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Thread: Kelly's Furnace Build Log

  1. #101
    I cant see from the pictures, but I got the carbon sheath t-shirt. It doesnt last when exposed to the hot air. Now if it was buried inside refractory, maybe. I too tried it inside an electric kiln.
    Visit me: WWW.HandcraftedLanterns.com
    "Sometimes by losing a battle you find a new way to win the war"
    -- Donald Trump --

  2. #102
    Quote Originally Posted by jagboy69 View Post
    I cant see from the pictures, but I got the carbon sheath t-shirt. It doesnt last when exposed to the hot air. Now if it was buried inside refractory, maybe. I too tried it inside an electric kiln.
    My furnace is air but not forced air, essentially a kiln and thus far I haven't had any reason to operate above 1800F. I was expecting them to be fine at that temp in air however I was thinking some occasional BN spray coating might be helpful in both services. I expect the carbon sheaths to be reduced during metal contact albeit slowly and at $2 each I view them as a consumable. They have a 1/4" wall so just a matter of how far I let them go before replacement. It's very early days but after a couple hours in air at 1800F and 20 minutes in molten aluminum contact at 1400-1700F it appears unchanged. When I removed it from the aluminum melt at 1700F the sheath was cherry red.

    The sheath on the TC inside them is Inconel 600. The other K-Type TC that I use directly in air is also Inconel 600 and it has been in air service at 1800F for about 20 hrs. Both see thermal shock to extent caused by removing them from furnace service and exposing them to ambient air.

    Best,
    K

  3. #103
    Quote Originally Posted by jagboy69 View Post
    I cant see from the pictures, but I got the carbon sheath t-shirt. It doesnt last when exposed to the hot air. Now if it was buried inside refractory, maybe. I too tried it inside an electric kiln.
    I'm going to have to retract my statement about the sheath being unchanged. After your post I went out and took a closer look and think I'd actually have to say there has been significant reduction of the sheath after a relatively short time.

    So it's the oxidizing environment that reduces it not the metal contact? I was thinking at 1800F the reduction would be minimal and thinking that being maintained at furnace temperature would reduce thermal shock.

    I may have to rethink the active control through molten metal contact/temp and revert to momentary contact to measure metal temp. This (carbon sheath reduction in oxidizing environments) might also be worthy of adding to the sticky because even though I expected them to be expendable, a couple hours of service is a little too much so.

    I believe commercial metal contact probes most commonly are SiC. May have to look harder at BN coatings too, perhaps on sacrificial Inconel sheath?

    Best,
    Kelly

  4. #104
    You got it Kelly. Ya see I tried this stunt with my junk pottery kiln that I use for wax burnout. I drilled a carbon gouging rod out and stuffed my TC down in it. Secured with a chunk of kaowool, I thought I was good to go. After just a few cycles, the carbon vanished and exposed my TC and cooked it. The gang here told me it IS the oxidizing environment that erodes the carbon. Like it or not there is some air movement inside an electric kiln, sure it wont blow your hair back, but it's there and probably accelerates the process as mine was installed horizontal and was eaten away I think on the top half of it.

    Now I have a SS 6buck probe. I'm expecting that will be the ticket. FWIW, I still use the carbon rod with a TC for plunging into a pot of bronze and that is holding up well. Weird I know.
    I do need to pick your brain sometime about how you sorted out your coil setup. (ohms, length, diameter, watts, etc...) Was that covered somewhere here?

    Jason
    Visit me: WWW.HandcraftedLanterns.com
    "Sometimes by losing a battle you find a new way to win the war"
    -- Donald Trump --

