Molten metal stirring motor

[bpost1958]

Would a motor from a microwave turn table work to slowly keep the molten pot constantly stirred? Is there any drawback to constant metal movement during casting??

[DLCTEX]

I would suspect it would stir oxidized metal into the melt, without flux to combine it back into the alloy this would not be good. The dross needs to float to the surface and be skimmed off, then the melt needs as little exposure to air as possible. My dos centavos. DALE

[montana_charlie]

I might consider that motor for use in building a case annealing machine. But, I think constantly stirring lead is a bad idea.
CM

[454PB]

Yup....stirring is bad!

That's why most of us have gone to bottom pour pots instead of the old fashioned ladles[smilie=1:[smilie=1:[smilie=1:

[Sundogg1911]

i was at a gun show about a year ago and a guy was selling an old Saeco pot with some type of long shaft motor. I asked the guy if it worked, but he was selling it for someone else and didnt know. I really didn't see any advantage to it

[carpetman]

Yes stirring a pot of molten metal is a very tiring task and anything to help relieve this terrible burden is needed. Has anybody got any ideas how this could be a wind driven deal? It's been pretty windy here lately.

[montana_charlie]

Yup....stirring is bad![smilie=1:[smilie=1:[smilie=1:
I said 'constantly stirring', Whistler.
CM

[Bret4207]

Rotissery motor, but I'm a ladle man and I stir every time I dip.

[EDK]

There was an article in BLACK POWDER CARTRIDGE NEWS awhile back about one of the super accurate shooters years ago and all the antics he did for cast bullets. He had a motor to stir the mix and a thermometer mounted to monitor the temperature. He cast 10 boolit lots and checked the weight...if one was out of spec, all ten got thrown back into the pot.


:cbpour: :redneck: :Fire:

[bpost1958]

My theory was to keep stratification out of the equation. Since the boolits I cast are cast from by-guess-by-golly school of metallurgy; I seldom see two pots of matching material. :roll::???::castmine:

[Nueces]

bp, if, by stratification, you mean gravity separation of alloy metals, I think you can relax. I know a lot of old CB info included cautions about this, but it was one of those pre-urban-myth things. Bullet alloys are chemical combinations, not mixtures or 'mixes', and do not separate into component metals by gravity when melted. The stirring done when fluxing helps put the metal into contact with the flux, but is not needed to 'mix' the metals. Good for us, huh?

Now, if you let a heat stay molten for a long time, there can be some selective oxidation at the free surface, usually resulting in loss of tin fraction as the dross is removed. And, when cooled, some component metals can precipitate out of the crystalline structures, but this occurs only at the scale of the metal grains (single crystals).

The art and science of CBs went through a sort of dark age, with a lot of oft-repeated hooey in the books and periodicals. Sites like this have helped end that unpleasantness.

Mark

[grumpy one]

Nueces, way back in 1935 Frances Weaver proved that in the high-antimony alloys such as typemetals, the liquid metal does stratify enough to matter. She published photomicrographs of etched samples of stirred-during-cooling and not-stirred-during-cooling ingots, and the difference was highly visible. She used a stirring motor in all her cooling curve solidification tests, and consigned the previous work without stirring motors to official oblivion.

I don't know at what antimony level the advantage from stirring becomes worth thinking about. Certainly the antimony does not rise to the surface, but the alloy closer to the surface has higher antimony than the alloy at the bottom of the pot.

[Nueces]

Hi, grumpy one. I attempted to qualify my remarks by saying "bullet alloys." Not too precise, really.

A bit of googling failed to turn up Weaver's research; have you any links or references I might access? I'd like to know how deep Weaver's melts were and to what she attributed the gradient of antimony concentration.

Thanks for the post, Mark

[grumpy one]

Hi, grumpy one. I attempted to qualify my remarks by saying "bullet alloys." Not too precise, really.

A bit of googling failed to turn up Weaver's research; have you any links or references I might access? I'd like to know how deep Weaver's melts were and to what she attributed the gradient of antimony concentration.

