How Much do Alloy Bullets Soften Over Time

[Jeff82]

I've been thinking of taking off from the range over the winter months and dedicating myself to making bullets for the upcoming year. Then I started to wonder how stable the hardness of cast bullets would be over time. I cast with two alloys; one is 7% antimony and 2% tin, the other is 3.5% antimony and 2% tin. I low temperature anneal the bullets after casting at 375-degrees for one hour. The 7%/2% bullets harden to about 14.2 bhn. The 3.5%/2% bullets harden to about 10.8 bhn. I'm measuring about a week after casting. So I'm wondering, if I don't shoot some of these bullets until a year later, would they have become softer?

--Jeff

[RogerDat]

Or harder over time?

[DougGuy]

More than likely they will gradually get harder and might possibly grow a few tenths of a thousandth in diameter.

You can cast, lube and size at the same time, but wait a few months and try and resize some of those or size some that you didn't run through a sizer when they were cast. About have to break the handle off the press to push them through after sitting!

[Jeff82]

I didn't realize they'd be likely to hard or widen. Does it make sense to cast large batches of bullets for use over time? Sounds like I should just mix the alloy in advance and then cast as needed.

[cbrick]

The demise of the HT'd bullet is highly over rated. I did a completely accidental test of age softening several years back. I had convection oven HT a couple of 100 round boxes of RCBS 35 200, placed them in old Sierra bullet boxes all properly labeled with date, alloy, HT temps and final BHN, they were 30 BHN. I placed them under the bench and promptly forgot them. 10 years later while cleaning things out I discovered them and thought they would be putty by now. They were still 26 BHN. So yes, they did age soften but . . .

Higher percentages of tin will increase the age softening time curve, the bullets in my test were 2% Sn. High enough percentage of Sn (5%+) will limit the amount of HT hardening that can be achieved.

I've never noticed a significant increase or decrease in measured diameter, nothing worth noting and any such changes were in the tenths of a thousands or . . . Insignificant.

So Jeff, feel free to cast away, just don't get carried away with the casting while the Pack is playing. :mrgreen:

Rick

[DougGuy]

If your dies, brass, and cylinder throats will handle them at whatever diameter they harden off to, I see no problem with loading large batches. I have some that are 25yrs old and still as deadly and accurate as the day they were loaded.

But I often do make mention of this hardening and growing when I am dealing with customers who send me their cylinders to ream the throats. I like to see a light drag fit of a boolit they send as a sample, but I like to know there is enough room there that if they sit on those boolits for a year, or two years, or ten years, they will still chamber and fire the loaded rounds without issues.

When I did the throats in my own SBH I didn't have a good .4325" reamer, I only had a .431" and had to take it the rest of the way with a hone that I made myself. I had some boolits cast by one of our members out of 50/50+2% at my request, and I loaded up some and used those as a gage, kept fitting them in the throats until I was happy with the fit. Wouldn't you know that after about 6 months, a dozen or so of the rounds I used for a gage, would not chamber. I have since redone those throats, and gotten a .4324" reamer custom ground, but just saying, I had heard about and read about cast boolits actually growing as they sat and hardened off, there was my proof that it happens.

[geargnasher]

I've experienced the frustration of getting to the range only to find my revolver cartridges wouldn't chamber, but that was a long time ago when I was unaware of such things as age-hardening or growth. The cartridges chambered fine a week after the bullets were cast and air-cooled, but a month later they would not. I was running pretty snug cylinder throat fit to begin with.

Oven annealing might result in a little re-hardening over time, but I'm unaware of anyone who's tested this, usually oven hardening is the technique used, sometimes with a slight 'draw' used later to soften slightly from the max oven hardness, but generally the goal is to reach a hardness greater than air-quenching yields.

Gear

[Yodogsandman]

I've always thought you should test an air cooled alloys' BHN after 3 weeks time.

[cbrick]

The time/hardening curve is percentage of Sb dependent. A low SB percentage will take longer to achieve final BHN than a higher percentage. The majority of hardening gained will be after the first several days, after that there will some minor additional hardening over several weeks at which point it levels off and stops. For my long range match revolver I size to mild snug fit in the throats to where they have to be tapped through the throats. If they are any too large I cannot chamber a round. After sizing they are convection oven HT to a final BHN of 18, dried, lubed and loaded. After a year or more these same mild snug bullets are still a mild snug fit and chamber just fine.

