Cryo treating my blade at a later date

I’m a semi pro knife maker using mostly high performance steels such as CPMS35VN and CPM154 for fix blades and my flipper-folder design. I live in a fairly remote community in eastern Washington. Dry ice availability for cryo treating is an
hour each way. I try and get ready 3-4 knife blades at a time to cryo treat but often that doesn’t work. Question to the guys in the know. Can I say build my folding knives and at a latter date cryo treat them. Understand I would have already hardened the blades @ 1950 deg and tempered @ 2 two hour sessions @ 400deg. I assume I might be able to take the knife apart, cryo the blade and then re heat treat. Whats your thoughts? Can I do that?
 

Edwardshandmadeknives

Well-Known Member
I’m a semi pro knife maker using mostly high performance steels such as CPMS35VN and CPM154 for fix blades and my flipper-folder design. I live in a fairly remote community in eastern Washington. Dry ice availability for cryo treating is an
hour each way. I try and get ready 3-4 knife blades at a time to cryo treat but often that doesn’t work. Question to the guys in the know. Can I say build my folding knives and at a latter date cryo treat them. Understand I would have already hardened the blades @ 1950 deg and tempered @ 2 two hour sessions @ 400deg. I assume I might be able to take the knife apart, cryo the blade and then re heat treat. Whats your thoughts? Can I do that?
No. Cryo should be as soon as possible after quench. That austenite that remains after quench that we are trying to convert to martensite will tend to stabilize over time, and resist conversion if we wait too long. I know they say you can do a temper first, then cryo, then temper, but it should really be done directly after quench. Austenize, quench, cryo, temper, temper. You wait, and it won’t convert, and you have to start over with heat treat from the beginning. If it is something that you can’t do, you are better off switching to a steel that doesn’t require cryo. If all that retained austenite doesn’t get converted, using “high performance “ steel really just means that you are making some expensive scrap iron.
 
No. Cryo should be as soon as possible after quench. That austenite that remains after quench that we are trying to convert to martensite will tend to stabilize over time, and resist conversion if we wait too long. I know they say you can do a temper first, then cryo, then temper, but it should really be done directly after quench. Austenize, quench, cryo, temper, temper. You wait, and it won’t convert, and you have to start over with heat treat from the beginning. If it is something that you can’t do, you are better off switching to a steel that doesn’t require cryo. If all that retained austenite doesn’t get converted, using “high performance “ steel really just means that you are making some expensive scrap iron.
Thanks for answering so accurately. What you say makes complete sense and I knew it probably wouldn’t work. Can I use these CPM steels without cryo and still expect a reasonable RC hardness? 57-59 RC. Or is cryo and absolute requirement.
 

Edwardshandmadeknives

Well-Known Member
Thanks for answering so accurately. What you say makes complete sense and I knew it probably wouldn’t work. Can I use these CPM steels without cryo and still expect a reasonable RC hardness? 57-59 RC. Or is cryo and absolute requirement.
Well, it all boils down to how much retained austenite you have after heat treat. I use CPM154 for all my stainless stuff, so I’m pretty familiar with it. The problem you run into is, the martensite finish temperature (where the austenite has fully converted to martensite) is well below zero Fahrenheit. You “can” heat treat it without cryo and expect a decent hardness, but the retained austenite over time can convert to martensite which is then untempered. Also, the boundary between areas of austenite and martensite is highly unstable, and the combination of the two issues leads to a good chance that the blade can develop cracks down the road. If you want a good example of how much cryo can affect the steel, try heat treating two samples. After they are quenched, stick one in a vise and bend it. As quenched steel should shatter like glass, but I’ll bet you that your sample will bend like mild steel, even if a file still skates off it. Cryo the other, and try the same test. It should break just like you would expect hardened steel to. I know that many high alloy steels have two tempering options, a high temp option that doesn’t use cryo, and a low temp option that does. The high temperature option uses the higher temps to force conversion of austenite, the low temp option uses cryo to force conversion. Usually doing the high temp version is at a small sacrifice to toughness and corrosion resistance. I’ve not used S35V, so I don’t know if that is an option for you. I have not seen that option for CPM154, and have always stuck to the low temp tempers that Crucible recommended. It’s a really great steel, but unless you find a recommended high temp temper option, do not skip cryo, and do it immediately after quench.
 

jmforge

Well-Known Member
"Cryo" typically refers to VERY cold treatments like LN. Some would tell you that dry ice bath is NOT "cryo." "Cryo" is considerd to be treatment using a liquid that has a boiling point below -130F. Dry ice, aka frozen carbon dioxide, sits around _109F. BUT it is cold enough for some steel. It is most certainly cold enough for say AEB-L and I would guess any low alloy carbon steel, but a number of years ago at Blade a rep from Bohler-Uddeholm told me that it didn't get quite cold enough for the optimal cold treatment for Elmax, etc. It was like a 20F aka "minimum" cryo temp IIRC, so I would think that using dry ice, while not "adequate" would be better than nothing. Another interesting thing that I was told was that you don't need to submerge the blade in -320F LN. You just need to get it cold enough. Point being that even if you are "low" on LN as in not enough to submerge the blade, you still probably have enough.
 
