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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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  #1  
Old 12-31-2009, 11:25 AM
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Question Liquid Nitrogen questions

Ive got a source, now I need a dewar - what size should I be looking for? How much capacity is enough?

Once I get one, do I just tie a SS wire around the blade and submerge it in the liquid or above it or how exactly do I introduce it into the dewar?

Thanks gentlemen-


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Old 01-06-2010, 08:38 PM
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You can go about the dewar one of two ways...... Either purchase a small one, that only holds enough Liquid Nitrogen to do one blade at a time. Even when sealed a small dewar will only last a few days before it all boils off. A larger dewar will last longer, but you also have the added expense of refilling it with a larger amount. The other thing that you have to think about is that most dewars have very small necks, and you'll likely wind up getting whatever one has a large enough neck diameter to accommodate the blades you intend to put in it.

My setup is as follows: Dewar that holds 56lbs of liquid nitrogen...its about 1/2 again the diameter of a 5 gallon bucket, and about 6" taller. The neck is 1 3/4" dia. When I first got mine, a fill would last about 1 1/2 weeks before it was all gone.

To prolong the nitrogen I built a plywood box, insulated with 1 1/2" foam insulation, including a lid that was lockable because at the time my daughter was about 8 years old. The box is just the right size for the dewar to fit into, and once it's in there I fill the air space with vermiculite. With this setup a fill will last about 3-4 months.

To use it, drill a small hole in the tang of the knife blade, run some SS tie wire through and secure it well. Gently lower the blade into the Nitrogen (make sure you have on welding gloves and a face shield!) Place the plug back in the dewar and leave the blade for at least 4 hours. Once the time has elapsed, remove the blade and place it between a couple of pieces of ka-wool and allow it to warn up to room temp slowly. I've had blades literally shatter when I screwed up and laid the blade directly on a steel workbench! Once its back to room temp, retemper the blade at 25F higher than previous tempers. Afterward you can finish as usual.

WARNING: Be very careful when dealing with liquid nirtogen! It will literally boil as you put the blade in, and if even a tiny drop hits your skin, it's like having a white hot piece of steel on you!


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Old 01-07-2010, 12:12 PM
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Awesome info, Ed! Thanks!


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Old 01-08-2010, 11:39 AM
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Yeah - Thanks Ed!

Could you just hang it up with the wire in room temperature to cool as opposed to in kaowool?


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Old 01-09-2010, 08:21 AM
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You could, but it's "chancey". When I first started with cryo, I had several blades that cracked when I took them out of the dewar and placed them on an aluminum rack to warm up. Most of them I got away with, but enough cracked that it made me rethink what I was doing. On a few blades, the cracks were so minor that I didn't find them until I was hand finishing....and I saw what looked like a fine hair on the blade....but I eventually understood it was a crack when I couldn't sand it out.
I went to putting the cyroed blades in ka-wool, and haven't experienced a crack since.


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Old 01-09-2010, 09:45 AM
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I use the same method as Ed when I use LN.

But I lay the blade on a wooden workbench to warm up. Never had a blade crack.

My dewar is set up the same way except my cannister was placed in the box, then expanding foam was sprayed into the box until it was full. It's in there permanently. Until the box was made I worried a lot about having the dewar turn over in the car and spilling the LN out into the car.

I seldom use the dewar now because I have to drive about 60 miles round trip to fill it. I only make one or two folders at once, so I use dry ice from the grocery store or Walmart (if they have it).
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Old 01-09-2010, 11:57 PM
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Good info. I have a Dewar that I have not used yet but will soon need. I think mine is double walled aluminum and about the size of an old time milk can. It has a vacuum between the walls. If a pointy blade got dropped into it wouldn't that punch a hole in the inner wall and ruin the Dewar? Any way to prevent this from happening?
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Old 02-12-2010, 09:47 PM
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One really easy way is to make a basket that just fits inside the neck and goes almost to the bottom. This way if you drop a blade it will stop at the bottom of the basket.


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Old 02-13-2010, 07:40 AM
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My dewar came with a wire mesh basket that hangs inside.
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