dry ice treatment

leifjl

leifjl

Joined
Nov 27, 2011
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I've got a blade in a2 and a blade in d2 ready to be h.t., which I was going to do tomorrow morning, and I've been reading up on dry ice/alcohol baths to achieve a more complete transformation of austenite to martensite. I've got a good grasp of the general process, however there are some conflicting points I keep coming across and would like some more clarification from someone with experience with the process.

Some conflicting info I've gotten:

Quench, then sub zero, then temper as usual
Quench, then temper, then sub zero, then temper as usual
Quench, then snap temper, then sub zero, then temper as usual

I think snap tempering means to throw into a fully heated oven for a short time, maybe 10 min., but I'm not even sure on that either.

Also, how long do you hold the part in the dry ice bath for? I've read 1 hr. per inch of largest cross section but don't know how to really translate that for a knife.

Thanks,
Leif
 
For A2 and D2, you need the snap temper.

For A2 -Pre-heat to 1450F and hold for 10 minute. Ramp to 1750F and hold for 20 minutes. Quench in aluminum plates, then cool to room temp. Place in a 300F oven for 10-15 minutes to stabilize the blade, then cool to room temp in running water. Put in the dry ice alcohol bath for about 30 minutes, thgen allow to return to room temp. Temper twice at 400F. This will give you a tougher and harder blade.

For D2 - the pre-heat is 1500F, austenitize at 1850F, do the rest the same as A2, and temper at 450F.
 
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Stacy, I have heard some say that a "snap" temper before first cryo kind of defeats the purpose of cryo by locking, for lack of a better word, the retained austenite that you are trying to get rid of into a form which more difficult to get rid of. ON a realted matter, I for one, canot get my oven to cool down fast enough to do a "snap" temper anyway, so why should I just give the blade a quick dunk in the cold stuff while I wait? Another comment that I heard was that the whole thing about having to soak in the cryo overnight is a bunch of hooey and all you need to do is get the temperature stabilized because the conversion reaction happens VERY fast, perhaps in seconds.
 
A snap temper used to be the norm for most steels going into sub-zero or cryo. Now, only high alloy, NON-STAINLESS tool steels are recommended a snap temper. Stainless steels do not usually need one.

Just set your kitchen oven to 300F and go do your HT. When the blade comes out of the quench plates, take it inside and stick it in the oven for 15 minutes. Take it out, cool off in running water, go back to the shop, and stick it in the cold dunk.
 
Thanks for the info stacy. I feel more confident about this now that I have a clear answer.
 
I am single, so I could do that kitchen oven thing, but my shop is not at my house. :D
bladsmth said:
A snap temper used to be the norm for most steels going into sub-zero or cryo. Now, only high alloy, NON-STAINLESS tool steels are recommended a snap temper. Stainless steels do not usually need one.

Just set your kitchen oven to 300F and go do your HT. When the blade comes out of the quench plates, take it inside and stick it in the oven for 15 minutes. Take it out, cool off in running water, go back to the shop, and stick it in the cold dunk.
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The snap temper is not an exact temper. A $10 used toaster oven with a piece of 1/4" steel plate on the shelf would work fine.

Another method is to try and pull the blade from the plates at around 350-400F, and let slowly cool in still air from there. This would allow some auto-tempering to happen as it cools.




Keep an eye on Craig's list and at the second hand stores for a small convection oven, or the top part of an old Over/Under electric oven. These make a great addition to the shop. They can be converted to run on PID control and will make excellent temper ovens. Placing a plate of 1/4" steel on the top and bottom racks, and putting the blade on the middle rack, with the TC on it, will get very good temp control and evenness of heating. It will take about 30 minutes to come up to heat, but once it is soaked, the oven will hold temp well.
 
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jdm61 said:
I am single, so I could do that kitchen oven thing, but my shop is not at my house. :D
Click to expand...

I have been happily married for 3 weeks short of 9 years and my wife has no issues with me tempering knives in the kitchen oven. I give them a quick wash with dish soap and water to get the Parks 50 off because I don't exactly want my food to taste like that (didn't worry much about that when I was doing knives out of O-1 and quenching in warm olive oil, my wife said "are you making the kitchen smell like french fries again?") Just put an old cookie sheet under the block you are using to hold your knives edge up (with supports keeping the knives spine down edge up) and a second cookie sheet on the top shelf above. Preheat the oven for at least 20 minutes beyond what it takes to get up to temperature before putting the knives in

-Page
 
You do NOT need a snap temper for D-2 before cryo.
If you do a snap temper before cryo...there is a noticeable decrease in performance.
Also, I have done a lot of A-2 and can't tell any difference with cryo.

