Hardness vs Mould material

[Spindrift]

I currently have two alloys in use, one of which is nuclear medicine lead, with 2% tin. I have cast loads of bullets with this alloy, with different kinds of aluminum moulds.

Air cooled hardness (day 1) is around 11, increasing to about 14-15 after a couple of weeks. With water quenching, I have got BHN 15-17. I have tested for hardness a bunch of times, different kinds of bullets in different calibers. Alloy performance has remained predictable.

The other day, I cast my first HP- bullets with this alloy, from a MP brass mold (311-180). This is not only my first HP- mould, but also my first non-aluminium mould. During the same casting session, I also cast some lee CTL312-160 with exactly the same alloy, when the brass mould needed a little break. All bullets water quenched, except a few «rejects», that were dropped on a towel and air cooled.

Today, I did hardness testing (monitoring alloy consistency).
Air cooled bullets, day 1, BHN 11 (as expected)
Water quenched bullets from 2- cav aluminum Lee mould: BHN 15,4 (as expected)
Water quenched bullets from brass MP- mould: BHN 20,9 (What??).

It must, obviously, be about heat. Nothing mysterious. But quite a surprising effect, anyway.

 

[358156 hp]

This is interesting. Next try doing everything the same, except without water quenching, segregate the bullets, and see what hardness you come up with. You don't want to water quench hollowpoint bullets anyway.

 

[Spindrift]

You don't want to water quench hollowpoint bullets anyway.

Good point. Based on my previous experience with this alloy, I didn’t expect much increased hardness by water quenching. Bit of a surprise. Will look more closely into this.

 

[Rick]

You don't want to water quench hollowpoint bullets anyway.

In addition to that if your increasing BHN by quenching there must be some antimony in your

nuclear medicine lead.

My purest lead is carefully sorted SWW, with 2% Sn added they are 8 BHN and stay fairly stable at that. They will not quench harden.​

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[popper]

By quenching I assume water dropped from the mould. Brass holds temp longer so alloy is hotter when dropped?

 

[Ian]

Probably hustling more with the HP so alloy is getting shucked and quenched while hotter.

 

[fiver]

yep the faster the quench [the hot temp to cold is] the more BHN it will gain.
same as heat treating to a higher temp, using colder water, or even just casting in a cold breezy garage.
it's all about the speed of the quench.