BHN of Air Cooled Versus Water Cooled Bullets?

[Jeff82]

I remember reading that in addition to reading BHN there is a difference between adding hardness by increasing antimony content and increasing hardness through water dropping bullets. Although the BHN could be the same, the water dropped bullets will be less brittle and more maleable. I'm not sure I have this right, and am wondering what the implecations are for bullet performance. I'd think that a water cooled bullet of the same BHN would obdurate better at lesser pressures, but would be less likely to perform well at higher pressures than an air cooled higher antimony content bullet.

Is this true? Can anyone expand on this?

[sqlbullet]

Well, this is an area where I have less experience. I have miles of data related to the effects of heat treating lead/antimony/tin alloys, but all of my data dealt with the same alloy.

Let me start be re-stating your question to be sure we are talking about the same thing. Your question is this: Are BHN and malleability directly linked independent of alloy. The answer is no. Bullets of the same BHN can have vastly different characteristics depending on the composition of the alloy. BHN is a measure of hardness, not tensile strength.

Therefore, your premise is correct. Water-dropping a low antimony alloy to achieve a certain target hardness will result in a bullet with different a different tensile strength than a bullet that has the same hardness via high antimony content.

That said, I tend to disagree with the idea that malleability is needed for proper obturation. Malleability is the ability of a metal to endure a plastic deformation without cracking. As I understand it the obturation we desire occurs within the elastic deformation range, not the plastic deformation range. If you apply enough pressure to the base of a properly fit bullet to obtain plastic deformation of the base, then most likely accuracy will suffer and leading almost certainly occur. Ideally the internal ballistics pressures will not exceed the alloy's elastic deformation limits.

All of that said, most of us don't have ready access to equipment to measure tensile strength. Oh how I wish I were back in engineering school doing labs. I could have paid my way offering to test bullet samples.

Since we can't easily directly measure malleability, we infer it based off of tables of hardness for known alloys. And, these work well enough for most purposes. Lee publishes such a manual in their handloading manual, and it works reasonably well for the typical wheel weight type alloy we generally use.

Others will be along and straighten us both out I am sure.

[popper]

BHN is a measure of hardness, not tensile strength. Not quite right, but close. Tensile strength is the molecular bond strength. Elasticity is the ability to return to the original shape after force is removed. Plastic deformation occurs when enough force is applied to cause a permanent deformation, generally referred to as exceeding the tensile strength. BHN test applies a force to deform and measures the deformation. Some metals fracture (brittle) when deformed, others don't (malleable). Atoms move around (have mobility) even in solids, much more mobility in liquids. Sn,Sb tend to 'clump' by mobility when slowly cooled and the clumps are brittle. WD or HT will freeze (reduce mobility), so clumping is minimized. To answer the OP's question, adding Sb will make it more brittle (and 'harder), the CB will not expand as easily to fill the bore, but will hold to the rifling better. It's expansion at the terminal end will be less. AC will harden some with time, WD will harden much more and faster.

[runfiverun]

the reason for water dropping a "softened" alloy is to retain the malleability of the low antimony content alloy.
yet increasing the external hardness enough to withstand the trip down the bbl.

i use a 50-60% [after settling down] increase as a rule of thumb.
an 11-12 bhn boolit waterdropped will settle out at about 18-20 bhn.

[geargnasher]

Jeff, you have it exactly right according to most of my experience, but I've also had it go all over the map. One alloy I made with foundry-certified metal was 3% antimony, 1% tin, .125% arsenic, and the balance lead. It air-cooled to about 13 bhn, but water-quenched to 28 bhn after aging. A similar alloy sourced from isotope shielding containers only made about 19 water-dropped after a month but was still 12 air cooled and stable in a week.

Wheel weight alloy compositions that get more of their "hardness" from calcium than from antimony (antimony around 2%), behave similarly to 4% antimonial alloy, provided you have about 1% tin present.

Stick with the understanding you have for a guideline, but expect the occasional head-scratcher, and hardness-test everything on a minimum 3-week timeline for both air-cooled and heat treated metal.

Gear

[popper]

the reason for water dropping a "softened" alloy is to retain the malleability of the low antimony content alloy. yet increasing the external hardness enough to withstand the trip down the bbl. Yup, exactly. Lower 'additives' usually means lower $$.
expect the occasional head-scratcher That too. The real fun is getting the alloy/load/application to work good. Gear, R5R and many others have much more experience doing that than I. I can follow a recipe, but my wife gets real good results with her baking.