Common in bullet casters vocabulary are words such as hardening, hardcast etc. Not to likely as deeply set as these terms are that it will change but to understand how a lead (Pb) antimony (Sb) alloy is "hardened" the correct terms must be used. What we are doing by heat treating or quenching from the mold is changing the grain boundaries within the alloy, one way to greatly enhance this effect is to use a "grain refiner". With a grain refiner present in a Pb/Sb alloy the alloy is "strengthened" well beyond the binary alloy. Many things can be used in a Pb/Sb alloy as a grain refiner but since most bullet casting is done with scrap alloys and the most commonly used grain refiner in the metals industry is arsenic (As) it's incorrectly thought that As must be present to strengthen the alloy. Many things can be used as a grain refiner in a Pb/Sb alloy. Sulphur, copper, arsenic are examples.
The smaller the grain boundaries in a Pb/Sb alloy the stronger the alloy will be. When you pour a Pb/Sb alloy into your mold and it begins to freeze (solidify) the grain boundaries are at their smallest and over time the grain boundaries expand. This should sound familiar to anyone using a high Sb alloy and the final bullet size is larger than a low Sb alloy from the same mold. What quenching from the mold or oven heat treating does is suddenly freeze the grain boundaries in place resulting in a stronger alloy. Grain refiners have a great effect on this process, a Pb/Sb alloy with as little as 0.25% As can be strengthened considerably more than a Pb/Sb alloy alone. Some have experimented with adding copper to their casting alloys resulting in much stronger bullets but it's doubtful that the percentage of copper increases strength (hardness) much even though copper is harder than lead but copper is a grain refiner.
The smaller the grain boundaries the more difficult it is for the boundaries to "slip" or move thus a stronger bullet. With the use of a grain refiner and quenching in cool water our bullets are made stronger. Given enough time the effect is reversed as the grain boundaries very slowly grow, commonly known among bullet casters as age softening. Ever notice that your properly sized and stored bullets 5 years later have grown by .001" or so and are somewhat softer? This process is slow, slower than I expect many think it is. I discovered a couple of boxes of bullets that I had oven heat treated to 30 BHN, the boxes were all labeled with alloy, date, BHN, heat treat temp etc. They were 10 years old and I thought they would be putty now but . . . They were still 26 BHN.
The smaller the grain boundaries in a Pb/Sb alloy the stronger the alloy will be. When you pour a Pb/Sb alloy into your mold and it begins to freeze (solidify) the grain boundaries are at their smallest and over time the grain boundaries expand. This should sound familiar to anyone using a high Sb alloy and the final bullet size is larger than a low Sb alloy from the same mold. What quenching from the mold or oven heat treating does is suddenly freeze the grain boundaries in place resulting in a stronger alloy. Grain refiners have a great effect on this process, a Pb/Sb alloy with as little as 0.25% As can be strengthened considerably more than a Pb/Sb alloy alone. Some have experimented with adding copper to their casting alloys resulting in much stronger bullets but it's doubtful that the percentage of copper increases strength (hardness) much even though copper is harder than lead but copper is a grain refiner.
The smaller the grain boundaries the more difficult it is for the boundaries to "slip" or move thus a stronger bullet. With the use of a grain refiner and quenching in cool water our bullets are made stronger. Given enough time the effect is reversed as the grain boundaries very slowly grow, commonly known among bullet casters as age softening. Ever notice that your properly sized and stored bullets 5 years later have grown by .001" or so and are somewhat softer? This process is slow, slower than I expect many think it is. I discovered a couple of boxes of bullets that I had oven heat treated to 30 BHN, the boxes were all labeled with alloy, date, BHN, heat treat temp etc. They were 10 years old and I thought they would be putty now but . . . They were still 26 BHN.
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