Author Topic: Oil fired crucible furnace  (Read 51997 times)

Offline vtsteam

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Oil fired crucible furnace
« on: July 15, 2013, 08:01:58 PM »
This is an extension of the Sawed Off Cupola thread I started earlier, since that furnace is no longer being tried as a single charge charcoal cupola.

Its now an oil fired crucible furnace, so I thought it deserved a new thread.

Recent events:

Yesterday I had a missrun in pouring the mold  -- the pour was cold. The burner I made wasn't able to feed fuel at a high enough rate to get the iron fluid, though it did melt. Three hours of heating finally resulted in the fuel running out and  forced pour. The result -- no casting, only a sprue.

Today I took the burner off of the furnace and checked it out. The same burner more or less had successfully melted my first iron a couple weeks ago, but that one leaked oil. The burner was a Kwikie burner design. This atomizing burner was set too far back in the pipe housing in a 4" thick refractory lining (by design), so the spray impinged on the pipe walls and dripped out.

I re-built it with a longer and larger diameter spray tube extension that brought the nozzle 2" closer to the furnace barrel. Otherwise the design was unchanged. But new delivery problems arose -- traced at first to a bad silver braze joint. With that repaired I tried yesterday's melt. That was the missrun.

Today, after taking it out I couldn't see anything wrong with the burner. So I figured that the nozzle was just screwed in too close to the jet orifice to allow enough fuel to pass. I opened it a quarter turn and re-installed in the furnace. The difference was dramatic! It was back to being a jet engine!

So with that worked out at 3:30, I thought I'd give it another try before dinner. I broke up some cast iron, loaded the crucible and fueled the burner with an container of old diesel fuel -- about 3 gallons worth that I found in the shed.

The burn went great! I molded up a flask while the furnace was heating, with a very healthy roar. I had to stop the burn a couple times to add iron, as I couldn't fit it all in at once. I needed about 7 pounds for this casting.

Finally the metal seemed readyafter dipping a rod in and having it come out pretty clean -- a little thin shiny slag and that was it.

When I went to lift the crucible, it stuck to the wall and plinth, and I couldn't get the tongs around it. The crucible seemed fairly plastic -- temps must have been high! Finally I maneuvered it off of the wall, but carried the plinth out with it. When I set it down in  the sand to skim off the slag the plinth released, and I pushed it out of the way. it was then easy to lift it in  the shank and pour. Very fluid this time, I filled the mold, and still fluid the excess splashed a little out of the ingot mould while pouring.

Great pour! Man iron is exciting! A real high to get it to melt and fill a mold.

All in all, it took an hour and a half exactly from lighting the furnace until the pour was done. I'm sorry I didn't measure the fuel beforehand, but I think it took around a gallon or gallon and a half. Just a great day!  :ddb:

Pictures when I can get them up.
« Last Edit: July 15, 2013, 09:19:20 PM by vtsteam »
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #1 on: July 15, 2013, 09:12:56 PM »
Plinth comes free of crucible ...


« Last Edit: January 14, 2018, 10:08:46 AM by vtsteam »
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #2 on: July 15, 2013, 09:14:55 PM »
The pour....

« Last Edit: January 14, 2018, 10:09:26 AM by vtsteam »
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #3 on: July 15, 2013, 09:16:48 PM »
All in, time to shut it down....



« Last Edit: January 14, 2018, 10:10:04 AM by vtsteam »
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #4 on: July 15, 2013, 09:18:07 PM »
The shake out.....


« Last Edit: January 14, 2018, 10:10:36 AM by vtsteam »
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #5 on: July 15, 2013, 09:20:45 PM »
First two pieces of iron....


« Last Edit: January 14, 2018, 10:11:30 AM by vtsteam »
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Offline tekfab

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Re: Oil fired crucible furnace
« Reply #6 on: July 15, 2013, 09:27:08 PM »
I think i'm going to have a go at making a Kwikie burner, after all i've got 600 (yes 600) gallons of used oil to dispose off so expect many questions !

