Heat Treating thoughts, based on Scientific Evidence

To JeffinNZ, yes bullets are through hardened.

To everyone:

So I am following up on my class oral presentation and my starter post regarding cast bullet heat treatment. So my initial thoughts of somehow directing Antimony to where and how I want it in the Lead-Antimony solution .... well it was a thought, not a principle. And it was for all intents and purposes not possible. It is but you might as well float a lead boat across the Atlantic as it would be easier to do. It would take a lot of work to produce the ONE bullet with perfect conditions during casting and well, its a great thing to have the Fed put some money into like exercising rabbits and giving them rubdowns after to see if they recover better. Its that difficult. But it could be done. I digress, as usual.

I have now dedicated MUCH effort, time, etc into the study of heat treatment of non-ferrous metals of many types. I have a pretty good grip on what LEAD and ANTIMONY do to and with each other in a bullet making material. Finally!

My class dissertation was on cost reducing techniques and performance improvements to cast bullets through heat treatment process. I DID NOT mimic Dennis Marshall!! Just ask the class.

In reality, very low Sb in the alloy has but one way to increase compressive strength, very well known and certainly covered by a multitude of folks in the last 30 plus years. But I have another item that has never been covered that I had a small epiphany over during tonights class presentation I made.

So “we” know as a fact that Arsenic is a grain refiner in the Pb-Sb alloy. Add a small amount arsenic, its like adding Vanadium to steel. It makes very small grain size and small is better than large in all metals (at least where strength is concerned). So it goes with lead too. Lead and antimony grain size matter. The arsenic DOES NOT make a bullet metal heat treatable.

This is a common misconception. The phase diagrams will clearly dictate this is true as will some actual testing of lead and antimony all by themselves in a heat treat environment. This has been documented in numerous govt papers, by Wiljen and by Dennis Marshall (and I am sure countless others). What Arsenic does is act as a grain refiner, significantly compounding the measured response during heat treatment. If you add arsenic to any lead antimony alloy, HT or not, it will act as a grain refiner. As per Wiljen, so will sulfur and a couple other elements. Heat treatment is NOT predicated by arsenic presence.

What I have seen zero documentation on is control of grain growth during the casting process.

Heres the crux: Big grains are bad and they form during slow cooling. Small grains are good and they form during fast cooling. Stupid convoluted grains (my word for really super strong grains) form during rapid cooling of arsenical alloys for sure! Arsenic creates minute nucleation points and those dendrites ram into each other and mess each other up real bad during rapid cooling. Thats bookoo strong alloy subsequently!

So what?

Since lead is at a very high temperature at room temp and of course REALLY hot at 200-300 degrees and its a fact that grains are STILL GROWING at 200-300 degrees, the longer a cast bullet is allowed to remain at the elevated temp, the larger grains will grow!! Regardless of your intent to heat treat or not. Matter of fact as far as I understand, that lead grain growth continues at room temp, forever! Just REALLY, REALLY SLOW!!

Of course the measurement of that is clearly defined by many folks: Water Quench your alloy and it goes say from AC BHN of 10-11 to a WQ BHN of 16-18. Not bad. For the water quench work you put in, you get some strength increase. But was that a form of heat treat or simply a method to TRUNCATE grain growth. I believe its grain growth! I have believed that quenching a bullet from the mold was a mild form of heat treatment. Its NOT, at least I don't believe it is. Its a grain growth inhibitor. So by adding arsenic to any alloy and adding a quench from the mold as rapid as possible (reducing it to room temp or below) will form VERY fine grain Pb-Sb microstructure.

So what??

So when you cast a bullet and let it air cool, its grain growth is LARGER, even in the presence of arsenic than it could be. If we quench the bullet, grain growth is halted quick(ER).

Then we heat treat the bullet. Bullet is in the lower transformation temperature zone and is a solid solution (not a slurry yet). Well the grains start growing at this temp!!! So we soak that bullet that we air cooled (maybe it was at 200 degrees or higher for upwards of 30 minutes on a hot summer casting day, may only 20 mins on a cold winter day). Thats 30 minutes of grain growth BEFORE you have heat treated (maybe you wait a month to heat treat!). Now you stick the bullet in the over at nearly 500 degrees, the zone where all hell breaks loose on the alloy and grain growth begins again. Regardless of the arsenic you added to refine grain in preparation for maximizing the strength improvement, your screwing up the material by allowing significant grain growth----- TWICE!!

