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Philip Lee 10-20-2003 04:57 PM

Cryo Treatment
 
Hi all,

Greetings from Fiji.

I m a part time knife enthusiast based in Fiji. I have made many knives. I work with the following steels, 5160, 1095, W2, and L6.

Generally speaking, when one talks about Cryo does it necessarily mean liquid nitrogen - i don't have access to it.

I was just wandering if the househld deep freeze could be used instead as a make shift cryo treatment. If i can use the freezer than how long does the steel need to be in the freezer.?

Thanks in advance.

Philip Lee

Jamey Saunders 10-20-2003 05:08 PM

I use my freezer for no other reason than this -- it can't hurt. I don't know what benefit you'll get from it (meaning that I don't know if it adds much more to the blade than simple quenching), but you won't hurt the steel. It's worth a try. I'm sure it's not as good as Liquid nitrogen, but if it's all you've got...

fitzo 10-20-2003 05:31 PM

the cryo is a debatable phenomenon. there are supposedly two aspects to it: conversion of retained austenite to martensite, and secondly carbide rearrangments in high alloy steel. the first is a fact, the second still a debate.

i have HT'd stainless at home for years, and it is notorious for retained austenite. i also have a rockwell tester to allow me to follow hardness as a measure of austenite conversion.

what i do know is that a SS blade coming out of an oil quench with a Rc of 58 will show a hardness increase of 3-5 points if cryo'd. my experiments on ATS and 440C have shown that it would continue to climb for a week or so if using a home deep freeze, before it leveled off. 12-16 hours at -78C in a dry ice acetone bath would do the same, as would 6-8 hours in liquid nitrogen. in my experience, all hardened to about the same rockwell. thus, i think it is a RATE phenomenon to some extent, so the lower the temp the higher the rate of conversion.

now, metallurgical literature speaks of things going on with the carbides that may be beneficial. this seems to take the ultralow temps of LN to achieve.

thus, my unstudied conclusion would be that, for a simple steel, home freezing for some length of time (I use a week) will suffice to transform unretained austenite, but lower temps are necessary for high-alloy steels.
BTW, simple steels should be snap tempered or tempered once before chilling to prevent cracking. stainless is apparently cryo before temper. i've never had a SS blade crack.

hope this helps


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