Tracking BHN of heat-treated COWW over time

BHuij

Active Member
Hey everyone--

I'm trying to push the envelope on what's possible with heat treated, plain old clip-on wheel weight alloy, since I get it for free. I wanted to really carefully document my experiments so I could share them with the community and give back a bit. I just finished my first set of experiments, and learned a lot. I sat down to write up my results, and when I was done I had roughly 17 pages, which will definitely not fit into a forum post. So I have linked a PDF for anyone interested. Basically I was heat treating bullets at various temperatures and then tracking their BHN over time. The kicker is that all of this heat treating was done after powder coating, so I could find out if PCing interfered with the heat treat, or if the heat treat destroyed the powder coating. See page 12 for the actual data.

Full write-up: https://drive.google.com/file/d/1zzZaz1oWxb6VsOB4RrVN-YFijxWAZ9ff/view?usp=sharing

TL;DR Version for those who have lives and don't want to read the whole thing:

  • You can powder coat, let the coat cool and cure, and then heat-treat up to at least 425 degrees for 90 minutes (where I maxed out) without ruining the coat (at least using Smoke's Clear PC). This gives you the advantages of a heat-treat/quench AND powder coat simultaneously.
  • My pure COWW alloy, at least, was able to get up into the high 20s for BHN using this method with 9mm projectiles. With .223 projectiles I was able to get up to nearly BHN 35. All of this is after powder coating.
  • My COWW alloy appears to get pretty close to stable hardness by the end of 7 days, but just to be safe, I'm going to continue giving bullets a full 14 days before shooting them. Your COWW may reach stable hardness sooner or later than mine based primarily on arsenic content. But I think I've confirmed, at least to my own satisfaction, that 14 days is a good universally safe amount of time to wait before loading/firing.
  • I have come up with a few more questions I want to answer through experimentation about heat treating and quenching--such as "does it matter how cold the bullets get when quenched, or just how fast the bullet temperature drops below a certain threshold?" It seems like one or both of those things changes your BHN after heat treating, since my smaller bullets got to higher BHNs with the same heat treatment as larger bullets.
  • The Lee hardness tester introduces some inherent human measurement error, particularly in the higher end of the BHN range it is capable of measuring. For future experiments, I'm going to be averaging at least 5 bullets for each "reading" to counteract that, as well as tracking standard deviation to help me know how confident I can be in my data. But 1 bullet per test just isn't enough for really useful results.

Anyway, thought I'd share. More to come as I continue to tinker. Posted this on the other forum as well (and Reddit).
 

Ian

Well-Known Member
Couple observations:

Convection ovens make about a very significant difference in the effectiveness of heat treatment, ask Brad about finding that out for himself. That would be my first suggestion for test v2.0.

The rate of aging has to do with antimony content, grain refiner content, and tin content. Less antimony, longer age. More tin, longer age. More grain refiners, harder and faster age. When antimony gets below 2% with minimal tin present (1% or less, preferably 1/2%), three weeks is a good time to wait before shooting. If the tin is about 2% also, it might take longer, depending on grain refiner presence.

I do have a question: Why not quench after the cure coat? The polymer is cross-linked at that point, and only the thermal effects of softening is what makes 400 degree bullets slightly sticky, so there seems no harm in quenching right then and being done with the whole process, with much less potential for thermal damage to the coating. Up to .30 caliber bullets can be quenched just fine after a 30-minute bake in a convection oven at 410-420F (my oven fluctuates as it cycles on and off during the bake) and reach bhn of 24 or more (I've done this very thing). The coating survives the quench just fine, although it does tend to insulate the bullet slightly and prevent maximum BHN from being achieved, although this factor isn't as significant as it might first appear. 24 bhn is as hard as you will ever need to go, in anything, and powder-coated bullets do just fine at over 200,000 rpm at air-cooled WW hardness. I consider rotational velocity to be a meaningful factor to cast bullets because of drive side engrave wear being the principle limiting factor of accurate velocity....after eliminating launch problems. Powder coat enables significantly higher accurate velocity (20-30% at least) from a given system, alloy, and natural bhn, so I cannot imagine needing anything over about 18-20 BHN core hardness when powder coating. Also, hard cores tend to put more stress on the coating as the bullet engraves.

With PC, you still have to get the alloy, fit, and propellant thing right, and have good castings, etc. etc. The two things the tough, slippery, but thin PC bullet surface does for us is helping the bullet find the bore center at the end of the throat with less damage, and giving a little extra protection and friction reduction to the drive side of the land engrave where HV stresses wreak havoc.
 

BHuij

Active Member
Great info as always Ian, and this will help inform future experiments for me.