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J. Doyle

Dealer - Purveyor
I don’t use or heat treat stainless steels so I might be talking out of turn here. I have however studied this topic to no small degree and have discussed it with a few who I consider quite knowledgeable on the topic.

My understanding in simplified terms is that cold treatments/cryo are a continuation of the quench, and should be thought of as such so that the blade goes from austenitizing temp and cools to recommended cold temp in a relatively smooth even process without unnecessary/unreasonable pause or delay.

If true, then tempering/snap tempering or any other step in between cooling and cryo makes absolutely zero sense as it would be interrupting the complete quench process.

Also on the topic of true cryo with LN vs. acetone/dry ice slurry:

It would seem that if a steel calls for true cryo temps, then just using acetone/dry ice would not be enough. With any other stage of the heat treating we would never say that "austenitizing calls for 1500 F. but 1350 F is good enough" or "tempering should be done at 400 F. but 475 F. is good enough"

I truly hope to be corrected if my understanding of this is incorrect.
 

Edwardshandmadeknives

Well-Known Member
I don’t use or heat treat stainless steels so I might be talking out of turn here. I have however studied this topic to no small degree and have discussed it with a few who I consider quite knowledgeable on the topic.

My understanding in simplified terms is that cold treatments/cryo are a continuation of the quench, and should be thought of as such so that the blade goes from austenitizing temp and cools to recommended cold temp in a relatively smooth even process without unnecessary/unreasonable pause or delay.

If true, then tempering/snap tempering or any other step in between cooling and cryo makes absolutely zero sense as it would be interrupting the complete quench process.

Also on the topic of true cryo with LN vs. acetone/dry ice slurry:

It would seem that if a steel calls for true cryo temps, then just using acetone/dry ice would not be enough. With any other stage of the heat treating we would never say that "austenitizing calls for 1500 F. but 1350 F is good enough" or "tempering should be done at 400 F. but 475 F. is good enough"

I truly hope to be corrected if my understanding of this is incorrect.
True. I have been unable to find a TTT chart that shows the MF for CPM154 so I don’t know what the real temperature that is needed to finish transformation. I believe I had read somewhere that it was around -90F. Using LN versus dry ice would be one way to be sure that you are at least reaching this temperature. I did use some dry ice with denatured alcohol, and checked the temperature and it said that it was -116F. That would be plenty sufficient if the MF temp truly is -90. LN is pretty much a guarantee that no matter what the MF is, you will reach it.
 

Edwardshandmadeknives

Well-Known Member
Here is a link to a good read on CPM154 with various austenizing and tempering temps, with a little info on heat treating it without cryo. I haven’t tried heat treating it without cryo, so some testing may be necessary.
 

jmforge

Well-Known Member
May I ask what steels absolutely require cryo? I don't have the means and would like to avoid those.
I ran into a guy in Florida who was selling multi thousand dollar folders made from crazy steel like CPM S90V and not using cryo. His attitude was that the people who bought these weren't ever going to use them so no biggie. That bothered me. but I guess that it can be done.
 

jmforge

Well-Known Member
Here is a link to a good read on CPM154 with various austenizing and tempering temps, with a little info on heat treating it without cryo. I haven’t tried heat treating it without cryo, so some testing may be necessary.
I don't remember which steels he did this to, but Larrin did do a test with LN, dry ice and his freezer and said that all three helped to some degree. BUT with some steels, perhaps ones that require a bit more cold, the dry ice was closer to the freezer than the LN.
 

Edwardshandmadeknives

Well-Known Member
I don't remember which steels he did this to, but Larrin did do a test with LN, dry ice and his freezer and said that all three helped to some degree. BUT with some steels, perhaps ones that require a bit more cold, the dry ice was closer to the freezer than the LN.
Right. Unlike heat, more cold isn’t going to hurt it, so it’s always better to go with more cold. Dry ice does help a lot, it’s got a far better conversion rate than just chucking it in your freezer, but the LN is the best you can do. A 10 liter Dewar is only about $400, the LN is actually cheaper than dry ice, and mine will hold liquid for about a month. To the OP, if you are doing stainless, you might want to look into getting a Dewar. Plus, it’s fun to freeze weird things in LN and smash them:))
 

Edwardshandmadeknives

Well-Known Member
May I ask what steels absolutely require cryo? I don't have the means and would like to avoid those.
Ya, pretty much any stainless stuff. D2, and A2 should have it, although they CAN be heat treated without it. Dozier’s work is a good example. There is no real need to avoid them, you can have someone heat treat them for you, or you can get into cryo for probably less than a grand. Once you start working with high alloy steels though, a real heat treat furnace is a must. Don’t even waste your time trying to do it in a forge.
 

