Casting ingots: speed cooling Vs. air cooling

[Jim]

I cast ingots in a mini loaf pan that sits in a baking pan. The baking pan has about an inch of water in it. This speeds the cooling of the ingots.

Yesterday, I deliberately left the baking pan dry as I had no need to speed cool this cast of ingots. I noticed a significant difference in the appearance and sound of the ingots when dropped on a concrete floor.

Below is a pair of ingots, one right side up, the other upside down, that came from the same cast using water to speed cool them.

http://floydpics.files.wordpress.com/2012/05/dscn0806-e1338327252962.jpg

The next photo is of two ingots that were cast without cooling water. They were allowed to air cool.

http://floydpics.files.wordpress.com/2012/05/dscn0805-e1338222193184.jpg

The ingots that were speed cooled had a slightly higher pitch of sound when dropped on a concrete floor. Also, the air cooled ingots retained that gold color as it was when it came from the smelter.

Anyone care to speculate on this? Court is in session.

[Defcon-One]

Speed cooling is another word for HARDENING. They ring higher because they are harder. Clearly there is some Antimony in your alloy.

The gold is a concern to me. Probably running a bit hot, but I'd flux better before pouring ingots next time.

[Trey45]

A "ring" in the sound when dropped is indication of antimony/tin alloy present in the lead. From my understanding, colors like you're getting, including blue/green/orange yellow is indicative of a high lead content. As for why the same alloy sounds and looks different when one is air cooled and the other isn't has me confused. Normally you'd gain hardness if you actually water dropped, these never physically touched the water, so I can't speculare if these are hardened or not. This thread bears watching, I'm curious myself.

[thegreatdane]

I concur with the speed cooling = hardening of the ingots. I'll be doing the same on my next smelt due to volume and speed required.

[Defcon-One]

The faster a lead alloy containing Antimony cools, the harder it will be! That is the whole point behind water quenching/hardening. When you cool it faster with a water bath, it will be harder relative to the slow cooled metal, regardless of whether the water actually touches the lead or not. It is the time it takes to cool that makes the difference at the molecular level. (Faster = Harder, Slower = Softer!)

I often water quench my ingots after they come out of the mold to cool them down faster so that I can handle them sooner. It doesn't really make a difference in the end product since it will all be re-melted and cast into bullets later.

The Gold color is indicative of a very hot pot or it could also be some kind of contamination. (Copper?) Controlling the temp. (No higher than 700 degrees F) and fluxing aggressively with sawdust should clean that up.

[Trey45]

I've always been under the impression that in order for hardening to occur, the cooling had to be rapid, very very rapid in order to lock the metal at a molecular level. Such as, water dropping bullets straight from the mold into a bucket of water, that's a very rapid cooling. The method Jim is using isn't as rapid as dropping semi molten ingots into a water bucket. Granted it's faster than air cooling, but is it really fast enough to achieve hardening?

[Dan Cash]

The ingots are hardened. Try a hardness tester.

[John Boy]

The difference for solidification of the ingots is due to how the antimony/tin crystal structures form. In the Lyman 4th Edition of the Casting Handbook, there is a whole multi page chapter on the subject

[clodhopper]

The gold color is just a light film of oxidation that formed while the moulten lead slowly cooled.
The lead in your "speed cooled" quenched ingots did not have a chance to form the film as they hardened much faster.

[Defcon-One]

The ingots are hardened. Try a hardness tester.

Thanks Dan, you are right! If you can hear a difference, then they are harder (Hardened to some degree!). It is as simple as that.

Why? The metal clearly contains Antimony, no idea how much but enough to make a difference. I would guess over 2%. If it was derived from many cast bullets that were made using Clip-On Wheel Weight alloy then it has some Arsenic in it as well. Not much, but enough to make a difference.

Antimony basically is the hardener, which means it promotes finer grain structure in the solidified metal. The faster the metal cools, the finer the grain structure, which equates to harder metal.

Arsenic helps promote hardening by causing the grain structure of quenched metal to be stable or even contract over time. So it widens the curve toward the harder end of the scale over time. This is why some metals age harden and adding Arsenic increases that potential.

If you cool the metal slowly at room temperature it will be relatively soft, as you reduce the cooling time it will gradually get harder (Finer grain structure). Jim, this is what you are seeing in your water pan cooling method.

If you drop hot lead into a pot of cold water, you cool it extremely fast and get the maximum hardening affect for any alloy. That is why they water quench in the first place! That is also why COLD water is often used. To increase the temperature differential and promote rapid cooling. (They are going for the maximum benefit.)

Lead/Tin alloys will water quench harden to some degree, but no where near as much as alloys containing Antimony. The problem is that Lead/Tin alloys do not harden over time, they soften and gradually go back to their softer state as the grain structures increase in size over time. So the small benefit of quenching is lost in days or weeks which makes it a waste of time.

Your quick cooled ingots sound different because they are harder. They are harder because you cooled them much faster and inadvertently hardened them! [smilie=b:

[jlala504]

I usually skim off the gold oxidation, should I leave it in with the ingots? Does it make a difference?

[alfloyd]

"I usually skim off the gold oxidation"

If you skim the gold oxidation off, it will come back.
You then skim it off again and again and soon you have NO lead left.

Leave in on as it does no harm and when you melt the ingot to make boolits, flux it back into the alloy mix. This way you loose less lead.

Just my way of doing things.

Lafaun

[Echo]

It's my understanding that the gold oxidation is tin oxide, not lead oxide. I believe lead oxide is blue-purple. In any case, I wouldn't skim the oxides - rather smelt at a low enough temperature so that the oxides don't form as fast, and smelt w/saw dust to get the Sn back into the alloy.
And I want to STRONGLY advise that you lose the straight water! If you want to speed-cool, then put an old towel in the pan and soak it with water, and set the loaf pans on that. MUCH safer!

[Alan in Vermont]

And I want to STRONGLY advise that you lose the straight water! If you want to speed-cool, then put an old towel in the pan and soak it with water, and set the loaf pans on that. MUCH safer!

Must be you don't understand what Jim is doing. The water is on the outside of the loaf pans, the lead is on the inside. Your damp towel will dry out in about 10 seconds and do no further good. It might even get hot enough to burst into flame and that might spread to any nearby grass or brush and before you know it the WHOLE COUNTY might go up in flames!!!!!!!!!!!! One can never be too careful!

I would tend to suspect that Jim is pretty capable of taking care of his own affairs and I can't see where you should be STRONGLY advising him on anything he's doing.