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Thread: How much TIN is needed?

  1. #21
    Boolit Grand Master



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    The Tin In Your Cast Bullets

    Here are a few excerpts from,

    From the NRA Publication "Cast Bullets" by Col. E.H. Harrison USA (retired)
    The Article "The Tin In Your Cast Bullets" by Dennis Marshall, Pg 130

    During casting, the tin portion of a lead/tin alloy divides; part of it is retained within the lead grains which make up the bulk of the bullet while the remainder segregates to the grain boundaries as lead/tin eutectic. While the lead grains are still hot, immediately after casting, they dissolve a large amount of tin, However, the solubility of tin in the solid lead decreases with temperature, and when a bullet cools its lead grains are left with an excess of tin which must be rejected from solution. In the process of being rejected, tin is trapped inside the lead grains and is unable to make its way to the grain boundries, Thus constrained, tin forms particles or precipitates in the midst of the grains.

    Permanent deformation, such as engraving a bullet by the rifling, occurs by movement of imperfections through the grains. Lead can be made stronger if the movement of these imperfections can be blocked. Tin precipitates act as barriers to the movement, and thereby help the bullet resist deformation.

    Precipitation hardening depends on a mumber of complex factors. An important one of these is the strength of the precipitates; stronger precipitates harden better. Unfortunately, the strength of tin is on a par with lead. Imperfections moving through lead grains meet relatively weak tin obstacles. Strength imparted is due to other factors such as the shape of the precipitates.

    Tin plays a considerably different role in type metals; improving not only the casting properties but also contributing to the strength. Unlike lead/tin alloys, the microstructure of type metals does not contain weak tin precipitates. Instead, tin combines with antimony during solidification to form an intermetallic compound, SbSn, which is approximately the same hardness as antimony.

    SbSn is one of three possible constituents in the microstructure of type metals; the other two are lead and antimony. For alloys like linotype, part of the strength is attributed to precipitation hardening of the lead constituent; i.e., hard antimony and SbSn precipitates form within the lead to strengthen it. Compared to tin, these hard precipitates are very effective in hardening lead. The remainder of the strength is imparted by large antimony and SbSn particles.

    Within certain contraints, raising the tin and antimony content increases hardness. However, the greatest hardness increase per unit alloy addition occurs when antimony and tin are added in equal proportions. The effect is most noticeable when the combined antimony and tin content exceeds about 12%. The microstructure of such alloys consists of precipitation hardened lead and hard SbSn particles; there are no antimony particles present. If cost were no object, the tin content of linotype could be increased to 12%, matching the antimony content, and increasing the hardness about 2 BHN.

    For tin/antimony ratios greater than one, SbSn is the principal hardening agent, but a fraction of the tin will precipitate as in lead/tin alloys. As the tin content increases, so do the tin precipitates with no corresponding improvement in strength.


    So, how much tin to add to your bullet alloy? It's generally accepted that todays WW contains at best 3% antimony so if you add more tin than 3% your actually softening the alloy and increasing its rate of age softening.

    Hope this helps explain it.

    Rick
    Last edited by cbrick; 12-03-2009 at 12:58 PM. Reason: typo
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  2. #22
    Boolit Master
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    That all makes exactly perfect sense. I just did not have it all articulated so elequently.
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  3. #23
    Boolit Buddy
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    My sincerest thanks...

    Marlin Hunter,
    I apologize if I have steered your thread off course. I hope your questions have been answered, as I'm sure the posts here have answered several similar, yet different, questions.

    HORNET,
    Thank you for directing me to the classic/sticky thread. You're right- it IS a long thread! Will take me a while to digest and sort, as it is material that will require repeated reading (probably several times) in order to gain full comprehension.

    lwknight,
    Thank you for your input and experimentation. I sincerely appreciate the fact that you do your experiments to keep alive the idea of "when in doubt, try it out, and see what happens".

    cbrick,
    Your reference and translation of such has been of considerable help. I sincerely appreciate it, as I find your posts very informative and helpful.


    Once again, to all, thank you.

  4. #24
    Boolit Master Marlin Hunter's Avatar
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    Quote Originally Posted by rob45 View Post
    Marlin Hunter,
    I apologize if I have steered your thread off course. I hope your questions have been answered, as I'm sure the posts here have answered several similar, yet different, questions.



    Once again, to all, thank you.

    Thats OK, i DON'T MIND. I learned something too.
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  5. #25
    Boolit Grand Master



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    Hijacked? Not here

    I don't think the thread was hijacked.

    Marlin Hunter asked:

    Quote Originally Posted by Marlin Hunter View Post
    Can anyone confirm the minimum amount of tin needed for pure lead with some antimony and NO arsenic? If the % of antimony makes a difference, then consider 6-8%. thanks.


    rob45 asked:

    Quote Originally Posted by rob45 View Post
    My questions are:

    1. Can anyone explain what is actually happening concerning the effect of tin as it pertains to alloy integrity? I am not interested in the effects on casting characteristics, nor am I concerned with hardening/softening characteristics. I want to know why the bullets are holding together better after impact. The additional tin seems to help counteract the effect of the antimony; is this what is actually happening, or am I way off base?

    2. Are my findings in line with what should be happening "in theory"? If not, maybe I need to reconsider my approach.

    3. Keeping in mind that these alloys contain some amount of antimony, is the above quoted 6-8% the starting point of actual brittleness, or is it the accepted standard of what is considered to be brittle for our normal range of utility? My reason for asking is this: If my WW metal mentioned above only has 3.2% Sb, why does it consistently shed more weight than 10:1 lead/tin alloy? The two alloys are nearly identical in initial hardness, but the 10:1 cannot be quenched or heat-treated to a high enough level of hardness to prevent leading. However, I did indeed run a test between the two in their as-cast state at 2200 fps just to see how they held up on impact (and dealt with the leading afterwards!) The WW retained 62% of its original weight; the 10:1 retained 83%.


    This thread follows the original post and question better than most threads. No hijacking here.

    Rick
    "The people never give up their freedom . . . Except under some delusion." Edmund Burke

    "Let us remember that if we suffer tamely a lawless attack on our liberty, we encourage it." Samuel Adams

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  6. #26
    Boolit Grand Master JIMinPHX's Avatar
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    I get good results from 1% added to recovered trap/skeet shot.
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