Age (quench?) hardening in Germany

[cbrick]

It's not unusual for me to get questions from the web site, today I recieved this from Germany. If I am reading this correctly his results are backwards giving a higher (quenched I think) ingot hardness than his boolits. Age hardening is a direct result of how rapidly the alloy cools and the larger mass of the ingot should cool somewhat slower than the less mass of the boolit. Before I send him an answer I thought I would see what the brain power here thinks of his letter.

Rick

Rick:
congratulation for your web pages, always a tremendous source of information! As an avid bullet caster the pages regarding casting, alloys, mould design, lubes, sizing etc. have been read over and over again. Especially Glen Fryxell's book has been my favorite literature.
That said I would like to ask you guys whether you could help me answering a question regarding lead alloy and BH. The question is related to the mechanism behind age hardening.

Let me explain: Using different alloys (for different applications) it is my habit to test BH over a period of time to see how much age hardening will lend to a 'final' BH value. Testing is done with a home made fixture using a cantilever system acting on a 6 mm steel ball via a weight. For multiple tests (over time) I cast a small ingot using short angle sections of aluminum fixed together with a c-clamp. Dimensions are 17mm x 18.5mm x 40 mm (roughly 5/8" x 3/4" x 11/2"). The aluminum is preheated to get smooth surfaces by dipping it into the molten alloy. For every testing 3 indentations are made and from the average the BHN is calculated. Duration of load is 10 seconds.

More or less by chance I made some measurements on a bullet cast from the same alloy from which one ingot was tested. I got a different reading. Somewhat irritated I ran a series of parallel measurements on small ingots and on bullets cast from the same pot at the same time. The results from one alloy are summarized in the enclosed diagram. As can be seen, the BHN of the bullets are lower than those derived from the ingot. My question is: Why that?

The alloy used is range scrap, mixed from a handgun range and a rifle range at a ratio of approx. 1 : 1. I assume that the alloy contains tin and antimony with a higher percentage of the latter. Maybe some arsenic is also present. The small ingot starts at BH 8.5 and increases to 16.7. With the bullets, the increase is much less, from 9.1 to 12.4.

It is not unusual to get age hardening. What puzzles me is the difference between the ingot and the bullets: It's the same alloy, cast at the same temperature (from the same pot) and tested at the same time. The only difference I can imagine is the rate of cooling after casting. I suspect that the ingot cools faster than the bullet. The bullet is contained in the hot mould at least as long as the sprue takes to solidify. The small ingot (although much heavier than the bullet) has only a thin sheathing of aluminum which will dissipate the heat rapidly.

To prove that theory (or refuse it) I would like to know how the rate of cooling affects age hardening. The general effect is quite clear: Solubility of Sb (or SbSn) is temperature dependant. During cooling antimony will precipitate from the (then over-saturated) solution. But what's the time dependancy: Rapid cooling equals coarse cristals (dendrites), slow cooling fine ones? Coarse dendrites yield high BH? Is it that way, that quenching works? Or is it the other way around that rapid cooling results in fine dendrites (but lots of) which support the lead matrix much better, giving high BH?

From the literature I have (including Lyman's Cast Bullet Handbook #3) I was not able to get some conclusion. I hope that you or someone from LASC will take the time to give me an answer (I understand that you do not offer a forum). It will be highly appreciated.

Looking forward to hearing from you.
With Best Regards

Karsten

[MtGun44]

One issue is the 6mm ball. With this large a ball, I think there are edge effects in the boolit
testing that invalidate it. T get valid hardness values, you must assume that you are compressing
the material as a whole, far from the edges rather than displacing it sideways as you bulge out
the boolit.

I think if the test is done with a 2mm ball or so, the results will be more consistent.

Bill

[runfiverun]

my thought were also along the same lines but for a slightly different reason.
you have less mass for the ball to push against and the boolit is giving way to the pressure.
whereas the ingot can take three impressions and full impressions.
using a smaller ball would equalize things better.

the thinking about the faster cooling is correct also and if he measured surface area versus mass he might have his other answer.

[RobS]

I've had the opposite effect when testing a flat meplat of the boolit, say a 45 caliber .320" flat nose, where there is an adequate amount of surface area to test. This scenario yields a little harder BHN test on the boolit meplat than on a lead muffin pan ingot that is approximately 2"X3"X1". The BHN of the air cooled ingot is typically softer because of a slower cooling effect due to the mass. I have found that given time though both air cooled pieces of lead from the same alloy typically stabilize out to the same BHN. That has been my results. Water quenching another story of course.

