Water quenching cast bullets
I have read that dropping your freshly cast bullets in a 5 gallon bucket of water will harden them significantly. However if you put them through a sizer this will decrease some of the hardness. Just what is being hardened in the quenching process? Just a thin outer layer of the bullet? I would think that the whole bullet would be hardened by this process.
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What hardens Lead when quenched is just that there are lots of incorrect Tin and perhaps Antimony atoms lost in the crystalographic latice of the Lead, plus some small islands of various precipitated compounds around the boundaries of or in Lead crystals. Both the incorrect atoms and the small islands of various Tin and Antimony compounds restrict deformation of the Lead lattice at room temperature. If quenched quickly, more of the incorrect atoms will be trapped in the lattice, and the islands will be small. The effect of aging on precipitates is for them to grow. My observation in the Lab 30 years ago was that over time, precipitate effect on resistance to deformation grew for a while until the precipitates reached an optimum size, then gradually fell off.
The effect of rapid freezing should be strongest at the edges, and lesser in the interior of a casting, but I don't know how you would get at the interior of a casting to determine what the property gradient was, because of the Recrystalization effect I mention below.
During the high temperature period of cooling right after freezing, those incorrect Tin and Antimony atoms migrate to Lead crystal boundaries. The incorrect Tin and Antimony atoms distort the Lead crystal lattice, and it is a lower energy state for them to be at the crystal boundaries.
All the metals I am familiar with have some Recrystalization temperature at which they spontaneously reorganize their crystalographic lattice in order to reduce stresses in it. For purposes of Cold Work and elimination of the effects of Cold Work by Recrystalization, Lead acts as if is at high temperatures. That temperature for all the metals I am familiar with is about one-half of the melting temperature, expressed in degrees absolute: degrees Rankin or Kelvin. Lead and its alloys are about at that temperature when at room temperature. That which fuels the recrystalization is the stresses in the lattice. Supply any Cold Work, and the recrystalization should take off, and you will lose most of the effect of improper atoms, plus precipitates, plus cold work.
There are other hardening mechanisms at work in many other metals when you quench them, such as in the Iron-Carbon system, but none of them are useful in Lead-Tin-Antimony.