Talk to me about “dryo” treatment

[Drew Riley]

I know it’s been mentioned in a handful of previous threads, but I’d like to know more about everybody’s “dryo” setups.

I’m thinking of getting some AEB-L to play with and would like to cryo treat it myself after heat treat.

I’m leaning towards denatured alcohol and dry ice, unless there’s a better reason to use acetone.

Also, I’m thinking of a plastic igloo cooler or similar, but I was thinking of lining it with a steel pan or something more durable and then filling the voids with spray foam, or even just setting the pan on a styrofoam block in the bottom of the cooler. I’m thinking this will help keep the cooler’s plastic from cracking.

Is there a good ratio to use? If anyone wants to share their experience or setups, I’d appreciate it.

 

[fitzo]

No reason to stick to acetone. People use acetone, alcohols, kerosene, antifreeze, all sorts of stuff. It all gets down to the same temp.

Denatured alcohol may get pretty thick seeming, but not a worry. The proper ratio of dry ice to liquid is "sufficient dry ice that some is still left in the pan when you take the knife out." I add dry ice slowly to the pan of solvent, let it bubble and boil and do it's thing. Keep adding slowly until it no longer boils. Put your blade in. Add a bit more dry ice.

My "pan" of choice is a long lidded pot that you can cook a whole fish in. I put it on the floor and make the bath. It frosts. I finish the "cryo," fish the blade out, and leave it all to warm back to room temperature on the floor. Save the solvent for next time.

 

[Stuart Davenport Knives]

No issues at all with denatured alcohol. I use one gallon and about 5lbs dry ice for one knife (the minimum it comes in at the grocery store). If I'm heat treating a bunch of knives in a day, I will double up on both, slowly adding more dry ice as the next knife is ready for the "dryo". I just use a round igloo jug. I think it's 3 gallons. I sort of bust up the dry ice if it's a solid brick, put a bit of dry ice to begin with in the igloo, and slowly add the denatured alcohol. Watch the eyes, it is sort of a violent reaction. If I notice that the bubbling slows down a lot, or is not present at all, I carefully add more dry ice. I do mean carefully, small amounts. Don't drop a big chunk or it will bubble out of the igloo and most of your denatured alcohol will be all over the place. Ask me how I know that one! I have a wire that goes across the top, with little wires hanging off of the main wire where I just thread it through a tang hole. I don't let the tip of the knife rest on the bottom, always making sure the knife is free hanging. Hmmm, there is a joke in there somewhere. The knife doesn't need a "soak at temp" per se. It just needs to get to the temp, about -100°F. I let each knife soak about 30 minutes. After that, remove knife, let it come to room temp and start your first temper. I suppose I could keep the denatured alcohol in the igloo, but I always pour it back in the aluminium cans it comes in. Make sure that the dry ice has completely evaporated out of the alcohol before torqueing down on the lid, otherwise you've just made a bomb!

 

[Alex Topfer]

The one time I did it I used an old baking tray i didn't like. I used excess dry ice, i have no other use for it and it's pretty cheap
There are more sophisticated solutions, but if you're going to put a lot of effort into it why not just get a liquid nitrogen dewer?

 

[Drew Riley]

The one time I did it I used an old baking tray i didn't like. I used excess dry ice, i have no other use for it and it's pretty cheap
There are more sophisticated solutions, but if you're going to put a lot of effort into it why not just get a liquid nitrogen dewer?

I've thought about getting a dewar in the past, but the LN is the prohibitive part. Either it's hundreds of dollars to fill a dewar, or the local welding supplies act like they've never heard of the stuff before. Last time I asked for a quote for 5 or 10 liters of LN, they guy said he'd have to call me back. After calling one more time to touch base, I gave up. I think they just wanted to deal with the farmers and larger customers who are spending the big bucks.
Dry Ice and DA are relatively cheap and widely available, and I can get nearly the same results as the LN from what I understand. Besides, I really don't do a whole lot of stainless anyway, so I figured I'd just give this method a shot.

 

[Stacy E. Apelt - Bladesmith]

Same here, Dry Ice and Denatured Alcohol work fine for me. I have used the same gallon of denatured alcohol ($13 at HD) for years. The dry ice comes from the grocery store 5 minutes away. IIRC, the dry ice runs me around $10 to do a whole batch of blades.

