cryo treatment in home freezer

J

John Frankl

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Oct 16, 2001
Messages
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Hello All,
I remember recently reading a thread on cryo treatment, specifically of 52100 and 5160, using a home freezer. But now I can't seem to find it. Any help will be appreciated.
Thanks,
John
 
People seem to react to this idea with either scepticism or sarcasm, but I've tried it once and think it made a big difference.

Put the blades in the bottom of a plastic (not styrofoam) cooler, cover them with dry ice (I used 2 lbs), cover that with acetone, tape the lid shut to reduce evaporation, and open it up 12 hrs later when the dry ice has completely evaporated. Then temper as normal.

I'd be interested in hearing whether there's anyone else who has tried this. I'm going to do it again soon, we'll see it I still think it works.

Dave
 
a word of caution though on the acetone/dry ice method...add the acetone to the dry ice. Don't drop the dry ice into the acetone. I have heard it can explode if you do. Let us know how it turns out.
 
My blades go into the deep freezer between tempering cycles to the point when removed they will stick to you fingers (no...I never put my tongue on one).

Years ago an old machinist told me that he got better and longer performance out of his cutting tools when he put them in a snow bank after heat treat.

I have never done comparative tests with this...but my blades hold a great edge and the freezer is right next to the stove.
 
cryo in a freezer? Sounds funny.
cryo is in liquid nitrodgen, at a extremely low temmp. In a freezer, I know it is supposed to help, but only at a minimal scale. just think about it, how long you wanna let a knife freeze at -20 °C when a cryo-treat is at -270 ° C ? for 10 days?

In any case, i do freeze my 52100 and 5160, all tiny bits help. But I doubt you'd notice the difference in cutting if you did'nt do and let the blade cool down to outdoor temp (5 degrees °C here) and let it wait there for a week to do the next temper run.

Greetz and take care, bart;
 
Any cryo treatment you give to a blade will help. The colder you get, the better. I'd STRONGLY recommend substituting kerosene for acetone, though, as kerosene is far less flammable. And, you can re-use it. Definitely not worth burning the house down.
Don't forget to temper at least once after cryo. 2 or 3 times is better.

RJ Martin
 
Do you cryo between EACH temper or only once before the first tempering?

Thanks; rlinger
 
Cryo treatment should be done directly after hardening. What you want to do with the cryo tratment is to change retained austenite to martensite and the best time to do that is up to 2 hours after quenching.

Every bit of cold temperature may help, but the ideal temperature is around -100° celsius, which is about -148° fahrenheit. And there are two other interesting facts.
First you should be informed that going below that temperature only produces a higher risk to crack the steel. So working with liquid nitrogen doesn't make a lot of sense over wotking with dry ice.
And second it is important to know that once you reach the temperature, the changing from retained austenite to martensite happens with the speed of sound. Quenching the steel for many hours doesn't make any sense at all. About 30 minutes is all you need.

Achim
 
Achim and All,

Thanks. What about a home freezer? I know it would not be ideal, but would bringing a blade down to about 10 degrees F have a small but positive effect on converting retained austenite to martensite? Also, what if this is done later, say weeks later? Will some austenite still turn to martensite? And, if so, should an additional tempering cycle follow the freeze treatment?

Thanks,

John
 
John, although my memory is hazy, I seem to remember that Ed Fowler puts his blades in his freezer "right next to the ice cubes" between tempering cycles and does not cryo at all. It is hard to argue with Ed's heat treatment and the quality of his blades. I have just begun experimenting with using the freezer. My freezer gets down to zero.

Fox
 
So after a cryo treatment, the martensite doesn't revert back to austenite? Even if retempered?
 
Actually, cryo performed after a single temper, or "snap temper" (a temper lower than the actual temnper temperature but, high enough to relieve stress-generally, 300F)appears to offer the best benefit. In the early days of cryo, steel manufacturers stressed the need to cryo right after quench. This tends to produce larger grain size, and, more internal stress. If using LN2, the effect of the cooling is so strong that you can cryo at any point after the first temper-days, weeks, months and you will still convert retained Austenite to Martensite. You can tell this is so because race car teams cryo entire engine blocks and other components months after they have been manufactured. What is important is getting that just quenched steel right into the tempering oven while it is still about 150F so that the martensite responds well to the initial temper. By tempering the martensite formed during quench before performing the cryo treatment, you are helping the steel to not be overstressed during the cryo treatment.
For simple steels, (high carbon), the Mf (Martensite finish) temperature is not nearly as low as with highly alloyed steels-it is the alloy content that pushes the Mf temp lower. This is why you can get good conversion of retained Austenite to Martensite with, say, 70 degrees of additional cooling (70 deg=RT, most freezers are about 0 deg F). For highly alloyed steels (BG42, any CPM alloy, etc)you'll get more benefit as the temperature decreases.
Remember, the desired microstructure in a finished blade is tempered martensite, not martensite. You only need to cryo one time, and follow it with one or more tempers.

Hope this helps,

RJ Martin
 
Go RJ, this is great. Now if I get this straight you are saying with simple carbon steel to:

Snap temper before the steel cools below 150,

Cryo after the snap temper and then go to the normal temper cycle(s).

If freezing in the home freezer, how long should it stay?
 
Peter: For carbon steel, I'd just temper it normally, as the temper range is probably around 400-500F. Snap tempers are more frequently used when the tempering temp is considerably higher. Yes, you want your tempering oven at temperature while you harden your blade, and you want to temper it immediately after quench-when you can just tolerate the heat of the still warm blade in your bare hands.
As to duration, I use 24 hours in the LN2-that's probably enough for a freezer treatment. Better to go too long than not long enough.
If you have access to a hardness tester, check your blade just before you cryo, then, afterwards and see if the hardness changes.

Don't forget the tempers after cryo!!

RJ Martin
 
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