This would (Ill say should since this is a theory which remains to be proven- but I will try) significantly reduce the overall strength improvements found during the precipitation hardening process due to EXCESSIVE grain growth.

Remember lead is hot at room temp, antimony is slowly traveling out of the solution and basically out of the bullet over time (at and below 6% antimony typical, not the real juicy high alloys). So since room temp is hot, 200-300 degrees is like having steel at bright red hot temps (over 1000 degrees). If this were steel, we are risking losing strength to grain growth for the same reasons. Translate that to 300 degree bullet material, same thing occurs.

So the thought: What if you water quench an alloy that you will ultimately heat treat (again low antimony below 6%)???

That first quench doesn't just make a bullet harder than air cooled for giggles, its setting the grain structure for optimal heat treatment. If the grain is GOING TO GROW during heat treat, you should try to ensure that the SMALLEST starting grain is obtained prior to the heat treat. Does that make ANY PRACTICAL SENSE??

So ideally, water quench from as hot a temp as the bullet will drop from the mold. Take it down with ice water potentially, so as to keep those few seconds of grain growth at 150-200 degrees in the core halted as early as poss.

Then IMMEDIATELY (like you do with Aluminum) send the bullet to heat treat oven. The longer the microstructure has to change due to the nature of the base metals, the less strength you will derive. Using the highest temp possible for the shortest period of time possible (alloy dependent!), soak that bullet and quench as usual.

Of course the aging process must kick in for a few days or weeks to build the final peak strength, as that precipitation process is very slow following the quench. But nonetheless.... it will happen. I think its a GOOD PLAN!! Or a BETTER PLAN

My second what if is this: Artificial acceleration of aging process like is done as a tempering for Aluminum. Yes I said aging and tempering in same sentence. Alum is hardened by aging at elevated but low temps (which is tempering since we are no where near any transformation temps). In a parallel to the Pb-Sb alloys, precipitation hardening occurs. 250-350 degrees for aluminum.......

but is there a temp above room temp that would move the aging process along in a single day or a matter of hours for a bullet material??

Thats the second experiment. I am a very patient person and could care less about how long a bullet after treatment takes to go from 13 BHN to 23. If its 2 weeks, that time is typically 2 years faster than Ill ever shoot the bullet given my crappy California lifestyle of paying everyone else's bills vicariously thru my taxes. But its a good what if!! If you can age aluminum and other non-ferrous alloys very accurately, lead alloys should/will be no different.

Is there a benefit? NO, NO, NO. Just go find something to do like load your favorite cartridge for a week or two. When your done, so will the bullets. But from a science perspective, it should and does matter that there is a possible accelerant process for aging.

None of this matters if all you want to do is shoot really rich antimony alloys. they need no processing. Linotype! Hard as nails right from the start and really over time, its not getting any softer (although since ALL phases of Lead/Antimony solutions are present in ALL ALLOYS, there will be some bit of precipitation hardened alloy that will shift over time and weaken the microstructure). it won't be but maybe 2 BHN points, hardly worth discussing. Just keep shooting the GODO STUFF then. But if I have stated anything true, then WE CAN MOVE INTO using VERY, VERY low antimony alloys and generate HIGH ALLOY compressive strengths with them. LOW Sb alloy is FAR cheaper than HIGH. And mastering the HT process SHOULD offer great flexibility in the FINAL WORKING BHN for a given alloy. Meaning if you use ONE ALLOY, you can shift its strength values anywhere from 10 BHN to 25 BHN WILLY NILLY if you have control of the HT process, the cast quench process and potentially SOME TEMPERING after HT to actually AGE the bullet to either a HIGHER BHN quicker or to REDUCE the BHN to lower than the HT process alone yielded.

If the J bullet folks make thin skinned jackets, thick skinned jackets, hard core, soft cores, banana peel starter cuts, hollow points, soft points..... all to alter the terminal ballistics and to some extent maybe interior ballistics....

