cryo times for 'complete' conversion

[SteveS]

When using liquid nitro what's your recommended soak times?

I'm sure the austentite/martensite conversion percentage is asymptotic and after a while the soak time really doesn't make an appreciable difference.

I've heard everything up to 12 hours. The Crucible data sheets don't list a time, just a minimum temp of -112 (or something like that).

I had one S30V blade soaked for about an hour and got the same results as over night. At least from a HRC measurement.

Steve

[nate d.]

the conversion wait let me start over the remaining part of austenite that you are able to convert to martensite in the cryo quench process happens as soon as the material's temperature goes below the Martensite finish line on the TTT curve diagram. I have asked Crucible the fine and helpful folks i might add for a TTT diagram for 3V and both times they don't have one and as a result of this i just dunk blades in LIN and let them come to euqlibrium in the LIN and leave then in as long as i feel like it. I though the same thing you did that the final martensite comversion was time dependent till my proffessor who was helping me HT sat me down and calmly ask me a few questions and that SET ME STRAIGHT!!!!!!!!!!!

[SteveS]

Nate,

No kidding? It happens when the steel reaches temperature? It'll take me a while to digest that.....

Uum, Let's see during the 'real' quench the conversion is incredibly fast (I forgot, but it travels thru the steel in fractions of a second.) SO I suppose the continuing conversion in sub-zero is also very fast?

Is that what the proffessor's little chat was about?

Steve

[RJ Martin]

Steve: That's my understanding as well. Conversion is complete when the parts equalize at temp.

[Quenchcrack]

The transformation does start to occur as soon as the austenite is cooled below the Ms temperature. Note I said "cooled below", it does take some undercooling to get things started. The percentage of conversion of retained austenite to martensite is a function of temperature and NOT quench velocity. Soak only long enough to get the blade cooled down to the temperature of the cryo bath, then you can pull it out. It is usually recommended that the blade be given a quick, low temperature temper prior to cryo treating, something on the order of heating to 250F in the forge and air cooling. This will minimize the danger of cracks and distortion in the cryo bath. However, this snap draw will also decompose some of the retained austenite to things like lower bainite and pearlite. This will slightly reduce the hardness.

[SteveS]

Originally Posted by Quenchcrack

However, this snap draw will also decompose some of the retained austenite to things like lower bainite and pearlite. This will slightly reduce the hardness.

OH!!!!!! That explains it, well part of it. Long story, but on the last 2 batches, I wasn't getting the same (any) jump in hardness after the cryo as described in the . However, I was doing a snap draw. Hmmm.

Thanks,

Steve

[rlinger]

I leave mine in there 10 to 12 hours minimum. Many times up to 17 - 18 hours because I sometimes am not here to pull after 12 hours. Once upon a thread Mete suggested 10 hours is plenty long enough.

RL

[RJ Martin]

Steve: Well, turns out I was wrong. Even though diffusion is very slow at -320F, it does still occur, and it takes time. This is why 24 hours is generally recommended as a soak time for the cryo treatments.

Also, remember that the better the initial heat treatment, the less apparent effect the cryo will seem to have. But, it is still working it's magic on your steel.

When I first started heat treating A2, I oversoaked a few blades just above the high end of the Austenitizing range. After quench, they were only about 56 rockwell, which pointed out immediately that they were oversoaked-Cryo brought them up about 6 points! After tempering, they were right at 59 Rc.
Of course, I didn't sell the,-Just turned them into beaters for the shop, and, they worked fine. That's the great benefit of cryo-It tends to equalize minor inconsistencies in HT that you might not even know are present.

So, leave those blades in there for 24 hours, bacause that's best!

[SteveS]

Thanks RJ,

Looking at the CM154 data sheet, it shows what you experienced if you use 2000 degrees. My assumption is that the entire blade is austentite, but the quench brings it down so fast thru critical that very little (by comparison) austentite converts to martensite. The cryo becomes the major quencher.

Always stuff to learn!