Thanks for the post, Mark

Mark, her article is titled "Type Metal Alloys", by Frances D. Weaver (Mrs Harold Heywood), The Journal of the Institute of Metals, 1935, No. 1, Volume LVI, pp 209-240. It is a classic article - it laid down a substantial part of what is known about lead/tin/antimony alloys, following on from the other classic in the field, a 1930 article by Aoki, Osawa and Iwase (Kinzoku no Kenkyu, 1930, vol 7, pp 147-160) which I haven't been able to get hold of so far.

If you end up unable to get the Weaver article PM me an email address and I'll forward it. It's 3.64 MB, so your mailbox would have to be big enough. Meanwhile if you happen to track down the Aoki et al article, I'd really like to get a copy of that.

Weaver tested 80 samples ranging from a low of 2% Sn, 6% Sb, to a high of 12% Sn, 22% Sb, carrying out cooling curves and hardness tests on all of them. She said that several previous researchers had found that "segregation occurs with slow cooling owing to the difference in density between the crystals of the antimony-tin solid solutions and the lead-rich melt". I find this wording a bit murky. Presumably as pure lead (which has a higher melting point than the SbSn compound) crystallizes during cooling, it leaves the lighter SbSn still in liquid form, floating on top of the solidifying lead. Same outcome that she described, but happening in the opposite sequence. In her own experiment she used a "special cylindrical fireclay crucible 1 7/16 in. in diameter and 2 1/2 in. high, the space between the crucible and furnace being well packed with asbestos fibre. The molten alloy was covered with a seal 1/2 in. deep of high flash-point cylinder oil which prevented oxidation and also formed a heat-insulating covering." Each sample was 250 to 300 grams. She also describes her stirring mechanism in detail and provides a diagram.

Geoff

[montana_charlie]

She published photomicrographs of etched samples of stirred-during-cooling and not-stirred-during-cooling ingots, and the difference was highly visible. She used a stirring motor in all her cooling curve solidification tests, and consigned the previous work without stirring motors to official oblivion.


It may not happen, but I visualize some natural motion in a molten alloy. As metal heats up, I expect it rises toward the surface, and after cooling at the surface, I expect it to migrate back down toward the bottom. Even without the stirring while fluxing, or agitation when adding fresh ingots, I think the melt is being 'stirred' a little by convection, alone.

If metals can stratify as the alloy cools, that is something we casters would only experience while our bullets harden in the cavity.

I don't expect any of us are interestred in little stirring motors attached to our bullet moulds...so I have trouble understanding what revelance this 'classic article' about stratification during slow cooling has for us.

CM

[DLCTEX]

I think the seperation occured during cooling as differing metals solidify at different temps. As long as the metal mix is heated above the molten points of all the metals, the mix should not stratify, or at least would take time to stratify. If not we would have stratification in all our boolits that would be obvious. When American Magnesium was in operation at Snyder, Tx. the water from the lake was heated in a huge vat to evaporate off the water, which was condensed and returned to the lake, and then the remainder was allowed to cool slowly. The materials left were stratified and were broken up with a jackhammer and separated by layer to obtain separate materials, magnesium being the most desirable one. DALE

[grumpy one]

A bullet alloy typically consists of a solution of lead, antimony, and the compound SbSn. These constituents have different densities, which seems likely to lead to some vertical variation in their concentration if the molten alloy were undisturbed for a long enough time. However they would remain in solution; they would not stratify in the way Weaver's samples did unless they were allowed to solidify, as hers were. Furthermore molten lead seems rather viscous to me, so any upward drift of the lower density solutes would be rather gradual.

Weaver reported that the extreme stratification of solidified samples which she observed was associated with very slow solidification. Nevertheless this subject seems to me to have considerable importance in relation to the practice of switching off the furnaces of linotype and other type-casting machines when they were not in use. Sure, it reduced surface oxidation, but each time it was done it would have caused stratification of the melt, and this would have increased progressively each time, unless the melt were stirred during use - which I haven't seen reported as having been the practice. I don't know whether the widely-observed loss of tin and antimony from 'old' typemetals was due to this, but it seems likely that disproportionate amounts of those elements would have been found in the surface layers and thus been preferentially exposed to oxidation.

I've read on this board that many experienced casters leave their lead pots full between casting sessions, simply switching them off then remelting next time they want to cast. If this is done, they are relying on whatever stirring they do each time they remelt, presumably as part of a fluxing process or during ladle casting. If they do not stir they will experience stratification.

