View Full Version : Sub Zero Quench

08-08-2004, 12:21 AM
Sub Zero Quench.

Does this type of quench create qualities in the steel that are superior for knives?
What exactly are the qualities that are enhanced by this type of quenching?


08-08-2004, 12:51 AM
There seems to be a lot of debate on this topic. From what I've gathered, the main benefit to a cryogenic bath after hardening, is transformation of retained austenite to martensite. Ultimately this would give a slightly higher hardness and after tempering, a bit more toughness. It also seems that not all steels benefit, alot of stainless steels (or near stainless steels) seem to have retained austenite and will benefit from it. A basic steel (like 1095) doesn't benefit unless the original hardening wasn't good.

If memory serves, alloys with a lot of chromium see the most change. (This would explain why stainless apparently benefit the most.) I don't remember if I read that somewhere or where that information came from, so please take that with a big grain of salt.


08-08-2004, 03:16 AM
I repeat everything polarbearforge said, I've heard the exact same things. I'll note a couple things he didn't mention though. On steels it works well on, you can usually expect to see a raise in Rc hardness between 1/2 - 1 point. Also just in case you don't know this, it's not a direct quench from hardening heat like an oil or water quench. You'd harden like normal, temper once, put it in the sub 0, then temper again.

08-08-2004, 04:57 AM
All that Polarbear and AWP are correct in that what they said was it TRANSFORMS retained austenite to UN-tempered martensite and that I usually notice about a cardinal point of hardness increase after DEEP cryogenic treatment.

The correct term is 'cryogenic treatment', not cryogenic quench. The steel is QUENCHED immeditaly after austenitizing and it is not quenched in liquid nitrogen. I used to refer to the cryogenic treatment as 'cryogenic tempering' since it does transform retained austenite. Some professional heat treating services refer to it as that. It is wrong to refer to it as a temper since it DOES NOT temper the martensite. The retained austenite transformed to martensite by cryogenics is NOT tempered and all usual tempering should be performed after the treatment. The first tempering should be performed immediately upon the steel warming to room temperature.

To do it properly the steel should immediately be placed into cryo upon reaching room temperature after a snap temper (which is performed immediately after the steel reaches approximately, but no less than, 125 F after quench). The snap tempering after quench is a preference of mine and some skip the snap temper for high alloy steels and go straight to cryo immediately after the steel cools enough after quench.

The thing to remember about cryogenic treatment is: It will transform retained austenite into untempered martensite. That is ALL it does. This untempered martensite must be tempered, therefore all tempers except for a previous snap temper are performed after the cryogenic treatment.

It is not completely accurate to believe only high alloy steels benefit from cryogenic treatment. There are some that will not, 5160 may be one example. AWP is correct though about not hitting the heat treat 100 percent and that is almost always the case, even with the best of the best. God himself may be able to. Almost always the heat treat will miss by a percentage.

The answer to your question about "does it benefit knife steel": Absolutely YES and all other steels of high carbon content used for such things as tools, etc.


08-08-2004, 08:20 AM
I would agree with all that has been posted but have a question regarding the use of cryo treatment to salvage a less than optimum heat treatment. I don't know how it would do that. If you failed to austenitize at a high enough temperature, the structure would have retained ferrite and cryo won't do anything to it. If you left undissolved carbides, they would remain in the martensite and the cryo would not dissolve them. If you quenched too slowly, depending on the exact cooling velocity, you would get a mixed structure of bainite, pearlite, and martensite. Cryo won't fix that. Retained austenite is caused by high carbon content impeding the shear transformation of austenite to martensite and quench velocity does not appear to have anything to do with it. What kind of "salvage" work do you use cyro treatments for?

08-08-2004, 01:24 PM
AWP is correct though about not hitting the heat treat 100 percent and that is almost always the case, even with the best of the best. God himself may be able to. Almost always the heat treat will miss by a percentage.

I think you might be remembering something I said on a different thread. When I said it I was actually talking about multiple quenches as opposed to cryo. It seems like it's alot of the same steel that can benefit from multi-quenches and cryo, other then air hardening of course, I believe it's do to undisolved carbides and crystals that some alloy materials tend to create. A 100% perfect quench won't benefit from another quench, but it's kinda rare to do it that perfectly.

08-08-2004, 02:23 PM
Salvage the heat treat?? I looked again and can not find where any of us eluded to that at all.

Again, ALL cryo does for us is transform retained austenite to untempered martensite. That is it.


08-08-2004, 11:26 PM
A basic steel (like 1095) doesn't benefit unless the original hardening wasn't good.

I believe it was that statement that Quenchcrack was refering to. Just guessing.

08-09-2004, 12:41 AM
Well yeah, I can see that. By no means will cryogenic treatment bail you out of a poor heat treat.


08-09-2004, 06:10 PM
I believe it was that statement that Quenchcrack was refering to. Just guessing.

That could have been. There were several statements that could taken that way. I didn't mean it that way, though. :)

At home with my knives, I don't do any cryo at all. At work, I think I should be with some of the stainless parts I'm working with to take care of any retained austenite and to help provide dimensional stability.


08-09-2004, 09:08 PM
Yes, that is the statement I was confused about. I posted this same comment yesterday but it did not show up. Not sure why. 0]

I have spoken with engineers at Cincinnati Subzero and 300 Below about cryo treating D2 for an industrial application (roller dies) and both provided considerable data to show it did improve abrasion resistance. I was a bit more skeptical regarding the data on improving the sound of brass horns and the flight distance of golf balls when cryo treated. :smokin

george tichbour
08-09-2004, 10:17 PM
Fourty years ago when cryo treatment first was reported in the journals the explanation was that conversion from Austentic to Martensic steel took place during the cool down period from critical temperature and ceased when the temperature leveled off (at that time at room temperature). Someone asked what would happen if room temperature were lower, would the conversion continue below room temperature? They tried liquid nitrogen and abrasion resistance(wear resistance) increased significantly.

Current investigations confirm the % conversion of Austentic to Martensic steel is higher after cryo treating, in some cases in the high 90's range.

Anything that improves the product performance is good enough for me.

I have since spoken with an aerospace machinist who uses cryo treated aluminum for high precision parts because he can hold tighter tolerances with cryo treated materials than un treated material. His industry believed that the cryo treatment acted as a grain refining step which prevented creep caused by cutting tool stresses.

08-10-2004, 06:10 PM
Don't forget it's supposed to make disposable razors and panty hose last longer. :101