# Cryogenic treatment, with versus without



## ajhuff (Jun 24, 2013)

Rather than hijack the lovely Japonocentric thread I thought I would ask here. There were a lot of allusions to cryo being a commonly accepted practice among leading US makers but not Japanese knife makers. 

Cryo is relatively speaking a new technique or rather a newly _accepted_ heat treatment. Does anyone have a comparison between knives showing the difference between being cryogenically treated versus not? This was kind of coming up as a difference between the American knives and Japanese knives but I'm wondering if it is more a difference between the makers themselves than the actual knives.

Thanks,

-AJ


----------



## PierreRodrigue (Jun 24, 2013)

Its more common with powdered stainless steels, and guys using and oven to HT with the knives in a foil envelope. There is usually a gain of about 1 to 2 points (typically 1 and a bit) HRC when HT'ing in an oven is followed by cryo. It allows the conversion between more of the retained austenite to martensite to continue a little more. There is a good article here... http://metal-wear.com/Theory.html

Using molten salts, allows a better conversion of austenite to martensite from the start, so the benefit of a cryo cycle is less when measured as a Rockwell hardness, but there still seems to be an improvement in toughness, and edge holding from my less than scientific testing.


----------



## JBroida (Jun 24, 2013)

fwiw, i know a large number of makers in japan that use cryo


----------



## ajhuff (Jun 24, 2013)

JBroida said:


> fwiw, i know a large number of makers in japan that use cryo



Aha! The plot thickens! 

-AJ


----------



## Don Nguyen (Jun 25, 2013)

PierreRodrigue said:


> Its more common with powdered stainless steels, and guys using and oven to HT with the knives in a foil envelope. There is usually a gain of about 1 to 2 points (typically 1 and a bit) HRC when HT'ing in an oven is followed by cryo. It allows the conversion between more of the retained austenite to martensite to continue a little more. There is a good article here... http://metal-wear.com/Theory.html
> 
> Using molten salts, allows a better conversion of austenite to martensite from the start, so the benefit of a cryo cycle is less when measured as a Rockwell hardness, but there still seems to be an improvement in toughness, and edge holding from my less than scientific testing.



Pierre, you mean with molten salts to austenitize? What brings the difference in retained austenite between that and a normal kiln or such?


----------



## PierreRodrigue (Jun 25, 2013)

In an oven, the blade wrapped in foil and plate quenched, the foil partly insulates, and doesn't allow as fast a cooling when quenching. Also the heat from an oven is radiant, in salt its contact, and the tempreature more even through out the volume of the salt column (typically within 5 degrees). Tests in an oven have shown fairly substantial temp differences within the oven space. The blade when drawn from the salt, is insulated from the ambient air within the space you are working, by the coating of salt on the blade. This liquid layer also acts as a heat transfer medium when plate quenching. If quenching is a salt solution, or oil, the same is true.

Bottom line from testing, and by others way more scientific than me, heat treating in a liquid such as salts (or back in the day, in molten lead) allow a more thorough conversion from austenite to martensite, than forge or oven methods. Besides that, when heating in salts, there is little to no de-carbonating due to the absence of oxygen in salts.


----------



## ajhuff (Jun 26, 2013)

A salt pot allows for much more uniform heat distribution and therefore much more uniform heat transfer.

-AJ


----------

