Alloy Hardness

8/8/2019 Alloy Hardness

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Heat Treating Cast Bullets For Better Performance

By Mark Trope

WARNING!This article addresses issues that deal with reloadingfirearm cartridges. The information presented is a resultof our experimentation and experience. We offer noguarantee or warranty of any kind on the informationpresented and you should proceed with caution if youchoose to try the techniques or products that wepresent. Reloading is not an exact science; we have nocontrol over what you do or what you use. Therefore,you assume any and all risk involved.

Cast bullets give us new freedom when we reload.Moulds are available in an astounding array of weightsand styles. If that is not enough, and you have an ideafor the next great mould design, then the good folks atLee Precision would be happy to make the mould foryou. All it takes is time and money. Articles here atsurplusrifle.com have covered well the tools, suppliesand basic procedures so one can produce clean, filledout bullets. See Making the Surplus: Reclaiming WheelWeights. In that article, Jamie briefly mentioned two

methods to heat-treat cast alloy bullets. In this article, wewill look in depth at those procedures for improving thestrength and hardness of our cast bullets.

Cast bullets can be made from various alloys. Linotype,because of its casting and strength properties isthepreferredalloy. However, as hot type printingdisappears, and photo offset printing continues to grow,linotype gets harder, and, increasingly more expensive to

Page 1 of 11Heat Treating Cast Bullets For Better Performance

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come by. We need a cheaper, easily obtainable alloy.There is such an alloy, and its no further away then theneighborhood tire store.

New York, 1886 Ottmar Mergenthaler (1854-1899), a German born knife,tool and clock watchmaker, invented the Linotype

machine. Essentially, before the Linotype machine wasmade, printing was done by individually placing singleletters to form words, words had to be placed intosentences, etc. This machine was able to cast an entirelines o type (Linotype) into a molten, fast-cooling alloywhich was then used in the printing process. It was themost common way of printing until the use of offsetprinting came into place in the 1950s wherephotographic techniques made use of full size printingplates.

As world progress moves forward, lets be very thankfulthe lowly wheel weight is one item industry has notfigured a way to make from plastic! Wheel weights arestill made from lead alloy, and will be for at least theforeseeable future. Most tire stores will either give you abucket full of used weights or let them go very cheaply.

Lead and lead alloy hardness is rated on a scale knownas the Brinell scale. Just like steel alloys are rated on theRockwell scale. To begin with, lets look at several alloysand their ranking.

Pure lead, as used for muzzleloaders and black powdercartridge arms, has a Brinell hardness of about 5; wheelweights have a brinell hardness of about 8 or 9. Linotypehas a brinell hardness of about 22, making it an excellentalloy of bullet casting.

The reason we say about 8 or 9 is that wheel weightalloy can vary, depending on the producer and when thewheel weight was made. Very old wheel weights were abit harder than the newer ones. The same applies to socalled pure lead. Industry has standards, however a bit

of leeway is usually present.

The higher pressures and powder burning curvesassociated with smokeless rifle loads usually do betterwith a bullet of a higher Brinell number. Harder, strongerbullets are less prone to deformation and gas cutting (orhaving the gas escape from the side of a bullet) then verysoft ones. So, what we want is an alloy with the strengthand hardness of linotype, but the price and availability of




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wheel weights. Provided we are willing to take one extrastep or two in our procedures, wheel weight alloy willserve just as well as the expensive and elusive linotype!

Pure Metal vs. AlloyFirst off, getting a 100% pure metal is very unlikely.There is always some sort of impurities in the metal.Never mind the cost of a certified pure metal source. Analloy, on the other hand, is a combination of metals thatare put together in known amounts to enhance desiredcharacteristics of the final metal. Like antimony is addedto lead to give it more hardness, tin is added to give themetal more fluidity to be able to fill a mould better.Stainless steel is an alloy of iron, nickel and chromium,giving the iron hardness and corrosion resistance. Gold

jewelry usually contains an alloy of gold and nickel (thenickel making the softer gold harder).

Just as steel can be hardened or softened by usingvarious techniques, bullet casters can harden wheelweight alloy to perform like linotype in our rifles andmagnum handguns. These techniques involving quicklyquenching or cooling a hot bullet in water.