Mike Young

Offline awemawson

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Re: Oil fired crucible furnace
« Reply #7 on: July 16, 2013, 02:27:15 AM »
Steve

I used to sprinkle something on the plinth brick to stop it sticking to the crucible but for the life of me I cannot remember what. Can't have been sand as that would fuse at those temperatures   
Andrew Mawson
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Offline NeoTech

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Re: Oil fired crucible furnace
« Reply #8 on: July 16, 2013, 04:12:23 AM »
Would be awsome to see a cross cut or a surface cut out of that square block.
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Offline mattinker

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Re: Oil fired crucible furnace
« Reply #9 on: July 16, 2013, 05:45:22 AM »
A piece of card board or thick paper will leave carbon to separate the crucible from the plinth.

regards, Matthew

Offline awemawson

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Re: Oil fired crucible furnace
« Reply #10 on: July 16, 2013, 06:41:53 AM »
Steve

I used to sprinkle something on the plinth brick to stop it sticking to the crucible but for the life of me I cannot remember what. Can't have been sand as that would fuse at those temperatures

Ha ha - I've remembered ! Not sprinkled but put - Morgan Crucible recommend placing a piece of cardboard between the plinth brick and the crucible to stop sticking and that's what I did. Gosh the old grey cells must be dying faster than I thought  :scratch:
Andrew Mawson
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #11 on: July 16, 2013, 08:07:51 AM »
Thank you lads!   :beer: Yes, two layers of corrugated cardboard had been placed under the crucible, as well as a dusting of plumbago on the plinth. The problem seems to come when spilling a bit of slag when adding metal during the melt. Very fluid in the highest heat of the furnace It works down the outside of the crucible and slides into the space between the two. The cardboard carbon blows away near the glue line allowing it to take effect. Luckily it is spotty so tilting the crucible when skimming pulls it free.

The whole thing feels stiff taffy like when ready to pour -- including the crucible this time. Plinth sagged slightly and the crucible glued itself to the furnace wall and had to be pulled free as gently as I could manage, so I could get the tongs around it. Crucible is slightly distorted in shape today therefore. All very exciting when ready to pour!
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #12 on: July 16, 2013, 06:04:50 PM »
Would be awsome to see a cross cut or a surface cut out of that square block.

Here you are. The casting had a shrink cavity on the top face, and a couple of serious bubbles. These last two were hidden until the face was milled. The shrink cavity would probably not be a problem, since this piece was intended to be bored out fairly heavily, but the bubbles may have spoiled it. We'll see.

This casting was not as soft as the last one -- it made life a little difficult for the HSS mills, but was machinable. Plenty of graphite came out.


Clean face:





Shrink cavity and bubbles ( upper face)





How it is supposed to go -- both get bored out:






« Last Edit: January 14, 2018, 10:14:24 AM by vtsteam »
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Offline NeoTech

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Re: Oil fired crucible furnace
« Reply #13 on: July 17, 2013, 05:12:02 AM »
That looks really nice and solid, no pours.. =)

I imagine you didnt do "crack" test wedges like ironman does in his vids?  Still very nice looking.

could that shrinkage been avoided if you had used some form of reservoir between the molding cavity and the sprue?
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #14 on: July 17, 2013, 08:33:45 AM »
Thnks NeoTech.

I'm trying to figure out how to avoid the shrink depression and bubbles next time. Iron is a little different than aluminum, but similar.

As a problem for me, shrinkage is less bad -- I can often plan a casting or work around it.

Big bubbles like those (6-8mm deep) are a serious problem. While I had shrink depressions sometimes in aluminum, I didn't have anything like those bubbles. Small ones once in  awhile maybe, and sometimes very fine gas porosity if the melt was stirred too much and it was a humid day and older scrap, but nothing like like these big bubbles.

Possibilities:

1.) Maybe the sand was rammed too hard (I did vent it with a wire, but I could try less ramming pressure)

2.) Maybe the sand was too moist (hard for me to judge -- it's new sand, Bentonite instead of fire clay, and this is iron instead of aluminum, so it's very different than what I'm used to. I could try molding drier.

3.) Maybe air got in while pouring. I used two arc shaped gates (instead of my usual one) and they were pretty deep this time compared to last when I had the misrun. So they channeled the iron very fast. The sprue was 1.25" dia and dowel shaped, not tapered. When I poured I tried to pour fast, but it really drained fast so the sprue did not look choked during the pour. It probably should have.

I could make up a tapered metal sprue cutter -- like the real molders use, and/or reduce the size of the sprue, reduce the number of gates, or reduce the depth of the gates in order to keep the sprue more easily choked with iron during the pour.

4.) Shrinkage depression --- the cope was 3.5" deep, and the pattern was entirely in the drag, so there was 3.5" of head pressure. I could build up above that to increase head pressure.

Or I could add a riser to feed hot metal as the casting shrinks. Although with such a small casting and a 1.25" dia sprue, I thought the sprue would feed it.

