Random Notes on Heat Treating Wheelweight

[mtngun]

As mentioned in another thread, I decided to "upgrade" my heat treating process by doing it in my shop heat treating furnace rather than in the kitchen oven. The heat treat furnace has a PID temperature controller so in theory I should be able to safely dial in the maximum possible heat treat temperature.

What is that maximum heat treat temperature? In theory, it's 486 degrees F for wheelweight alloy. But wheelweight does not have a distinct melting temperature, it begins to melt at 486 F but may not completely melt until 575 degrees or thereabouts. And the composition of wheelweight varies so to be sure I placed a sample bullet in my heat treat furnace and gradually walked up the temperature, looking for signs of melting:

463 degrees -- bullet OK
466 -- OK
469 -- OK
..... and so on
493 -- OK
500 -- a tiny "spot" appeared on the base band. It might be a microdroplet of molten lead. It wasn't there before.
515 -- droplet gets bigger
525 -- droplet gets bigger
535 -- droplet gets bigger
540 -- entire bullet looks fuzzy instead of shiny
550 -- a second droplet appears on the base
570 -- bullet gets even fuzzier
575 -- bullet fell over, partly melted.

The phase diagram for wheelweight, from page 120 of the NRA Cast Bullets book, showing that wheelweight begins to melt at 486F and should be completely melted at 575 - 590 F, depending on the antimonial content of your wheelweight :

I had originally planned to heat treat at 450 - 460 but after testing the sample bullet it appears that I can safely go much hotter. I'm going to heat treat a sample bullet at 470, 480, and 490, and see how hard they are. In the meantime 480 degrees appears to be safe in my furnace.

[mtngun]

Test results, more results will be posted as data rolls in. This is for clip-on wheelweight, measured with my homemade hardness tester.

490 degrees
-- after 4 weeks, 28 BHN

480 degrees
-- after 1 day, 28 BHN
-- after 1 week, 30 BHN
-- after 3 weeks, 30 BHN
-- after 4 weeks, 28 BHN
-- after 6 months, 29 BHN
-- after 38 months, 15.8 BHN (10mm / 150 kg / 30 sec test) wuuuuut? I have no explanation, just reporting the facts.

470 degrees
-- after 4 weeks, 27 BHN
-- 475 degrees after 38 months, 19.3 BHN

430 degrees
-- after 1 day, 26 BHN
-- after 1 week, 25 BHN
-- after 3 weeks, 23 BHN
-- after 4 weeks, 29 BHN
-- after 6 months, 19 BHN
-- after 38 months, 18.1 BHN (10mm / 150 kg / 30 sec test)

So far I am not seeing any red flags that would prevent me from shooting bullets 1 day after heat treatment. I found some old notes for 430 degree kitchen oven bullets that listed the following values:
-- after 4 weeks, 27 - 33 BHN
-- after 2 months, 34 BHN
-- after 3 months, 31 BHN
-- after 4 months, 33 BHN
-- after 6 months, 28 BHN

And I found 3 different batches of bullets from years past, with the usual 435 kitchen oven treatment:
-- after 9 years, 19 BHN
-- after 10 years, 21 BHN
-- after 10 years, 21 BHN

The next question will be "do 34 BHN bullets shoot better than 26 BHN bullets?" If 34 BHN bullets don't shoot better, then there's no need to worry about how long they have aged.

OBSERVATION on long term aging: even though BHN shortly after treating is pretty consistent from one batch to the next, not so for BHN after 3 years, ranging from a low of 15.8 for one batch to 21 BHN for other batches. Realistically, 15.8 is fine for most hunting and plinking applications so there is no need to worry about it for everyday shooting. For serious target shooting where you want a consistently hard BHN then best shoot 'em before they are say 3 or 4 months old.

[mtngun]

The tests in the previous post will take some time to complete, in the meantime I have already learned some things:

-- 480 degrees in the shop oven gains about 5 BHN compared to 430 degrees in the same shop oven
-- it appears that my kitchen oven(s) run a little hot, judging by how bullets treated at 430 degrees in the kitchen oven were harder than bullets treated at 430 degrees in the shop oven. (To add to the confusion, that was a different kitchen oven in a different house!)
-- it appears that it is perfectly OK to shoot heat treated bullets after aging only 1 day. No need to let them age for several weeks.
-- if you let heat treated bullets age for 10 years or so, the hardness will fall to about 20 BHN. That may or may not be a problem for your application.
-- Dunno why my current batch at 480 degrees can't top 30 BHN? Maybe I need to add some arsenic or sulfur?
-
CONCLUSIONS AND PLANS:
-- 480 degrees in my shop oven will be my new standard treatment.
-- go ahead and shoot heat treated bullets that have aged for 1 day or more.
-- try adding arsenic or sulfur to see if that increases hardness.

[mtngun]

Here's an aging chart by Dennis Marshall, borrowed from page 124 of the NRA Cast Bullets book.

Marshall was reaching 33 to 36 BHN -- but his alloy had 5% antimony. My clip-on wheelweights are likely 3% antimony.

Note that some of his bullets reached peak hardness after about 1 day, while others took up to 4 weeks to reach peak hardness, depending on the heat treat temperature.


CONCLUSIONS AND THINGS TO TRY:
-- try adding an additional 1% antimony to my wheelweight. I don't have any antimony laying around so it'll take me a while to follow up on that, it'll probably be a winter project.

[mtngun]

More data on how the BHN changes with age. Fortunately I have some bullets that were oven treated in 2005.


The 12 year old bullets were probably treated in a kitchen oven at 435 degrees, as was my habit at the time. The others were treated in my heat treat furnace at 480 degrees. All were tested using a 10mm ball and 150 kg load. Your mileage may vary depending on alloy, oven temperature, BHN tester, etc..

[Bob]

It looks like I've been too conservative in my bullet heat-treating. I must admit that I never did test my "mutt" range scrap alloy(s) to the melting point, I just tried a couple of different temp settings, and stopped when I got the results I thought I wanted. There's obviously more available.