades and leave them there while I get the HT oven ready. They only need to sit in the bath for about 5 minutes, but I leave them in until The oven is set up and heating … about 30 minutes ( It doesn't matter how long). Remove the blades, wipe them off, let them warm to room temp, and put tin the oven for HT. The DI bath can be left until it stops bubbling nd poured through a funnel back into the can. Leave the cap a bit loose for a day, shake the can to release any carbonation, and cap it back tight. It can be used over and over again as well as for epoxy clean-up tasks that require alcohol (alcohol is the proper solvent for uncured epoxy).

Fish pans can often be found at the thrift stores for a few bucks ( I paid $1.00 for mine), or #20 on Ebay, but even a new one isn't expensive. https://www.amazon.com/Norpro-Stain...G4/?tag=bingshoppinga-20&linkCode=df0&hvadid={creative}&hvpos={adposition}&hvnetw=o&hvrand={random}&hvpone=&hvptwo=&hvqmt=e&hvdev=c&hvdvcmdl={devicemodel}&hvlocint=&hvlocphy=&hvtargid=pla-4584482454994369&psc=1
 
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I still get the feeling that some of you still are confused at the story. I too am a metallurgist , retired but with years of practical experience. So many ask for 0% RA yet with some steels that's not even possible . What we are really looking for is a very stable structure . We do know that some applications do better with some RA for toughness .BTDT not by theory but actual usage. The heat treater must know his business and how to translate theory to the real world. I'm still looking for careful definitions of cryo , sub-zero, and things in between. As for stability , remember that cryo tweaks the crystal structure to permit the formation of eta carbides . Without time for this to happen. Without some time eta will never appear ! Cryo is a slow diffusional process , martensite process is fast. get a good microphone -- you can hear martensite [ speed of sound ] but not cryo. If an outside HTer does better , send it to him.
 
Stacy , we call it a steamer or poacher , a long narrow , covered pan great for poaching of steaming fish. An excellent quick way to cook fish . I don't know the metallurgical term but the GF associates it with the term " What are you doing now ?? "
 
The eta carbide change is not something that happens.
 
I'm very new at this but was in your boat about a year ago so I've been working with AEB-L and S35VN exclusively as their HT recipe on Alpha doesn't require LN. There are other steels out there well that don't require LN. You can start with those. S35vn is awesome. AEB'L is great to but I always have to deal with warps.
 
Larrin said:
The eta carbide change is not something that happens.
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so Larrin from your -
Cryogenic Processing of Steel Part 1 – Maximizing Hardness
"... The transformation was complete around -100°C, therefore even with dry ice nearly all of the unstable retained austenite was transformed..."

so if I'm reading your writeup correctly, we have what I like to call - the point of diminishing returns.
In other words there is only a marginal benefit from a cryo treatment for maximizing hardness?
90% conversion for DI and 92% for a sub zero cryo based on the martensite formation figure chart.

I don't have alot of experience with Stainless HT. Only having just done a Damasteel HT.
You can see on their HT guidelines you can see that deep freezing is defined as -110 F or dry ice bath. No mention of cryo.

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SLC_Blades said:
Are these treatments REALLY necessary to achieve an acceptable hardness on stainless knives?
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I think the answer is, it depends on what specific stainless you specify...and you will certainly need a sub-zero treatment of DI, but probably don't need the cryo, unless you can see that the steel manufacturer specified it.

so perhaps a more narrowly focused question is appropriate - Is cryo really necessary to achieve an acceptable hardness for XXXX steel? :)
 
The difference between dry ice and liquid nitrogen is small in terms of converting retained austenite with "normal" heat treatments, assuming the time delay is small between quench and cold treatment.
 
HSC /// said:
so perhaps a more narrowly focused question is appropriate - Is cryo really necessary to achieve an acceptable hardness for XXXX steel?
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I think this is perhaps a very good question. Sandvik shows 3 types of HT - room temp, -5°F (home freezer), and -95°F (Dry Ice). Sandvik maintains LN doesn't provide any advantage over Dry Ice treatment.
Larrin said:
The difference between dry ice and liquid nitrogen is small in terms of converting retained austenite with "normal" heat treatments,
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Larrin (and Mete) are two of my "go to" folks for knowledge. BUT - as has been mentioned the amount of RA isn't the only thing in the equation of making a good blade HT.
I appreciate ALL ya'll for sharing good knowledge - it's the way I grow.

Ken H>
 
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