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[machinist] OT: Want to reline a muffle furnace for heat treatment: any tips?.
- From: Lawrence London <lfljvenaura@gmail.com>
- To: machinist@lists.ibiblio.org
- Subject: [machinist] OT: Want to reline a muffle furnace for heat treatment: any tips?.
- Date: Wed, 4 Dec 2013 14:16:26 -0500
http://www.practicalmachinist.com/vb/general/ot-want-reline-muffle-furnace-heat-treatment-any-tips-276190/
Thread: OT: Want to reline a muffle furnace for heat treatment: any tips?.
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OT: Want to reline a muffle furnace for heat treatment: any tips?.
I scored a small laboratory muffle furnace yesterday with the coiled heating wire embedded in the lining of the furnace. Anyway the walls appear to be a lightweight zirconia insulation which has been damaged by a lot of misuse and by the removal of the old heater wire. There's some Kanthal A1 wire and a digital controller on the way but I'd like to know if anyone's done this repair before?.
I'm hoping to place the new coils in position and then grout over them with high temperature grout. So I'd like to know of any high temp grout products or recipes I could use to reattach the wires to the walls. Aluminium oxide seems to have a suitable high temperature (2000 deg C, 3000+ deg F) and is reasonably thermally conductive: I could mix with some clay and water to make a thick paste. -
12-03-2013, 12:17 AM #2
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Replacement refractory material that one can cut to fit and 'groove' for the wire is available. I'd suggest NOT grouting-over, however.
Originally Posted by SAG 180
I scored a small laboratory muffle furnace yesterday with the coiled heating wire embedded in the lining of the furnace. Anyway the walls appear to be a lightweight zirconia insulation which has been damaged by a lot of misuse and by the removal of the old heater wire. There's some Kanthal A1 wire and a digital controller on the way but I'd like to know if anyone's done this repair before?.
I'm hoping to place the new coils in position and then grout over them with high temperature grout. So I'd like to know of any high temp grout products or recipes I could use to reattach the wires to the walls. Aluminium oxide seems to have a suitable high temperature (2000 deg C, 3000+ deg F) and is reasonably thermally conductive: I could mix with some clay and water to make a thick paste.
The wires move. A lot.
Bill -
Well the original was definitely grouted in, but you can see it failed and came loose as the previous operators tried to jam steel screws into the insulation to keep the coils falling. The grouting may be a chem lab thing as the other lab furnaces I've used in the past had embedded elements like this one, possibly for some corrosion protection. The zirconia insulation is thick enough to re-groove but soft and fragile so the grout seems to form a harder layer too. Originally Posted by thermite
Replacement refractory material that one can cut to fit and 'groove' for the wire is available. I'd suggest NOT grouting-over, however.
The wires move. A lot.
Bill -
I'd like to know more about this as well. I too have a muffle furnace in need of new coils, it's made from a ceramic tube with the heating element wrapped around the outside and grouted in. Since it's a smooth tube the elements must be embedded or kept in place by some other means to keep them from shorting out. I've looked at high temperature cements/mortar for fire places and similar, but I'm uncertain abut how it would handle direct contact with the elements.
I'm on a tight budget for the moment, so I'm hoping to avoid ordering expensive special products. From what I've read the grout could be something like zirconia or alumina bound with sodium silicate, perhaps something I could mix up myself. I do have a bag of fine and coarse chamotte clay (grog) and dry bentonite, any of those useful? I also have some premixed, unfired mullite (alumina and quartz) powder, but I'm not sure that would work either. Any good ideas? -
12-03-2013, 03:43 AM #5
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Can't be certain WHERE - may have been on PM. Originally Posted by SAG 180
Well the original was definitely grouted in, but you can see it failed and came loose as the previous operators tried to jam steel screws into the insulation to keep the coils falling. The grouting may be a chem lab thing as the other lab furnaces I've used in the past had embedded elements like this one, possibly for some corrosion protection. The zirconia insulation is thick enough to re-groove but soft and fragile so the grout seems to form a harder layer too.
Or not.
But the information passed by my wetware scanner within the past 12-24 months as to how to specify, select, source, and work the refractory materials, resistance wire, pyrometric equipment & timers, simple and complex-curve computerized, and everything else to rebuild or build from-scratch.
