Sneak peak at upcoming BHN experiments with heat treat/quenched alloy

[BHuij]

Hey everyone--

I've been trying to find ways to stretch what is possible with clip-on wheel weight alloy, since I get it for free. For a while I've been interested in the idea of heat treating and quenching for the BHN increase, but I have also had far better results in both pistols and rifles by powder coating than using traditional lubes.

Unfortunately, powder coating anneals the alloy and undoes any benefit from heat treating and water quenching. Until recently, I believed that heat treating after the powder coat process would destroy the powder coat. However, it seems like maybe it doesn't.

The LA Silhouette Club has a great article on heat-treating and quenching ternary alloys of Pb/Sn/Sb with trace amounts of As. It includes some limited data about how time and temperature affect the final BHN over time. But their data is very incomplete, and I really wanted to be able to predict with good accuracy what my BHN would be for the purposes of developing various types of loads in various calibers, particularly after powder coating.

To that end, I have started some testing to track the BHN of a bullet over time under a variety of heat treat/quench conditions, both powder coated and left bare. I have a lot of testing still to do, but I wanted to share my first-day results, as they have already been quite promising.

The alloy I'm using is plan old clip-on wheel weights. I made a batch of a few hundred bullets from the same pot of alloy, using the Lee .356" 124gr tumble lube mold that I shoot in my 9mm. As a baseline, 1-2 weeks after casting and air cooling, this alloy comes in at around 12.3 BHN, which is pretty standard for COWW.

I will be doing a bunch of test groups, tracking BHN every day for a week, every week for a month, and then a few more checks after longer intervals up to a year. These bullets will be heat treated at various temperatures for various times, and I'm testing bullets that are both left bare and powder coated before heat treating and water quenching.

Today I started my first measurements.

Bullet A was powder coated, but not heat treated afterwards. On day 1 (today), it measured 9.2 BHN. I expect over the coming week, it will stabilize right around 12 BHN just like the non-PC'd bullets did.

Bullet B was powder coated, allowed to cool to room temperature, and then put back into the toaster oven at 400 degrees F for 30 minutes before being immediately water quenched. This represents the "mildest" heat treating in my battery of tests, both in terms of time spent baking, and temperature. On day 1, it measured 10.1 BHN. This makes sense -- it's a tiny bit harder than air cooled bullets, and I expect it will harden further over the coming 14 days. How high the BHN goes before stabilizing will be interesting to see.

Bullet C was powder coated, allowed to cool to room temperature, and then put back into the toaster oven at 425 degrees for 60 minutes before being immediately water quenched. On day 1, it measured an impressive 19.3 BHN. I was surprised to see that much of a jump in BHN more or less immediately after quenching in water. We'll see how much higher it goes over time.

Bullet D was treated identically to Bullet C, but actually baked post-PC in the oven at 425 degrees F for a full 120 minutes before being water quenched. This represents the high-end of heat treating for my experiments, the hottest and longest time in the oven. Weirdly, this one measured out to 17.9 BHN. Still significantly higher than base alloy strength, but not as hard as the one baked for an hour.

My theory at this point is that at a certain point, it doesn't matter how long you bake the bullets - they will reach a "max" temperature in the oven (how long it takes to get there depending on bullet mass) and further time spent at that temperature will have no further effect. Most likely, the unexpected 1.4 BHN difference between the 60 minute and the 120 minute bullets is due to differences in time spent between taking out of the oven and quenching in the water, or simple measurement error (or both).

When I have more data collected after a few more weeks or so, I'll definitely post my full results here. At that time I'll give full methodology, procedure, etc., as I'm trying to be fairly scientific about this so my results aren't confounded. But just thought I'd share -- it looks like heat treating and quenching has a significant and immediate effect on BHN for COWW, even after powder coating. I didn't expect to be able to get 19+ BHN on day 1.