 

[Ken H>]

"Dryo" - ok now I understand. Using dry ice & alcohol rather than LN. I used "Dryo" for several years until I got the dewar. LN is cheap where I get it, started at $5 to fill a 10 liter dewar, but a yr ago the price increased to $8 to fill the same 10 liter dewar. 10 liter dewar tends to last around 6 wks, so LN is actually cheaper than dry ice for me. Plus it seems to get me about 1 extra Rc than DI does.

 

[Seedy Lot]

I've thought about getting a dewar in the past, but the LN is the prohibitive part. Either it's hundreds of dollars to fill a dewar, or the local welding supplies act like they've never heard of the stuff before. Last time I asked for a quote for 5 or 10 liters of LN, they guy said he'd have to call me back. After calling one more time to touch base, I gave up. I think they just wanted to deal with the farmers and larger customers who are spending the big bucks.
Dry Ice and DA are relatively cheap and widely available, and I can get nearly the same results as the LN from what I understand. Besides, I really don't do a whole lot of stainless anyway, so I figured I'd just give this method a shot.

Maybe try a medical supply store for LN2. I live in a fairly isolated area but between ranchers freezing semen and the hospitals I can get LN2 with little issue from the one welding/medical supply store.

When I first called them up Norco asking about getting a small amount of LN2 they quoted me a ridiculous price but when I showed up in person with a 10L dewar the price was reasonable.

 

[JoeBusic]

I don't use alcohol at all. Didn't notice any advantage, but used alcohol before. I sandwich the knife between Al plates just a bit larger and put it in the DI container. Pull out an hour later. Al plates spread the temp evenly very fast and there is no shock. Just in case, clamped knife is the freezer for 10min before dumping it in DI to make the quench slower and save some ice.

 

[Ken H>]

No reason to stick to acetone. People use acetone, alcohols, kerosene, antifreeze, all sorts of stuff. It all gets down to the same temp.

fitzo, speak to us about the difference in using a liquid/dry ice slurry vs just plain chunks of dry ice.

 

[fitzo]

fitzo, speak to us about the difference in using a liquid/dry ice slurry vs just plain chunks of dry ice.

Ken, I keep wondering about the same thing. It's all about complete contact. There is no reason a blade buried in powdered dry ice would be any different than a slurry. Chunked too large it may not contact everywhere the same, so that may be an issue, but I even wonder about that. My biggest concern might be that uneven chilling may cause warpage in the warpy alloys like AEBL.

We always used DI slurries in the lab, but then we were working with liquids and laboratory glassware and it made sense. I'm thinking powdered DI would work fine. I would gladly run the experiments. Unfortunately, I am a little out of the shop for the (near) future. (Turned out that "No xxxxxxx" diagnosis they gave me was incorrect.)

 

[Ricky-O]

Is it important to get an insulated container for the alcohol and dry ice, such as an igloo, or can any container work? For example, would a 4" PVC pipe work?

 

[Stacy E. Apelt - Bladesmith]

I remember this from my days at Virginia Chemical. We had a cryo plant. In the case of dry ice, heat does not transfer the same amount from a gas atmosphere as it does from a liquid bath because the temperature change is caused by the sublimation. Sublimation is endothermic, and draws energy (heat) from surrounding matter to fuel the phase change. If the blade is in liquid and the liquid has the dry ice in it the heat is energy is drawn from both almost equally and both will be at the sublimation temperature. If the sublimation happens in air, the heat comes from the air and gaseous CO2 is released. The blade does not transfer as much heat to the air as it did to the liquid (gases are good insulators). Thus, if only gaseous air and CO2 touches the blade the blade temperature will be higher than in the liquid that has solid dry ice sublimating in a gas. Even when the blade is in direct contact with solid dry ice it is actually only in contact with a thin layer of gaseous CO2. The heat transfer is still mostly from the air.

 

[Stuart Davenport Knives]

Ricky-O, the igloo (or similiar) will keep the slurry cool for longer than a PVC pipe. But with that said, the goal is just to get the knife to -100°F. It does not need to soak, other than to make sure the knife is at the temp. So, no, I don't think an insulating container is necessary, unless maybe you have quite a few knives to heat treat. In that case, you would want to keep the slurry cool as long as you can. Or just keep adding more dry ice to your PVC.

