Cryo treatments:tougher or brittler?

J

Jeff Michaud

Joined
Sep 9, 2006
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Howdy. Does anyone have any experience with cryogenically treating steel to make it harder? I'm wondering specifically about dipping the steel in liquid nitrogen. A friend dips his D2 blades in -300 degree nitrogen after heat treating, claiming that it freezes the molecules at the fastest rate possible making the steel harder. It sounds good in theory, but I wonder if it would cause the steel to be more brittle and prone to breaking? D2 is already pretty kick ass, wouldn't you think? He swears by it. What do you all think? Thanks.
 
Cryo freezing seems to be common practice in the knife world, It seems that it adds a point or 2 in the Rc scales.

I havn't tried yet but some very respected and knowledgable knifemakers use this method and swear by it.

IIRC i think D2 is also generally cryo'ed after the first temper by alot of makers.
 
It isn't so much about freezing the ,olecules, since they don't. It's about converting austenite to martinsite, or simple terms, getting everything in the blade that will form fine little carbides to do so. It requires at least 100 below zero and works on air hardening steels, like D2. Or the stainlesses, or A2.

When the blade is quenched, with these steels in air, there is unconverted austenite left in the blade. The cryo cycle forces that conversion to martinsite. That's the good, tough stuff.

Typically a cryo cycle makes the blade harder and tougher. To heat treat air hardening steels without a cryo cycle is to do a dis service to the customer and produces less than the best possible knife.

My 2 cents worth, I hope it helps.

Gene
 
Wow. I'm always facinated at the sophistication of steel. Thanks for the insight. So it's not overhardening them to the point of being brittle?
 
Embrittlement is dependant upon heat treat with no regard to the cryogenic component. Cryogenics transforms retained austenite to martensite (the structure you desire as a knifemaker). It will have no effect on toughness or embrittlement. Tempering the steel properly is essential regardless. Embittlement is caused by enlarged structure which cryogenic treatment does not effect. As an example: soaking the steel too long in heat treat can cause embrittlement.

Cryogenic treatment is good for all hardening steels and especially beneficial in high alloy steels (your stainless blades). Cryo just after quench and the steel has acheived hand warm. Temper afterwards or snap temper immediately before cryogenics and then do your tempering.

rlinger
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One thing to remember - The austenite that gets converted into martensite in cryo is UNtempered martensite ( and thus is more brittle). All cryo treatments must be followed with a temper cycle.
 
rlinger said:
... As an example: soaking the steel too long in heat treat can cause embrittlement...
Click to expand...

Don't mean to knit pick Roger, but this is one of those bits of misinformation that is growing and spreading wide enough to become detrimental. I realize that you use it only as an example and that it may not reflect your personal opinion, but just to clarify...

Soak time does not cause grain growth. Overheating from lack of temperature control causes grain growth, a concept that we have firmly established on this forum with threads dealing with soaking for outrageous times with no negative effect on grain size. Extended soaks will result in higher hardness values but that is dealt with in tempering, and after tempering the more complete solution one could find higher strength values in the steel. With more thorough tempering the toughness will increase as well. A recipe that I have taken to heart, and has yet to fail me, is to go for maximum hardness and strength in the quenching and then add the toughness with the temper.

Some embrittlement can follow cryo if one does not temper the newly converted martensite, but perhaps the argument could be made that things just sort of equal out to the mixed microstructure that could result without cryo. Any steel that has enough excess carbon, or heavy alloying to stabilize the austenite at room temperature could benefit from the more complete conversion that cryo could offer.
 
Thanks so much everyone. This stuff is so facinating. This forum RULES! I'm going to go chisel some cement with my D2 cryo blade now that I know it's not going to snap off and poke my eye out.
 
Steel has no molecules??
Beware of the atoms Jeff, they are the little guys that go for the eyes!!;)

Mike
 
Metals have atoms, crystals, grains and metallic bonding but NO molecules nor molecular bonding [that's reserved for plastics !!]
 
I miss the point of that article . Are they making smelly metals ??..The last time I researched carbides I found that while most have metallic bonds , some have molecular bonds .But those are carbides not metals.
 
I think it was simply something they did to look at the bonding nature. The main point was noticing the pi electrons in an all-metal compound. Metals don't usually exhibit that "covalent bonding" characteristic with each other, so it was novel. Inapplicable to knifemaking, of course, but mentioned only in case you had interest that there were actual molecules created entirely from metals.
 
I started a thread about this a week or so back. I don't know if anyone really read this article, but there do seem to be quite a lot of diverse opinions on the efficacy of cryo treatment. The article below casts some doubt on the absolute need for cryo and does document increased embrittlement following cryo treatments.

http://www.airproducts.com/NR/rdonl...019GLB.pdf#search="cryogenic quenching steel"

This was about A2, but I have also seen a post on one of the knife forums by Bob Dozier, where he claims that if you need to cryo D2, you are not heat treating it right. You may not believe him, but he does have a certain success with D2 knives.

The point of this is that the matter is still open to debate.
 
Jeff & anyone else I might have seemed like a smart a** to Please forgive me.That was not my intent :foot: (gosh how many times have I used that icon :) ) I made the statement about metals not having molecules based on this article http://www.knivesby.com/robert-cella-1.html , which I found very interesting. We are indeed very lucky to have the likes of Kevin Cashen , Mete , fitzo and so many others who post answers to questions like Jeff asked. The only gripe I have is that when any of these guys post I usually spend the next few hours with a Dictionary and Google trying to figure out what it is they just said. Thanks Guys ;)
 
David, I like smart a**, it is more fun than pragraphs of huge words that are dryer than a popcorn fart. A few wise cracks and fun between the $5 words makes things easier to read. God help us all if any of us ever take ourselves too seriously to have a little fun. ;)
 
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