Pottery kiln for HT

[jdm61]

I was dropping off my Paragon 24 incher for repair and the guy doing the work had a number of kilns in his shop of varying sizes. I eyeballs one that was a standard smaller top loading pottery kiln with a 22.5 inch inside depth and maybe 12-14 wide. I have seen some guys use ones like that for tempering (thinking of James Helm) and I would guess that you could do the bit where you cut a slot in the top and hang say hawks down though the slot and "seal" the top with firebricks. Lets go a step further. Could you build a salt system out of such a kiln so that you could use a last tube larger than the typical 3-4 inch diameter? I ask because I can get a used kiln like this rebuilt to as new specs by this guy with a one year warranty for around $800.

 

[Stacy E. Apelt - Bladesmith]

The problem with converting a kiln to a high temp salt pot isn't the kiln …. it is the controller circuits needed. It is a lot more complicated than just sticking in a salt tube and dropping a TC in the salt.
The Evenheat salt pots are not cheap, but the engineering and control circuits ( and multiple fail safe features) are worth it. Look at the control features and you will see why:
https://usaknifemaker.com/evenheat-sb-718.html
A complete setup with tube, salts, and accessories will run around $2500 delivered.
For a high temp pot, you want a 4" to 6" tube, preferably with extra heavy wall thickness ( schedule 80). It needs to be stainless steel and the welds need to be professional grade with pre/post heating. You can get away with a much less robust pot for low temp salts.



I have converted a kiln into a low temp salt pot with good results. This will work fine with blades austenitized in a normal HT oven.

Here is how I converted a small kiln to a low temp salt pot:
The kiln was about 24" across and 16" deep. The chamber was about 16X8".
I make a 4" wide heavy wall stainless pot 16" deep.
I cut a hole in the kiln top that the tube would pass through.
I made a collar of 2" inswool to fit around the tube on the top. Basically a 10" circle with a 4" hole in the middle. I also cut a 6" circle to place over the pot top while in use.
I cut a 4" hole in the center of a piece of 10X14" stainless 20 gauge sheet metal. I bent the long ends down as 2" legs, making a 10X10 table to sit over the wool collar.
I left the old mechanical controller in place and set it on HIGH/MAX. I made a separate controller box with a ramp/soak PID and wiring a socket to the SSR controlled by the PID. The kiln power cord was plugged into the socket. This made the PID control the kiln. The same controller box could be used to control a toaster oven or other low temp device like a quench tank.
A monel clad type J TC was in the low temp nitrate salts. The one I got from Ebay came with a 90° bend near the wired end. It was rated to 700F.
The PID was programmed to start by slowly heating to 100F, then 200F, then 300F, and finally to 400F ( or the desired target). It takes about 1 hour to do this evenly and safely. You don't want to start with the kiln trying to heat the pot of solid salt directly to 400F or you may get a volcanic eruption.
I filled the salts to about even with the top of the wool collar on kiln top. This put about 10-11" of salt in the tube.
The salt pot worked fine for quenching, but wasn't particularly better than oil, with one exception. You could hold the blade in the pot at 400F for any reasonable time. You can straighten any warp or twist easily, as the blade is still rubbery soft austenite. Just put back in the pot as it cools a bit, wait a minute, and pull out to straighten more. Once it is straight, just hang in air and let it harden.
It was good for tempering, but not particularly better than an oven.
It was good for heat bluing fittings and damascus blades. A quick dip of the blade at 475° to 500° won't affect the temper. You get a very nice rich blue color.
I got a rubber cap that fit the tube for when it was not in use. The salts are very hydroscopic, and will absorb water from the air if not kept tightly sealed. Any absorbed water will steam off as the salts slowly heat, which is why the slow ramp rate is required.

 

[jdm61]

Well, the problem is that I wouldn't want a 4-6 inch tube. I would want an 8-9 inch tube. A low temp setup would be nice for making lower bainite. So what about modifying the kiln to use as a regular "atmospheric" furnace with the slot in the lid for the high temp application?

The problem with converting a kiln to a high temp salt pot isn't the kiln …. it is the controller circuits needed. It is a lot more complicated than just sticking in a salt tube and dropping a TC in the salt.
The Evenheat salt pots are not cheap, but the engineering and control circuits ( and multiple fail safe features) are worth it. Look at the control features and you will see why:
https://usaknifemaker.com/evenheat-sb-718.html
A complete setup with tube, salts, and accessories will run around $2500 delivered.
For a high temp pot, you want a 4" to 6" tube, preferably with extra heavy wall thickness ( schedule 80). It needs to be stainless steel and the welds need to be professional grade with pre/post heating. You can get away with a much less robust pot for low temp salts.



I have converted a kiln into a low temp salt pot with good results. This will work fine with blades austenitized in a normal HT oven.

Here is how I converted a small kiln to a low temp salt pot:
The kiln was about 24" across and 16" deep. The chamber was about 16X8".
I make a 4" wide heavy wall stainless pot 16" deep.
I cut a hole in the kiln top that the tube would pass through.
I made a collar of 2" inswool to fit around the tube on the top. Basically a 10" circle with a 4" hole in the middle. I also cut a 6" circle to place over the pot top while in use.
I cut a 4" hole in the center of a piece of 10X14" stainless 20 gauge sheet metal. I bent the long ends down as 2" legs, making a 10X10 table to sit over the wool collar.
I left the old mechanical controller in place and set it on HIGH/MAX. I made a separate controller box with a ramp/soak PID and wiring a socket to the SSR controlled by the PID. The kiln power cord was plugged into the socket. This made the PID control the kiln. The same controller box could be used to control a toaster oven or other low temp device like a quench tank.
A monel clad type J TC was in the low temp nitrate salts. The one I got from Ebay came with a 90° bend near the wired end. It was rated to 700F.
The PID was programmed to start by slowly heating to 100F, then 200F, then 300F, and finally to 400F ( or the desired target). It takes about 1 hour to do this evenly and safely. You don't want to start with the kiln trying to heat the pot of solid salt directly to 400F or you may get a volcanic eruption.
I filled the salts to about even with the top of the wool collar on kiln top. This put about 10-11" of salt in the tube.
The salt pot worked fine for quenching, but wasn't particularly better than oil, with one exception. You could hold the blade in the pot at 400F for any reasonable time. You can straighten any warp or twist easily, as the blade is still rubbery soft austenite. Just put back in the pot as it cools a bit, wait a minute, and pull out to straighten more. Once it is straight, just hang in air and let it harden.
It was good for tempering, but not particularly better than an oven.
It was good for heat bluing fittings and damascus blades. A quick dip of the blade at 475° to 500° won't affect the temper. You get a very nice rich blue color.
I got a rubber cap that fit the tube for when it was not in use. The salts are very hydroscopic, and will absorb water from the air if not kept tightly sealed. Any absorbed water will steam off as the salts slowly heat, which is why the slow ramp rate is required.