Having said all that. I have also found D-2 to perform well without cryo, but it likes a way more toothy edge.
It doesn't like a high polished edge, it needs it's micro serrations. This is all based on my experience and personal testing. Also another smart dude who edited his post on this thread. FWIW D-2 is my go to EDC, skinner, and folder steel.
 
Ok, we have Stacy saying essentially "yes snap temper makes it better" and JBS saying "nope doesn't make a difference"

I certainly believe each is describing their experience correctly, but that leaves the rest of us wondering how to treat A2.

Anybody have actual data they can share?
 
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What Stacy described for A-2 is spot on.....I'm just saying I haven't seen any performance differences with cryo.
I don't believe A-2 has as much trouble with RA as D-2.....this is my opinion based on my experience. I use a lot of A-2 for choppers and such, although lately I have been liking 3V as a replacement.
I would also like to point out, I don't do the sub-zero with dry-ice. I use liquid Nitrogen. We may be comparing apples and oranges here.
 
http://www.metalscience.com/images/wear_resistance_graph2.jpg

This seems to indicate a significant difference in performance can be had. I understand that the temp. noted on the graphs are not obtainable with dry ice but that is what I have at hand. If I can get a bit more retained ausenite to transform into martensite then I've already done a better h.t. than I normally would've been able to pull off without the sub zero bath. Ultimately that is my, and probably everyone elses, goal.
I think the snap temper is a small trade off for the security of not having to deal with warp or micro cracks developing. You put it in the tempering oven just long enough to relieve some stress but not long enough to start converting a significant amount of retained austenite to cementite.
I'm just a novice heat treater and could be wrong about a bunch of what I said so lets just say this is my opinion, and I hope I got the terminology straight. ;)
 
The ASM Heat Treater's Guide suggests stabilization at 300F for 10 minutes before sub-zero treatment for D-2 and for A-2. I am sure you can get by without it without cracking the blade, but the recommended procedure says to use it. Since the OP is a less experienced maker, I went with the advice from the people who wrote the book on HT.

Dry ice or LN shouldn't matter, as what we are doing here is mainly ending the conversion of Austenite, which happens at about -100F. The LN will help with some carbide conversion, but the austenite ended at -100.

I doubt it would make a big difference in a knife whether you use the snap temper or not.

That chart seems to show a HUGE difference between Cryo and Sub-zero treatment. I haven't hear of it being that much before. I would be interested in seeing the article it is part of, as these statements can be misleading without the context it is made in.
 
Dry ice or LN shouldn't matter, as what we are doing here is mainly ending the conversion of Austenite, which happens at about -100F. The LN will help with some carbide conversion, but the austenite ended at -100.

When the concern is a cutting tool, carbide conversion is critical. Maybe this is the apples and oranges.
I believe there is a difference in performance concerning the snap temper. Some think it's risky to skip. I see no risk as long as care is taken in handling.
Example: drop it on concrete right out of the quench....will it shatter ? Maybe....maybe not.
Don't drop it.
The charts are standards, I won't argue that. What I will argue is quoting things for facts when new data hasn't been studied....or tested.
I have talked to several reputable heat treaters about the subject, most do a snap temper simply for insurance. They don't want to take a remote chance of ruining someones blade. Totally understandable. Doesn't mean it's the best practice for the individual who wants the best product possible.
 
Actually, the guy from Bohler-Uddeholm USA DID say that it mattered. He said that steels like M390 and Elmax show the real gains at around -120F and don't really need the -300F treatment. the bad news is that -120 appears to be about 20-30 dgrees cooler than what you can reasonably expect with dry ice.
JBS TOOLMAKER said:
Dry ice or LN shouldn't matter, as what we are doing here is mainly ending the conversion of Austenite, which happens at about -100F. The LN will help with some carbide conversion, but the austenite ended at -100.

When the concern is a cutting tool, carbide conversion is critical. Maybe this is the apples and oranges.
I believe there is a difference in performance concerning the snap temper. Some think it's risky to skip. I see no risk as long as care is taken in handling.
Example: drop it on concrete right out of the quench....will it shatter ? Maybe....maybe not.
Don't drop it.
The charts are standards, I won't argue that. What I will argue is quoting things for facts when new data hasn't been studied....or tested.
I have talked to several reputable heat treaters about the subject, most do a snap temper simply for insurance. They don't want to take a remote chance of ruining someones blade. Totally understandable. Doesn't mean it's the best practice for the individual who wants the best product possible.
Click to expand...
 
I thought the dry ice technique was supposed to be used with acetone? Is there much of a difference between using alcohol or acetone with dry ice?
 
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