I have been experimenting with HT alloys for at least 25 years across many calibers and several alloys, any "growth" I've ever experienced was from minor to insignificant. If yours are growing larger to any significant amount I don't know why or how. Any possibly growth I've experienced was measured in tenths not thousands.

In one test I did with a lower percentage of Sb (about 1%) using the same HT technique the bullets did reach the same final BHN as the 2-3% Sb alloy but it took 3 weeks instead of the normal 3-4 days.

Rick

[Jeff82]

If I understand the advice I'm getting, I think for my 3.5%/2.0% 10.8 bhn bullets I'd probably be OK casting a year's batch in advance as long as I don't size them in advance. My 38 Specials aren't very sensitive to bullet hardness, although my S&W Model 14 is extremely sensitive to bullet diameter. I think I'd probably be best off just mixing ingots for my 7.0%/2.0% 14.2 bhn bullets and casting as they're needed, since my 357 marlin is sensitive to bullet hardness.

[RogerDat]

I'm getting a different conclusion from reading this. Cast, air cooled will change to become slowly harder over time, with insignificant change in diameter. But will be much harder to size later due to being harder. I would think this would support cast away, size and store BUT make sure they are not sized to a super snug fit, go for moderately tight fit.

Heat treating (HT) they will get slowly softer over time. But unless you require a very tight range of hardness and plan on letting them sit for many years they should be way harder than the stated 14.2 for the next decade even with a less rich alloy.

I am a bit puzzled by annealing the 91/7/2 alloy to soften it. As cast that alloy should be around 16 BHN according to the alloy calculator using the rotometals formula. Annealing seems an extra step when reduction in antimony would yield the 14.2 as cast and those would harden over time. Or heat treat to above required hardness so that amount of reduction over next year or two won't matter.

[cbrick]

I'm getting a different conclusion from reading this. Cast, air cooled will change to become slowly harder over time, with insignificant change in diameter. But will be much harder to size later due to being harder. I would think this would support cast away, size and store BUT make sure they are not sized to a super snug fit, go for moderately tight fit.

Assuming a Sb alloy they will age harden over a few weeks, how much depends on the Sb percentage. The higher the Sb percentage the quicker they will reach final BHN. They will grow very little.


Heat treating (HT) they will get slowly softer over time. But unless you require a very tight range of hardness and plan on letting them sit for many years they should be way harder than the stated 14.2 for the next decade even with a less rich alloy.

They will age soften very slowly. Unless your shooting CB bench rest matches any change over a few years will mean very little.


I am a bit puzzled by annealing the 91/7/2 alloy to soften it. As cast that alloy should be around 16 BHN according to the alloy calculator using the rotometals formula. Annealing seems an extra step when reduction in antimony would yield the 14.2 as cast and those would harden over time. Or heat treat to above required hardness so that amount of reduction over next year or two won't matter.

Unless the Sb alloy was either quenched from the mold or oven HT annealing would be pointless. It cannot be made softer than the air cooled BHN of the alloys that make up it's total. If you have an air cooled 16 BHN alloy no amount of annealing will make it softer than 16.

Rick

[Yodogsandman]

I have a coffee can of boolits cast from an RCBS 30-180 SP in 1999 of COWW and 2 1/2% Sn, sized .310" and still lubed with Lyman NRA 50/50 alox/beeswax formula. Originally I shot them from my 308 and now I'm shooting them out of a 30-30 win. No difference that I can detect.

[RogerDat]

Unless the Sb alloy was either quenched from the mold or oven HT annealing would be pointless. It cannot be made softer than the air cooled BHN of the alloys that make up it's total. If you have an air cooled 16 BHN alloy no amount of annealing will make it softer than 16.

Rick

That makes sense, can't be made softer than it is. Then I'm puzzled by the OP's stated 14.2 BHN from a mix that I think should be around 16 given 91/7/2 alloy specified. I assumed annealing mentioned was responsible. Brass and copper do soften so I assumed that was the cause for the lower BHN given for that alloy. Good to know lead alloy does not act the same way.

