Cryogenic

Has anyone here experimented with the cryogenic process on knife blades? There are amazing claims about this process adding to the edge and toughness ability.
The process involves slowly freezing (below -320 deg. F) and slowly thawing back to room temp. The simplified theory is that the molecules normally are unevenly spaced, when you freeze the metal you pack the molecules tightly, and upon slowly thawing out the molecules space out evenly.
Has anyone tried this process and where can I send a blade to have this done. What is the cost? I would like to try it out myself.
Thanks!
 
Jerry Hossom

Jerry Hossom

Joined
Aug 1, 1999
Messages
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Cryogenic quenching isn't slow at all. When you put the blade into liquid nitrogen, it gets cold pretty fast. Same when you take it out. It does make a lot of difference in the results, amking the blade both harder and tougher. Ideally, the cryo treatment should occur before tempering. Crucible Steel recommends always tempering after the cryo on all their steels.

Paul Bos does my heat treating and cryogenic quench is a normal part of his process. He only does air hardened steels though. Those are normally most steels with 5% or more chrome.

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Jerry Hossom
www.hossom.com
 
Here is what Tom Mayo had to say:

tom mayo
Member
Posts: 922
From:waialua, hi, usa
Registered: Jan 99
posted 01-09-2000 06:12 PM
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cryo is absolutely necessary for complete marstinite transformation and needs to be done after full hardness and before tempering, not at the end of the cycle. if you send your blades to paul bos he will do it that way EVERY time...at only $60 for 20 blades...plus return shipping and a modest handling (packing) charge. hes been doing mine for 18 years....the iconel boxes he puts the knives into that slide into his furnace cost more than all the heat treating equipment most knifemakers have....do it right the first time.....send it to him.
------------------ www.mayoknives.com

I tend to believe Tom. Walt


[This message has been edited by Walt Welch (edited 03-19-2000).]
 
Tom has pretty much hit the nail on the head.
High carbon, high alloy materials are very sluggish in their transformation to martensite. Lack of complete transformation is commonly called retained austenite.
Typically, materials will respond to cryogenics with a 1 to 2 point HRC increase.
You must temper after cryogenics to temper the newly formed martensite. The snake oil claims come from the misbelief that cryogenics will enhance any and all alloys. If there is no retained austenite, there is no reason to do it. Since most martensitic stainless steels have retained austenite, it is safe to say cryogenics is doing something positive.
 
I have never completely understood why steel companies are so lukewarm in their support of cryogenic quenching. In every steel I've ever used, not all of which are considered stainless but are air-hardening steels, cryogenic quenching makes a significant difference in edge holding and capacity to handle the stresses that are important to knife blades.

A2 is certainly not considered stainless, yet in recent tests, a sword I made in A2 (heat treated by Paul Bos) cut cleanly through a shank of beef including 3" of very hard bone. The edge which was shaving sharp before the test was unharmed. The same evaluator, testing another smaller blade in A2, heat treated by the recommended schedule, chipped out and rolled its edge on much softer lamb bone, and never could sever the leg of lamb. While edge geometry might account for some of this, the improvement achieved with better heat treating is profound and undeniable.

I have also noticed that even though the cryo treated blades are harder by at least 2 points, they also seem to be less brittle. I can easily flex a 12" ATS-34 blade at Rc60-61 at least an inch by hand. I can flex a 12" CPM-3V blade at Rc60 over 2-inches. I have a 20" ATS-34 machete of 3/16" steel I can flex over 4-inches. This parameter, IMO, is a key factor in blade performance.



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Jerry Hossom
www.hossom.com
 
Cryo doesn't always increase hardness. It might, but, the closer the initial heat treat is to ideal, the less retained austenite there is to transform. If the heat treat was off, you can see substantial hardness increases, indicating less than optimal HT initially.
Even if there is no increase in hardness, the microstructure is still refined and the properties improved.

Also, I believe that it is best to temper once before cryo-it lowers the stress in the steel and rsults in a better grain structure.
Ed-can you please comment on this?

Also, you need to sharpen the knife after cryo-there is about .001 of material on the surface that isn't affected by the cryo. Sharpening removes this material.

Cryo is the only way to go. I wouldn't buy a knife without it.

Rj Martin


 
RJ, I accept the hardness change issue. I was comparing my own heat treating (carefully done but likely less than great) to what I got from the same steel treated by Paul Bos. It jumped 2 points from mine to his. I assumed it was in the cryo, probably not.

The CPM-3V I had done, however, was Rc60 which is about 2 points higher than Crubible cites in their literature.

This though is why it is very difficult to compare or evaluate steels when they are from different makers and/or heat treated less than optimally.

Thanks for the clarification

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Jerry Hossom
www.hossom.com
 
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