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.