cryogenics

D

DeadeyeLefty

Joined
Jun 2, 2007
Messages
610
No, not freezing the head of your dead dog...:D

I'm wondering about the pros and cons of cryo tempering and looking for recommendations. Does it enhance all steels or just some particular alloys. I've done some reading on the process, but I'm looking for a first-hand perspective. Does the shape of the part make a difference, ie knife blanks as opposed to gears or tools ?

Personal recommendations for Canadian shops would be preferred, but I'm not too picky....
 
Cryo converts retained austenite to martensite. So it is good for steels that retain austenite such as high alloy and stainless.

Some die steels are designed to retain austenite to reduce dimensional changes (martensite is less dense, so hard tools grow). We don't care about little dimensional changes in a knife, we want martensite. Dimensional changes in a gear is bad.

It probably should be done as part of the quench. If you wait until after temper, much of the austenite will have stabilized and won't convert.

The soak takes hours, not minutes.

You should always temper after the process to temper new martensite.

Based upon my reading of this subject, it increases the hardness and strength of the steels that benefit from it by increasing martensite, but it makes them less ductile.
 
Nathan, you've got it right ! Anything beyond that is mostly hype.
 
mete said:
Nathan, you've got it right ! Anything beyond that is mostly hype.
Click to expand...


It's an old subject, gone through more than once
but the search function sucks, which complicates hunts.
It needed you or Kevin, if you could spare the time
And at last so you did, and this time with a rhyme.
 
If ever there was a topic that should be in a permanent F.A.Q. on any knife forum this one is it. I would not want to do a search for it due to the overload one would get as I believe there has been one form or another of this same question asked almost every two weeks for years here alone.

Simple answer- yes it works, for converting retained austenite. Beyond that you will get a cacophony of input about it wondrous effects, until you eliminate all but data from people who are not trying to sell you cryo services, then there is nothing but crickets chirping.
 
What's wrong with chirping crickets? :p:D

Especially, if it helps make a better blade! :)
 
Finding proof the cryogenics works that isn't tainted by those that stand to make a buck is tough - but it does exist. I think the best study I stumbled onto was done in the circa 1930's that showed strong evidence cutting action in knives (in this case industrial knives) was significantly improved. I don't believe guitar strings sound better or tennis strings work better if given the cold dunk. Years ago, I shot a lot of trap and I had the barrels on my shotgun cryo treated. I can't say that I shot any better but I felt better about my equipment and that was worth it to me.
 
Nathan and Tracy, that's exactly the kind of info I'm looking for, bearing in mind I'm no metallurgist by any stretch. I'm interested more in parts with a more mass, like gears and such and figured this would be the place to ask. Everything I've read bears out your point that it's described as a magical process that makes everything better.

For the record I did a search, but came up with a white screen with an error message at the top. I usually try a search first because I know how annoying that is but I also know that the BF search function isn't 100%.

Rob - profile edited, thanks. I'd like to hear your thoughts.

Again, thanks guys.
Brian
 
I have quetion
crucible recomends -112F between 2 tempers for S30V .
do you guys think it would be better to do it befor
the first temper? and would liquid nitrogen at
-321 °F convert more austenite to martensite.
would it be worth the investment to get a liquid nitrogen tank.
thanks
 
"crucible recomends -112F between 2 tempers for S30V .
do you guys think it would be better to do it before
the first temper?"

Seams likely, but they did develop it as a knife steel, so I don't know why they would recommend it this way if another is better. It would be great if someone from Crucible could chime in.

"would liquid nitrogen at
-321 °F convert more austenite to martensite"

According to charts I have seen for other steels, yes, to a small degree

"would it be worth the investment to get a liquid nitrogen tank"

According to charts I have seen for other steels, no, probably not. But the convenience may be a good reason.

It is time and temperature dependant. An apparently unbiased paper I've read on the subject indicate there are negatives to cryo such as development of alloy lean carbides in grain boundaries. The decreased steel toughness caused by these carbides more than outweighs the slight increase in abrasion resistance caused by their presence. This issue and the issue of retained austenite are probably also effected by other aspects of the heat treat, making this a complex subject. So the short answer is, a dry ice bath is probably just as good as a liquid nitrogen bath. There appears to be little difference, and what little difference there is may not be all advantage.
 
Some temper before cryo but it must be a "snap temper" of 300 F. Higher temperatures will stabilize the austenite. The second temper then is 400 F or higher. In the old days we called it "sub-zero quench" at about -100 F rather than -300 F.
 
U can get a stainless steel "Made in China" thermos, in ~2L range, with a wide throat,
for something like $20 these days. Your local friendly welding supply shop (u befriend them by renting a gas cylinder and/or buying a flow meter/regulator) should not have problems
occasionally filling this thermos with LN, for a few bux and this is how you can try it.

Do read up on safety issues with LN. They boil down to NOT EVER storing/transporting it
in a CLOSED container. There must be a way for the vapor to escape.

LN is rather cheap, as it is a byproduct of them "distilling" Oxy out of, literally, thin air :)
 
I wish I could site the studies I eventually found. I can't. I did come up with them via a lot of google activity so they are out there. The metalurgists seem to split but most will say there is at least some improvement in the cutting edge durability. Once you hit -96, the conversion seems to start and going colder moves it along. I think there is a finite amount of martensite to be had so you don't get "more" at LN temps, you just get it all. Studies called for 8 hours soak time. I think most that doing home cryo are using LN dewars from 20liters to 30liters in size. They come up on ebay all the time. Expect to pay $200 to $300 for a dewar. I fill my 30liter for $65 or so and it lasts at least a month. Longer if it's in the winter.
 
hardheart said:
http://img19.imagevenue.com/img.php?image=80728_travel2_122_1020lo.jpg
Click to expand...

img.php


In practice, I have not noticed anywhere near that much wear resistance improvement. The blade is a few points harder for the same temper because it has perhaps 20% more martensite. That's a great improvement for a knife blade if you can tolerate the toughness tradeoffs. But more than doubling the wear resistance? Perhaps it is a non linear thing. Passes some threshold?

I *think* I can tell the difference between two identical blades where only one was treated (I have done this), but it ain't night and day. These cut tests, using very constrained motion in a lab, don't always seem to reflect the reality I observe. Like Crucible's data that show S30V has way better edge retention than D2? Come on....


A chart I've seen showing different temperatures show a different rate of change of conversion, but the ultimate results weren't that different. Did they give the soak times used for these results?

There does appear to be a lot of contradictory data out there. But the metallurgists here have always indicated that anyone telling you there is this huge improvement is prolly trying to sell you something.
 
I asked one of the Crucible guys about that at a seminar last April. I typically soak overnight. His reply was that once the steel reaches the cryo temp, transformation is instantaneous. The long soaks are not required The question of when the steel reaches that full cryo temp is another matter, but I go with a 2 hours soak now. I could probably get by with 15 minutes, but I like to err on the side of caution.

Gene
 
I am glad you included that Gene, I have tried to point that out in the past and you would think I had blasphemed in some way:rolleyes: Which kind of shows how little understanding there is in the differences of the martensite transformation. Diffusion takes time, so operations like soaking for quench and tempering need a hold time to be effective. Martensite doesn't form this way at all, it forms via instantaneous shear driven mechanisms, hence all that matters to it is temperature. A short wait can allow the temp to equalize and for the strain to build, but the actual transformation is over in the blink of an eye.
 
I've been bothered by that too .There may however less thermal stress taking it down slowly, then bringing it up slowly.Then of course we enter the MYTH ZONE !!
 
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