Does powder coating a bullet "restart the clock" on the BHN increase?

[JimGnitecki]

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

 

[Riomouse911]

I’m not a metallurgist, but a wild guess is no.

My guess is because you are not rearranging the bullet alloy molecular structure enough baking it to 400 degF, it still remains solid (solid-ish?) and it is the powder coat material that is melting.

The molecular structure of the bullet alloy is altered greatly when you heat it high enough to go from solid to fluid, and then let it cool back to a solid in the mold and quench it in water (if you do that).

Again, just a guess from an average Joe. Maybe some of the more scientific folks on THR have the academic answer for you.

Stay safe.

 

[nettlle]

I'm not a metallurgist either but when it comes to hardening and tempering metals they have to have carbon in them which lead does not.

 

[41 Mag]

That is something that I have wondered about but never checked. To me it is akin to annealing, but I don't have the evidence to make a valid judgment.

I can say however, that for the most part all of my coated bullets are usually loaded weeks or even months later. If there is anything going on with them it has probably already done it.

Now there are some who not only quench when cast, but also as some as they come out of the oven from coating. Me, I don't do either. Everything I cast has done just fine simply air cooling from both.

 

[Hooda Thunkit]

As for being a metallurgist, both my (engineer) Dad and one sister are so educated and certified. I try not to listen to them when they talk shop.

450° F, depending on the specific alloy, can be very close to the melting point of a bullet. Keep in mind, the eutectic alloy of Sb/Sn is about 60% Sn, and melts at around 360° F (think solder).

My short answer - yes, heating to 450 degrees can at least partially restart the hardening cycle.

 

[AJC1]

I quench both out of the pot and out of the oven. I don't believe powder coat is hot enough for molecular movement in lead. I don't know why your going to 450, I use 400 for 20 minutes and quench to cool them off. More concerning would be the work softening when sizing.
The basis for my thoughts is heat treating. If you heat treat boolits, there are temperatures and times associated. That is the direct purpose of that process, leaving me believing less has no real effect considering the discussion on how tight the band is to be effective.

Heat treating cast bullets and lead - antimony - arsenic alloys

 

[JimGnitecki]

I quench both out of the pot and out of the oven. I don't believe powder coat is hot enough for molecular movement in lead. I don't know why your going to 450, I use 400 for 20 minutes and quench to cool them off. More concerning would be the work softening when sizing.
The basis for my thoughts is heat treating. If you heat treat boolits, there are temperatures and times associated. That is the direct purpose of that process, leaving me believing less has no real effect considering the discussion on how tight the band is to be effective.

Heat treating cast bullets and lead - antimony - arsenic alloys

The Eastwood instructions on the powder package call for 450F until the powder has melted (that is when its appearance changes from "matte" to "glossy"), then lower the temperature to 400F. I am following those instructions.

Jim G

 

[AJC1]

The Eastwood instructions on the powder package call for 450F until the powder has melted (that is when its appearance changes from "matte" to "glossy"), then lower the temperature to 400F. I am following those instructions.

Jim G

Never read them . After all of the loads of bacon low temp coating videos I never worried....

 

[KKJ13]

I got distracted when powdercoating one time and by the time I got back, all of the bullets sagged into puddles. So, I think I restarted the clock on those ones.

 

[JimGnitecki]

I got distracted when powdercoating one time and by the time I got back, all of the bullets sagged into puddles. So, I think I restarted the clock on those ones.

That sort of "proves" that the oven heat during powder coating IS sufficient to undo the hardening, doesn't it?

Jim G

 

[doubleh]

I have never worried about it. Since I took up powder coating my bullets leading has disappeared so problem solved at least in my case. Soft lead mushrooms better than hard lead so I would think it will be better in a hunting situation.

 

[JimGnitecki]

I have never worried about it. Since I took up powder coating my bullets leading has disappeared so problem solved at least in my case. Soft lead mushrooms better than hard lead so I would think it will be better in a hunting situation.

Yes, when Richard Lee in Modern Reloading recommended matching the BHN to the peak pressure, he was talking about ACCURACY, not terminal expansion. Terminal exonasion requires softer bullets that can mushroom aggressively. That is more important in hunting than absolute best accuracy.

Jim G

 

[JimGnitecki]

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 G

 

[JimGnitecki]

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