Now when tested on a smaller surface area such as thinner drive bands, the lead alloy flows more and displaces outward toward the lube groove/crimp groove yielding a larger indentation which will then translate over to a softer BHN on a equal chart vs an ingot with a larger surface area to be tested. Even from the same boolit described above, testing on the meplat vs the drive bands can yield different results. All this of course when testing from the same alloy.

Rick, the person in your email never stated where the boolit BHN testing was done i.e. on the thin drive bands where the alloy could flow easier or if he was testing on a large meplat where metal flow would be similar to the ingots. I also agree with the individuals above that using a smaller ball bearing could help especially on tested areas of a boolit where the alloy could flow more easily such as the drive bands or even the rounded side of a boolit that could have been filed a little for a small flat surface.

[cbrick]

I suspected a problem with the testing method but info was a bit skimpy, as an example he never mentioned quenching until like the fourth paragraph so I could only assume all samples were quenched. Even if all samples were quenched the ingots would cool slower than boolits and thus be softer. Also he never gave the weight of his "small" ingots.

Instead of replying to him via email I'll wait until we get a few more responses here and then send him the link to this page, maybe he'll join up.

Rick

[RobS]

Sounds like a win, win Rick.

[Bigslug]

No definite answers, but a couple more questions that might help with the scientific method.

It is our gut instinct that the ingot cools slower than the boolit. On the surface, this makes sense as there is a great deal more mass to an ingot.

BUT:

A boolit is poured into a mold that is pre-heated to about 350-400F and kept there until it solidifies. There is a great deal of effort spent to keep the alloy liquid until proper fill-out is attained. Only then is the boolit set free to air-cool - - usually on a towel or some other insulator.

With an ingot, we don't particularly care about mold temperature or pour technique. The mold is usually room-temperature or at whatever temperature it cooled to from the last pour. This will result in faster cooling of the surface than with a pre-heated mold. Also, a thin aluminum ingot mold might conduct heat outward - cooling the ingot even faster. The ingot pour itself gets a lot more exposure to open air on the way in than a boolit pour does. Then there's the surface area aspect. It might be that an ingot has a harder outer shell, where a boolit's hardness may be more uniform.

I'm thinking that while the ingot is "hot" for a longer period of time than the boolit in the "I REALLY don't want to pick that up with my bare hands" sense (due to sheer mass), it may actually spend less time at the critical temperatures where crystal structure and hardness are determined.

Like I say, not facts, just theories to explain his results - though I tend to agree that his testing methods may be part of the issue. He might want to compare the findings from his rig against an established hardness tester.

[williamwaco]

As can be seen, the BHN of the bullets are lower than those derived from the ingot. My question is: Why that?

I have noticed that air cooled bullets and ingots poured from the same pot at the same time will not immediately test the same hardness.

Two examples

Reclaimed bullet cores.

Ingots measure Lee BNH 11.4
Bullets cast immediately before emptying the same pot measure Lee BNH 7.8
Ingots cast from same alloy deliberately limited to 1/2 inch Thick measure Lee BNH 9.2


Same alloy as above with addition of Linotype
69 ounces above alloy 21 ounces Linotype
Ingots measure Lee BNH 13
Bullets measure Lee BNH 10.4

All bullets in above test were Ballisti-Cast #651 .358 158gr SWC
Immediately means as soon as they are cool enough to handle comfortably.

. . . test BH over a period of time to see how much age hardening will lend to a 'final' BH value.

I have an opinion on this subject, I sure would like to hear Karsten's results.


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[cbrick]

See how this turns out, this was with the email. I had to convert it to a pdf file and then to jpeg. Kinda hard to see and the whole thing didn't fit the page but this goes past 80 days. BHN on the left & number of days across the bottom.

Rick

[Pepe Ray]

Hardness measuring before absolute temperature stabilization is useless.
No matter if air cooled or emersed in water, rather iced or not, the sample should be stabelized to room temp before attempts to measure.
Better to spend your time measuring the temperature of the sample.


Or not.
Pepe Ray

[williamwaco]

See how this turns out, this was with the email. I had to convert it to a pdf file and then to jpeg. Kinda hard to see and the whole thing didn't fit the page but this goes past 80 days. BHN on the left & number of days across the bottom.

Rick

Interesting.

Agrees pretty will with my observations but still no definitive answer to why are the ingots harder than the bullets?

It seems to me that the bullets should be harder because, even air cooled, they cool off MUCH faster than the ingots.



Some of you metallurgists experts please feel free to chime in here.



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