Role of Arsenic

Glen

Moderator
Staff member
We had a discussion on heat treating cast bullets over on facebook, and I threw out my speculation on what the role of arsenic is. I thought I would share it here and see what you had to say.

Carbon in steel forms iron carbides, which are MUCH harder than steel. There are several different phases of iron carbide, and the properties of the steel depend on which iron carbide phase is present, and how much of it is there. The purpose of heat treating steel is to convert one iron carbide phase into another. Antimony in lead alloys is roughly analogous to the iron carbides in steel, in that it is much harder than the surrounding matrix and that the properties of the alloy are dependent on how much is present, and what form it is in. That's pretty much where the analogy stops. Iron carbides are insoluble in steel, and always form a separate phase. Antimony is slightly soluble in lead, and like many things, the degree of solubility changes with temperature. At room temperature, antimony is soluble in lead to the tune of less than 1%. At molten lead alloy temps, antimony is much more soluble (e.g. 20-30%, depending on the alloy and the temperature). As many experienced casters can tell you, getting pure antimony to dissolve in lead alloys is a major pain because it dissolves so slowly. Why would antimony rich phases in a cast bullet be any different? I believe that the role that arsenic plays in the heat treatment process is that it forms an intermetallic compound with antimony, and the arsenic-antimony adduct is much more easily soluble in the lead matrix (much like the tin-antimony intermetallic adduct, e.g. Lyman #2 alloy). IF the arsenic-antimony adduct formation is roughly a thermoneutral reaction, then it will also be reversible, meaning that the arsenic atom could "pop off" after the adduct had migrated into the lead rich phase. If this model is accurate, then arsenic would be essentially serving as a shuttle to facilitate transport of the antimony into the lead during the heat treatment process.
 

Glen

Moderator
Staff member
I don't know that it's right, it's just that for heat treatment to be successful, one needs arsenic to be present. Not a lot, just a small amount, and the amount of hardening is not proportional to the amount of arsenic, which indicates that the arsenic is probably playing a catalytic role in the hardening process. That got me thinking about it, and the model above is what I came up with.
 

Ian

Notorious member
I always understood As to act as a "grain refiner" in ternary Pb/Sb/Sn alloy, as long as the alloy is not in a eutectic ratio. The reduction of shear planes makes a tougher material. Now, why we see a dramatic "hardening" effect from As only when rapidly quenching the alloy, I haven't been able to explain. The model of Sb forming an intermetallic compound with As and achieving higher solubility in the alloy makes a lot of sense.

My question, then, is why does As have more of an affinity for Sb than the other elements in solution?
 

Glen

Moderator
Staff member
Actually, for this model to work, the As doesn't have to have more affinity for Sb than the other elements in the alloy, it needs to have more affinity for Sb than Sb does (i.e. it is serving to break down an antimony crystal and drag it into the other regions of the alloy).
 
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RicinYakima

High Steppes of Eastern Washington
"If this model is accurate, then arsenic would be essentially serving as a shuttle to facilitate transport of the antimony into the lead during the heat treatment process."

Or could we say that it aids the more even deposit of Antimony crystals rather the forming of long dendrites? Making the solid solution more homogenous?
 

Glen

Moderator
Staff member
Yes, the distribution of antimony would be more uniform. The antimony structure would still be dendritic, but instead of long heavy needles, it would be composed of many smaller, shorter needles. The spatial distribution within the alloy would be more uniform, but it would still be composed of two different phases -- the disordered solid solution (i.e. lead-rich phase), and the crystalline antimony dendritic phase -- but the smaller domain size would help to make things more uniform.
 

RicinYakima

High Steppes of Eastern Washington
Therefore, if more uniform, it would test "harder" using our crude ball penetrating hardness testers? Slower cooling allows longer and more spaced out "long heavy needles" of antimony reading "softer" on our testers?
 

Glen

Moderator
Staff member
Yes, exactly. The smaller needles mean more needles, and more crosslinking. Think of it this way, if you poured a 20x20 concrete slab and put one piece of 1" rebar right through the middle of it, that slab wouldn't be very strong. But if you used the same amount of steel, in the form of wire mesh, the slab would be much stronger.
 

fiver

Well-Known Member
Actually, for this model to work, the As doesn't have to have more affinity for Sb than the other elements in the alloy, it needs to have more affinity for Sb than Sb does (i.e. it is serving to break down an antimony crystal and drag it into the other regions of the alloy).

this.
this is how I see it working.
think about the other compounds that do the same thing.
sulpher works the same way, and I believe even tin in a very low amount allows the same thing to happen.

if you break open a piece of lino-type you can plainly see the grain structure, but something else with a small amount of As or sulpher will have a completely different appearance with a much tighter grain structure.
I think..... what the tighter grain structure does is trap pieces of antimony throughout the alloy [think more like a spider than a blob, like the atoms are strung out] making it act like there is more of it there.
this shows up especially in quick quenched alloys.
 

pokute

Active Member
Without doing some tedious calculations that have long since flown from my memory anyway, I assume that Arsenic has an atomic radius that "fits" in the interstices of the lead and or tin lattice and introduces strain energy into the solid solution that would otherwise leak away via annealing. If I was still able to do the calculations off the top of my head that I could do in 1982, I could probably prove this. Fortunately a side-effect of early onset Alzheimers is not caring that you don't remember.
 

fiver

Well-Known Member
I don't think sulpher has the same hardening capability but it has some.
imo sulpher is really good at attaching to carbon [and other impurity's] bulking everything up and allowing it to float out of the melt.
it doesn't stay in the melt in the amounts arsenic does.
but it is better than nothing.