What alloy is this?

[labop]

I got some 1" plates about 2 feet square that were used in sheilding for medical a application. I'm sure it was medical because also got some of the 36# isotope capsules and the guy says they came from the hospital. Anyway as cast the stuff tests Brinell 7 but age hardens to HB 15. The issue is it takes a 10-12 months to get there. So all this time the hardness is changing. Alloyed with tin, antimony and some mag alloy shot for the arsenic it age hardens in 3-4 days as with any other pure lead I have used. What is the cause for this to age harden so slowly?

[454PB]

Are you heat treating, or are you saying it age hardens with no heat treating?

Alloys will tend to age harden slightly if air cooled, but an increase of 8 BHN is not normal.

[labop]

I agree this unusual. This is from aging no heat treat. I have run three heats of this with the same result over the last year. A semi-quantitaive EDS scan done on this doesn't show anything there except lead. Must be some trace element present.

[sqlbullet]

I have seen this before in isotope lead. The samples that generated the chart here (http://fellingfamily.net/isolead) were tested again 24 months later.

Series 1 increased in hardness from 10.53 to 13.94. This is not a particularly unusual result
Series 7 increased in hardness from 11.23 to 16.73.
Series 13 increased in hardness from 11.77 to 17.74.

The other series which received some form of heat treatment either from the mold or in an oven also changed, but the change was more in line with what we expect and report here with the changes being less than 30%.

The only difference was the casting temperature and the method of cooling for these samples. They alloy was all from the same batch and all the slugs were cast on the same day. This does seem to indicate that casting temperature has an impact beyond bullet weight, diameter and texture. At least with common isotope lead alloy.

[labop]

Some interesting data from sqlbullet. Thanks. Have you checked to see how uniform hardness from quenching is compared to oven heat treat? I'd like to try quenching from the mold on some RCBS 6.5mm 140 grain if I can get consistent hardness.

[sqlbullet]

TL&DR: I don't see any correlation in uniformity of hardness and treatment method. It is relavent that I maintained a constant lead temp in the pot and a constant cadence in casting. If you are letting your pot temp wander or you vary your cadence you may have different results, just as you might if you don't uniformly get oven treated bullets into the quench.

Also, regardless of method, the harder the resultant bullet, the greater the deviation and ES.

Here is the data.

For the three batches that were quenched from the mold:
650 degree batch had a deviation of 1.05 and an extreme spread of 2.4 on a average hardness of 23.77
700 degree batch had a deviation of 1.15 and an extreme spread of 4.2 on an average hardness of 24.29
750 degree batch had a deviation of 1.67 and an extreme spread of 6.3 on an average hardness of 32.41

For the oven quenched batches:
650 degree batch had a deviation of 1.45 and an extreme spread of 5.6 on a average hardness of 16.44
700 degree batch had a deviation of 1.24 and an extreme spread of 3.9 on an average hardness of 20.46
750 degree batch had a deviation of 1.29 and an extreme spread of 4.5 on an average hardness of 24.87

For the batches that were quenched from the mold AND then oven treated
For the oven quenched batches:
650 degree batch had a deviation of 0.94 and an extreme spread of 2.7 on a average hardness of 17.38
700 degree batch had a deviation of 1.14 and an extreme spread of 3.7 on an average hardness of 21.83
750 degree batch had a deviation of 1.54 and an extreme spread of 4.5 on an average hardness of 25.33

In my experience, backed by the above data, I can achieve the same uniformity and higher hardness by a uniform cadence and a slightly higher temp pot. Each caster has his own rhythm and your mileage may vary.