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.