WAY CANT WE MASTER taking a cast bullet and bending it to the EXACT form we want. Meaning we accurately define the bullets terminal performance and also its interior ballistic performance?? Why can't we define the perfect bullet like Sierra has, or Berger? I could care less about the void it has in it, the frosty finish, the lumpy base, etc. Thats all crap. Fix your mold or your technique and that usually gets taken care of. I am asking why we can't have a super hard bullet with massive tolerance of high pressure that when it hits bone acts like a wheel weight, with LESS ANTIMONY than WW alloy has in it?? Have the best of both worlds and all points in between. Cal it the Utopia alloy. Thats basically what the ASM does with other metals. They take an alloy and use heat and quench to shift its properties to EXACTING specs. I think we can do that with a bullet.

I didn't say it was worthwhile. This is “discussion". Thought provoking. It probably means nothing to everyone but me. Like moldy bread did to a doctor way back when. Its just moldy bread, it has no medical benefits.

So far I have found the RICH alloys have very few uses except fixing bullet fit problems, leading problem (but they start as many leading probe and they fix). I have found that the Heat Treated alloys lead less, have sufficient ductility to deform when you want them to and NOT when you don't need it, and will withstand the working pressures you define. So a part can be hard, but not tough. The high alloy bullets are HARD and strong, but not TOUGH! For game, we would like a HARD, STRONG bullet that is TOUGH. It will mushroom because its tough and not fragment. Thats really where heat treat makes sense. Its about getting the strength UP to xx KSI but still have ductility and TOUGHNESS. You can't do that with 15% antimony. It WILL fracture. Linotype for example. We actually benefit MORE from a heat treated, fine grain microstructure bullet at a modest BHN than we do a straight alloy bullet at same BHN. Antimony is out best friend, but its use is also the absolute enemy. It destroys your pocket book when you use much of it. I think a little dab will do just as well. A VERY SMALL DAB!!

Lastly I spent some time contemplating BHN vs compressive strength charts that Lee and others have put out. I realized that there is a correlation to MY personal experience and something I read. This is only profound to me. Im sure that ALL of you have concluded this and forgotten about it 40 years ago when I was eating light bulbs out of my childhood toys when mom wasn't looking.

So if a bullet is at 20 BHN, say it will supposedly YIELD (deform) at 30ksi and will stay elastic at or below 27ksi. If your bullet fit is poorer than it could be, or other factors such as propellant burn rate, etc produce LONG pressure curves with relatively low transients, by using a weaker bullet you may actually net better performance.

For instance: that 20 BHN bullet is in a cartridge that has a large throat, larger than you would like for perfection. Say you use a slow propellant like a rifle powder in 4895 class for sake of velocity and terminal performance. Its pretty long in pressure curve with not much POP at the first bit or burn (the opposite of Bullseye which is all POP and just about ZERO duration). That bullet won't have an opportunity to obturate (hit the plastic region ) for long enough or at all. It started its motion not fitting and will end its motion in barrel, still not fitting.

But take a bullet down at 15 BHN (clearly understated and rated for our 27++ksi service), use with the same propellant and same curves. Now the bullet may be low on strength by 3-4 ski, but it got an initial POP in the butt obturating the base (boy thats a can of worms I shouldn't have opened!!) potentially causing a better fit. Now maybe I used a bad example with a 3-4ksi differential, but if charts are accurate (and they are) then using a bullet slightly weaker than the working pressures dictate may very well be the absolute optimal condition.

Ive never considered this before. If the chart says 34ksi loads need X BHN, then I made a bullet AT LEAST X BHN if not a bit more. Accuracy though, as I think back, was just as good if not better, with slightly softer materials than the official X BHN material! I think this is some evidence of truth in the above statements. Too bad lead alloys cant work harden!

So anyway there is my sole contribution to the cast bullet society at large. No articles. It may all be BS. Sound neat though. Wont help you shoot better, probably bored you to near death 'cause you crossed that bridge same time as Moses crossed the Red Sea. But thats what the CBA wants and needs, articles, right? Not recipes for tuna salad, but an article having to do with a cast bullet.