Also had sight of why ceramic kilns do not convert well to heat-treating of metals, the pros and cons of bottom-lift vs horizontal doors, gas vs electric, inert (and carburizing) gas source, atmosphere, purge..... IOW the whole nine yards of a Maxim's linen-belted ammo feed.
Worth a deep-dive to find that info, as it has been very well covered, and in fine detail.
That could save considerable re-inventing of the wheel in time, money, and disappointing side-trips.
Bill -
A Maxim with nine yards??? Vickers were far more popular with the tea drinkers
The whole nine yards - Wikipedia, the free encyclopedia
A Browning later on, sure.. Seperate links... Tossed out wing chutes...
First World War.com - Weapons of War: Machine Guns
And since Brits were running Brownings in aircraft... A 27 ft long fabric belt, is a bit ungainly.. Around aircraft..
Some late Maxims ran links (Turkish for example) Non disintegrating tho..
Back OT.. heating elements are wear items.. grouting them in place, seems like the already short life, would be shortened more..
Looks like it expands at ~ 2x the rate of normal steel. -
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Welp....... moving right along, I ran some experiments with a mix of alumina and clay and made up a mix and applied it directly to some old heater wire out of the furnace. The mix is interesting as it seems to have thixotropic properties: if you leave it alone, it's very thick but a little vibration and motion and it liquefies and can be troweled easily. Anyway being impatient, I troweled it onto the coil and connected it to the variac and gave it some erg potential after a few minutes it sizzled a bit and smoke came off just like those "green" bendable heating elements before they are ready to use. After about 5 minutes it was glowing bright yellow and with no smoke at all. The mix behaved very well, troweled smoothly, went solid once you stopped moving it and was ready to bake out while wet. There was no spitting when heating but after cooling it was too friable, but had very little clay in it around >5% so I reckon if I boost the clay content and stir in some zirconia wool fibres I should be all set to go with a suitable mix for the next stage of experiments.
Once I get the ramp soak controller and maybe a cooling fan installed it should be up to the task.
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12-03-2013, 05:35 AM #9
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The Vickers I suspect we both refer to WAS a 'Maxim' (improved). By patent origin, anyway. Originally Posted by abarnsley
A Maxim with nine yards??? Vickers were far more popular with the tea drinkers
The whole nine yards - Wikipedia, the free encyclopedia
A Browning later on, sure.. Seperate links... Tossed out wing chutes...
First World War.com - Weapons of War: Machine Guns
And since Brits were running Brownings in aircraft... A 27 ft long fabric belt, is a bit ungainly.. Around aircraft..
Some late Maxims ran links (Turkish for example)
In any case, not speaking of disintegrating link belts. I've linked a fair share of them. Their length is arbitrary, extensible on-the-fly by a competent assistant gunner, (amongst gravel-agitators, anyway..) and permissive of tear-off to stop runaway from cook-offs.
The canvas or linen belts of WWI (and earlier) had tabs at each end, were not amenable to tear-off, were fixed-length.
I do not 'personally' know if they were 9 yards - the few museums where I've seen them displayed were not touchy-feely places..
Period-correct examples also appeared in the interior shot of Strelnikov's armoured train in 'Dr. Zhivago', and surviving examples (or photos of same) turn up online from time to time..
Roger Ford is a good read BTW .. regardless of belt length..
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12-03-2013, 07:12 AM #11
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Aluminium oxide is supposed to be moderately thermally conductive, but at what rate after I've got through with it?. Originally Posted by Fulmen
Mark: That looks promising, keep up the good work. Any thoughts on thermal conductivity? -
Your guess is as good as mine. Other than ensuring a dense, non-porous coat I don't know. OTOH it could become more subceptible to cracking or put more load on the wire, not sure if that's worth the gain in conductivity.
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High Temperature Adhesives and Epoxies, Ceramics, Insulation, Epoxies and Epoxy seems to have a lot of the stuff.