One problem with a riser is that it takes even more metal in the melt, and I'm starting to approach the maximum comfortable capacity of my crucible already at 7 pounds. I had to add metal twice during the melt. A sprue or riser would add another pound and a half or so.

Anyway those are the ideas I've had so far.


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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #15 on: July 17, 2013, 07:44:59 PM »
Checked out the crucible and furnace more closely this morning. The crucible really took a beating last time when it got stuck to the furnace wall, and is no longer usable. Here's a picture of the used one and a new one.

Besides the chips cracks and shape distortion, the walls had thinned quite a bit. I sure hope I'm not going to get only 3-4 melts out of a crucible or this is going to get expensive fast. I don't think the 3 hour marathon the other day did it any good. Also I really had the blast up at the last 20 minutes of the last pour -- maybe that was too much. Could also be to much air early on, or maybe even the soda ash flux. I could try a slightly more reducing flame.

Anyway, hope to get more pours out of the next one.


« Last Edit: January 14, 2018, 10:15:43 AM by vtsteam »
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #16 on: July 17, 2013, 07:48:28 PM »
The furnace lining on the other hand is doing great. It is just somewhat more glazed than it was. Starting to look like pottery.


« Last Edit: January 14, 2018, 10:16:27 AM by vtsteam »
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Offline NeoTech

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Re: Oil fired crucible furnace
« Reply #17 on: July 18, 2013, 02:54:41 AM »
Hmm, if the extra volume of metall is a problem, i would prob. just cast the piece a bit thicker so not that much extra added material was needed and then just cut it down to size and hopefully get the imperfections out that way. =)

Isnt there someting like a flux for cast iron? Wouldnt that solve the gas inclusions problems?
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Offline awemawson

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Re: Oil fired crucible furnace
« Reply #18 on: July 18, 2013, 04:02:22 AM »
You always get the crud and voids at the top of a shallowish casting unless you make gating to avoid it. I'd be tempted to make up the mould vertically with an extra sacrificial bit to machine off, and also increase the head of the pouring basin
Andrew Mawson
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Re: Oil fired crucible furnace
« Reply #19 on: July 18, 2013, 05:14:04 PM »
A couple of suggestions from the things I have learned in the past:

1. All crucibles are not created equal, some are rated for ferrous metal, and some are not.  Crucibles have a maximum temperature rating, and for cast iron work, you should be using a crucible that is documented for operation at around 3000F, ferrous-metal-rated (generally the clay type, but not all clay types).

2. According to the crucible literature I have seen, the burner should enter the furnace wall at the centerline where the crucible sits on the plinth, and the burner should not impinge on the crucible, but rather the burner flame should enter to one side of the crucible.  This entry point is also important to get the flame to swirl around the crucible and distribute the heat evenly without overheating any one spot on the crucible.

3. The crucible literature mentions that the crucibles should be carefully lifted from the lower part of the crucible, with full support around the crucible.

4. There is a book called "Basic Principles of Gating and Risering" by the American Foundrymen's Society, Inc. that is very helpful in designing sprue systems that do not aspirate air, and gate and runner systems that fill the mold evenly while skimming off slag, and risers where needed for shrinkage, as well as venting to relieve gassing from the cores and mould itself.  Venting is extemely critical if you are using water-based sand and core mixes.  Oil-based sands do not work very well with cast iron since the metal is so hot it tends to wash and errode the sand.  I have seen some use oil-based sand with cast iron with some success though.

I have heard reports of people using quality cast-iron-rated crucibles for 100 pours or morre.  There is no reason for a good quality crucible to fail prematurely unless you use it in excess of its temperature or metal-type rating.

The soft fire bricks that have a lot of air entrained in them will not hold up to cast iron temperatures.  The hard fire bricks apparently will hold up to cast iron temps as you demonstrate, but the hard bricks may need a little more insulation behind them since they will not insulate as well as air-entrained bricks.

Good luck.

Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #20 on: July 18, 2013, 07:02:36 PM »
NeoTech, there is a machining allowance, but it wasn't enough. I would like to avoid taking off 6mm, however through better molding practices, if I can.

Andrew, I think you're right it would have been better to orient vertically, but I'd have needed a deeper drag. I do think more head on the pouring basin would have helped. I'm thinking maybe ramming softer might also relieve the gas, and reducing sand moisture, maybe. Also, there was no crud -- just a depression and bubbles, so feeding and gas/moisture relief were the problem.

