Super-Wheelweight from Rotometals Superhard ?

[mtngun]

In search of the ultimate hard cast bullet for high velocity loads, I want to try making "Super-Wheelweight" with 6% antimony rather than the usual 3% antimony. Enter Rotometals "Superhard" ingot containing 30% antimony.

Formula for 6% antimony, assuming your clip-on wheelweight contains 3% antimony:
1 part Superhard
8 parts clip-on wheelweight

I also want to try boosting the arsenic and to that end I'll add reclaimed shot to the 6% alloy. Assuming that the 6% alloy contains 0.17% arsenic (NRA Cast Bullet manual page 127) and that the reclaimed shot contains 1% arsenic (typical but it varies tremendously) then the following mix would boost the arsenic to 0.3%, which I'm told is near optimal (NRA Cast Bullet manual page 126):

Formula to boost arsenic from 0.17% to 0.3%:
1 part reclaimed shot containing 1% arsenic
7 parts wheelweight or Super-Wheelweight

or Formula for 6% antimony and ~0.3% arsenic:
1 part Superhard
1 part reclaimed shot
7 parts clip-on wheelweight

Since we don't know the exact chemical composition of either wheelweight or relclaimed shot, these proposed formulas are not super precise but should get us in the ball park.

[mtngun]

I made a batch of 6% antimony and ~0.3% arsenic:
1 part Superhard
1 part reclaimed shot
7 parts clip-on wheelweight

density = 2730 grains per cubic inch

I had predicted that the density would be 2752, so my mix may be a bit "rich."

Compare that density to other alloys:
Reclaimed shot -- 2719
Clip-on WW -- 2794
WW + 2% tin -- 2780
Lyman #2 -- 2703
5% tin -- 2776
91/6/3 -- 2700
Linotype -- 2597

When I attempted to heat treat some sample bullets at 480F in my shop furnace, about half of the bullets melted partially. That doesn't make sense because in theory the melting temperature of 6% antimony should be the same as for wheelweight.

Update: I put a sample bullet in the center of my shop oven and slowly walked up the temperature in 5 degree increments.
430 to 470 F: no issues
-- 475: surface texture changed from shiny to fuzzy
-- 480: very slight slumping at the base of the bullet.

So 480F is definitely too hot for this alloy. Next time I'll try heat treating this mix at 465F. I have no explanation for why this mix has a lower slump temperature than wheelweight, but I can't argue with the data.

[mtngun]

Heat Treated @ 465F
-- 32 BHN after 1 day
(Compare that to 25 BHN for clip-on WW treated in the same batch.)

Heat Treated @ 480F
-- 30 BHN after 1 day.
-- 36 BHN after 1 week.
-- 40 BHN after 1 month
-- 37 BHN after 5 months
-- 27.8 BHN after 20 months
-- 23.2 BHN after 24 months

[mtngun]

I found this chart among some 25 year old notes. It came from volume 4 of the ASM "Metals Handbook," unfortunately my notes do not specify the page or the edition. The content of the metals handbook changes from one edition to the next so if you go to a university library and look for this chart in the latest edition of the Metals Handbook, it might not be in there.

Anyway, the takeaway is that the optimal antimony content for heat treating is 6% (alloy 615A on the chart). Adding more than 6% antimony actually decreased the heat treated hardness. Hence my attempt to make "Super-Wheelweight" aimed for 6% antimony.

The hardness "HV" refers to Vicker's hardness, not BHN. There is no straightforward way to convert HV to BHN that I am aware of.

[mtngun]

Using a USB microscope to measure the indentation on Super-Wheelweight after aging one month. As you can see the indentation is not perfectly round so it is measured in several different directions and the results are averaged, and I try to err on the conservative side. This one ran from 0.081" to 0.085". I called it 0.084" which is 40 BHN.