Edwardshandmadeknives

Well-Known Member
@Edwardshandmadeknives Do you think the fact Crucible recommends dry ice for CPM154 suggests their belief is the Mf is above -109F?
That is quite likely. Like I said earlier, I believe I had read somewhere that it was around -90F. I wish that they had TTT charts available for some of these steels so it was easier to see exactly what is required. I doubt that LN is necessary for that particular steel. But, then again, more cold won’t hurt it.
 

fitzo

Gold Membership
That is quite likely. Like I said earlier, I believe I had read somewhere that it was around -90F. I wish that they had TTT charts available for some of these steels so it was easier to see exactly what is required. I doubt that LN is necessary for that particular steel. But, then again, more cold won’t hurt it.

I agree, TTTs and CCCs on a number of these modern Crucible steels would be really nice. With my restart, I have planned on using CPM154 along with my ancient stock of 440C and ATS34, but have found that the jerk that runs the local Airgas won't dispense LN into a Dewar. I haven't been successful finding another source. Fortunately, dry ice is still available one town over.

Either way, the conversion of austenite is a continuous curve so every little bit helps. Even that trip to a chest freezer converts a little bit more after main quench, albeit insufficient.
 

Edwardshandmadeknives

Well-Known Member
@fitzo ya, finding people who will dispense it can be a pain. Last time I got it at a place called “Central McGowan”. They sell welding supplies and stuff like that, as well as dry ice. My “real job” involves installing medical gas piping, and we use a lot of nitrogen for that. Having a license for doing that work probably helps with procurement of the LN. The problem with a lot of nitrogen is that it is used for medical uses, and because it is a medical product, the dispensing of it is pretty tightly regulated. Plenty of people can get and use it in gas form, but not so much with the liquid stuff. It’s not so much that they “won’t” dispense it, but rather they really “can’t”. It’s a medical thing. Now if you look up people that do artificial insemination for cattle, one of them can probably hook ya up. (With nitrogen, that is) ;)
 

fitzo

Gold Membership
I bought it at the same place for years before I got sick, but in the intervening time, Airgas acquired the welding business in addition to the gas/LN sales. And the new guy isn't the old guy and he made his decision, "No," as if it were a personal decision. What you say, though, is interesting: perhaps there have been laws passed during my off time that affect the sale. I'll have to look into it. Thanks for the idea.

I spent my working life in chemistry labs. It spoils one, as there was always LN readily available somewhere on the same floor. I'm so used to having access I forget the nanny state may have acted out and passed restrictions.

@KenH mentioned the cattle breeders to me and I am still hunting. The local feed supply doesn't have a clue, but I know they have to be out there. Lottsa wealthy horse people and their equestrian estates not to mention there have to be some farmers left out there in the county.
 

Edwardshandmadeknives

Well-Known Member
@fitzo there isn’t any law preventing you from buying LN. Anyone can buy it, but if your supplier has a big ol’ tank of it that is earmarked for a hospital, then more than likely they have the say over who gets to use it. The Airgas guy could sell you a few liters of it, but then he is going to be jeopardizing whatever agreement they have with the primary user of that nitrogen. He isn’t going to risk losing a bazillion dollar contract over a few bucks worth of nitrogen. When he said “no”, like it was a personal decision, it probably was, based off of that reason.
 

fitzo

Gold Membership
@fitzo there isn’t any law preventing you from buying LN. Anyone can buy it, but if your supplier has a big ol’ tank of it that is earmarked for a hospital, then more than likely they have the say over who gets to use it. The Airgas guy could sell you a few liters of it, but then he is going to be jeopardizing whatever agreement they have with the primary user of that nitrogen. He isn’t going to risk losing a bazillion dollar contract over a few bucks worth of nitrogen. When he said “no”, like it was a personal decision, it probably was, based off of that reason.

That would make sense, of course, and not bother me. Pity if it was the case and the guy wouldn't explain that. Thanks for this thought!

I was under the impression they filled the industrial Dewars (100L?) on site, though. The only time I was with the guy who filled my 30L back in the day, it was out of a 2" hose and not a small industrial tank.
I'll find some eventually. About twenty miles north is the next dying industrial town and they have a couple welding shops in addition to the Airgas. And more farms, and breeders. Hopefully I can find it when the time comes. Next blade (an ulu) will be dry ice/acetone as it's bigger than the neck on the Dewar anyway.
Thanks, again, for the comments and knowledge! I appreciate the insights.
 
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