Author Topic: Oil fired crucible furnace  (Read 66982 times)

Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #100 on: July 27, 2013, 05:38:46 PM »
Metal is poured. Just waiting for it to cool. Seemed to go pretty well. Mostly skimmed it in the furnace. I did give it a quick skim just before pouring while it was outside the furnace. Greensand was dry.  Riser filled, Well vented. Used thick Iron stock for the melt.  Total time from lighting furnace to pour was 1 hour 20 min. That included shutting down at 1 hour to add metal to the pot, and one time to scrape slag. Also of course making up the mold.

I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #101 on: July 27, 2013, 09:31:56 PM »
Bit of a delay for dinner and family movie time.

So, shake out showed familiar problems. But not just similar problems, almost identical patterns of problems to a prior pour. And THAT was interesting. Take a look:

This pour:




And the earlier pour:

« Last Edit: January 14, 2018, 11:19:07 AM by vtsteam »
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #102 on: July 27, 2013, 09:41:48 PM »
So I'm not too upset about this one. Because it is a clue. So what is it trying to say?

Well, what am I looking at? Two sets of flaws in almost the same patterns.

The flaws appear almost entirely in  the central channel. And they don't appear in the middle of that channel, but in 2 groups either side of the center.

Now if these were gas or steam bubbles, we would expect them to be beside the channel, not in the channel itself, because they would tend to go to the highest point in  the casting. In fact it would be pretty hard to get them to stay under a ridge of sand, which is what the channel is as a mold surface.

So why are they there and why are they in the pattern they have?

Well lets try to draw them as groups on top of the photo.
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #103 on: July 27, 2013, 09:50:51 PM »
« Last Edit: January 14, 2018, 11:19:57 AM by vtsteam »
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #104 on: July 27, 2013, 09:58:34 PM »
These holes are in the flow lines going from sprue to the riser.

Why are they where they are?

The central channel sand ridge is slowing and trapping them before the freeze. These holes are slag covered bubbles. There is no way that increased sand permeability or vents are going to get rid of them. They are solids. They are lighter than the iron, so they don't appear on the bottom of the casting -- they are near the top, but the channel blocks them.

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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #105 on: July 27, 2013, 10:13:27 PM »
So where do they come from?

Well one likely possibility is of course the metal scrap. And thin radiator metal has a lot of surface area, with rust and maybe even some sand inclusions from coring inside. So it could generate a lot of slag -- being thin it also could oxidize badly in the furnace atmosphere. .

But now we get to an interesting distinction between the two different pours with the same patterns. The first one was poured with thin radiator scrap, but the second one was poured with only five chunky pieces of thick cast iron with very little rust. That metal melted quickly and was skimmed three times -- twice in the furnace and once outside just before the pour. So you would expect clean metal.

But something I noticed this time is another clue. There was a lot of slag the first two skimmings. And it was the old pasty frothy thick kind I'd had with the thin radiator metal. But that was odd, because the prior melt, with the very same kind of metal, in the very same crucible poured with no slag.

So what's the difference?

Well one thing was different.

The crucible was new last time.
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #106 on: July 27, 2013, 10:23:22 PM »
Suppose the crucible itself was generating the slag. A particularly sticky frothy hard to remove cottage cheesy slag.

That might make sense because a crucible is supposedly made of refractory materials. If they melt, or even more interestingly chemically react with molten iron, you'd expect it to be a pretty nasty type of slag -- almost a solid, not fluid at all, since it is close to its solid temperature. It's not like glass or something else with a lower melting point. And a chemical reaction between the iron and the cruciblecould produce a frothy gassy quality. It would also mean that there was an afiinity between the iron and the slag. They might be hard to separate from eachother. Mingle.

If you think about it, where did the crucible substance go, as these got thinner and thinner each melt? Was it really that they sloughed off on the outside where the heat was greatest. Where the flame was?

Or were they dissolving into the iron melt?

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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #107 on: July 27, 2013, 10:36:41 PM »
Let's take a look at an earlier photo of two of the crucibles:





« Last Edit: January 14, 2018, 11:21:00 AM by vtsteam »
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Offline dsquire

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Re: Oil fired crucible furnace
« Reply #108 on: July 27, 2013, 10:43:34 PM »
Steve

I'm wondering if there is any way to measure the crucibles to determine if they are getting thinner on the inside or the outside?

