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Old 04-15-2017, 01:23 PM
samuraistuart samuraistuart is offline
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Join Date: Dec 2012
Location: San Antonio Texas
Posts: 163
I don't know of many knife makers at all that do CONTROLLED drop in temperature to -300F with LN2. I am not a metallurgist, but would say that is not needed.

Terminology, as with many things, is important so that we are all on the same page when discussing things. Generally speaking, us knife makers have 2 terms we use. "Sub zero" which refers to the dry ice slurry temps of -100F, and then "Cryo" to refer to LN2 temps of -300F. Shallow cyro, deep cryo, as Jay uses them.....that's fine too. But like I said, most knife makers who cryo are lucky to find/afford a dewar with a mouth wide enough to insert a large blade. Very very few would invest in a machine that drops the temp to -300F in a controlled manner.

And just to add to this discussion about exactly WHAT we are trying to accomplish with "cryo" or "sub zero". By FAR, by FAR, the biggest reason to use SZ/Cryo is the elimination of retained austenite. As a steel is quenched, the austenite state is transformed to martensite. However, some steels have a martensite finish temperature below room temp. O1 is NOT one of them. 52100 is NOT one of them. A2....yes. D2...yes. And of course, stainless steels and higher alloy tool steels like AEB-L, M4, etc. If you see gains in HRC reading by employing SZ or Cryo with 52100....you need to re-think your 52100 heat treat. A2 and D2 have enough retained austenite that a sub zero dry ice treatment will help to convert much of that. Cryo, LN2, will convert nearly 100% of the RA. I think many aren't getting this concept.......the MAIN reason by FAR, again, just to stress it, of employing SZ/Cryo operations is the elimination of retained austenite. The 2nd benefit is the formation of super small "eta" carbides that will precipitate upon tempering....but this is ONLY happening with LN2 temps of -300F and SOAKING for a number of hours.

Back to Sub Zero. As I mentioned, there is no eta carbide precip going on with sub zero dry ice temps. All that is happening with dry ice is RA elimination. And martensite forms at the speed of sound. Literally......at mach 1, thereabouts. There is NO NEED TO SOAK A STEEL when using sub zero dry ice for hours. Once the steel reaches the dry ice temperature, the martensite is converted, instantly. Soaking in dry ice any longer does nothing. Nothing good, nothing bad. I leave my knives in dry ice slurry until the dry ice has evaporated (overnight).....but again, the transformation we are after happens at the speed of sound, at the instant the steel reaches the dry ice temperature.

About tempering.......the ONLY time you should employ a snap temper BEFORE cryo is IF you are concerned about warping. Tempering stabilizes retained austenite....it's just a fact of life. If you TEMPER BEFORE CRYO, you are severely hampering the ability of the sub zero/cryo treatment to do it's job......change RA to untempered martensite. A PROPER quench is done in a continuous temperature drop. For a carbon steel like D2, that means it should be quenched from it's hardening temp of 1850F all the way down to room temp, then IMMEDIATELY into the SZ/cryo chamber, then back up to room temp, then temper. If you temper before cryo, you are interrupting that quench cycle, and stabilizing RA....which again sort of negates the whole purpose of sub zero /cryo. I know what the "books" say, but NONE of those "books" are talking about making the best knife, but rather bearing, dies, mold, etc.

If you are seeing changes in O1 HRC readings, or 52100 HRC readings by doing sub zero or cryo, your HT is off. The Mf of those 2 steels is well above room temp. Sub zero will not make either of those steels tougher. However, Cryo with LN2, due to the precip of the eta carbides, not only makes it tougher, but slightly more wear resistant. NOT 400% more as is claimed by cryo companies selling cryo products.

Last edited by samuraistuart; 04-15-2017 at 01:29 PM.
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