The first technique involves using an oven to heat thecast bullets to just below melting point, and maintaining

that heat for a period of 30 minutes to 1 hour, thenquenching the bullets in water. The hardening processstarts as soon as the bullets are immersed in the water.After being removed from the water, the hardeningprocess continues for a period of two days, by which timethe bullets have reached their maximum hardness.Typically, people use their household oven to heat thebullets. The second method involves getting the bullets,while still hot, right from the mould into a container ofwater. Of course, there is a lot more to both methodsthen the brief description I have just given. In this article

we will examine both methods of heat treatment in detail,and see what each has to recommend.

But, before we get to the actual nuts & bolts of heattreatment, lets have a short discussion on chemistry (Ipromise it will be short). If you hear someone laughing inthe background, it is my old college professor; He saidwe would need this stuff one day!




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When you heat an alloy or pure metal, the molecules inthe metal become mobile, the more the metal is heated,the more the molecules can move around. Once themetal is allowed to cool, the molecules in the metal willreform into a solid mass and settle down into distinctcrystalline structures. Hardness is directly linked to thecrystalline construction of the metal and how themolecules of different metals in the allow come together.Wheel weights, are composed roughly of 95.5% lead, 4%antimony, and 0.5 % tin. Tin is added to increase thefilling out of the mould. That is, it allows the molten leadalloy to flow smoothly and fill the mould completely byreducing the surface tension of the lead. Antimony isused to give some molecular level hardness to the lead.Too much antimony in the lead alloy and the metal canactually become brittle enough to break on dropping. Toget the best results, both tin and antimony must be

present.

When the lead alloy is in a melted state, the smallamounts of tin and antimony are dissolved into the lead.If this alloy is cast into a bullet, and then allowed to coolslowly, the tin and antimony will have time to precipitateout separately. If, however, the metal is quenched, theantimony and tin is trapped within the lead crystalstructure, making it harder.

Even after the quenching, the bullet will age harden at

room temperature. That is, over a period of time, likedays, the metal will continue to get harder on the Brinellhardness scale. In industrial situations, metal can be agehardened faster by keeping it at a heated temperature(much lower than the quenching temperature) for aperiod of time.

A word of caution:

Lead can be introduced into the body in two ways, either

through ingestion, or inhalation. Wearing gloves andwashing hands takes care of the former, to address thelater we need to do a bit more. One can either cast outdoors, or have a positive ventilation system. I chose todo the later. I acquired an old exhaust fan from a gasfurnace, and mounted it right above my casting furnace.It exhausts directly out the window. Another note. Whencasting, one part of the technique involves removing thedross, or dirt and undesired impurities from the moltenalloy. This dross contains lead oxide, which is




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particularity toxic inhalation hazard. Dispose of thispromptly; do not allow it to accumulate in your castingarea.

Fumes from flux being drawn up by exhaust fan.

Some necessary casting equipment: wool glove,folded towel, plastic hammer, moulds, container offluxing material, matches for smoking Lee moulds,

old mess kit tray for sprue collecting, and an ingotmould.

The quench-from-the-mould heat treatment process isvery simple. I drop bullets from the mould unto a foldedtowel while wearing a 100% wool GI glove liner. I thengrab the bullets and drop them into a water filled plastictray I got from the Dollar Store. The tray has a foldedtowel in the bottom. The water in the tray does not need




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to be ice cold, just insure the water is 3 to 4 inches deep.The first few bullets that are cast will not be the best norideally hot enough for consistent heat-treating. To testthis, I drop the bullets into one corner of the tray andlisten for a sizzle sound. Once the sizzle sounds thesame for bullets as I cast, then I can assume that I amgetting the same hardness treatment on the bullets. Onlythose bullets, the ones that sizzle, are used for shooting.The few bullets in the Non-Sizzle corner will beremelted later. After a casting session, all the sizzlebullets are spread out on a towel to dry. They will reachmaximum hardness in two days. This is the method ofheat treatment I always use. It is simple, quick, effectiveand cheap.

SAFETY NOTE

NEVER TOSS A WET BULLET BACK INTO THEMOLTEN POT OF ALLOY. THE DAMPNESS WILLCAUSE A DANGEROUS EXPLOSION OF MOLTENLEAD TO OCCUR.

Plastic quench tray with cloth pad and water

The oven method is a bit more involved, yet some swearby it. Simply drop bullets from the mould unto a foldedtowel and let them cool. Next, preheat an oven to 450degrees and place a metal tray (which will NOT be usedfor food) with bullets in the oven and let them heat forone hour. After that time, get them into a container ofwater quickly after removal from the oven. They too willrequire 2 days to reach full hardness.