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your problem is that ...
well
throw the whole damn thing out
get some k23 insulating fire bricks, cut ends so they form a circle. cut grooves into firebrick using router or old table saw. instead of trying to coat element, hold in place with pieces of kanthal wire made in to staples.
by coating element with an insulating medium, you are increasing the temperature of the element inside the refractory. this will lead to element failure. your coating would have to be a very uniform thickness, or you end up with a spot that is insulating better, a hotter spot. this is where the wire fails.
btw, for heat treating purposes, you can get away with nichrome wire and spend a lot less
sorry it's not written well, tooth extraction and vicodin = me tired and fussy -
12-03-2013, 08:46 AM #15
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I've been working on glass and pottery kilns for a few years. Most info i get says to avoid getting the wet castable refractories on the coils because it can cause hot spots. If the whole thing is coated it might not matter. Pragon kiln and Euclids are the two places I get my info about lining and element repair
Replacement Elements for Kilns, Annealers and More. Euclids
Ceramic Kilns, Glass Kilns, Pottery Kilns and Many Other Specialty and Custom Kilns 1
I get my castable refractory from a local steam boiler repair shop. I have made a mould to cast the bricks with the channels in them. I let them air dry for at least a week before i fire them, then cut them to fit. Pins to hold the coils in the broken out channels are simple pieces of 303 stainless welding rod -
For a furnace with internal elements I would probably follow Snowmans advice and forgo the embedding completely. This is common for both laboratory use and pottery, so I doub't there is any need for embedding except for very special uses (corrosive atmosphere and similar). But in my case I must have something to keep the coils in place, and the ceramic tube is both too hard and to thin to fix it with wires. The only other option I can think of is to wind the tube with rope or plastic tube and build up with a fireclay to make channels for the coils. This would expose the coils to the insulation (a fine, unidentified red-brown powder), I'm not sure if there is any risks of adverse reactions there.
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12-03-2013, 11:44 AM #17
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SAG:
You don't say what make and model of muffle furnace it is. If you can determine that info you may be able to buy new elements and just drop them in.
The one I have has elements that are encased in the refactory walls. No seams except at the corners. So the elements are well protected. I can buy replacement wall panels with new elements from the manufacturer. I have also posted here before the wiring diagram/schematic and parts list for digital process controller and SSR setup I made for mine. Search for it or I will dig it up later.
-DU- -
12-03-2013, 12:11 PM #18
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+1 on Snowmans advice. I made a HT oven about a year ago using the K26 bricks which are very easy to groove. Being a little anal, I used a dull 5/8" endmill and grooved them in the mill. Wound the wire over a piece of 1/2" CRRD in the lathe and then experimented with an ammeter to get the length right per the current carrying ability of my electricals. Luckily, the mortar I used was the only thing that held up to the heat. Several other high temp grout products I used failed miserably. I would recommend using mechanical means of holding the wire in the grooves which will allow it to "float" to some degree. I'm happy with the end result. Used woven fiberglass for the door seals.
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12-03-2013, 12:15 PM #19
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That's a topic certainly worth thinking about. I use an old Thermolyne electric furnace for non-HSS heat treat, and the one thing I hate about it is that it has an ordinary "fridge" type horizontal swinging door. Open that sucker at 1750F and you'll want to be damned sure you have a latch (not supplied by OEM!) to keep it open so it doesn't swing shut when you're reaching into the chamber. Originally Posted by thermite
the pros and cons of bottom-lift vs horizontal doors
If I ever clear about 75 items off my to-do list, I may turn the door on that thing into a vertical lifting door to keep the hot face away from the operator. -
At one time we ran 15 front loading kilns in a brazing operation. (Our customer had a lab furnace for R&D but I didn't want to spend that kind of money.) We rebuilt 3 to 5 a day.
As has been mentioned - Find a good supplier of ceramics for kilns.
To make elements - stretch out an old element to get the wire length. Cut a new piece of wire to that length.
Get a 1/4" rod and chuck it an electric drill. Use a leather glove to guide the wire on as the rod rotates. Leave tags of a few inches on each end. When the coil is wound pull it to length. (It has been years but, as I recall, a 1/4" rod gives a coil that has an inside diameter of about 3/16".)
DO NOT USE GOOD TOOLS TO WORK FIRE BRICK. It will rapidly ruin woodworking tools. Get a fire brick saw and use hack saws. Use old screwdrivers as chisels.
If you use power tools remember that the dust is very abrasive and thus hard on bearings, etc. Also hard on lungs.
Build flat bottom grooves in your walls or, better yet, grooves with a trough for a bottom. Lay your coils in these and pin them. Use lots of pins since hot coils grow and will sag if not supported.
Basically I am agreeing with and just adding a bit to those above.
Reply
- [machinist] OT: Want to reline a muffle furnace for heat treatment: any tips?., Lawrence London, 12/04/2013
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