To answer your question, I decided to separate the PC cure step from the HT step primarily because I've dialed in on a PC cure temp/time that works great for me, and I didn't want to mess with something that already worked. Since I have zero signs of damaging the coating from a second HT process, I'll save changing that particular part of the process for future experiments.
 

fiver

Well-Known Member
i think it's pretty handy info to know you can cook a umm pre-cooked PC bullet, and predict the outcome.
when I make something I like to focus on the task at hand and knowing how it will affect the next step helps me pay attention to what is important.
not just important to right now but what can be done to smooth things down the line.
if I can re-cook and shoot for an across the board 19 BHN with predictable results from batch to batch I'm happy to know I can do that.
 

BHuij

Active Member
I'm really optimistic that I can eventually get a point where I can confidently cook any PC'd bullets (even ones that were previously heat treated to a different BHN) and decide on the final BHN by varying oven temperature. My next set of tests is trying out some lower oven temperatures, since everything from 400 to 425 that I tested ended up in the 24-26 BHN range more or less. Harder than I generally need.
 

Brad

Administrator
Staff member
An hour is plenty of time for heat treat.
You can easily get a specific BHn based on temp. Once you know the temp to use just set it and go. I have a small convection oven I connect to my PID with the thermocouple thru a hole I drilled in the side. Temp doesn't vary more than a degree either way.
 

BHuij

Active Member
Yeah, on my list to find a cheap way to get air circulating in my existing oven. A convection oven isn't in the cards anytime soon. Out of curiosity, what HT temp is getting you 18-19 BHN?
 

Ian

Well-Known Member
With straight WW I have no idea. With 50/50 WW/purish scrap and no tin, drop straight into water from a 415-425-degree mould, they get about 19-20, same thing with straight WW goes to 24-26 depending on caliber. What I did find recently is that even 350-60 will draw the hardness right out of HT'd bullets, down to as-cast air-cooled hardness, in only 20 minutes.

What I'd like to see is ACWW with a pinch of extra tin, like 1% max extra, coated and treated to final 16-18 bhn.
 

Ian

Well-Known Member
Perfect. So we'd need to use a lower cure PC if we wanted to do it in one shot, or cure in one step per powder requirements, air cool, and adjust hardness later at will by heat treating at whatever temperature is deemed necessary. This looks to be VERY doable.

I have both 400 and 350 powders, 10 minutes @ PMT. If the 400 polyester TGIC powder is cooked at 375 for a longer time, like 30-40 minutes, it achieves full cure. That longer time at lower temp might be good for getting a better quench out of heavier bullets.
 
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BHuij

Active Member
Yeah big fatties (like probably almost anything larger than 44) I would guess are not heating through all the way in 20 minutes, or maybe even 30-40. I'm using 60 minutes for the HT now, may shorten it for smaller projectiles in the future when I can afford to play fast and loose with independent variables :D
 

Brad

Administrator
Staff member
I tried 30 minutes and they came out the same no matter the temp. Went to an hour and BINGO, big differences.
Anything over an hour is just taking up time.
 

Brad

Administrator
Staff member
I'm no metallurgist but I think it is more than just getting a good heat soak. We need to give the crystals time to change.
 

Rick

Moderator
Staff member
I tried 30 minutes and they came out the same no matter the temp. Went to an hour and BINGO, big differences.
Anything over an hour is just taking up time.
confused-face-smiley-emoticon.gif

Seems I've heard that someplace. If I'm not mistaken I think it was possibly John had tried a 1/2 hour also with similar poor results.
 

BHuij

Active Member
I didn't even bother trying that short of a time. I also figured there would be a certain point when letting it bake for extra time wouldn't make any difference (although I didn't know when that cutoff point was). The data seems to bear this out. At least for 9mm bullets, an hour seems to be the same result as 90 minute. I did some for 2 hours previously and I don't think it changed anything there either, although I wasn't being very careful about my data at that point.

Anyway for me I'll do 60 minutes for anything up to at least 124 grains. Probably double check with a quick test sometime later to see if my fat 30 185 grain bullets need longer than an hour or not.
 

Ian

Well-Known Member
I can already see I need to get on your bandwagon and do some supplemental testing. Since you want to stick with straight WW, and I wouldn't mind that a bit, (at least not unless my rifles don't like that alloy as well when powder coated and shot at HV as they like soft 50/50, air cooled), I can do that, or run some straight COWW and some 50/50 tests concurrently.
 

Brad

Administrator
Staff member
I don’t think over an hour is needed. I was doing 300 gr 44 cal bullets.
If the oven is already hot then the bullets will do all they need to do in an hour.
 

Rick

Moderator
Staff member
When I was doing all of that heat treat testing for the sake of uniformity I didn't start the hour until the pre-heated oven had reached it's pre-set temp after putting the bullets in. Opening the door to put them in lowers the temp considerably so I didn't start the hour until it was back at it's set temp.
 

Kevin Stenberg

Well-Known Member
Brad = What alloy did you use for testing 375 for 18/19 BHN?
I will prob. have more questions after i read BHuij's book.