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An interesting note is that the temperature of the waterused in the oven heating quenching method DOES havea positive effect on the hardness of the bullet, unlike thein the mould-to-water method. For the oven method, usewater that is as cold as possible to obtain the bestresults, however, if you are doing batches, you will needto replenish the cold water as you change batches toobtain consistent results.

Homemade tray with bullets ready to go into oven

Bullets being quenched in sink after oven heating

In the past there was no affordable, ready-made tool thata caster could use to get a direct Brinell reading. Therewere a few tools one could get to do a comparativemeasurement, however they never received wideacceptance. Lead Bullet Technologies, (LBT) changed allthat.




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Lead Bullet Technologies, (HSC 62, Box 145, MoyieSprings, ID 83845), headed by Veral Smith, began toproduce an affordable, accurate Lead Hardness Testerthat read in direct Brinell units. The LBT tool is simpleand foolproof. All measurements taken in this article weretaken with my LBT Lead Hardness Tester.

LBT hardness tester with instruction sheet

LBT has a complete line of products for the bullet caster.The LBT Hardness Tester currently sells for $90.00 directfrom LBT.

Lee has recently introduced a Brinell hardness tester.The Lee unit is quite a bit different from the LBT, howeverthe Lee unit sells for much less. While I have not had thechance to try the Lee unit, I look forward to trying one inthe near future.

Method of heat treatment BHN two days after Trt.

Quenched directly frommould

30

Oven heated, quenched 35

No heat treatment 9

On a separate set of 5 bullets, I obtained a rate ofhardness aging for the oven treatment. At the initial point

right after quenching, the BHN was about 20-22, afterone day, the hardness had increased to 30-35 BHN. Afinal hardness test after 2 days gave a reading of 40-41BHN. In comparing the test above where the 2 dayhardness lists as 35, the most likely reason is that whenusing only 5 bullets, the quenching water started outcolder and remained colder for the length of time Iallowed the bullets to quench.




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As the above table clearly shows, plain wheel weightalloy can be dramatically improved with the simplestprocedure. Oven heated bullets showed the mostimprovement, however, the use of a household appliancethat is also used for food may not be wise. Given thatlinotype has a Brinell hardness of 22, and the easymethod of direct quenching yields a hardness of 30, thatis an increase of 26%. A very easily attainable result.

Bullet quenched directly from mould shows a Brinellhardness of 30

Oven heated bullet shows a Brinell hardness of 35




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An untreated bullet shows a Brinell Hardness of 9

There is one more point we need to cover concerning

heat treatment of cast bullets, that being the sizing ofbullets that have been heat-treated. If you were tosize .001 below the as cast diameter (which is what isnormally done in sizing), it removes the heat treatment ofthe bullets and softens them. There are a few ways towork around this characteristic.

If your mould casts the correct diameter for what youwant, simply size and lube at least .001 above the as-cast diameter of heat-treated bullets. An example wouldbe your .38 caliber mould casts bullets at .357. Usea .358 sizer and the heat treatment should not be

affected. If you have a mould that casts grossly oversizedbullets, use a two step process by casting the bullets,allowing them to slowly cool to room temperature. Thensize and lube them down to the correct diameter that youneed. After that, use the oven-quenching method toharden the bullets. Realize, however, that you will needto lube them again after quenching as the heat willremove the lube.

Linotype will allow the most casting ease of all alloys,however, the cost and effort involved in getting linotype

hardly justifies the result, when straight wheel weightalloy will surely suffice. By adding a bit of 50/50 (50% tin,50% lead) bar solder to wheel weight alloy I could haveincreased the castablity of the wheel weights. However,for the purpose of this article I used straight wheelweights. I can wholeheartedly recommend this simple,inexpensive procedure to dramatically improve thestrength and hardness of your cast bullets.




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Copyright 2003, 2004 Jamie Mangrum

WARNINGThis article addresses issues that deal with reloadingfirearm cartridges. The information presented is a resultof our experimentation and experience. We offer noguarantee or warranty of any kind on the informationpresented and you should proceed with caution if youchoose to try the techniques or products that we present.Reloading is not an exact science, we have no controlover what you do or what you use. Therefore, youassume any and all risk involved.

By Mark Trope


Alloy Hardness

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