Radial Hardness - Water Cooled ?

[youngda9]

Does water quenching a boolit increase

the outer hardness only
the entire hardness equally
the outer hardness with less hardness increase as you move inward towards the center of the boolit


I believe the answer may be #3 since the outside of the boolit would see the greatest speed of cooling and thus benefit the most from the hardening process. The center would cool faster than air cooled, but not as fast as the outside of the boolit that is in contact with the water.

Perhaps someone can explain the metallurgy behind hardness increase with water quenching boolits.

If the answer is #3, has anyone ever water dropped a cast boolit to increase hardness...and then cut the boolit in half and measured hardness from the edge towards the center in 0.1" incraments? (I'm an engineer and I like data !)

[BABore]

Both #2 and #3. It depends on the boolit temperature when it hits the water and the boolit's diameter. It leans more towards #3 as you get into 0.358 boolits and bigger. The hardening occurs as it locks the antimony and arsenic crystals on the exterior of the Pb/tin grain structure by rapid cooling. The trick to getting a softer core (ideal hunting boolit) is to be barely above the threshold where hardening can occur. As the boolit is quenched, the exterior rapidily draws the heat out of the core, through convection, so it is below the hardening threshold. You need to maintain a steady mold temperature and cut the sprue at a certain point. For me, that's where you are just tearing a bit of sprue out.

It's quite hard to section a boolit without work softening it exactly where you have to test. The best method I've found is with expansion media like saturated newspaper. I oven heat treated my alloy, 50/50 WW-Pb, at various temperatures and gave them a 2 week cure time. Hardness values were taken from each lot, then they were shot into media with the same load. With the data I got, I adjusted my water drop cadence to duplicate the temperature curve. For my alloy, a 425 to 435 F oven temperature will produce a softer core in my 41 to 458 cal boolits. Higher temps reduce the effect. All temps used produced a 20-22 bhn surface hardness.

[RobS]

Now that's very good useful information, excellent.