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Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by JTknives, Aug 3, 2016.
JT - you're doing good work!! Looking forward to your testing. I think this is going to be GOOD!!
I'm dying to know -- has there been any progress on this? Anyone else attempt to build a fluidized sand bed?
I got the parts all ready to finish up. But buying a house and building a shop go in the way so the build was put on hold. But the shop is almost done and once it is I will be finishing this prototype up rather quickly and I will keep everyone informed.
When I first heard about salt baths about a month ago, I thought to myself, why not use sand? It went on my to-do list, and now this thread pops up out of nowhere. Glad to see great minds think alike!
Got WAY too much on my plate at the moment (like a belt grinder build), so I will join this quest when the time is right.
I'm waiting on this one too... it's way more appealing than hi temp salts, and more versatile as well. A person could austenitize, temper, and perhaps even quench some steels in the same type pot. It could be used to make bainite etc. And no danger of molten explosion, no salt corrosion, no adjusting chemistry, no futzing with tapered stainless rod when cooling, none of that.
It is a very interesting subject for sure.
Do you have some comparison figures about the heat transfer rates between athmosphere, fluidized sand bed and high temp molten salt?
From what i have seen, the main differences between heat treating with salts and sand are all about heat rates and oxydation....plus safety issues, the latter being one of the main reason why the average shop would tend toward sand.
Let me dig out my book and I can get you all this data. I had it when I first designed this sand pot but I can't remember it off the top of my head right now.
I made a small version and it works . But weed burner I use was weak .It take long time to reach 800-820 degrees Celsius .And it was the maximal temperature I achieved . But it works perfectly... From two inches above the bottom /Right above the place where the flame from burner enters/ and to the top of sand I have stable/constant temperature . Then I discovered that without sand I can quickly warm up and maintain a stable temperature of 800 degrees ......... Now I'm working on electric version with 4 inch tube , but without improvisation .Much safer , no more that scary sound of burning gas ....Sorry I have just a few pictures .. . The first one is during construction .
I think this is the right thing
Without sand, forced air and being electric you have just made a muffle heat treat oven. You have to have air forced through the sand from the bottom to fludize it as well as create the even temps. The "boiling" effect of the sand is what creates the even temp. By even temps I'm talking within +- 3°C. The tricky bit is injecting the gas "air" just right so it creates and even bubble matrix. The other real tricky bit is as the bubbles forum at the gas ports aka jets or bubblers you don't want a pressure drop in the air supply. This causes problems with the sand chugging which is the sand going up and down and pulsing. The. After that you need to design the jet so the sand can not get into it and weep back into the manifold. This is actually rather tricky to do and even proper systems that use bubble caps that are shealded can weep. It has to do with the drop in pressure when a bubble forums and the surface collapses and kinda shoots sand twords the zone of low pressure. I was looking at X-rays last night of bubblers that had this problem. Was cool to actually see the sand in action and caught in a picture. Any way I have a few ideas on how to deal with these things in a sand pot. I hope to be back on build soon.
Oh and about the heating rate of sand vs salt, it's rather quick. MUCH faster then just a atmospheric heated forge. According to the chart a 1" round steel bar will heat to 1830° in around 6min from ambient temp. But the same bar heats to the same temp in salt in around 4min. Compare those to a atmospheric furnace and that is 14min. If your wanting actual numbers here is what the experts hav to say. "Relative heat-transfer coefficient range for various furnaces"
salt bath = 500-1200 W/m²·K
fluidized bed = 500-700 W/m²·K
Forced circulation furnace = 150-200 W/m²·K
So as you can see salt is fast but so is sand when compared to a normal furnace.
I work on electric sand pot JT .On picture is also with AO sand and with forced air . Air regulation was not a problem, I have a precise tool for that. Little tricky was to find how much jet and size of jet ..... I think I've experimented enough and that the electrical will turn out properly .
I haven't been around here much for quite a while, but I'm back to keep watching this thread... Any new progress?
Yeah really, Id like a winter project. This looks doable for me
getting closer guys, i will keep you updated
I look forward to your progress!
I've ordered parts to put one of these together myself, just waiting on a few things to arrive. Hopefully I'll have something to contribute when I get around to assembling it.
Bump. (No offense Bruce.)
Slight update, I've got a piece of 316 stainless pipe and most of the other things I will need. I'm at the point now where I just need to tig the pot together, and pick up refractory to line the shell.
Since its going to be electrically heated, I've needed to figure out a way to secure the elements. Looking at my evenheat it looks like grooves which the elements are just pressed into. My idea was to line the shell with a couple layers of inswool, and then make a cardboard form for the inside cavity, with a spiral the size of the element made down the outside with twisted paper or something along those lines. Slide that down inside with about an inch of space between it and the wool, and pour in a locally available equivalent of Mizzou castable refractory. If it works as it does in my mind I should be able to go in with the cutting torch and a real oxidizing flame, and burn all the cardboard and paper out once it is cured, leaving a groove for the element. Now I'm sure there would be some carving and patching involved to get it to fit perfectly, but does anyone think that might work or am I just completely insane?
That should work as long as it's stiff enough to hold form when you pour. I'd probably want to do a cardboard tube wrapped around a PVC pipe or something to be sure as the cardboard is going to suck up some moisture and lose rigidity. But then you could slide the PVC out easily and burn the cardboard out like you planned.
ETA also I don't know if you planned 120v/240v but figure that out so you can plan coil starts/stops or whether you want a 2 lead spiral, etc.
Good idea on the PVC. I've picked up the components to wire it 240v, but I still have to do a bit more reading on how it all goes together before I pour the liner.
While I haven't cast a round liner for coils ( yet), I do cast round liners.
First, if you cast refractory in a tube filled with wool, it will crush the wood down a lot. you need an inner sleeve of sheet metal tubing inside the wool. Stove/vent pipe works great. Wrap the stove pipe with the wool and slide the assembly in the shell.This is your insulation layer. Cast the refractory liner on that.
The inner core mold needs to be strong, too. Wrap heavy paper around a piece of appropriate size PVC pipe. Use masking tape to hold it snug. Use sisal rope for the spiral groove model material. Tack it in place with Tightbond ( or similar) wood glue before pouring the castable refractory. Place the core mold in place and space it with some small blocks of wood on each end.
IMPORTANT - The tube needs to be sealed on the down end when pouring the refractory, or the refractory will blow out and end up all over the floor. Pour about 2" of refractory in the space brtween the core and the insulation section and let it cure for a while until it is solid. Then mix and pour the rest. When the refractory has cured for a few days, pull out the PVC, then rip out the paper. If the rope comes out easily, pull it out. If not leave it in place to burn up in curing the liner. Fire the liner with any method you wish to cure the refractory. I would start with a torch and dry it well, then fire at 1000F or higher to finish. I have heard of people filling the tube with burning charcoal, adding more as needed for an hour or so, and letting it burn out overnight. I haven't tried that, but it seems sensible to me.