It sounds to me as if stratification is a natural hazard that we should be aware of, and take steps to avoid.

[Nueces]

Thanks for the good poop, Geoff. This one statement you quote (and some subsequent remarks) indicates to me that stratification was observed to occur during very slow cooling, something we casters don't need to worry about because we keep our metal heated during use. And, after switching off the pot, what does it matter? The metal has to be melted again before further use, so the slow-cooling thing doesn't appear to be relevant to our activities. I remain confident that a hot molten alloy does not exhibit any significant gravity separation at all.

Does that make sense, or have I missed something (else!)?


She said that several previous researchers had found that "segregation occurs with slow cooling owing to the difference in density between the crystals of the antimony-tin solid solutions and the lead-rich melt".

Geoff

I'll poke around for those articles and let you know if I locate the second one. Thanks very much for the offer of a copy of the first. Ain't email great? Let you know also if I need to take you up on that.

Mark

[grumpy one]

Mark, I think the stratification on cool-down is a problem if you leave the pot full when you finish with it, unless you stir it pretty thoroughly the next time you use it. This is an issue that is probably mostly of concern to bottom-pour casters - ladlers probably stir the pot enough to overcome the problem, in the course of their casting activity.

I've just been doing a whole bunch of cooling curves using my Lee Dripomatic as a furnace, and I have to remember to stir it up after I've taken my data and remelted the pot, before I pour it into ingots.

[Nueces]

Geoff, I don't think any stratification would survive after melting. The only mechanism identified by Weaver, et al, occurs during slow cooling. Are we to think layering cold alloys in a pot and melting them could result in differing compositions by height? I don't think hot alloy behaves that way.

As a practical matter, ladlers stir whenever they dip, as you say, and bottom feeders disturb the metal when opening the valve, adding metal and fluxing. There is also a constant heat flux from the pot, which would create convection. I'll bet pot stratification in molten alloy, if it exists at all, is no more than a second-order effect, which can be ignored for our purposes.

One way to look for it would be to bottom pour from one melt, while disturbing the alloy as little as possible, then test for percentages to see if they vary with alloy depth in the pot.

If you can show it's a real concern, you can likely claim an honorary PhD in boolitology, while complicating the lives of us unbelievers, who'll now have to stir while muttering your name. :drinks:

Mark

What have you found from your cooling-curve measurements?

[grumpy one]

I use cooling curves as a way to establish actual composition of alloys. Weaver published a chart of liquidus temperature, and another of BHN, on axes of antimony content and tin content. When you know your liquidus temperature for an alloy you can look it up on the chart, then look up hardness on the other chart. If they both point to the same alloy composition you can be pretty confident about it. My process is to do that for each of my base, stock alloys and use a spreadsheet to calculate what constituents and proportions to use to produce the specific alloy I want. Then I do cooling and hardness tests on the specific alloy as well. When I first started doing that I got some inconsistent results, which always turned out to be due to me having omitted to test one of my stock alloys, due to being overconfident that it really was what it was supposed to be. Having now refined my understanding of what is really in my stock alloys, I no longer seem to get any surprises from the cooling curves on the specific alloys I mix up. Incidentally one of the quickest ways to test whether your stock alloys are what they should be, is to use them to mix up a eutectic and do a cooling curve on that.

[Nueces]

Incidentally one of the quickest ways to test whether your stock alloys are what they should be, is to use them to mix up a eutectic and do a cooling curve on that.

Hey, that's cool, good idea. I just picked up some linotype pigs from a print shop, be a good idea to test it as you suggest, no mixing required for this stuff.

Mark

[Lee]

Just a thought here, and not meant to stir things up([smilie=1:)...
The electronics type guys have been using wave soldering pots and setups for years. Of course with a 63/37 tin/lead alloy, but they use stirrers to produce a "hump" wave motion to wash the underside of a stuffed circuit board. I don't recall hearing any negative issues with that type of set-up...
Just my $0.02..............................Lee:wink:

[grumpy one]

Thanks Lee, you just made my day. Being deeply embedded in Australian colloquial expressions I was having difficulty for a while figuring out why somebody would bother to solder a circuit board if it was already stuffed.

From here on the day will be all downhill.