Cryo Treatment - How to?

[Allen Rice]

I have a number of questions about how to get started in Cryo treament. I'm about to embark on getting the requisite equipment (Dewar) to do Cryo treatments as part of the HT process. After quite a bit of searching I found one thread from last year http://knifenetwork.com/forum/showthread.php?t=21121 that somewhat addressed a few of my questions about how to do it.

Much of what I have read in published material about Cryo treatment has the, to be treated, items slowly brought down in temperature over a fairly substantial amount of time and conversly slowly raised in temperature at the end of the treatment time.

Is this necessary or preffered for blades? How would one go about regulating this rate of change? Or, should I not worry about it and just try and minimize the thermal shock by precooling as much as possible and just slowly place the blades in the LN?

Any links to any tutorials on how to do Cryo treatment would be much appreciated.

Regards,

Allen

[SteveS]

Allen,

I've heard about the slow cool/warm cycles. I haven't used them. Just lower it into the tank, wait 12 hours (or 2 or 24 or ...) and lift it out. You'll get the rise in hardness.

BTW I did a little test the other day. Just set a sample of ATS34 on a block of dry ice and waited 4 hours. The hardness when from ~56 to 62. That works too. In fact, Crucible's listed cryo temp is actually dry ice, not LN. LN is more convenient 'cause it'll stay in a good tank for months. But if you are only doing a couple of blades a month, use dry ice.

My 2 cents.

Steve

PS If you are buying a dewer, besure to check the neck opening size.

PPS I bought a cheap lab dewar that holds 5 liters for about 4 days. I fill it up when I have a batch to do. It works for me.

[tonn]

As I understand sub-zero :
Austenite to martensite transformation has its starting temp & end temp. When cooled below end temp little or nothing should happened!?


So You must find out those temps(at least end temp for cryo) With carbon steels end is not below -80C. That means - no LN?

Hope our scientists will switch on and explain that!(& translate it to english )

[Allen Rice]

Steve,

Thanks for the reply. Interesting results using the Dry Ice. Once I get my Ray Rogers hardness tester completed I will have to do some experimenting like that and chart the results.

Do you do any precooling before placing in the Liquid Nitrogen? If so, what process? What about the rate of immersion? Sorry for all the questions but the more information I have about what others experience the better prepared I'll feel when doing it myself.

Allen

[SteveS]

Good questions Allen - I should know I had the same ones!

I've gone from room temp into LN and from the home freezer - no difference real difference I've found (but I don't have an electron microscope either).

I've dipped slow and fast. ( don't think slow is a good idea, but no real reason other than "Why freeze the tip and have the handle warm?")

I've asked about the dry ice/acetone mix. That's only to make better heat transfer, not make it colder. So if you're leaving something that thin on ice for 8 hours, acetone probably doesn't matter.

Just lay it on a block of ice and be done with it. LN is cool (so to speak), but the setup costs are high. I'd say if you are only treating blades once or twice a month, use dry ice. Above that, then a tank is much handier.

Steve

[RJ Martin]

Tonn: Cooling below the Mf temperature is not harmful. Liquid Nitrogen will be cold enough to yield maximum conversion for any steel. You do not need slow cooling. that is for large/irregular parts to prevent cracking from thermal stresses. Knife blades are thin and uniform and they rarely crack during cryo.

Steve S: If you are getting that much of a hardness increase during cryo, I'd check to see if you're overheatng or oversoaking during HT.

[SteveS]

RJ,

That hardness range was for ATS-34 heated up to about 2000. It matched, almost exactly, what the crucible sheets (for CM154 obviously) say should happen (http://www.crucibleservice.com/datas...TOKEN=43494783).

So, I don't think there was anything wrong with my process. Actually I used that austentizing temperature specifically because of that strange behavor (evidently massive amounts of retained austentite.) That way I could easily tell (measure) if the dry ice thing had the desired effect.

In the same test batch I had S30V. It doesn't exhibit that behavour on the test sheets or my results.

Steve

[edit to add] that same data sheet says: "To completely transform any retained austenite, a freezing treatment with dry ice at -100?F (-74?C) ..." Not that I should question Crucible, but it's nice to know if I use the same temps and dry ice I get the same kinda results.

[SteveS]

RJ,

Oh yeah - I should clarify. That was for my dry ice test. I've never used that temp for an ATS-34 blade. That behavior doesn't look like a heathly way to treat a bar of steel. That's why I've always used 1950.

I use 2000 for S30V.

Steve

[RJ Martin]

Steve: Agreed, not healthy. Try 1950 for S30V, too. Works great.

[tonn]

[QUOTE=RJ Martin]Tonn: Cooling below the Mf temperature is not harmful. Liquid Nitrogen will be cold enough to yield maximum conversion for any steel. You do not need slow cooling. that is for large/irregular parts to prevent cracking from thermal stresses. Knife blades are thin and uniform and they rarely crack during cryo.

I didn't mean it's harmful.Question for me is: if temperature range for martensite transformation is from A to B then cooliling below B should stop the transformation ..or what?

[RJ Martin]

Tonn: Yes, if you cool continuously from the quench temperature to below the Mf (martensite finish) temp, the transformation is completed. Your steel will have the highest percentage of martensite possible, assuming the preceeding heat treatment was correct.

[mete]

Tonn, yes , theoretically anyway. The basic rule is that the more complex the steel [the more carbon and other alloying elements] the more critical the times and temperatures in heat treatment and the more likely there will be retained austenite .However when we look at a specific alloy, 52100, for example ,the Ms is480F and the Mf is 235F. We should then have no retained austenite but in fact you will get some. Most of the retained austenite that will be transformed with cryo will do so by -100F. Other factors include austenitizing temperature , the higher it is the more retained austenite......So don't bother with cryo for 5160, it might be a good idea for 52100, but definitely do it for S30V.

[tonn]

Thanks, RJ & mete!
I have a very basic understanding of cryo.
My question is: If time & temperature is important, does it stops the transformation, when cooled rapidly far below Mf?
If reaching Mf is enough for complete transformation, is there any reason to hold it in freeze any longer?

Reason for asking is:
I don't make many knives in one year(10-15) so liquid nitrogen is not practical solution(hard to get here & expensive too)
I have a Cold Spray for sub-zero. When sprayed to blade it gives instantly -55C. Should I keep it in freezer after spraying or is it as good as it gets?

[Gary Mulkey]

Tonn,

As it was explained to me by one of the Crucible metalurgists was that the time for dry ice was a minimum of six hours to get the best tranformation. This should give you 99.5% martensite which is the best you can hope for. If your temperature doesn't reach that of dry ice (if I remember is approx. -150) then the time required is longer. (I have read where Viking swordsmiths buried their blades in a snow bank all winter.)

Gary

[mete]

I don't know what your Cold Spray is but you want a uniform cooling .Though the martensite transformation is not a diffusion process it might be a good idea to follow the Crucible advice.