How many steels need the cryo/dry ice phase of the heat treat process?

jonnymac44

jonnymac44

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Sep 27, 2007
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Ok, I am on the verge of picking up an Evenheat because I want to do my own heat treating to save time and money.......then I thought about the cryo portion of the heat treat cycle. Is this a required portion of the heat treat process for all stainless steels? What about D2 or CPM-3V?

What do you guys think who have been doing this for a while? Is it a pain to go run and get dry ice every time you want to heat treat a blade? Is it worth it? It was suggested to me by someone in another thread to work in batches when doing heat treat to save on trips to get dry ice, but one of the main reasons I wanted my own oven, aside from being able to say my knives are sole authorship pieces, was to be able to heat treat a single blade whenever I want to speed up production on particular pieces. Having to do my own heat treating in batches defeats that purpose.

Any input on the subject would be great to help me decide whether or not to go for the oven. Heck, there's a mill that I'm looking at for folder work in the same price range so.........
 
I second the question.. It would be nice to have a list of steels prone to retained austenite....(most of the fancier stainless??)

At work, I freeze water pipes using liquid Co2 from 20# and 40# bottles to add valves, do repairs, etc...... Has anyone used one of these bottles with the diptube to make their own dry ice? Or even to just chill -100 degree slurry? I have been mulling over playing with a bottle to see if you could regulate the flow way down to allow a constant source of new Co2 to keep your slurry cold. In my application, I run through the 20# bottle in an hour which would never work for knives... I don't want to hijack Johnny's thread, just maybe trying to figure out an alternative for him and us knifemakers....
 
It seems that all stainless steels benefit from a true cryo treatment. I've had a few makers tell me they just throw their blades into a deep freeze for a few days and it does as good a job. I don't believe that. Frank
 
Brian,
No worries, not a hijack at all. I guess the main point of this thread is to ask the question "is it a good thing for me to get an evenheat right now or would I be better served by using Peters for a while longer?"

I know the processes involved in making knives, but maybe the heat treat portion of the equation is a little more involved than I had previously thought. I'd like all the info I can get on what it takes to do modern powdered steels and others before I make a significant purchase. When you factor in electricity costs, heat treat foil, dry ice, etc. is it really worth it to do my own heat treat?

Basically, what am I getting myself into trying to do it on my own?

Thanks!
 
Jon-
You might want to look at getting a tank of liquid nitrogen. A full tank would last a very long time and I think the cost is like $85in NYC so it would probably be much cheaper anywhere else.
 
jonnymac44 said:
When you factor in electricity costs, heat treat foil, dry ice, etc. is it really worth it to do my own heat treat?




Thanks!
Click to expand...

Peter's will do up to 20 lbs. of a single alloy for 72.00 plus shipping. If you can send him at 12-15 blades per batch then his price is pretty hard to beat in straight costs, compared to doing it yourself. Not to mention your time, including correcting warpage.
 
Great topic! I really want to know why a steel needs cryo, either that or a list would be great! :D

I hope you get the kiln, with the knifemaking skills you have you can pull off any heat treat.
 
Grumpy, that's something to look into, thanks!

Justin, I understand that, but when I add in more than one steel type or have steels with special requirements sometimes the price jumps up a bit higher. Also, shipping costs both ways and I always ship insured which can be pricey for a batch of fifteen blades when you value them according to selling price. All said and done a batch of blades is usually over $100 dollars.

Daniel, thanks for the vote of confidence buddy!:D
 
First the definitions
Sub-zero treatment - CO2 , about -100 F
Cryogenic treatment - Liquid nitrogen , about - 300 F
Sub-zero reduces retained austenite .
Cryo reduces RA further and creates a fine "eta" carbide which ads to strengthening, hardening.
The steel MUST be tempered after sub-zero or cryo.
Typically high carbon , high alloy steels can benefit from these treatments. The stainless knife steels such as 154CM will benefit but even some lower carbon non-stainless ones like CPM 3V will benefit.
 
Hey Jon,

Have you considered moving to high carbon for the sole authorship and sending Peter's the stainless? Personally, I don't put huge stock in sole authorship when it comes to HT, I want the best result, shipped out or maker.

EA
 
You might look around on Crucible Steel's website they have a ton of useful info geared specifically for knifemakers. Back in May I was able to go to the metallurgy seminar held at the OKCA knife show, and crucible was there to talk about there steels and how they apply to makers, to my understanding you can gain benefits with their cpm steels, such as a higher Rc but cryo is not a "necessary" step. I say go for the oven, it opens up a lot of possibility's and you can feel free to try different things you may not think are "worth" sending out. here is a link to a data sheet on CPM 154...... http://www.crucible.com/PDFs\DataSheets2010\Datasheet CPM 154 CMv12010.pdf
I am by no means an expert on the subject, just my $.02

Ben
 
So it seems that cryo is not a requirement, it just makes a knife a little bit better right? Are all the big knife brands such as benchmade kershaw and spyderco cryo treating all of their knives? I had always thought that an ametuer knife maker make perfectly good knives without a cryo treatment, am i wrong?


I had always wondered if you could use one of these knife ovens to bake clay. If so then I say there is one more reason to get an oven, you can do multiple crafts!
 
I expect those three manufacturers utilize a cold treatment of some sort. It is common in the cutlery industry when processing stainless.

You can make a perfectly serviceable knife without subzero. And a perfectly serviceable deer rifle can be purchased at walmart. However, if I buy a custom deer rifle, I expect certain things. I think most people who buy a custom knife expect the heat treat to be as good as it can be. At least above a certain price point.

