Cryo treatment with dry ice recommendations needed.

[Bob Duguay]

So I want to start HT Aebl and would like to use dry ice for the cold treatment. Been searching around and can’t find much info on the recipe for the slurry mixture. I came across a post here that mention this:

“There can also be more error in DI slurry since it has to be made. Some folks don't add enough Dry Ice or Alcohol and also don't allow it the time to boil down to the coldest temp.
If the DI slurry is still boiling like when it's first mixed it's not at temp. You can also not add enough Dry Ice and it won't be cold enough.

Some folks don't always check the temp of their slurry to confirm, they just expect it to be "-130F"

So since I would like to be successful at this can someone please educate me to do it properly?
So far the closest place that sells it is 45 min away.
Thanks.

 

[samuraistuart]

I don't know what that statement is all about. Dry ice is dry ice. It is about -110°F. You could just cram the blade inside a large chunk of it and call it good, but I think it's better to mix it with something like acetone or, what I prefer, denatured alcohol. It doesn't take much dry ice, but where I buy it the dry ice is in 5lb blocks (which is PLENTY). I'll give you my process.....

Since where I get it they sell it in a block, I hammer it and break it up into smaller pieces in the bag it comes in. Doesn't have to be a powder, just break it up good. Poor it into an igloo jug. Add about a gallon of denatured alcohol slowly. Add it all at once and it will boil over, so slowly pour it in the container. Insert the knife into the slurry, lay the top on (don't screw it down....or you'll have a bomb on your hands), wait about an hour, remove blade and let it come to room temp, then temper. You only need to let the blade get to the dry ice temperature, there is no need to soak it. An hour is plenty.

Since you're 45 minutes away, just bring an igloo cooler or something to keep it stored in for the trip back.

 

[JTknives]

A lot of gas/welding stores sell dry ice in a pellet form. That’s the best form to mix with your preferred liquid.

 

[Richard338]

It is pretty easy to use. It is used in chem labs all day long. You just want plenty of liquid to cover the piece. Slowly add chunks of DI and it will boil less and less vigorously as the mixture gets down to temp. You want some extra pieces of DI in the mixture, with it already settled down, and you are ready. If the DI pieces disappear while you work, just keep adding a few small pieces. Once you are done and it comes back up towards room temp, you can recover your liquid for next time.

 

[JTknives]

+1^

very similar how you make In ice water bath for calibrating temp probes.

 

[Bob Duguay]

A lot of gas/welding stores sell dry ice in a pellet form. That’s the best form to mix with your preferred liquid.

I don’t have one in my area but I will check that out. Thanks

 

[Hubert S.]

I'm one state down from you in SC and down here they sell dry ice at Publix and Kroger. I think it is around $1.50 a pound. Kroger sells the pellets, but last time I bought them they still required a hammer to break up.

 

[Jason Fry]

I affirm what the other guys said. I bust it up into smaller chunks, throw the ice and the blades in a cooler/ice chest/igloo, and pour in enough kerosene to cover it all. I tend to leave it overnight, but there's no real reason to. It does need to get down to equalized cold in the slurry/ice/blade mix, but past that I don't believe the time is significant.

 

[Stacy E. Apelt - Bladesmith]

The blade only needs to stay in the bath long enough to drop to the bath temperature. 10 minutes is plenty long enough. Longer does no good, or harm, so leave it in if you wish.

Small chunks work better than powdering it, as the powder turns into gas almost immediately (causing the boiling and foam over). Let the bath sit for about five minutes to drop to around -100°F.

 

[Tyson A Wright]

...Longer does no good, or harm...

I was under the impression that all the steps in the heat treatment cycle should ideally happen with minimal pauses - the only reason to soak at a temperature was to make sure everything that happens at that temp has time to happen, but longer than that would be counterproductive. I would have thought that, as soon as optimal cold treatment has happened, it should come out, come up to room temperature, then go into tempering cycles.

This is based on reading, not experience! I'm a total newbie to all this, so I'm just trying to see if that idea I had was incorrect.

Thanks,

-Tyson

 

[Stacy E. Apelt - Bladesmith]

At -100°F a knife blade will be just sitting at the bottom of the quench curve. From there the blade is taken to the tempering stage. A long delay before the temper cycles could stabilize RA. Days/weeks might make some difference in RA, but hours won't.