Muffle for Electric Furnace

J

jackw19

Joined
Aug 4, 2012
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After reading several posts on this forum written by some obviously erudite gents about radiant heat and over-temp excursions, I'd like to put a muffle in my electric kiln to shield the object being heated from its glowing elements. I have a an Olympic front loading ceramic kiln that I've retrofitted with a PID control and argon injection. It's small (1/2 cubic ft) so it heats up reasonably fast; I typically use it for making plane blades, max dimensions 8" x 2-1/2" x 3/16".

The problem I see w/ making a muffle is that the kiln floor measures only 9" (deep) x 11" (wide) so if I start building a muffle out of 1-1/4" firebrick, which I understand shouldn't come closer than 1" to the elements in the side and back of the kiln, I'm going to start running out of room. Even with the blade going in on a diagonal, the thick refractory eats up available space in ahurry.

Could someone suggest another material for making a satisfactory muffle? Possibilities I've considered include kiln shelf tile or ceramic (alumina,etc) refractory tube of approx 4" diameter. Using kiln shelf material begs the question how one would hold the muffle together--drill holes for pins, or what?

I could bend a muffle from Inconel sheet; there are alloys in this family that certainly could take the degree of heat used to HT any knife steel. But what I wonder is whether such a muffle would itself glow red at HT temps--if so that might be as much of a problem as the glowing elements themselves. Or maybe Inconel 803 doesn't glow till well past 2000F?

I hope to avoid a solution that involves ceramic fiber. I know people here use it all the time for various applications. But I don't like the idea of the micron-sized fibers finding a new home in my lungs. Sure, a P100 mask would stop most of them, but what about all the fibers that land on the user's clothes, in the shop, get racked inside for the family to inhale, etc. Maybe I've gotten like Howard Hughes and germs, but I've been there, done that all the years working w/ asbestos.

Thanks very much in advance.
 
Inconel will work fine, but it's spendy stuff.

Pretty much anything that will take the temperature will do the job, including HT foil. You can even use used HT foil if you can recover it in big enough sections.

Ceramic fiber paper washed with clay-based sealer would also do the job with minimal risk of airborne fibers.

For plane blades, I'm guessing you are using Carbon steels, in which case 316 stainless sheet will take the temperature.

If you HT with the blades held vertically in a rack, you just need a multi-blade rack, put the blade(s) in the middle slots and the shields in the outer slots.

The issue you are trying to deal with is the differential between the temperature of the elements and the temperature of everything else in the HT oven. The math is a PITA to deal with if you feel the urge to understand/quantify it scientifically, but basically you just need to ensure there is no line-of-sight between the elements and the workpiece. Other than that, you want the shields to offer as little hindrance to air/Argon movement as possible so that the temperature can come up fairly evenly.

If you have ramp/soak capability on the controller, using a slow ramp will also prevent (or at least dramatically reduce) the radiative heating overshoot problem.
 
Try a section of old exhaust pipe. You can flatten it to make it wider if necessary. Muffler shops usually throw out lots of it so it is cheap and expendable.
Jim A.
 
I was thinking along the lines of line-of-sight, but it's great to have some confirmation.

Was somewhat fixated on a rigid solution--but, sure, why not a tent of HT foil?

One thing I don't understand however. All things radiate light of the same color at the same temperature (assuming they don't burn up first, etc), right. So why doesn't the HT tent glow at the same color as the coils. Well, it must be that the coils aren't putting out enough heat for the foil to reach their temp of 2600F, too much heat loss through the kiln walls and door. Have I answered my own question?

Thanks very much for your reply.
 
And of course the controller is turning off the coils when the temp at the thermocouple get to the setpoint.
 
The coils are the radiator - they radiate the energy. The blade is the receptor - it receives the energy. The air between them is the insulator, it slows and limits the amount of radiation that reaches the blade. Distance and density of the insulating medium determines the amount of energy reaching the receptor. In most HT ovens 6" or so of air is sufficient to ameliorate the uneven radiation and make the heating of the blade even enough for our needs. Adding a secondary receptor between the radiating coils and the primary receptor target ( the blade) will make the secondary receptor act as a secondary radiator. This will smooth out any hot spots as well as slow down the rate of heating of the primary target. It can be something with mass, like a metal or refractory tube/tunnel, or simply an opaque object that blocks direct radiation on the target. A foil tent would work. A foil wrap may be a bit too close to the target to completely eliminate hot spots. A metal pipe muffle will take care of energy absorption and distribution pretty well for our purposes. A refractory tube/tunnel will have the highest degree of smoothing effect.

In a metallurgical or laboratory oven, the chamber is separated from the coils by refractory.

In a knife HT oven, it isn't really necessary. If worried, stick the blade in a piece of pipe or place a double thickness foil tent over the blade.
 
I was more worried about the closed end of my 24 " Paragon oven with coils blazing on 3 sides and really big bowies getting the tip close to them, so I stuck a piece of half thickness fire brick up in the end.
 
So much wisdom here, so little time. Thanks gents.

If one hunts around on xbay or one of the sacred metal yards where they permit customer rooting (you fellows in So El Monte CA know where I mean), a drop a foot or two of 304/316 tubing in the 3-4" diam range isn't all that expensive. There's some really interesting reading about high-temp service of common SS alloys available from the Nickel Institute at https://www.nickelinstitute.org/~/M...tureCharacteristicsofStainlessSteel_9004_.pdf Both 304 and 316 look to be rated with plenty of headroom (1600-1700F) for the high carbon steels I'd likely be using. Those numbers are probably a conservative recommendation for a production environment, so for an occasional user and maybe turing on the argon I expect the max temp could be pushed a bit. If some fancy hi-temp air-cool steel was going in I'd likely just use foil. There are some numbers in the 300 SS series that are service-rated in air up to 2000F, but I'm not seeing any in the usual scrounging spots.

Another reason for preferring a rigid muffle is that by drilling a hole in its side the snout of a thermocouple could be introduced as part of a semi-permanent setup. That'd be getting pretty close to knowing the actual temp of the steel during HT, I think.
 
Just stick the TC in the muffle sitting on the blade. Run the leads out the door seam.
 
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