[popper]

Record the water temp as well. You will find that the heat treating time & temp actually allows the molecules in the solid to move around to get a stable alloy (like you have when first cast). Fast cooling 'locks' them in place. As the LASC experiment, try some in ice water, just cold water, etc. You will see the difference! Also try some with As (shot) treated the same way. As you are using WW, not foundry alloy, other's numbers may be different, but close. When quenching, put a screen in the water container for the boolits to land on so they get cooled on all sides the same. They should be in the water 5 min. to make sure cooled all the way through.

[JBinMN]

Interested to learn more about this, even though I am a "lube" guy & not a "PC" guy.
( Neither definition of "PC" for me, BTW. LOL )

Thanks for sharing your experiments.

[DrewSeeg]

Good stuff ... I just tried heat treating for the first time, half soft half lino, 45mins at 425 then ice water. Seemed to make a much harder bullet but I don't have a tester. Is there a reason why you are letting them cool and reheating vs just heating during the PC cure? Keep us posted!

[BHuij]

...Is there a reason why you are letting them cool and reheating vs just heating during the PC cure? Keep us posted!

I have always gotten the best results powder coating by curing at 400 degrees F for 20 minutes. Basically my gut feeling tells me that letting the PC cure and cool fully before throwing it back in at a higher temperature for a much longer time isn't as hard on the powder coat and will make for a better final product. I might be completely wrong about this though.

[DrewSeeg]

Well, I don't have any proof otherwise. Keep doing what works for you!

[kungfustyle]

Great. Will be following. Thanks.

[retread]

Interesting. Will be interested in the results. Thanks.

[BHuij]

I have revised my experiment--

Before, I was testing at 2 temperatures: 400 degrees and 425 degrees. Each of these temperatures was tested at 4 times: 30 minutes, 60 minutes, 90 minutes, and 120 minutes.

I was not recording my water temperature when dropping, and there were some differences in how long the bullets sat in the water before I pulled them out. Some sat for a few minutes because I was busy, others were pulled out almost immediately.

Preliminary results suggested that how long the bullets spent in the water, as well as the water temperature, are both actually pretty important to the final hardness. So I will now be standardizing per Popper's recommendation on 5 minutes in the water after the quench, and for the time being, doing all tests using room temperature water (something like 65 degrees, I'll measure before I do any more testing). In the future, I may experiment with different water temperatures, but I think controlling the oven temperature will be an easier, and no less effective way of reaching a given target BHN.

I also found evidence to support my hypothesis that the most consistent results come from heating the bullets for long enough to reach the target temperature all the way through, with no additional benefit from having them cook longer. 30 minutes appears to be too short of a time at least for my 124gr .358" bullets, and would result in different final hardnesses using different bullet types, (i.e. a 55 gr .225" bullet would heat through sooner than a 230gr .309 bullet). I suspect most bullets will reach target temp within 60 minutes, with maybe some larger ones taking as long as 90 minutes, so I will be testing both 60 and 90 minutes for each temperature, rather than 30/60/90/120. This cuts my time spent testing BHN each day (and my time spent preparing each test group) in half, and I suspect without sacrificing any useful or interesting data.

Finally, I have added in an intermediate temperature of 410 degrees F, so I'll now be testing 400, 410, and 425. The reason for this is, I made a small handful of .225" bullets for my AR-15 a few weeks ago, not as a part of this experiment. I gas checked/sized, then powder coated, then sized again to .225, and lastly heat treated at 425 for 90 minutes before quenching in water and letting them sit for a good 10 or 15 minutes. I checked the BHN when they hit their 2 week mark after heat treating, and they came in at a whopping 34.8 BHN! I suspect that is a lot harder than I would ever need for practical purposes, so 425 may be cooking hotter than I want. At the same time, I suspect cooking at 400 is probably not going to get me into the mid-high 20s, which I think I DO have a practical use for at some point. Hopefully 410 will provide a good medium. Furthermore, with 3 data points for temperature, I can probably come up with a reasonably good model to estimate the final "stable" BHN of a heat treated bullet given any temperature between 400 and 425, which should allow me to really dial in on a BHN by altering the heat I cook bullets at.