 

[fitzo]

I remember this from my days at Virginia Chemical. We had a cryo plant. In the case of dry ice, heat does not transfer the same amount from a gas atmosphere as it does from a liquid bath because the temperature change is caused by the sublimation. Sublimation is endothermic, and draws energy (heat) from surrounding matter to fuel the phase change. If the blade is in liquid and the liquid has the dry ice in it the heat is energy is drawn from both almost equally and both will be at the sublimation temperature. If the sublimation happens in air, the heat comes from the air and gaseous CO2 is released. The blade does not transfer as much heat to the air as it did to the liquid (gases are good insulators). Thus, if only gaseous air and CO2 touches the blade the blade temperature will be higher than in the liquid that has solid dry ice sublimating in a gas. Even when the blade is in direct contact with solid dry ice it is actually only in contact with a thin layer of gaseous CO2. The heat transfer is still mostly from the air.

Technically correct, but, IMO, splitting hairs. In the real world, I doubt it will matter, especially if it's in pulverized dry ice. Be an interesting experiment, though.

 

[Ricky-O]

Ricky-O, the igloo (or similiar) will keep the slurry cool for longer than a PVC pipe. But with that said, the goal is just to get the knife to -100°F. It does not need to soak, other than to make sure the knife is at the temp. So, no, I don't think an insulating container is necessary, unless maybe you have quite a few knives to heat treat. In that case, you would want to keep the slurry cool as long as you can. Or just keep adding more dry ice to your PVC.

The reason I asked is that 3 gallon igloos have an inside depth of about 10 inches, but what if I want to make a knife with a blade length of about 15 inches. Even on a standard chef knife the steel might be 13 inches long, which wouldn't fit in a 3 gallon igloo, although I think they'd fit in a 5 gallon igloo; but that's a lot of alcohol.

 

[Ken H>]

especially if it's in pulverized dry ice.

Pulverized to being like sand would help, but I've never done much more than small pebble size chucks of DI. That size certainly does need a liquid to help with heat transfer. A temperature probe gets a good bit colder in the DI/alcohol slurry than just chunks of DI. Been a long time since I tested DI with a temperature probe.

 

[fitzo]

Pulverized to being like sand would help, but I've never done much more than small pebble size chucks of DI. That size certainly does need a liquid to help with heat transfer. A temperature probe gets a good bit colder in the DI/alcohol slurry than just chunks of DI. Been a long time since I tested DI with a temperature probe.

I am thinking of this in terms of the blade. There are several relevant points that have come to me, thinking of this more specific to knives. How cold does the neat dry ice get a buried blade? Does the temp lowest temp achieved pass beyond Mf? Does the likely gentler cooling slope without the slurry affect the conversion? How much retained austenite remains with the neat versus slurry cold treatment? Another question that arises may be whether, with let’s say -70C versus -78C, if the difference is significant, at which point does one say it’s time for LN?

A lot of ifs, should one not want to make a bath. An interesting metallurgical question otherwise. I’ve used pulverized in the lab to get <-70C, I remember, but that was a tad more scientific and tech than a knife making shop. Sounds like a nice paper for Larrin.

 

[Alex Topfer]

The reason I asked is that 3 gallon igloos have an inside depth of about 10 inches, but what if I want to make a knife with a blade length of about 15 inches. Even on a standard chef knife the steel might be 13 inches long, which wouldn't fit in a 3 gallon igloo, although I think they'd fit in a 5 gallon igloo; but that's a lot of alcohol.

Those containers are something like polystyrene sandwiched between layers of plastic. So you can insulate the pipe if you need it to stay cool for longer.
A meter of 150mm pipe is ~18L (~4.5gal?), and that's kinda overkill. Half of that is probably enough, or 75mm pipe if you're doing bundles of rapiers

 

[tattooedfreak]

The reason I asked is that 3 gallon igloos have an inside depth of about 10 inches, but what if I want to make a knife with a blade length of about 15 inches. Even on a standard chef knife the steel might be 13 inches long, which wouldn't fit in a 3 gallon igloo, although I think they'd fit in a 5 gallon igloo; but that's a lot of alcohol.

Dont forget that it wont be straight up and down, the diameter is probably 11-12 inches, which would give appx 14.5 inches of coverage on angle, while not ideal, its pretty close. However, why not skip all that hassle and get a styrofoam cooler that has the length you need, you only need to fill it up high enough to cover the blade laid on the spine.
Does the blade have to be straight up and down? Pretty sure it doesnt as long as enough coolant get reach all areas correct?