Making ZA27

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Doug_Edwards
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Re: Making ZA27

Post by Doug_Edwards » Thu Apr 01, 2021 1:01 pm

jscarmozza wrote:
Tue Mar 30, 2021 7:52 am
I've been making some pot metal castings and began to run low on metal, so I added a zinc ingot and some thin aluminum trimmings to the pot, melted it and continued to pour with good results. When I ran low again I did the same thing, but threw in a piece of 50/50 lead tin solder this time...that changed things! The mold filled beautifully, the casting finish went from bright shiny silver to frosted whitish silver, BUT, the casting was weak and broke into many pieces when removed from the mold. You could snap the casting easily just like a piece of chalk. Obviously it was the solder that made the difference, do you think I could dilute the problem by adding more zinc and aluminum to the pot?
Lead, tin, and antimony (plus others), are impurities in zinc that cause intergranular corrosion that will eventually cause the castings to break apart.
Mantua model trains with the die cast zinc boilers were known to break apart over the years by just sitting. This is the reason "pot metal" got a bad name. Some carburetors also suffered from this.

Toss everything that you have added the lead and tin to, including the pot you melted them in, and start over.

Regards,

Doug
http://www.precisionlocomotivecastings.com/
Building a 70 ton Willamette in 1.6"
Building a 80 ton Climax in 1.6"

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jscarmozza
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Re: Making ZA27

Post by jscarmozza » Thu Apr 01, 2021 5:47 pm

I just came in from the shop where I was cleaning up the window frame castings and saw your comment regarding pot metal. I'm disappointed to say the least, but It's better to find out that I have a problem now than after I finish the project. This is not the hobby for anyone who is easily discouraged, so I guess I'll be starting over again.

jscarmozza
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Re: Making ZA27

Post by jscarmozza » Sun Apr 04, 2021 9:16 am

I'm not trying to kill this matter but I'd like to share what I learned and observed in case someone out there wants to try something similar. Notwithstanding the long term corrosion affects that Doug mentioned, I think the addition of the lead tin solder required cooling to a temperature below that of the zinc aluminum before extracting the casting from the mold. The reason I say this is when I extracted the casting containing solder from the mold, both the mold and metal were still very hot and the casting broke up, the casting pieces could easily be broken into smaller pieces by hand; in retrospect I don't think it had fully solidified. I think this because the other day when I was cleaning up the shop, I picked up some of the pieces containing the lead tin solder and was unable to break them like I could when the casting was hot. When I poured these castings I had the steel mold on an electric hot plate, so the mold wasn't just warm it was hot, and it stayed hot all through the extraction and mold reassembly processes before it went back on the hot plate for the next pour. The mold was so hot I could only hold it for very short periods even with asbestos gloves (I know). I think the melt containing lead and tin needed to cool to a much lower temperature after casting to gain enough strength to successfully remove it from the mold. When I had trouble with the batch, I poured off the melt containing the solder into ingot molds made from angle iron and wound up with trianglular cross section ingots 1" on each side and 6" long. I clamped one of the ingots in a vise and gave it four hard hits with a lump hammer before it broke; it may not have any longevity, but it's strong. That's my experience, I hope it may be helpful.
John

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Re: Making ZA27

Post by Harold_V » Sun Apr 04, 2021 3:09 pm

What you're reporting tends to be common knowledge for some folks. That's called hot short. Some alloys don't have any strength when they're hot, and crumble readily. They can NOT be worked hot, yet may display excellent strength when at ambient temperature. Several of the copper alloys display that quality, although I am unable to provide the information that would help those to know which ones (because I don't know).

Don't ever apologize for providing helpful tips. If some folks take offense, they are the ones with an issue. None of us are born with knowledge, and we can often benefit from the endeavors of others. It's helpful to report mistakes as well as successes. Lessons are learned from all experiences for those with an open mind.

The one thing to keep in mind, here, is what Doug reported. Some combinations of elements are simply not compatible. They may appear to be stable, but over the long term will self destruct.

One other thing. Not all elements will alloy. Some elements simply will not combine with others and remain combined. Some will, yet can be separated from one another simply by the addition of a different element. A good example is silver and iron. They do NOT alloy, yet gold and iron do. If one finds iron with gold content, to separate the gold from the iron, all that is required is to melt the resulting gold iron alloy and introduce silver. The gold will readily transfer to the silver, leaving the iron free of gold. With sulfur present, when the resulting metal is poured to a cone mold, it readily stratifies, so there's a complete and clean separation of the iron from the gold silver alloy. Gold and silver are readily separated from one another chemically.

Yeah, I know---a lot more than you care (or need) to know, but it can be useful information for that time when you're faced with a similar issue.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

jscarmozza
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Re: Making ZA27

Post by jscarmozza » Mon Apr 05, 2021 5:15 pm

Harold, do alloys result from a mixture or chemical reaction? Depending on the answer, more questions to follow.

Harold_V
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Re: Making ZA27

Post by Harold_V » Tue Apr 06, 2021 1:23 am

I'm not a chemist, so my answer may be a bit squirrely.
So far as I know, alloying isn't a chemical reaction--it's simply a matter of melting elements and allowing them to combine. Many do so readily. From my point of view, the only chemical reaction might be the source of heating.

When it gets down to steel and cast iron---that may not be true. I just don't know. Carbon does not have a liquid state, but it is readily absorbed (as a gas) by iron and kept in solution when it cools. If the amount is lower than a little less than two percent, the result is steel. If it is greater, some of the carbon precipitates as graphite, at which time the resulting alloy is cast iron. It can be white or grey, depending on cooling rate, silica content and percentage of carbon. That, in my mind, would be a chemical reaction. Like I said, a bit squirrely, due in part to my ignorance.

That said, separating elements once combined, that's a different matter. While some will readily separate, others will not. As an example, once silver and gold are combined, they will not separate easily, although the required process is quite simple. Same for copper alloys. Some folks get the mistaken idea that because gold is so much heavier, all that need be done is to melt them and allow them to cool, with the gold settling on the bottom. That doesn't happen.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

jscarmozza
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Re: Making ZA27

Post by jscarmozza » Tue Apr 06, 2021 10:26 am

Thanks, you answered all my questions.
John

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