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METALLURGY OF BULLET CASTING Lead alloy metallurgy for bullet casting may be better understood knowing the casting principles. These principles are melt fluidity, pouring temperature, velocity of the melted alloy pour, adequate sprue puddle to insure the liquid column of alloy stays liquid until the mould cavity is completely full. Explanation of these terms follows below. Fluidity. The liquid must be quite fluid to pour well. Fluidity is a function of temperature as well as alloy content in lead which is 'sluggish' and requires a high temperature to achieve acceptable fluidity. Addition of a very small percentage of tin, about 0.5 to 1%, will greatly improve the fluidity. As alloying metal content increases the melt may become more fluid. If tin is not present then WW and any antimonial alloy is a bit sluggish and oxidation occurs faster. Pouring temperature. In the above explanation pure lead requires a temperature of 775F to 800F. The small amount of tin added will improve the fluidity as well as lower pouring temperature perhaps 75 degrees. More tin more fluidity. Velocity of the pour. The pour must be rapid to lose as little melt temperature as possible. As soon as pouring starts the liquid stream starts cooling or losing temperature. A slow pour may prevent the bullet cavity from filling completely. Sprue puddle. The sprue puddle is the 'reservoir' of melt that feeds the bullet cavity in the mould blocks. There must be a continuous puddle of melt which maintains a liquid column as the bullet solidifies in the mould. Large puddle lots of melt. Bullet solidification. Bullets solidify from the bottom and side of the mould cavity first and as long as the center is liquid the bullet does not shrink significantly in the mould. It has to 'shrink' slightly when cool in order to get it out of the mould. If the column of liquid metal extends up all the way into the sprue puddle until the cavity is completely filled there is little probability of any shrinkage voids within the bullet. This is the most important concept of bullet casting. Any voids will be on the bullet centerline if the sprue hole centers over the cavity and has minimum effect on balance. If not, adjust it. Venting of mould blocks. The mould cavity has air in it when the pour starts. Mould blocks have various ways for the air to be vented or 'forced' to escape from the cavity as it fills. Venting may be fine lines across or in a pattern on the faces of the blocks that are in contact, as well as out under the sprue plate by a slight 'relief' line where the blocks meet in the top edge. Clogged vents, poor bullets. Do not use sharp hard steel implements to clean. A chisel pointed hard 'lead' pencil (actually carbon) is the best tool to use if the mould faces get lead 'chips' adhering or stuck to them. Common bullet casting alloys. Pure lead for muzzleloaders and paper patched bullets (new book available). Lead/tin alloys with ratios from 50/1 to 20/1 used for BPCR (black powder cartridge rifle) and low velocity handgun bullets. Tin is very expensive. WW (clean smelted scrap wheelweight) is a low antimonial lead alloy with trace tin and arsenic. An alloy that is very economical with unlimited application. Lyman #2, 5% tin, 5% antimony and 90% lead, very expensive alloy. "Magnum Metal" & variations is 2% tin, 6% antimony and 92% lead, about equal to Lyman #2. Linotype which has the absolute best casting properties, lowest casting temperature, high alloy hardness and is very expensive. WW with proper metallurgy heat treat will equal or exceed all the properties of these other alloys with the exception of pure lead and the very low tin/leads like 40/1 at much lower cost other than the bullet caster's labor and time. Casting equipment and accessories. First requirement is a pot to melt in. Pots range from simple small cast iron heated on a gas burner to electric bottom pour or ladle pour. You get what you pay for however low cost equipment well taken care of and used properly may last extremely long times. The smaller volume pots do limit production sometimes. Mould blocks are available in endless variety, style, ease or difficulty of use, how the metal is poured. Materials may be steel, cast iron, aluminum alloy, brass or bronze. Each material has it own best operating temperature range and this is found by trial and error. Melt pouring methods. Simplest is either the bottom pour electric with built in nozzle or an appropriate straight pour ladle. A bit more cumbersome are those ladles that require mould rotation, sticking the ladle nose or nozzle into the sprue and then rotating the entire assembly upright and gauging the pour. Accessories: A lab grade thermometer to know the true temperatures to get good bullets for each alloy, a knocker or gloved hand to cut the sprue when ready, a small electric hot plate to warm the mould blocks on to avoid a bunch of bad bullets being cast to get the blocks to proper temperature, and various other tools and products peddlers sell. The powder scale is fine to weigh bullets on and segregate into groups, a good no smoke no flash no odor non-toxic clean powder flux is helpful when smelting and alloying. Literature and recent magazine reprints as well as some good books are available from me and on my web sites that is backed by 60 years of metallurgy and casting experience. Hardness testing. This has become more and more misunderstood as penetrator type testers have become common. Comparative hardness testing is quite simple, requiring only a vise, a ball bearing and two different ingots with smooth surfaces. Place the ball bearing between the 2 ingots and squeeze in the vise. The smaller indent is the harder alloy. If pure lead, Lyman #2 and Linotype are available then use these as standards for comparison. Lead is Bhn 5, Lyman #2 15, lino 22. These are real Brinell numbers, Bhn, not guestimates. I use a drill press, 10 mm ball and 100 kilogram load with a dwell time of 10 seconds and the Brinell formula to obtain these numbers. Penetrator testers use an entirely different metal principle of displacement. The more accurate penetrators have used comparison with the Bhn standards given above which have been used for at least 100 years. That does not mean the hardness they measure is exact, it is a comparison. As long as comparison testing using the vise and ingots or penetrator unit is for the bullet casters own information the results are certainly accurate enough. Use a simple inexpensive comparison test first. Bill Ferguson
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