Sand Pot, could it be a salt replacement.

JTknives

JTknives

Blade Heat Treating www.jarodtodd.com
Joined
Jun 11, 2006
Messages
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Ever since I watched the peters heat treat vid I was fascinated with fluid bed heat treating. I have been kicking around some ideas and being that the heat treat oven is just about done I guess that means it's time to plan the next project. I had acquired all the stuff for a salt pot but decided to do an oven. But now I was thinking and think a small scale sand pot could work. And I have just about everything to test it out.


My thought is to take my salt pot tube and weld a base on it and then use 316 stainless tubing and weld it to the base. Cut a hole in the top of my verticle forge cover plate and suspend the tube just like you would a salt pot. My thought is to maybe coil the 316 tubing around the sand pot and then come up and out the forge top. I have about 35gal of silica which is just refined silicone dioxide aka sand. It has a melting temp of over 3000°. The stuff I have is very very tini balls, lab grade silica.

So I would fill the tube with the "sand" and fire up the forge and start heating the sand tube. Next I would pump gas into the 316 tubing which causes the "sand" to bubble and create a very even heat. The gas can be nitrogen like peters heat treating uses or argon or if you don't mind scale it could just be air from a compressor. My first test will be from an air compressor and if it works I will go with a shealding gas. With nitrogen you now have an environment that will not oxidise the blade. The gas will need to be regulated to a low flow so you eather don't create a flaming shotgun air cannon and also don't cool the sand to much. But if the gas going down the tubing has enough time to warm up before entering the sand pot I think this will be mitigated.

So what's your thoughts, am I bat shit crazy?
 
Picture of the sand pot from peters heat treat

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If the melting point of the sand is over 3000 F. what are you going to do with it? That's way too hot for anything knifemaking related.
 
Unfortunately Im at work and can't see the picture. You aren't melting the sand right, just using the forge to get it to whatever temp you need? You will obviously need a temp gauge of some sort, possibly more than one to make sure the sand is at even temps throughout. I understand the thought process behind it, I just question the gas flow process and having even heat at temp.
 
I see now....you're wanting to use the sand in solid form....before it melts. Interesting idea.
 
i use a sand pot for tempering, but it is inside an oven and just keeps the blade at even heat.
i see keeping uniform heat a major issue, i don't think just injecting gas is going to circulate the sand enough. what is advantage of this scheme vs. using a muffle inside a regulated gas or electric furnace?
scott
 
I was wondering how that sand setup worked. I was talking a while back with the guys from Metallurgical Solutions in Providence,RI. i asked them"why Providence?" and the guy told me that RI was he only state in the immediate area that would approve a major molten salt setup. It sounds like the sand setup is a "safer" replacement for salt, or even worse the old school lead/mercury setups like you see on the videos from Sheffield and Solingen.
 
I am only familiar with sand as a low temp medium. I had not read about its use for high temp austenitization. Maybe I'll call Brad and ask.
 
OK, just had a nice chat with Brad at Peter's Heat Treatment.

The sand pot is called fluidized bed technology. It was invented by Sunbeam in Meadville ( the same town as Peter's), back in the 1970's.
I knew of fluidized solids from some research on soil becoming in a fluid during an earthquake. Many people don't know that a particularized solid like sand is nearly as hard as concrete when it is contained in a chamber ( like a tube). You can not insert a knife very far before the sand acts as if it was solid rock. The sand needs to become a fluid to allow the blade to enter. This is usually done by vibration, but in the case of the HT sand pots it is done by passing a gas up through the sand column.

The "sand" - The sand is a pure grade of 120 grit aluminium oxide. As said earlier, the meting point is 3000F. It also does not react with air and other gases, so it stays pretty much the same at high temperature. At 1500-1600F it looks like glowing lava, but is still separate particles.

The pot - The pot is a 12X24 inch chamber with a porous bottom and open top. The bottom of the sand tube is made from a diffusion plate. This is basically a plate with many tiny holes that are smaller than the sand. This allows gasses to be pumped up through the sand .. more later on that.

The heat - Peter's and other industrial users have the pot heated bu electric coils. Brad said that there is no real reason that it couldn't be done with a gas burner on a small setup.

Control - There is a TC stuck down in the sand in the area where the blade will be. It controls the heat sourc to maintain the sand at temperature. It is very tight control, and drift is small.

The gasses - Four gasses are used at Peter's.
Air is used to fluidize the sand for insertion of objects , cooling, and for some specific HT results. For a knife maker, it would be used to make the sand act as a fluid by bubbling up through the sand.
Nitrogen - Nitrogen is used for oxygen exclusion to lower decarb and scale. It is pumped up from the bottom at 150-170 CFM through the pot.
Propane - Yes, propane! This is used to add carbon in carburizing tools and parts. This would not be a hobby technique or really of use to knifemakers.
Ammonia - It just gets weirder, doesn't it. Ammonia is pumped through the sand for nitriding of a blade or object. This coats the blade surface with a hard protective coat of nitride. Ammonia is very poisonous and this should not be tried at home.

The big advantage - Safety! The pot is not molten, so it won't flow.
Other advantages - The sand is not corrosive or soluble, so it does not need much care or maintenance. The sand will not react with water. Even a drop of sweat will make a mini-eruption in a salt pot. Brad said he has poured a quart of water in a sand pot to show how safe it is ... all that happened was a big puff of steam. Clean up and maintenance is simple. Cool things down ( they pump air through once down to 1200F to cool faster), dump it into a screen to sieve out any debris or lost knife blades/parts, make any needed repairs to the pot, refill ... ready to go. Clean - the sand does not coat the pot and everything around it like salt does.