Thanks again,

Steve

[Quenchcrack]

RJ, with all due respect, I must politely disagree with your comments about soaking at cryogenic temperatures. Tempering is a diffusion controlled process. The transformation from austenite to martensite is a diffusionless shear process that is dependant only upon temperature, not time. However, leaving it for 24 hours will certainly guarantee the blade has been sufficiently refrigerated and will do it no harm.

Remember that the quench velocity does not have any influence on retained austenite. Retained austenite is caused by high carbon content (and alloy content) that impedes the shear transformation to martensite.

[RJ Martin]

Quenchcrack: I have read published data that stated both cases as being correct. So, today, I called Scott Devanna. Scott is one of the CPM gurus over at Crucible, and I wanted to hear his answer. He says soak time in the LN2 definitely matters, particularly for some of the very highly alloyed CPM steels. Apparently, the MF temperatures for these alloys can be very low, and, even with the soak you can't get 100% Martensite, but you can reach well into the 90's.
So, the second answer I gave is the one I got straight from him. I'm inclined to stick with the 24 hour deep freeze. Like you say, it can't hurt. As an Engineer, I'm fine with that. If I were a Scientist, I'd probably want absolute proof......

[fitzo]

Do you guys feel there is any truth to the claims of other things besides conversion of retained austenite going on that may be more time dependent? I read an e-paper on a site called "cryogenius" or something like that which spoke of things like "eta-carbide rearrangements" and didn't know if it was gibberish-science or based in fact. I believe the author was speaking specifically about high alloy steels.

Thanks for any opinions.

[Quenchcrack]

RJ, well, I learn something every day! I wonder if it has anything to do with the way the CPM is made? I looked this up in 4 different books and all of them said time at low temperature was not an issue ONCE YOU GOT THE PIECE DOWN TO CRYOGENIC TEMPERATURES. I wonder if it just takes time to get the blade down to -300F? Although, the CPM grades are extremely high in alloys known to retard the shear transformation. So I guess what I should have said was that for carbon and low alloy steel, soak time is not critical. For very high alloys, soak time is critical. Sound right?

Eta-carbides: I believe these are the carbides that knifemakers have been precipitating for thousands of years but nobody could see. Eta carbides are formed at low tempering temperatures and are so small they can only be resolved with a Scanning Electron Microscope. They are a slighly different chemical composition from the Fe3C found in pearlite, etc.

Ms and Mf temperatures: The Ms temperature for a given steel is usually known fairly accurately. The Mf is much more ambiguous as it is difficult to know when the transformation is actually complete. Some books us the M99 temperature which is a more conservative temperature that represents a 99% transformation. There are empirical equations to calculate Ms, M50, M90, and M99 but they are probably only good estimates...

[Mike Stewart]

Hi Guy's,

I have been Cryogenically Treating my blades since 1987.

In every test I have ever done the 24 hour process has yielded a better and stronger blade that holds an edge better. I do 500 blades at a time on the average batch and get on average a 1 point increase in rockwell. I then do a low heat stress relief at about 275F for an hour. I don't get broken blades and for semi-production knives I get exellant results.

I really don't think that there are too many folks out there that have been doing Cryo-Processing longer than I have. I've had years to test this stuff and I'm not a good debater. I'm going to keep doing it the same way. I never tell anyone how to make their knives but I have never had a problem with the long soak method. If it ain't Broke...........

Mike..........

[RJ Martin]

Mike: Sounds like you have a good track record. I have been cryo treating almost as long as you, but, if you're doing 500 blades at a time, you have lots more data than I do!
Generally, 300F is suggested as a stress relief for the newly formed Martensite, so, 275 is right there. I do my tempering after the cryo.
Quenchcrack: Yes, I am wondering if it doesn't take quite a while for those blades to actually reach -320F. And, the CPM alloys probably complicate the process a whole bunch. For simple steels, the soak time is probably not critical. And, yes, my understanding of the eta-carbides is that those are ultra-small carbides that precipitate during cryo. They help wear resistance and toughness.