420 Sub Zero Quench

mhawg

mhawg

Joined
Jan 10, 2003
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Is this some kind of BS procedure or does it do what is says?

"Cold Steel has always sub zero quenched the blades that they have manufactured in Japan, and they are now adding this often omitted, yet crucial, process to the heat treatment sequence of the new blades they are making in Taiwan. Typically, stainless steel blades suffer from retained austenite after they have been heated to critical temperature and then air or gas cooled. The big drawback of this procedure is that it doesn't get the blades cold enough to assure complete transformation from an austenite to martensite grain structure. Only by freezing - sub zero quenching - stainless blades to 120 degrees below zero (Fahrenheit), will the austenite be completely transformed to martensite. This assures the blade will be as hard, strong and wear resistant as can possibly be."
 
Also known as cryo treatment or cryogenic tempering, if either of those ring a bell. Its more effective on some steels than others. Carbon steels don't retain austenite, so its not an issue there.
 
I have read that carbon steels that have more than 60 points of carbon do retain austenite. However, they do not need cryo treatments. But instead, all the austenite is not transformed to martensite until the blade temp is brought below room temp. I would think overnite in the freezer would help transform some of that retained austenite.

I did started a small test on c1095 with oil quenched samples that went into the freezer for 9 hours. The hardness on all three samples was right around 64 HRC.
 
Thanks. Now I'm getting both sides of the question. I've just read some posts that seem to claim that this steel is substandard and possibly not a worthy quality knife steel.
 
I don't believe it is. At least I wouldn't choose it. Works better for other non-knife stuff. 420 would make a tough (stainless) blade though.
 
In metallurgical terms, cooling to -120F is "cold treatment", while cooling to -300F is "cryogenic treatment". The names can be a little confusing since medical cryo is only -120F.

There is never 100% conversion of austenite to martensite. Some is always retained. The freezer will convert more than room temperature, but it would take years in the freezer to get as much conversion as is acheived by going colder. The greatest percentage is reached at -120F. How much difference varies quite a bit according to steel and heat treatment. Colder temperatures than this do not cause greater conversion but does other things.

Most of their statement is correct... with the exception of the last sentence.

Daniel Watson
 
Cryo, deep or shallow (-100 for what I consider shallow) is the real deal. There is a small notification for you upon cryo that some do not observe: There will be a very fine (a mil or two) of soft material left on the surface. This is, as far as I know, an anomyloy not yet explained. It is not important to us knife makers except to be able to expect it and to know that sharpening of the edge should be done afterwards (which, of course, would happen after all HT and temper anyhow). This, what I point out, is rarely mentioned by others during any HT/Cryo arguements but very true non-the-less.

Roger
 
Roger,

The "skin" you describe is only present when the steel is actually in contact with the LN2. If the steel is chilled to the same temperature but does make contact, then there is no soft skin.

Daniel
 
Originally posted by Laredo7mm
I have read that carbon steels that have more than 60 points of carbon do retain austenite. However, they do not need cryo treatments. But instead, all the austenite is not transformed to martensite until the blade temp is brought below room temp. I would think overnite in the freezer would help transform some of that retained austenite.
Click to expand...

Unfortunately there is not enough Carbon in the 420 to take full advantage of the process!
 
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