Does heating bullets during powdercoating re-start the age hardening process?

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  • Last Post 07 November 2023
JimGnitecki posted this 04 November 2023

I have seen charts like this one:

which seem to say that if I cast bullets that have a target BHN = 20, they might take up to about a week to get to full hardness.

But, if I in the meantime powder coat them by heating at 450F for a couple of minutes to melt the powder, and then at 400F for 20 more minutes to bake the powder, does this cause the hardness to again "fall" to where it was right after casting and "re-start the clock" on the hardening process?

Jim G

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JimGnitecki posted this 07 November 2023

In prepration for today's range session, I analyzed my bullet quality as objectviely as I could. As I am still an amateur in bullet casting, my Extreme Spread on cast bullet weight is between 0.9% and 1%. My standard deviation is 1.2 grains (=0.25%). I'll get better with time.

Today's range session was encouraging:

Average velocity was 1393 fps, which is about a 35 fps increase versus the softer heavier bullets that were cast with softer alloy.

At 200 yards, the Labradar said they were still reliably supersonic, which means I'll be able to use my Shotmarker electronic tareget system with the Pedersoli rifle out to 200 yards. 225 yards MIGHT be possible, but the remaining velocity there is right around the speed of sound, so the Shotmarker might not catch some of the bullets at 225 yards.

Accuracy was improved today in 2 ways today over prior range shooting sessions:

1. The best 5-shot group fired today was 1.25" at 150 meters = 164 yards, which translates to 0.73 MOA at that range. Slowly getting closer to my target of 0.6 MOA.

2. There were no "wild" shots at all. Just groups not as good as the 1.25" group, but NO "flyers". That is a very significant improvement.

So, higher BHN did make a noticeable improvement.

This was with BHN = 16 (so far) and .459" sizing. Need to next try BHN = 20.

I will also, separately, try a batch of bullets sized to .458" versus .459", since the .459' did make a marginal improvement over the prior .460" in a prior range test, That makes .458" worth trying, and the Lee .458" sizer insert and punch did arrive a couple of days ago.

Jim G

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JimGnitecki posted this 06 November 2023

I THINK I might see why powder coating does not restart the age hardening process: The thermal conductivity of Lead bullets is too low.

All 3 of the alloy constituents I am using have low THERMAL CONDUCTIVITY. Lead and Antimony both have thermal conductivity that is only about 10% of Copper's thermal conductivity. Tin has about 20% of Copper's thermal conductivity and the 2% Tin content in my alloy is too low to make much difference in the alloy's thermal conductivity.

So, exposute to 450F for just a couple of minutes, followed by 20 minutes exposure to 400F, simply does not allow enough time to meaningfully heat up the entire mass of a big 473 grain bullet.

A smaller and lighter bullet might heat up more meaningfully, which is why some bullet casting shooters might have indeed had a "hardening restart".

Jim G

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JimGnitecki posted this 06 November 2023

Hmm, VERY interesting result today: I powder coated a batch from the bullets that were cast Friday, selected a sample bullet from the newly powder coated batch, and used the Lee Hardness tester to compare it BHN to the UNcoated bullets. The original uncoated bullets were as of 58 hours after being cast showing a BHN = 15.5. The powder coated sample, just 13 hours after being powder coated, showed a BHN = 58. So, within the reasonable error range likely with a Lee Tester, it's the same as the UNcoated bullet sample.

 

Maybe 450 degrees F for 2 minutes followed by 400 degrees F for 20 minutes, is not enough to cause a "restart" of the hardening process?

 

Jim 

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JimGnitecki posted this 05 November 2023

I am afraid to try more heat and then water drop, as I am trying to follow the Eastwood instructions that come right on the powder package. They say to heat at 450F until the powder melts (i.e. when its appearance becomes glossy versus matte), and then lower the temperature to 400F and bake for 20 minutes more. 

Jim G

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Hornet posted this 05 November 2023

To give you something else to confuse the issue...  the miscreants that I know who are powder-coating are usually running softer alloys ( like 4:1 WW/Lino) and leave the oven at about 450° for the entire bake time ( ~20 minutes after shiny).They then water-drop and get BHN's well into the 20s with a much shorter aging time. Just another option...

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JimGnitecki posted this 05 November 2023

OU812: Yes, I have a Lee Hardness Tester, and its pocket microscope is mounted in an aftermarket stand to make it much easier to use and get accurate readings reliably.

Duane: The alloy mix I am testing now is Tin 2% / Antimony 13.5% / Lead 84.5%. This is supposed to get me to BHN =20 to finally answer the question of whether BHN can make a significant accuracy difference or not. Bullets made with this alloy WILL be for paper only,.

Interestingly, this morning, 1 full day after they were cast, these bullets were still only at BHN = 15.5. They have a ways to go to reach BHN 20. Noting that, and the above comments about powder coating restarting the hardening process, I am now planning to powder coat them tomorrow and let them age a bit.

The chart I posted above suggests that it will likely take "about" a week to fully harden after the powder coating has undone any hardening that HAS occurred so far

Jim G

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Duane Mellenbruch posted this 04 November 2023

If the ratio of the alloy has a greater percentage of tin over antimony, it will age soften over time.  If the ratio of the alloy has a greater percentage of antimony over tin, it will age harden. I am not addressing the heat treat and quench factor if that is employed.  Powder coating does "re-start the hardening or softening process, it does not "Stack" hardness.

From the LASC Site

  • Antimony hardens / strengthens lead. It helps the alloy flow and fill out the mould with better, sharper detail. It is extremely brittle and if the bullet is for other than paper punching the antimony should be held to about 6% of the alloy. Antimony is what enables the heat treating of lead alloys.
  • Tin adds both minor strength and minor hardening to bullet alloys. It reduces the surface tension of the melt allowing the metal to flow and better fill-out the fine details of the mould. Tin provides dross protection up to about 750o. Tin reduces the melting point of lead alloys plus the higher the percentage the more it limits the amount of heat treating that’s possible. Higher percentages of tin cause faster age softening.

 

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OU812 posted this 04 November 2023

Not all alloys harden over time. Do you have a hardness tester? I have the Seco version. It's expensive, but works good.

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JimGnitecki posted this 04 November 2023

Sounds like the answer to my question is "Yes", the powder coating process will restart the clock. cry

Jim G

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OU812 posted this 04 November 2023

To anneal any lead alloy: Place bullets in oven at 350 degrees F for one hour, turn oven off and allow to cool in oven. The lowest possible hardness will be obtained with a very slow cooling rate. Per Veral Smiths book, Jacketed Performance With Cast Bullets

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Spindrift posted this 04 November 2023

I've never done proper hardness tests over time, after powder coating. But I've noticed that the process indeed "re-starts the clock" to some degree.

My practical approach, is I wait 4 weeks after coating before shooting.

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