  5. #105
    When heated to the proper temperature Carbon burns, ok really in this case it oxidizes rapidly. It just takes heat carbon and oxygen. It's not even that the atmosphere in the furnace is an oxidizing environment (an environment with more oxygen in it then standard outside environment). With a burner you can in theory pull in more air (oxygen) then the flame uses... this is an oxidizing flame. You don't get this condition in the electric furnace. In the case with the kiln and the electric furnace the oxygen level is the same you just increased the temperature and the carbon becomes more reactive (it's why charcoal is a great cover for copper based metal...it's an oxygen sponge) it reacts with the o2 in the furnace an makes CO2 (and other compounds). When it's in contact with the molten metal the source of O2 is removed (or significantly reduced) thus the reaction is less. The carbon will react and go into solution in the metal but at a much slower rate than it reacts with the O2 in the furnace. The SS an Iconel covers are much less reactive to the O2 till the temps get much higher. At Iron temps your going to see them start to react faster, but that's closer to their melting points too. But they are both more reactive to the molten metal, they go into solution much more rapidly than Carbon does. So plunging them into the melt pool and leaving them ....They will have a short life.

    You best bet is to plunge the carbon as far into the melt pool as you can. and have SS or the like above it. You'll still have erosion from the melt pool to the SS.. Or only insert the Carbon TC into the furnace when you want to get a metal temp. (as you melt more you'll know how long-ish your furnace takes to melt each metal...so dunk it a few times near the end to dial in the temp for pouring).

    I hope this clears things up more than it confuses things...

    CBB

  6. #106
    Thanks guys. I'll post a picture of the carbon sheath reduction.

    I really cant precisely say the exposure time and average temp for sure, though it was maybe 20 minutes in metal contact and several hours at not more than 1800F in air for the active melt time. However I left it closed up in the furnace along with crucible after several melts. The furnace is well insulated and the temp slowly degrades over hours, but at most probably another 10 hours before it was into the low/mid 100sF. The bare Inconel 600 TC has many hours at temp in air so looks like it is a winner in that regard.

    In any case, I have plenty of spare carbon sheaths and agree it will need to become a momentary molten metal contact only probe. In fact, other than a plunge though the lid to make sure I'm at least at target pour temp (and as suggested, after more melt experience with the furnace probably not necessary), may just as well just overshoot on temp, skim and measure metal temp outside the furnace until it recedes to pour temp.

    Best,
    Kelly

  7. #107
    Quote Originally Posted by jagboy69 View Post
    I do need to pick your brain sometime about how you sorted out your coil setup. (ohms, length, diameter, watts, etc...) Was that covered somewhere here?

    Jason
    The short story on that is you will trying to be measuring a resistance comparable to or less than that in the probes of a consumer grade multimeter, and that won't be successful. If you search the net or YouTube there'll be methods with shunts and various other things. I tried them....still complete nonsense readings.

    I bought the resistance wire from here:

    http://psh.ca/index.php?cat_id=159

    ...and is discussed more here in this build thread.

    http://www.alloyavenue.com/vb/showth...l=1#post187793

    You buy it by the pound. You may need to calculate the approximate ft/lb, but if memory serves, I just told them how many feet I wanted and they charged me by weight. They provide a resistance figure in ohms to the fourth digit. I measured mine out with a tape measure and based on ohm's law and the voltage and amperage shown on the panel meter on my rig, their stated resistance figure was dead nuts on, and even agreed with the small 3-4% change in resistance as temperature increased.

    You also need to iterate a bit to converge on the right gauge wire, your coil diameter and over all wire length (total resistance). Then divide your overall length by Pi*D for number of coils, take your number of coils times the wire diameter and this is your close wound coil length. You want a between a 3:1 and 5:1 stretch from a close wound coil so that's where your leeway is to make to make the overall coil length fit the groove in your kiln.

    Best,
    Kelly

  8. #108
    Quote Originally Posted by Robert View Post
    Outstanding. I will be curious to see the longevity of the carbon sheath. Robert
    I get the feelin you knew where that one was heading ;-)

    Best,
    Kelly

  9. #109
    Here 'tis next to an unused sheath.



    Best,
    Kelly
    Last edited by kcoffield; 05-12-2017 at 02:17 AM.

  10. #110
    Yeah your lucky it hasnt eroded through yet. Mind did and it was lights out for me. Thanks for the lead on the wire. I've got some pre wound coils for my old cat piss kiln, but not the right length. I'll fix it up soon.
    Visit me: WWW.HandcraftedLanterns.com
    "Sometimes by losing a battle you find a new way to win the war"
    -- Donald Trump --

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