I know I'm not saying much but I am following every post. Keep up the good work.  :D :D

Cheers  :beer:

Don

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and your better best

Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #109 on: July 27, 2013, 10:48:37 PM »
The one on the left is the first one I tried, the one that got leathery on the last pour and distorted. On the right is the second crucible I tried. I's had one pour, which I believe had no flux. They both show some surface pitting, and a spill of slag down the sides. But no sign of the refractory actually fusing or melting. No sags or hot spots. You can tell by the way that neither had the flame directly playing on them -- no local discoloration. The flame swirls around the crucibles in the furnace, as it should.

The main difference between the two crucibles, is that the one on the left was fluxed with sodium carbonate - soda ash - and on its last pour it received a fair handful of that. The soda ash was quite effective at dissolving the slag in the melt, despite the fact that it received only thin radiator metal. In fact that flux was so effective that in truly liquified the slag into a syrup consistency -- the only time this has ever happened in all of the melts I've done.

So it is a powerful flux, capable of liquifying the really difficult slag I've had. It also coincidentally affected the whole crucible, made it soft.

But was that a coincidence. Or were both the slag and the crucible the same substance?
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #110 on: July 27, 2013, 11:05:13 PM »
Don, I think that for sure there is thinning on the inside, and especially visible at the top where hot metal and gas and slag all come together. I'm sure there is some erosion on the outside. But I just have a feeling that the significant part of it is occurring on the inside.

Now here's a couple statements to contemplate. One from BCS:

"These "A" shape crucibles are formed from what appears to be clay, fire clay grog and a smattering of silicon carbide particles. "

Yet these crucibles are identified as "Clay Graphite" crucibles. Why silicon carbide?

I should hasten to add I did not buy my crucibles (6years ago) from BCS. But I suspect they may be from the same source manufacturer.

And the second statement I have to find a source for, but I've seen it fairly commonly in a few references -- that silicon carbide is attacked by molten iron.

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Offline awemawson

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Re: Oil fired crucible furnace
« Reply #111 on: July 28, 2013, 03:01:51 AM »
Steve,

Are you venting your moulds as well as giving them a riser and poring basin?

I used to insert a 'needle' made from 1.5mm gas welding rod on about a 1" matrix all over the pattern area. The trick is to push it in so it ALMOST touches the pattern. In fact if you do touch the pattern you just get little pips on the casting to file off.

This greatly helps with escaping gasses giving a free passage to atmosphere avoiding the metal.

Andrew
Andrew Mawson
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Re: Oil fired crucible furnace
« Reply #112 on: July 28, 2013, 11:16:57 AM »
Hi Andrew. Yes I have been venting all of them, and especially took pains on this last one. It got 30 vents with wire, Fifteen each side. And I even pushed them through to contact the pattern. The vents on the drag side left pips, those on the cope side did not. The cope is only 2" thick these days, so perhaps there was insufficient hydrostatic pressure to push it up the vents. It did rise up the riser however.

I don't believe these slag bubbles will dissipate through a vent however. They are hollow solids, not just gas pockets. That is why they collect at the bottom of the groove in the cope side of the casting, instead of dissipating, or rising to the higher edges and corners of the casting.

I cannot imagine permeability is the problem:

The cope is 2" thick, the sand is 60 mesh of good quality, the mold was vented 30 times, the moisture content was a measured 2.5% +- .05%, and the clay is quality bentonite (probably sodium type).

The only thing I can do at this point to increase permeability would be to reduce bentonite percentage -- which I have already initiated by mixing up new sand at 4% bentonite last night. I've given it overnight to age, but my impressions were that it was way under-bonded, and maybe unworkable even at 4% moisture -- at least yesterday -- we'll see how it feels today.

I really do not believe sand is the problem. But am trying to be open by trying this option. I think it is a slag problem, and that is where the solution is going to lie.
I love it when a Plan B comes together!
Steve
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Offline awemawson

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Re: Oil fired crucible furnace
« Reply #113 on: July 28, 2013, 11:25:11 AM »
Personally I think you need to work on keeping the slag away from the mould! Can you form a pouring basin system that has a longish horizontal run before plunging into the mould with a skimmer bridging it to leave the slag behind. Or even decant the iron into a previously heated pourer.

