Lead Too Brittle?

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jchiggins posted this 01 May 2018

Been working on load development for a 6.5mm Swedish Mauser using the Lee 170gr cruise missile, which is fairly long at 1.25".   In experimenting with COL and press setup, I've been assembling dummy rounds and taking them apart using an impact bullet puller.  For the most part I've been using the same bullet over and over.  Well, yesterday when using the impact hammer the bullet broke in half with half of it staying in the brass.  The bullets are at BHN 13, but the alloy is a mystery since I make it up from what I get from a recycler where I can pick through their bins which include a variety of alloys.

I'm trying to figure out if the alloy is too brittle or if this is something that could happen in the normal course of subjecting a bullet to more than it's fair share of stress in an impact hammer?  It usually doesn't take more than two or three whacks to dislodge the bullet, so I'm thinking that isn't that stressful.  If the bullets are too brittle, I wonder what effect that is having on accuracy.  The cross section of the broken bullet is crystalline; the bullets are water quenched. 

Any thoughts on this would be appreciated.

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Duane Mellenbruch posted this 01 May 2018

Long skinny bullet, drop into cold water from a very hot mold, perhaps while still a bit fluid?  Try holding some of the other bullets with pliers and see if they fracture in the same area.  Probably fracture through a lube groove which would be the weakest point.  Is the bullet allowed to drop onto soft cloth in the bottom of the pan or just strike something solid when fresh out of the mold? 

A quenched bullet at BHN 13 seems a bit unusual as it is a little softer than expected.  What age when tested?  What else can you tell about this that may give a bit more information.  Weight appropriate for the nominal cast bullet weight or heavy or lighter than other alloys? 

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jchiggins posted this 01 May 2018

Duane - your questions prompted some research with interesting results.  I have cast this bullet three times. The bullet that broke was from the visual reject pile, so am not sure which batch it was from.  However, using your idea of bending with pliers (the rejects) showed that some snapped relatively easy, others not so.  Surprised me. Set up a test using a "V" block sitting on a digital scale with a drill press holding a screw driver bit serving as an arbor to apply pressure on a bullet. Got some good info.

The first batch was cast at "casual speed" (I'm a new caster) and water quenched in 5 gal bucket with no padding.  I later found out this caused a number of bullets to be bent.  At any rate, this group snapped at an average of 196 lbs  pressure.

The second batch I cast much faster because i thought that would create harder bullets when quenched because of them being hotter out of the mold.  The bucket had some rags on the bottom.  This group snapped/broke at 180 lbs average.

The third batch I cast slower, counting until at least 5 before cutting the sprue; sometimes longer.  I was trying to not bend bullets.  The bucket had carpet padding in it and the resulting defect rate was significantly improved.  This group bent before breaking and this occurred at an average of 157 lbs.

The bullet small sample average weight is 170.6 gr out of a 170 gr mold.  The fly in the ointment is the first two batches were cast from the same alloy source; the last source batch is the same BHN, but was smelted at a different time.  I do not have a thermometer; so I leave the setting on the Lee bottom pour to about "680 degrees" ... whatever it may really be.

Your questions and what they prompted makes me think quality control/consistency is the culprit. Need to work on methodology!  

BTW - usually measure BHN the day I cast the bullets.

Duane, thank you for your help.

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BudHyett posted this 01 May 2018

Pressure and heat are interconvertible, the bullet going down the barrel is a fluid mass somewhere between a solid and a liquid. At the relatively pressures of cast bullets, the bullet will act more like a solid mass. But, there may be enough pressure to reform the bullet. 

As to the question of the bullet splitting, that may well be an incipient fracture formed during the cooling cycle. The contraction during cooling is bidirectional, the inside of the bullet will attempt to shrink outward while the outside shrinks inward. The mass of the bullet mold retaining the heat causes the resultant action that the inside does not shrink outward due to pressure of the mold. However, there can be stresses during cooling.

If there is a void from the lead pour, that may develop what you are seeing. Examination of the two split surfaces that shows a rough surface will indicate there was a void causing the fracture. Regulating your pour flow to be thin to go through the hole and steady can stop this.

I know this, because I have had the same problem. 

Country boy from Illinois, living in the Magical Pacific Northwest

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John Alexander posted this 02 May 2018

jchiggins

This isn't complicated as far as getting shooting results are concerned.

If the first fracture surface is more or less prependicular to the axis of the bullet and looks crystalline it WAS a brittle fracture.  If it had been a ductile fracture the fracture surface would likely have been at about 45 degrees and the the pieces would have had reduced diameters at the fracture surface ("necked down").