Don't think of it as a unimportant add-on. Think of it as completing the quench. The Mf for most complex steels is below room temperature. Stopping before you complete the martensite conversion promotes retained austenite which plays hell with fine edge stability. Think of those areas of RA as little perforations like you'd see in a sheet of postage stamps. It isn't about getting that last point of HRC, it is about avoiding the structures that cause that loss.

For simple steels, I wouldn't worry about it. For stainless and D2, it can make a real difference. I have confirmed this with my own testing.

I did a thread a while back illustrating my testing methods that illustrated the difference: http://www.bladeforums.com/forums/showthread.php/839148
 
jonnymac44 said:
When you factor in electricity costs, heat treat foil, dry ice, etc. is it really worth it to do my own heat treat?
Click to expand...

I've done the math and based purely on cost, the answer is "hell no". You forgot to mention Rc testing... add in another several hundred dollars for a tester and whatever time it takes to calibrate it and learn to use it properly. Then again lots of makers, maybe even most, simply plug in their oven, follow the directions and call it good.

When you factor in hard-to-define things like being able to HT one blade whenever you feel like it, the ability to try and compare different "recipes", the cache' of saying you do it yourself... the answer is "maybe".

I'm struggling with this myself and have been for quite some time. Frankly if I had the cash on hand I'd put it into a mill or better grinder rather than a HT set-up (which as you rightly point out, involves a lot more than just buying a kiln).

EDIT: as far as cryo goes, I'm convinced that it's just plain silly to pay significantly more for high alloy steels and not make the most of their potential by completing the quenching process properly. At that point you might as well stick with 1084, a campfire and a bucket of used motor oil, it would be a more efficient use of resources.
 
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OKCA knife show .That Crucible was there certainly shows that they are interested in the knife business !!
 
Just to explain a bit about what the sub-zero and cryo process does:

When simple steels with a carbon content near the eutectoid ( 0.84% carbon) are heat treated and quenched properly, the austenite is supercooled and passes the pearlite nose. It drops to about 400F where it starts converting to martensite. If the alloy mix is simple, pretty much everything converts to martensite and carbides by the time it gets below 100F. If there is a great excess of any alloy element, especially carbon, some of the austenite still won't have converted at room temperature.
To complete this process some people stick it in the freezer, which they claim does miracles, but that actually will complete the change on only a tiny fraction of a percent of the RA ( retained Austenite).....and basically does nothing.
It requires the steel to drop below -90F to complete the transformation. This is called a sub-zero quench.
Dry ice is -110, and dry ice and acetone/kerosene is -100. The acetone/kerosene mix works a bit better as it assures even contact with the quenchant. The transformation completes rather quickly, but most folks leave the blade in for a good while ( probably because the dry ice is not keepable, so it seems like less of a waste to keep using it until it is gone).

When you use liquid nitrogen the process is called cryo - short for cryogenic treatment. The RA is taken care of at -90F, but the carbides don't rearrange until near -300F. The exact process is a bit murky to me, but as I understand it, the carbides change their structure and arrangement into fine eta carbides. Guys like mete and Kevin like that stuff, so it must be a good thing :)

Whichever process is used, it is the completion of the quench, and must be done as a fairly continuous process. A long delay in cryo/sub-zero treatment will allow some of the RA to stabilize and it will never change. A delay in finishing the quench and getting to temper may also cause some blades to crack or break. A snap temper at 200-300F for 30 minutes to an hour can help avoid cracks in hyper-eutectoid steels. This is especially important if there is an overnight delay in getting the blade into the cold treatment. Most high alloy stainless steels are not at risk in cracking from a delay getting into the cold, but a snap temper is still a good idea.

Just to be clear, there is no terrible problem with a small percentage of RA in a knife blade. In some industrial uses, it is a benefit....but we knife guys like our martensite.

Does doing these extra steps make a bad blade good - NO.
Does not doing this make a good blade bad - NO
Will doing this help a good blade be better - YES, to a small degree.

The gain with sub-zero treatment should be lower RA and a slight gain in hardness.
The gain from cryo should give you a harder and slightly tougher blade.

You would not be able to tell if a finished knife was treated these ways by looking at it or "feeling" the edge.
Analytical and other testing of the blade before and after the treatment should show a small gain.
Just doing the normal shop testing on a finished blade won't tell what you gained by cold treatment, but testing your blades will tell you if they are done right.

It is that extra effort and slight gain that make a custom knife more desirable to many people.
 
Nathan, James, Stacy, Jawilder.....
Thanks everybody for lots of input and insight!!
I definitely want to build the best knife I can and not be the type to plug in an oven, follow some simple instructions and call it good.

I think I might hold off and spend some time studying the art of heat treating before I jump into this.

Thanks for all of the info everyone!!
 
It's funny that once science gets complicated enough, it's easier to refer to it as art :D. Just messing with you, Johnny.

Definitely take some time and read, re-read, and read again some of the great posts on heat treating here on BF's. Get familiar with the terminology. Get familiar with the mechanics of what is happening at each stage in the heat treating process (whether it's normalizing, thermal cycling, hardening/quenching, cryo, or tempering). Once I was able to have a working understanding of what I was doing and why, it made my attention to each step in the heat treating process that much sharper, and it helped me fine-tune heat treatments just a bit within the industry recommendations. However, don't let it intimidate you away from heat treating. A good oven and proper quenchant is all that is needed for many steels. And it's not too difficult to get set up to maximize HT for high alloy/stainless steels.

Keep your eye out on Craig's list or even here on the forums for a used LN dewar. They can be had for reasonable prices, and you don't have to have a giant tank as long as the opening and neck is large enough to allow blade access. LN at a air/gas supplier is pretty reasonable and lasts from several weeks to several months in a good dewar. Factor the extra cost of the LN into the price of your blades if you need to, but if you want to do this for a long time or at a higher production rate, it's worth it.

--nathan
 
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