Disadvantage - Not many. Mostly that there is a certain amount of drag out and sand gets all around the floor in the area. Just sweep/vac it up and toss in the trash.





From me:
With all this, I can see the use as extremely good for an advanced skill knifemaker. I have a gas fired salt pot under construction that will likely be changed to a sand pot.
While the diffusion plate wouldn't be all too hard to set up, a 1/4" stainless pipe down the side with upward facing jets at the bottom would work for our purposes. I think that pumping nitrogen or argon at the amounts Peter's uses will be an economic problem for home HTers.
 
Yes JT, you are batshit crazy. But that is OK in my book, because this is extremely cool! I'm interested to see how it will turn out!
 
I agree that JT is batsh!t crazy, and has the tests to prove it. In this case, he may have actually hit on a neat idea.

Now, .... if he can finish the other projects first ... that remains to be seen.

JT, send me a PM and I will pass on a few more things.
 
I have a couple of friends who work for a company that casts high detail engine parts. I've toured through their factory and they have fluidized sand beds that they use that are the size of a roll off dumpster with a hydraulic lid to help keep the heat in. It's a really amazing setup and and slightly intimidating to be looking down on hot fluidized sand from the catwalk. Theirs is not near this temp as it was not red hot.
 
Wow I was expecting to wake up this morning and have replays like "you'll shoot your eye out" lol

Yeah after after watching that video over a year ago maybe 2 I was mesmerised by it. Figured there had to be a way to do it small scale. I had thought about using a screen but figured a tube inserted into the bottom of the pot would be simpler and more robust for us simple folks. My only concerns where cooling the sand to much with the air injection but if the tube is exposed to the inside of the forge for a sec first I think it will preheat the gas a good bit first.

Peters in the video said he uses nitrogen but I would imagine thy make there own nitrogen on site so yeah you would need a lot of it. Electric heat I think for a big shop like there's is great but for a small scale shop like ours gas fired is quicker. You are able to pack a lot more btus into the tube faster with propane then electric so it should be more small scale friendly. What I think is brilliant is I think the gas solves another hidden problem. In a verticals forge the bottom is always hotter then the top. But pumping the gas in at the bottom will draw heat from the hotter area and rise up and heat the cooler upper section of sand.
 
Would their be any real need to diffuse gas through the pot throughout the entire process? Or just to get your blade in and out? Edit: Assume you're not trying to do any gas shielding and just using compressed air.
 
Something I hadn't heard of, very interesting. I've also got everything for a high temp salt pot setup I've been meaning to finalize for a while. I'll be watching this thread.
 
I'm with many of you, having never really heard of his before. I'm very intrigued!
 
Ok guys the prototype is done.

Started with a thick wall stainless tube
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Then drilled a hole in the base end of the tube to take the gas inlet tubing.
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Then I welded on the base cap, man I love welding stainless.
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Tube was inserted into the base about half way and bent up to run along the side of the pot.
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Cut a hole in the top of the vertical forge to holt the pot.
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All together with the ball valve and needle valve. Trust me the needle valve is a MUST, see the sand everywhere facepalm.
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It takes VERY VERY little gas flow, the needle is just cracked.
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Befor we go any further we must talk SAFTY. This set up is much safer then a salt pot but still has potential to hurt you. So thing I learned right away was you must have the air flowing befor heating the sand. I don't know if this is becaus of the silicon dioxide I used but until I test other sands I will say it's a must. What happened to me was I got the sand heated and cracked the air and this was befor the needle valve. No bubbling at all, then all the sudden all the red hot sand shot out of the tube and came raining down. The base of the sand close to the air had clumped from the heat held the pressure and then let go at once. This clump hit the roof and landed on my head and my arm. Which was immediately brushed off with ease. If this was a salt pot my shop would have been on fire and I would be rushing to the emergency room. I suffered mild burns on my arm and nothing more. But it was a real good lesson to learn and I will be looking into why it happened. One problem I think I have is this silicon dioxide I have is actually very very fine silica gel which is designed to absorb moisture and I think that caused the caking. I have had This drum of silica for a very long time and it had absorbed a lot of moisture I'm guessing becaus as it was heating up it was boiling and if I tried to add more it would shoot out. So we will not be using this stuff any more and will buy some aluminium oxide sand blasting media.
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Here is a little peak inside the sand pot showing its glowie goodness
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After running this test I discovered a few things, first the air is a must thought the entire time it's running. The air or gas creates a VERY even heating inside the entire length of the tube from bottom to top. If you turn the gas off then the lower section heats up much hotter then the top. Also it uses so little gas that I would not consider nitrogen or argon gas injection out of the possibility of being practical. I don't think I would the shealding gas the entire time. But you could do a switching valve setup that allows you to run with air while heating up the sand and then befor you insert the blade switch over to shealding gas for the blade heating. With the sand it takes very little time to heat the blade so I think very little gas would be used. I think if a cone was welded or added to the top of the tube then the sand would stay in the tube. It does poof out a little and after about an hr run time I lost about 2" but my sand was flower fine.

I compiled a video showing it working becaus the proof is in the pudding. I think it needs some refining and with all your guys input I think we can design a very good new heat treating option for home shop knife makers that is much safer and just as good as salt pots.

https://youtu.be/cMyCGiIj1f0
 
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I saw the vid. That is extremely cool. I believe you have me sold on it, instead of salt I may rig a long tube of sand down into my sword HT forge. The shielding gas option for a primarily air rig sounds indeed like the way to go.

Gnarly about the burn. You're right, if it was salt your shop might be toast and you could even be dead. This highlights the much greater degree of safety with sand. I guess it would be no problem to have high and low temp sand pots, either.

With the air, did you get a lot of scaling on the steel?
 
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