At this time, I will not be testing a "control" non-powder coated group of bullets that corresponds to each powder coated test. Despite anecdotal evidence I have read elsewhere, it doesn't appear that powder coating actually affects the heat-treat/quench process in terms of final results, at least using my process here which separates the powder coating step from the heat treating step (which is encouraging). This testing is fairly time consuming even with 7 groups of bullets to test daily, and doubling that to have equivalent non-PC control groups would push this experiment into being unrealistic to keep up with faithfully, given my 9-5 job and 4 month old son Besides that, since I have personally sworn off traditional lubes until such time as I get a lubrisizer, I'm really only interested in the results of heat treating powder coated bullets at this point anyway. Sorry to those of you who are not on the PC bandwagon, but also I kind of suspect that the results here will more or less generalize to non-PC'd bullet, so hopefully this data will still be useful to you.

The experiment will get its own post where I outline in detail my methods, etc. Just wanted to give everyone a heads-up here.

[fredj338]

I air cool when casting & then water quench out of the oven after PC. I am only going 15m, but it seems to give a little BHN bump, 3-4BHN.

[BHuij]

That sounds right in line with my findings so far. However, it looks like doing more "serious" heat treating after the powder coating is completed can get you way higher than the mid teens for BHN. A I mentioned in my above post, I got some projectiles intended for my .223 AR up to nearly 35 BHN after PC.

[Tripplebeards]

Last fall I did a little testing like you did. I started with coww. BH was around 13.4. I water quenched them dropping them right out of the mold into ice water and ended up with 33 BH. If I took that boolit and PC'd it, put in in my oven at 400 degrees for 20 min and immediately dropped it back into ice water I was around 15 BH. I learned that I did gain a little harder boolit by wq in ice water after PCing ( after I was told by many posters I would not,lol) I was going to heat treat them like you did as I have been baking PC on boolits at higher temps for a smoother paint flow and at longer times and have found the PC doesn't burn or turn dark but haven't gotten around to it. Actually I've been going the opposite and want a softer BC than my 13.4 BH and have been ac and GC my boolits so I get expansion and Kennetic energy transfer instead of a hard as a rock boolit zipping right through game.

Keep up with the good experiments. Looks like I'm not the only one who figured out you gain a little BH when wq after PCing. It's only 1 or 2 BH but its something.

[BHuij]

You should play with making soft nose bullets with two different types of alloy

[retread]

What effect on the PC when reheating for heat treating? Are you standing them up like you would when curing the PC or are you laying them loose in a basket or? If touching do they tend to stick together?

[BHuij]

What effect on the PC when reheating for heat treating? Are you standing them up like you would when curing the PC or are you laying them loose in a basket or? If touching do they tend to stick together?

As far as I can tell, the PC is not compromised. Part of my experimentation will be checking to see what passes the hammer test though. I am standing them up in a metal container, but it's difficult to stop them from touching each other when I quickly yank them from the oven to submerge in water. They do not appear to be sticking together at all. I believe the powder coat is a lot more "liquid" during its initial cure than it is during subsequent heat treating.

[dbosman]

Cast Bullet Association member Bill McGraw spot treats the noses of high temp quenched bullets for hunting accuracy and expansion. He uses a low temp torch.

You should play with making soft nose bullets with two different types of alloy

[BHuij]

Cast Bullet Association member Bill McGraw spot treats the noses of high temp quenched bullets for hunting accuracy and expansion. He uses a low temp torch.

Yeah that sounds like it would work. Submerge the bullets nose-up partially in water, with only the part you want annealed out of the water, hit with a torch for a little bit, boom; instant softnose.

[popper]

Yes, the powder is cross linked at the first cook and does NOT un-link. Test some dumped into ice water and you will see the difference.

[BHuij]

Been tracking data for a week so far and I gotta say the results are surprising. I think I'll need to do some follow up tests at lower heat treat temperatures later on, since even this early, more or less everything is above my target hardness.

[Handloader109]

Got a question. On the PC bullets, are you removing the PC and testing the lead directly?

Or are you testing through the PC? If second. The PC is getting brittle and altering your results.

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