I appreciate that you are working single handed - were you not your helper would need to hold the slag back as you pour. Can you devise some system a bit like a tea pot where the fluid leaves the pot from below the surface?
Andrew Mawson
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Offline NeoTech

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Re: Oil fired crucible furnace
« Reply #114 on: July 28, 2013, 11:43:00 AM »
with no idea about pouring melted iron.. but, have you tried those small nifty "mesh" thingys you see some people put in therem oulds.. like a small stainless mesh or something that sits in the inlet of the mould cavity supposedly to sort out the slag.. i imagine you conjour up one from fine mesh stainless.. Like those used in .. ehm ehm whats the word.. sand shaky sorting stuff thingymajigs.. you know.. ;)
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Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #115 on: July 28, 2013, 11:55:42 AM »
Thanks Neotech -- yes I've seen filters and read about them. And also gating is used to trap slag and sand impurities between the sprue and the casting.

But first I would like to concentrate on eliminating slag from the pour if possible. And to do that I have to find out where it is coming from. Ironman has written that there are 4 sources he's seen for new iron casters. Over oxidizing furnace atmosphere, too slow a furnace melt, badly oxidized thin metal, spending too much time skimming outside of the furnace before pouring.

I'd like to add to those the possibility that the crucible is contributing material to the melt, and this might be a particularly difficult type of slag, if it is indeed happening.
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Steve
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Offline tom osselton

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Re: Oil fired crucible furnace
« Reply #116 on: July 28, 2013, 07:55:39 PM »
 I found this take a look at the more information on casting flux it generates a pdf file.
Lol didn't put the link!! Mindfart or what!
http://www.vesuvius.com/en/end-markets/iron-steel/casting/ingot-casting/
Tom
« Last Edit: July 28, 2013, 08:47:36 PM by tom osselton »

Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #117 on: July 30, 2013, 10:33:27 AM »
Andrew and Tom -- sorry i didn't answer your last two posts, for some reason I didn't see them in "new posts".

Thanks guys. All helpful ideas and good practice for dealing with slag. But first, I want to get to the source problem -- if it's over oxidation in my heating and pouring practice, or if it's bad crucible materials entering the melt, I want to know that as a fact, rather than just surmising about causes. And when actually identified, I want to solve those source problems first. Then liquify and trap slag if needed.

Next step for me is making the tongs and shank to fit the new Morgan crucible. Also milling the existing castings to see whether the small flaws are more than skin deep. If the flaws mill out, I can use them. If not I'll re-melt them. They are basically thick clean ingots at this stage.
I love it when a Plan B comes together!
Steve
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Offline awemawson

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Re: Oil fired crucible furnace
« Reply #118 on: July 30, 2013, 11:18:13 AM »
Steve,

When I set up my 100kW induction furnace at my last place (**), I had to replace the permanently fixed crucibles in the two 'furnace bodies' (*) that I had. They were a special size, and I ended up having a batch made in Poland and shipped to the UK - they were pricey but not exorbitant. Researching crucible materials at the time I came to the conclusion that Alumina was the way to go as it was good for all the metals I was melting (Aluminium, Bronze, Brass, and Iron)

Morgan Crucible started life in Battersea on the south bank of the River Thames just across the river from where I lived in Chelsea at the time. When I was leaving school I was offered an apprenticeship there but declined it !

(*) Two furnace  'bodies' - one is a 'total inversion' model where the mould is strapped on top of the body, which is then inverted and the metal pours straight in. The other also tips but has a spout and allows direct filling of a mould. Both look rather like building site cement mixers with large 'steering wheel' handles for tipping!

(**) Still got it but haven't re-commissioned it since I moved here although I have run in a 3 phase 415 volt 160 amp per phase supply so I can run it direct rather than use the generator as I previously did.

Andrew Mawson
East Sussex

Offline NeoTech

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Re: Oil fired crucible furnace
« Reply #119 on: July 30, 2013, 11:20:09 AM »
Induction furnace... Oki.. you need to start a build thread on of those.. (been trying my hands at induction... i just blew out the fuse box ) ;)
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Offline awemawson

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Re: Oil fired crucible furnace
« Reply #120 on: July 30, 2013, 11:24:01 AM »
I was thinking the same as I typed that. When the Traub lathe is finally tucked up and put to bed, maybe that'll be my next project - it's time I got round to it. I even put up a fireproof extension to house it then filled it with other stuff   :bang:
Andrew Mawson
East Sussex

Offline Mayhem

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Re: Oil fired crucible furnace
« Reply #121 on: July 30, 2013, 11:39:14 AM »
What NeoTech said (with pictures, lots of pictures).

Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #122 on: July 30, 2013, 01:17:25 PM »
Yes I'd like to see that, too.

Just as an explanation of what I've done so far and why, I'm going to quote here from C.W. Ammen's "The Complete Handbook of Sand Casting" 1979:

"Melting cast iron in a crucible is actually so simple, it's a wonder more people don't do it.

"The usual practice incorporates clay and graphite crucibles. The iron, free from contaminants (anything other than iron) is broken into small pieces, the size of walnuts or a little larger. The crucible is charged (filled) with alternate layers of charcoal and iron, to which is added about two handfuls of soda ash. The soda ash can be placed either below or on top of the charge. After you have gone this far with this process, cover the crucible with an old crucible bottom and place a circular piece of corrugated cardboard between the base and support block to prevent the crucible from sticking. Start the furnace with the flame set to slightly oxidizing.. Gray iron melts at about 2327 degrees Fahrenheit. Although this figure may look large compared to that for the melting point of brass, you will find that the layers of charcoal promote rapid melting that can be done in not much more than the time it takes for brass. With a little practice you can produce high grade iron castings. (Old cast-iron steam radiators can be easily broken up to produce an excellent source of very fluid iron.)"

My first casting, and the most successful so far, followed this procedure exactly - including making a clay cover for the recommended "clay/graphite" crucible and incorporating charcoal, and soda ash, as well as the radiator source metal. Unfortunately the soda ash seemed to soften the crucible after only a couple of melts.

I own and have read probably 10 books on casting, including the extensive online US naval casting manual. Almost all provide alternate and even opposing views on all aspects of casting, including casting flaws, sand recipes, etc.

It's clear to me that the truest line above is "With a little practice you can produce high grade iron castings."

A little practice....that's what's more commonly called experience. There are no absolute recipes for success. Only a process towards it, experience, where you learn and discover the special requirements of your own equipment and materials and your own capabilities.

I love it when a Plan B comes together!
Steve
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Offline tom osselton

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Re: Oil fired crucible furnace
« Reply #123 on: July 31, 2013, 08:03:46 AM »
 I know I haven't cast anything but I have a theory :  The pics that you posted both look similar, as for your pic of the flow of the metal I think it is the position of the riser. I believe that the metal travels along the path you say but when they meet some goes into the riser (at a right angle) and some of it will go towards the center, the left side will circle clockwise and the right counterclockwise putting the defects basicaly in the same spot on both castings. You might want to try it with one gate and riser moving the riser to a corner so the flow is funneled into it. If you still want to use two gates you could have a main at the farthest point and another not as thick so it feeds at a slower rate. Like you said there are a lot of casting books but I can't  remember any that talked about the placement of sprue's or risers or the hydrodynamics that take place. I'd be interested to get Ironman's thought on this.

Tom

Offline vtsteam

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Re: Oil fired crucible furnace
« Reply #124 on: July 31, 2013, 11:51:34 AM »
Tom, my own understanding:

The main function of the riser is to feed metal to prevent shrink depressions or cavities. I haven't had a shrinkage problem since I started using one. But the riser does not reduce blow holes or slag inclusions.

Blow holes can be reduced by better sand permeability, lower sand moisture content, and venting.

Slag inclusions can be reduced by slagging the melt and gating. The ideal there is to prevent slag from entering the mold in the first place. Though once it is in a mold, gating can help trap it.

Excess or difficult slag in the melt can prevent complete slagging before the pour. Difficult slag can come from the metal used, excessively oxidizing furnace atmosphere, slow melting, and, in my opinion, the crucible materials.

Slag can be made easier to remove by fluxing the melt. But flux can deteriorate crucibles.

So, all of the above need to be juggled in relation to the materials and furnace you have, to give you the quality of casting you expect.

Not every casting purpose requires a perfectly cast part. I've seen casting flaws in commercial products, and in other individuals' work. A machining allowance is, in a sense, a way of dealing with the surface flaws and draft which are a necessary result of sand casting.

But in trying to learn how to cast iron, I want to try to achieve a no flaw casting, so I at least know the subject better. Or as well as I can.
I love it when a Plan B comes together!
Steve
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