This isn't surprising a lot of alloys at 13 BHN dropped hot in water will be brittle including wheelwrights.

My suggestion is to stop dropping them into water. 13 BNH bullet work great for a great number of applications and in other than a custom chamber with bumped bullets will usually shoot better than harder bullets.  You are not doing yourself a favor making hard bullets unless you are trying to shoot then much faster than the usual CB speeds. 

Hope this is helpful.

John

 

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Duane Mellenbruch posted this 02 May 2018

I can see two issues by your casting method.  First would be quenching an unknown alloy from the mold and testing for hardness within 24 hours will not be giving you the mature hardness.  It would depend upon the percentage of alloy in the mix as to how fast it would harden.  Secondly, would be why quench at all?  It would seem that might be introducing stress where it is not wanted.  Is this a necessary process, or just one of convenience?

Since you have a hardness tester, what is the natural hardnss of your cast bullets at 14 days?  What is the hardness of your quenched bullets at 14 days? 

Following the thought by BHyett, is there sufficient sprue puddle to feed the cooling casting or is the cooling and shrinkage pulling the bullet in half as it cools?  In your third session, the bullets bent without breaking.  That might be worth looking into in the future.  Did the extended cooling rate affect the strength of the bullet or was it a little different alloy?  Hmmm.  More to think about I guess.

 

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jchiggins posted this 03 May 2018

Gentlemen - thank you very much for your responses. After reading them and thinking about it, iv'e decided to do the following:

1. recast the bullets

2. will not water quench

3. Pay more attention to doing a better, more consistent job pouring the alloy.

I quenched with the idea that harder would "probably" be better, but the response about stress and potential for fractures makes sense, especially since some of the test samples broke easily....that really makes it clear. Some of the bullets were somewhat crystalline; one was very much so. 

This coming weekend I'll cast and then let them sit for a few days and then re-test.  BTW, I rechecked the BHN and it is at 13.8 now that a couple of weeks have gone by. 

I'm glad this situation occurred; it has been educational!

Thanks again for the guidance! 

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Qc Pistolero posted this 2 weeks ago

Coming in very late but maybe will get read and hopefully be helpful.

AS(arsenic)will cause that.And there is a lot of AS in small shot(7 1*2 and smaller).I've experienced it when a friend gave me 75#s of no 8 shot and melted it,water quenched and found a lot of cracks in the grease groove of 429421 bullets,some even easily breaking in half just from the sizing operation.

Reduce AS content;less than 1% of AS is enough.#8 shot can contain up to 4% of it(if not more,I'll have to go fetch my long lost references!)

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Brodie posted this 1 weeks ago

To the best of my knowledge "quenching" bullets began as a way to drop bullets without bending them.  People quickly learned that these bullets were harder than the same alloy simply air cooled.  If memory serves the first time I heard of it was in the late 60's.  A writer in "The Handloader" magazine reported quenching as a way to drop bullets from the mould without bending or denting them.  Back then everyone was looking for harder alloy. 

To harden a bullet by heat treating or "quenching" the alloy must contain both antimony and arsenic.  I don't know the chemistry of how this works, but it does.  As stated bird shot contains a lot of As, and wheel weights and most other scrap lead has some antimony in it.  The bad part of this practice is that the bullets become much more brittle as shown previously.  The part I have trouble with is your quenched bullets only increased in BHN by 0.8 points.  Usually Quenched wheel weights (clip ons, the older ones we used to get ) harden to 18 to 22 BHN, with heat treated  ones going as high as 33 BHN.  Such slugs can be driven very fast and react spectacularly when striking flesh (they blow up like varmint bullets).

B.E.Brickey

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jchiggins posted this 5 days ago

Well, I've learned a few things along the way........

The first few times casting, I did not have a thermometer, so set the dial on the Lee pot to 700.  Later, thanks to having purchased a thermometer, i discovered 700 deg. equated to over 900 deg.!  With improved techniques and more experience, bullet quality is much better.  Recently built a PID per the info on this forum and used it for the first time last week to cast some 429421.  That is a great tool!  Kept the temp very steady.   Reccomend anyone who doesn't have one to build one; its a very useful tool.  Cost about $30 for the Rex-c100 and 40amp SCR.

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Chickenthief posted this 2 days ago

Lead and tin will not fracture they will deform. Antimony on the other hand will fracture readily.

Add some tin to the mix and the problem will go away.

 

See chapter 3: http://www.lasc.us/Fryxell_Book_Contents.htm

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