Fluidized sand bath for tempering?

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Leon Husock

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Jul 17, 2019
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I've seen some stuff about fluidized sand for heat treating, which seems cool but not something I particularly want to tackle. But it got me thinking about using a tube of fluidized sand and a low temp propane burner for tempering, say something like a sword which is hard to temper with other methods. My plan was to take a piece of capped square tubing I have, drill a hole in the side at the bottom and run a piece of 3/8" pipe with a hole drilled in the top through it. Cap one end of the pipe and run low PSI compressed air into it to fluidize the sand, then point a propane burner at the tube and drop a thermocouple in the top. The goal would be to get it to hold around 425-450 degrees.

Does anyone have any experience with this sort of thing? Any reason this wouldn't work?
 
Any experience? Yes, but very little. Like I’ve used them once in school and mostly just remember fishing parts out of them. They’re pretty cool though. Very even heating and fast to heat the work piece. We were using them for copper alloys though.
 
Paragon makes a sand bath pot.

JT did a lot of experiments years back. I don't think anything came of it.

Search Fluidized Sand Pot using the custom search engine in the stickys.

Here's one:
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Review Thread 'WIP Fluidized Sand Pot'

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Hey folks Steel Pig Forge here,

Its been a long time since I have been on blade forums, but since I got the inspiration from here I thought I would give back a little bit.

Here is my fluidized sand pot build. Ill add pictures and descriptions and a parts list. Here is my warning though. This is serious stuff. Not to be played around with. I in no way am responsible for any injury or property damage that may come from someone attempting to replicate this build.

With that said, Ill be happy to answer any and all questions you have. Lord knows I had...
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Ok that is not what I was thinking of. The ones we used were fluidized in that compressed (hot) air was used to basically suspend the sand so that work pieces would sink into them. They were a low temperature tool for tempering or, in our case, annealing copper after cold rolling. They could not be used for austenitizing. Max temperature was in the 1200 deg F range. That there is terrifying. There wasn’t any bubbling or spitting to speak of.
 
Sand pots can be used for annealing, or tempering. It just depends on how high the heat is. The basic physics is the same.

Your mini-sand pot would have some issues. Read the treads on JT's journey getting the air flow and heating right. A torch on the side wouldn't work well.
 
Stacy E. Apelt - Bladesmith said:
Sand pots can be used for annealing, or tempering. It just depends on how high the heat is. The basic physics is the same.

Your mini-sand pot would have some issues. Read the treads on JT's journey getting the air flow and heating right. A torch on the side wouldn't work well.
Click to expand...
Gotcha, I was thinking it would function similarly to an oil temper tube (but less scary). I had skimmed the thread you linked but, I'll admit, my eyes glazed over at the complexity and I sort of assumed something that doesn't need to get above 450 also wouldn't need to be nearly so complicated.

My alternative idea for a sand-based temper was basically to stick a tube of sand into my heat treating barrel (55 gallon drum lined with kaowool) and run a low temp burner into it. The combination of barrel and sand would keep the heat even, and since I'm operating at relatively low temps the increased difficulty of getting a long blade in and out of the sand without fluidizing it wouldn't be super important.
 
The problem is that sand is basically a solid. You will have a very hard time pushing a sword down a tube of sand. Fluidizing happens when all the grains are in motion and act as a liquid. That takes considerable air flow from the bottom of the tube. With considerable air flow comes considerable heat loss .... you get the picture. Keeping it hot and keeping it moving takes just the right combo. A hose to the air compressor and a propane torch is not going to work well. And when we say "sand: we men silicon carbide grains. Grain size matters a lot, too.
 
Stacy E. Apelt - Bladesmith said:
The problem is that sand is basically a solid. You will have a very hard time pushing a sword down a tube of sand. Fluidizing happens when all the grains are in motion and act as a liquid. That takes considerable air flow from the bottom of the tube. With considerable air flow comes considerable heat loss .... you get the picture. Keeping it hot and keeping it moving takes just the right combo. A hose to the air compressor and a propane torch is not going to work well. And when we say "sand: we men silicon carbide grains. Grain size matters a lot, too.
Click to expand...
Hmmm you make good points. I don't have too much trouble getting a sword into a tube of sand just because it's pretty thin. But thinking about it, I could probably just use the same system I described above with the barrel furnace at absolute minimum temp and a tube to basically create an oven for heat treating, with the tube helping to prevent hot spots (the burner would go into the bottom of the barrel but go off at an angle and the flame wouldn't contact the tube). Could be the sand isn't actually necessary at all.
 
Many of us use a muffle pipe/tube for HT of long blades.
I have a stainless square tube with 1/4" thick walls that works great in the forge.
 
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