[runfiverun]

375 isn't annealing anything your just getting the whole batch to it's air cooled bhn.
steel molds run at about 375-f when casting in them.
now if your water dumping the batch from the oven then you are influencing the final hardness.

[Jeff82]

The reason that I low temperature anneal the bullets immediately after casting is to standardize the rate of cooling. Since I cast year round, temperatures can range from 90-degrees to maybe 10-degrees below zero, and while I try to maintain a consistent melt temperature, this can vary as well. I was having a great deal of difficulty achieving the same bhn from the same alloy mix while air cooling, and an even harder time when water dropping. By air cooling and then low temperature annealing I've been able to achieve very consistent bhns from batch to batch.

[youngmman]

That makes sense, can't be made softer than it is. Then I'm puzzled by the OP's stated 14.2 BHN from a mix that I think should be around 16 given 91/7/2 alloy specified. I assumed annealing mentioned was responsible. Brass and copper do soften so I assumed that was the cause for the lower BHN given for that alloy. Good to know lead alloy does not act the same way.

Just as a comparable reference, I have been loading a 93/5/2 alloy for many years since it seems to work well in most any handgun caliber I use. It checks at BHN 14 on the nose. I have checked some bullets cast from this alloy several years ago but never used and they have maintained the BHN 14 hardness.

[cbrick]

I think too many people are getting wrapped around the axle over this in applying it to the vast majority of cast bullet needs. For most uses/needs the effect is from minor to insignificant.

Air cooled Sb bullets will gain some hardness over a few weeks as they stabilize. After that over many years they will loose a bit of that. For almost all cast shooting this effect is insignificant.

Heat threated Sb alloys will gain hardness over a few days to a few weeks depending on the Sb percentage and then over years will slowly age soften. Again, insignificant for most cast shooting. If you need hardened bullets just give them the time needed to get there and shoot'em.

It's an interesting phenomenon with Sb alloys but beyond that most cast shooters will never see a degrading of their shooting by it.

Rick

[runfiverun]

The reason that I low temperature anneal the bullets immediately after casting is to standardize the rate of cooling. Since I cast year round, temperatures can range from 90-degrees to maybe 10-degrees below zero, and while I try to maintain a consistent melt temperature, this can vary as well. I was having a great deal of difficulty achieving the same bhn from the same alloy mix while air cooling, and an even harder time when water dropping. By air cooling and then low temperature annealing I've been able to achieve very consistent bhns from batch to batch.

that makes sense, when we water drop what we are really doing is influencing the rate of cool down.
by casting in wide temperature swings your rate of cooling can be influenced there too.
heck humidity can influence how an alloy acts when being poured from a pot.
is it enough to really matter in the long run [shrug]

[Jeff82]

"Just as a comparable reference, I have been loading a 93/5/2 alloy for many years since it seems to work well in most any handgun caliber I use. It checks at BHN 14 on the nose. I have checked some bullets cast from this alloy several years ago but never used and they have maintained the BHN 14 hardness."

That makes sense. When I cast at 92/6/2 I also come out at about 14.2, which is about the minimum antimony percent that I need to achieve decent accuracy with my Marlin 1894C, though I still need to use CGs. I add the extra 1% of antimony as a fudge factor for measurement error, etc. I've found that with air cooling and low temperature annealing its hard to increase the bhn much between 6% and 8% antimony, which is OK by me. My 5% antimony bullets usually have a bhn of about 13, but don't shoot well out of my Marlin when pushed to about 1,600 fps.

[Jeff82]

I think too many people are getting wrapped around the axle over this in applying it to the vast majority of cast bullet needs. For most uses/needs the effect is from minor to insignificant.

Air cooled Sb bullets will gain some hardness over a few weeks as they stabilize. After that over many years they will loose a bit of that. For almost all cast shooting this effect is insignificant.

Heat threated Sb alloys will gain hardness over a few days to a few weeks depending on the Sb percentage and then over years will slowly age soften. Again, insignificant for most cast shooting. If you need hardened bullets just give them the time needed to get there and shoot'em.

It's an interesting phenomenon with Sb alloys but beyond that most cast shooters will never see a degrading of their shooting by it.


I hope it comes down to that. I'm not too concerned about it for my 38-special bullets, I'm a little more paranoid when it come to my 357 carbine rounds.