However, look at Ironman's castings -- amazing stuff. Big wide plates, letter plates, muffler parts, etc, with super finish, 2 simple gates (like the ones I used/imitated) very low head -- copes are quite shallow, shallower than mine, simple small sprues, the plates oriented horizontally, and they come out finish quality, with no shrink depressions, no crud, no bubbles, crisp lettering. So it can be done. I'm just not doing it right.

MetalCaster, hello, welcome to the forum. Please introduce yourself, and tell us about your melting rig and projects. Do you cast in iron?

re. your suggestions/questions -- I used a 3000 degree rated clay/graphite crucible.

re. burner: the burner doesn't impinge directly on the crucible. The flame swirls around the furnace barrel all the way to the lid.

re tongs: the lifting tongs fit the crucible properly and it is lifted at the proper point in the bilge.  (Please see my earlier construction thread re. "Sawed off cupola" for all of these details.)

re. firebrick: The hard firebrick I used has very good insulating qualities since it is 4" thick. Thickness counts in insulation -- read about thickness in relation to R values in insiulation if interested. I believe this furnace probably does nearly as well as some of the 2" kaowool foundries, and I'm sure it would compete with castable refractory furnaces of lesser thickness for insulation value -- there's also packed sand/clay with airspace in this build. This furnace does not get hot to the touch on the outside for a very long time after a melt is underway, and retains heat after it is over overnight.

Soft firebrick is holding up perfectly in the lid -- it has a barrier of about an inch of sand/fireclay under.  Materials are not good or bad per se. -- the devil is in the details. Be careful of generalizations, like saying soft firebrick is unsuitable for use in an iron melting furnace, Specific cases,  application and actual experience can contradict a lot of the commonly repeated axioms on the internet.

re. mold venting: my moulds were vented, as I mentioned earlier.

re. core venting:  I wasn't using a core.

re. gating references:  I have a lot of foundry books illustrating a multitude of gating and risering schemes, methods, and theory but if you watch Ironman's youtube videos, you will see perfect flat simple castings in iron with the simplest double or single gates. We aren't casting a V-8 engine block here.

re. crucible misuse via overtemp, or wrong metal rating -- it's possible the final temp was too high, as I mentioned above. I think there are many other possible factors, too, though. Oxidizing or reducing atmosphere, corrosive flux, etc. And as mentioned also earlier, the reason for the crack and mishapen upper portion was that the crucible got glued to the furnace wall and had to be pried off. That might also have been partly because of overheating. My bigger concern is crucible thinning. If the crucible is not up to par, then okay, but if it's a result of a bad furnace atmosphere, or the sodium carbonate I used, or over temp, then those are things I can, and would like to correct.

Again welcome, and please introduce yourself. A fellow metal caster wold be a great addition!  :beer:





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Offline awemawson

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Re: Oil fired crucible furnace
« Reply #21 on: July 19, 2013, 02:31:29 AM »
Really you need silicon carbide crucibles for repeated use with iron
Andrew Mawson
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Re: Oil fired crucible furnace
« Reply #22 on: July 19, 2013, 11:42:14 AM »
According the rep, the Morgan silicon carbide crucibles are not ferrous-metal rated, and are not rated for iron temperatures either.

The Morgan clay-graphite unit is rated at nearly 3000 F and ferrous-metal rated.

Not sure about other curcible manufacturers.

VT-Thanks for the welcome, I am a lurker type, so will remain a wall flower.  Yes, I do cast iron.

Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #23 on: July 19, 2013, 01:29:32 PM »
Metalcaster. no fair commenting on others work without sharing your own!

Tell us about your rig, what you've cast in iron with it, how long it has been in operation, what kind of crucibles you've tried and what the results were of different crucibles.

Pics would be good, we love pictures!
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Offline awemawson

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Re: Oil fired crucible furnace
« Reply #24 on: July 19, 2013, 02:17:28 PM »
According the rep, the Morgan silicon carbide crucibles are not ferrous-metal rated, and are not rated for iron temperatures either.

The Morgan clay-graphite unit is rated at nearly 3000 F and ferrous-metal rated.

Not sure about other curcible manufacturers.

VT-Thanks for the welcome, I am a lurker type, so will remain a wall flower.  Yes, I do cast iron.

Yes I'm obviously remembering it the wrong way round. Old age is a terrible thing :(

All my crucibles are Morgan ones, but they've been in storage for 6 years!
Andrew Mawson
East Sussex