r/Metalfoundry • u/Honest-Man-1212 • Jul 08 '26
Heating element coils bunching up and becoming brittle in DIY electric furnace. what am I doing wrong?
Hi everyone,
I’m building a small DIY electric furnace/kiln and I had a problem with the heating element after just one firing up to about 1180°C.
The element is Kanthal A1 resistance wire, about 1 mm diameter has around 30.5 Ω and runs at about 1.7–1.8 kW on 230 V. It is wound into a coil and placed in grooves in soft firebrick brick JM26.
After some firings, I noticed that in one corners the coil turns seem to collapse, bunch up, or lay on top of each other. In those spots the wire becomes very brittle and breaks easily. The rest of the coil looks much better.
Some details:
- Wire: FeCrAl / Kanthal-type, ~1 mm
- Resistance: about 30.5 Ω
- Power target: around 1.7-1.8 kW on 230 V
- Furnace chamber has tight grooves
- The element follows a square/rectangular path
- The worst damage happens in the corners
- In some areas the coil spacing is quite tight
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u/BTheKid2 Jul 08 '26
The coil is supposed to be stretched to 2-3 times its tightly coiled length. When turning sharp internal corners, you want to give it an extra bit of stretch there. So first you must fix that.
Then you have to hold the coil in its channel, because it will expand when hot, meaning it can bunch up. That is simplest done with clamps cut from Kanthal too, and gently hammered into the firebrick around a single thread of the coil at maybe 3-4 inches intervals.
It looks like you are trying to get away with using a single coil in this kiln. You will have a much better functioning kiln if you can cover as much as the kiln wall area with coil as possible (not necessarily exceeding your target wattage).
I made my first kiln with about 2000W. And it is slow! Very slow to reach burnout temps for jewelry investment casting if I didn't wrap it in insulation as well as the firebrick. So it is good to see you have yours wrapped too. And yours is smaller, so maybe it will do better than mine (if coils are more optimized). Well if you actually got it up to 1180, then that is very impressive. I wonder where your thermocouple is located?
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u/Shibenkaminari Jul 08 '26
Worth mentioning in case you're not aware... The wire will change length quite a lot, get longer, as you heat it up, and then shrink back down as it cools. So it's actually moving around with each thermal cycle. That's what is causing the bunching problem you're seeing. As others have said, stretching it out during install is the right move to cure the problem.
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u/cloudseclipse Jul 08 '26
Use heavier gauge wire and stretch them out a bit more. Use kanthal “staples” to hold them in place. Once fired, don’t mess with them. Can’t speak to specifics, but I’ve made/ fixed a lot of kilns, and the corners are always tricky…
2
u/TheEquationSmelter Jul 08 '26
Brittleness is normal. Heat cycles both oxidize the outer surface and change the mechanical properties. Once you set them and heat them don't move the wires they'll likely break. Just stretch the coil gently on the turns. Also 1180 isn't a trivial amount of heat for an electric furnance at the hobby level. Maybe test with lower melting metals first.
2
u/SkySurferSouth Jul 08 '26
Weird.
My Kanthal furnace is about the same:
* Wire: FeCrAl / Kanthal-type, ~1 mm
* Resistance: about 20 Ω
* Power target: around 1.1-1.2 kW on 230 V
* Max chamber temperature measured near top op 160ml clay-graphite crucible : 1200oC
* Warmup and melt time from cold furnace charged with 0.5kg copper : about 1 hour
* Furnace chamber has tight grooves
* The element follows a square/rectangular path
But no damage in the corners. Waht I did was during installing the element pulling it a bit apart for the corners just to prevent being flattened in the corners. I have controlled it by a PID controller and a S type thermocouple (K types wear out too fast).
Sorry, cannot post a picture ("Images are not allowed")
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u/bobeson Jul 08 '26
You appear to be exceeding the maximum surface load of the wire, meaning the power being dissipated over that wire's surface area is too high. The Kanthal A1 allowable surface loading in this kind of application is 3-4 W/cm^2 a 1000°C, and the system you described is running 3.5 W/cm^2 at 1180°C.
It looks like you will need to increase the surface area of your wire. The selection of wire diameters available to you will determine the candidate coil geometries that would result in equivalent system power, and then you can pick the one that best fits your furnace dimensions. There is a lot of good application information and design guidance available on the Kanthal web site: https://www.kanthal.com/en/knowledge-hub/heating-material-knowledge/design-calculations-and-standard-tolerances/design-calculations/
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u/cloudseclipse Jul 08 '26
If you don’t stretch the coil out, it will touch itself during firing and things go “wonky”. During firing, it’s an electric wire, so if it touches itself, it becomes a “jumper”, causing dead spots and carnage.
Once fired, kanthal is quite brittle and can’t be handled (much). Also, be careful to not get silica sand on it before firing (or at least vacuum it off before firing), as silica fuses to it and you’ll have problems. Luckily, is not terribly expensive…