Work Harden Lead?

[Uncle Grinch]

A fellow caster and friend was discussing his experiments on lead hardness and posed this question....

Does lead work harden like other metals?

It seems he used his Lee hardness tester to verify some of his boolits and then checked the hardness of them after firing into the dirt backstop. They were substancially harder

I assume the process of firing them and then the subequent impact work harden them.

[Sundogg1911]

actually that should soften them. It's very hard to get an accurate reading on a fired boolit. they should be tested on a flat spot filed on a part of the bullet that was against the mould. Not the base that is againt the sprue cutter. Sizing a boolit will soften it some depending on how close the bullet size is to the sizing die. The more sizing you have to do the softer you make the lead. lead will also harden over time unless they are heat treated or water quenched, in which case they will soften. Lead based alloys will always eventually go back to their native hardness.

[Uncle Grinch]

actually that should soften them. It's very hard to get an accurate reading on a fired boolit. they should be tested on a flat spot filed on a part of the bullet that was against the mould. Not the base that is againt the sprue cutter. Sizing a boolit will soften it some depending on how close the bullet size is to the sizing die. The more sizing you have to do the softer you make the lead. lead will also harden over time unless they are heat treated or water quenched, in which case they will soften. Lead based alloys will always eventually go back to their native hardness.

Sundogg1911,

It's been my experience that water quenched boolits harden over time after which (many months....) they slowly start getting softer.

The boolits that my friend recovered were checked several times and all of them were harder after firing than before firing.

[44man]

I wouldn't call that work hardening. It would most likely be compression from impact if in fact they are harder.

[Linstrum]

It is not uncommon for lead boolits to melt when they hit a rock or steel backstop. The reason why the boolits were harder after shooting them than before is from the boolits being suddenly heated and then quenched. The heat is from both barrel friction and then the impact when the potential energy contained in the boolits from having been accelerated into motion by burning the powder is violently converted back to kinetic energy (in the form of motion or vibration of the metal atoms in the alloy, which appears to us as heat) when the boolits are suddenly stopped. The heat is from friction both inside and outside the boolit. Inside the boolits the metal is forced to flow and become deformed by the sudden deceleration and the friction between the atoms and molecules produces the heat. The faster the deceleration takes place the hotter the boolit will get until as much of the energy as possible from its motion is converted into heat. In physics the amount of time it takes to decelerate multiplied by the force it takes to decelerate is called its IMPULSE, which when measured can be useful to help estimate such things as its velocity or in some cases even what the alloy was! The physics of what happens is extremely complex because it is a semi-elastic collision with many, many factors involved. Lead has one of the lowest specific heats of any of the elements and consequently takes very little work to cause a substantial temperature rise.

Specific heat is how much work it takes to raise the temperature of 15.4321 grains of a certain materials exactly 1.8°F (one gram one degree Celsius) and for reference water takes one calorie of work to raise its temperature 1°C at room temperature, which means it has a specific heat of exactly 1. Pure lead has a specific heat of 0.031, meaning that it only takes about 1/32 the amount of work to heat it up 1°C (or 1.8°F) when compared to the same weight of water. In other words a butterfly sneeze is all it takes to warm up an aspirin-size lump of lead quite noticeably.

When the boolits finally stop after a few milliseconds they are hot but quite often will rapidly cool down from contact with the target material and air, which can have the same effect as quenching them when casting, although the temperature differential is much less than when boolits are just cast in this case because they obviously did not melt.

You can heat up a lead boolit yourself by hammering one out on an anvil with a single jack or other big hammer. If you try this, wear eye protection! You have to do it very, very quickly so the anvil doesn’t have time to suck the heat out of the lead by being in contact with it.

Pure lead will not harden to any substantial degree when worked like many other metals and alloys do. For the lead alloys that contain arsenic, antimony, phosphorus, and/or bismuth, they instead work soften due to the breaking up of the intergranular crystals responsible for the hardness that act in a way a bit similar to the gravel in concrete. The intergranular crystals prevent slippage between the lead crystal faces, tending to lock them together that in turn makes the metal harder. The alloying materials added to lead will continue to crystallize out of a solid solution at room temperature up to a point and are responsible for the increase of hardness over time. The elemental lead also continues to grow into larger crystals, but at a much slower rate, eventually softening the alloy by crushing the other crystals that get in the way. Like everything, it just takes time.

[Boomer Mikey]

Wow!

It looks like there's no substitute for a good education.

A good understanding of Physics and Chemistry unlocks the door to many mysteries.

Boomer :Fire:

[44man]

I always said this site has the smartest shooters!

[HORNET]

Wasn't there just a gripe on another thread that there aren't any good technical discussions anymore?[smilie=1:

[Uncle Grinch]

Wow!

It looks like there's no substitute for a good education.

A good understanding of Physics and Chemistry unlocks the door to many mysteries.

Boomer :Fire:

Looks like Linstrum gave me an answer...I think I get the idea, but maybe my friend who has a Mechanical Engineering degree and specializes in U.S. Air Force armement can comprehend all of it.

Thanks guys!!

[Jim]

Wasn't there just a gripe on another thread that there aren't any good technical discussions anymore?

If there was, I missed it. As far as I'm concerned, this is the only shooting forum on the internet.