Attempts to strengthen lead by reducing the grain
size or by cold working (strain hardening) have proven unsuccessful.
Lead-tin alloys, for example, may re-crystallize immediately and completely
at room temperature. Lead-silver alloys respond in the same manner within
Transformations that are
induced in steel by heat treatment do not occur in lead alloys, and
strengthening by ordering phenomena, such as in the formation of lattice
superstructures, has no practical significance in typical lead alloys. In one study of possible
binary lead alloys it was found that the following elements, in the order
listed, provided successively greater amounts of solid-solution hardening:
thallium, bismuth, tin, cadmium, antimony, lithium, arsenic, calcium,
zinc, copper, and barium.
elements have successively decreasing solid-solution solubility's, and
therefore the most potent solutes have the most limited solid-solution
hardening effects. Within the midrange of this series, however, are
elements that, when alloyed with lead, produce useful strengthening.
In most lead alloys,
heat treating and rapid cooling (quenching) result in a breakdown of the
supersaturated solution during storage (aging). Although this breakdown produces
coarse structures in certain alloys (lead-tin alloys, for example), it
produces fine structures in others (such as lead-antimony alloys). In
alloys of the lead-tin system, the initial hardening produced by alloying
is quickly followed by softening as the coarse structure is formed.
Adding sufficient quantities
of antimony to produce hypoeutectic lead-antimony alloys can attain useful
strengthening of lead. Small amounts of arsenic
have particularly strong effects on the age-hardening response of such
alloys, and heat treating and rapid quenching prior to aging enhance
The alloys with 2 and 4%
Sb (antimony) harden comparatively slowly, and the alloy containing
6% Sb appears to undergo optimum hardening.
castings from arsenical lead-antimony alloys offers an attractive
alternative method of effecting improvements in strength.
The alloy containing 2% Sb
clearly does not respond sufficiently to be considered as a possible
alternative. The 4% Sb alloy, however, attains a hardness
of 18 HV after 30 min, and the alloys that contain 6, 8, and 10%
Sb could be handled almost immediately.
There is a
great deal of information for the bullet caster in the article from "Key
To Metals". The article was written mainly pertaining to strengthening
lead alloys for the manufacture of lead/acid batteries but the principles
are the same for bullet casters that need to strengthen their alloy.
Points of The Article
Lead-tin alloys age soften
Antimony is an effective
method of strengthening lead.
Lead-antimony alloy can be
strengthened by quenching.
Small amounts of arsenic
have particularly strong effects on the age-hardening response of such
Heat treating and rapid
quenching prior to aging enhance these effects.
The percentage of antimony
greatly effects the amount of time for strengthening to occur when
heat treating or quenching. The alloy containing 2% Sb
clearly does not respond sufficiently to be considered as a possible
alternative. The 4% Sb alloy, however, attains a
hardness of 18 HV after 30 min.
point is that the metals industry refers to "strengthening" lead while bullet
casters refer to "hardening". Strengthening is precisely what we need as
bullet casters, the strength to withstand the pressure of our loads and
the strength to take and grip the rifling without stripping and yet, not
so "hard" as to not seal the bore.
Bullets cast of very hard
alloys seem to be quite the rage these days, especially with the
commercial bullet casters. Ideally bullet strength will match the pressure
and velocity of the load in the firearm we are using. Using bullets with a
higher BHN than is needed for the load can prevent the bullet from sealing
the bore causing gas leakage, gas cutting and leading. Too hard can also
cost us velocity and open up groups. Too hard can in
some cases be worse than too soft. For interesting, informative articles
by Glen E. Fryxell on alloy and bullet obturation use these links:
Alloys and Obturation and
A Few Comments
On Cast Bullet Alloys.
A note on the
Freedom Arms web site states that shooting too soft of an alloy for the
pressure/velocity will cause premature forcing cone wear in high pressure
rounds. To hard is not
good and can cause leading and to soft is bad for the revolver so how do
we know how hard is just right?
Alloys and Obturation:
experimentation has revealed the empirical correlation of 3 x 480 x
Brinell Hardness Number (BHN) (or more simply, 1440 x BHN) as an estimate
of the minimum peak pressure required for bullet obturation (the reason
for the "3 x 480" format is the number "4 x 480" also has significance,
and this format makes it easier to remember both formulae). Thus, a bullet
with a BHN of 24 (typical of commercial hard-cast bullets) will not
undergo plastic deformation and obturate until pressures exceed 34,000
This formula is
a guide to "minimum" pressure for a given load. It is not a set in
concrete rule, it is a guide, a starting point. In the 24 BHN example of
commercial hard cast bullets in Glen's article 34,000 PSI is very nearly
full power (maximum) 357 magnum loads (35,000 PSI) but keep in mind we are
looking for minimum not maximum BHN's. The 38 special, 45 ACP, 45 Colt
etc. maximum loads are less than half this pressure and 24 BHN is clearly too hard for
obturation. Even top end 45-70 loads are max at 28,000 PSI. By multiplying
the BHN number of your alloy by 1440 you can get a solid idea of whether
or not you need to heat treat your alloy and to what "starting" hardness.
Its a fairly common misconception that the 1440 formula is a maximum BHN
for the load and this is incorrect, we need to know at what pressure the
alloy starts to deform (obturate) and seal the bore. This formula WILL
NOT tell you what the max pressure that will cause leading is as some
believe, it is an approximation of a starting point. In a quality
bore/chamber with a "properly sized" bullet it is very possible to run the
pressure much higher than the formula suggests as a starting BHN without
leading, the FA revolver tests mentioned below proved it.
that we know a
minimum hardness as a starting point what about a maximum hardness?
This is a little more complicated and depends on many variables such as
the intended purpose of the bullet, bullet type (does a HP need to
expand?), cartridge, pressure, action type, rifling twist, chamber & bore condition
amount of freebore, velocity, bullet fit etc. Testing different BHN
bullets starting at the minimum and checking for leading, velocity gains
or losses and improvements in groups is the only real
way to know for sure. I did an extensive BHN test in my 9" Freedom Arms
Model 83 357 Mag revolver that took over a year and a half and fired hundreds of
rounds in five shot groups. Using only virgin WW
brass and all powder and primers from the same lot number and all alloy
from the same lot, I fired 5 shot groups at 150 meters scoped from the
bench (12x Burris). All bullets were sized nose first in a Star Lubrisizer
and lubed with LBT Blue. All loads were as identical as I could make them
changing only the bullets BHN by heat treating. Best grouping and highest
velocity was with bullets at 17-18 BHN. Testing started at 11 BHN with air
cooled wheel weight + 3% tin, various BHN's were tested up to 30 BHN.
Harder than 17-18 cost velocity plus groups opened up, softer than 17-18
and groups opened up. None of the loads caused any leading in the FA but
proper bullet fit is as or more important than alloy BHN. This revolver likes it's near max load with 18 BHN
bullets. Consider the best groups and highest velocity with 18 BHN bullets
and compare this to the 20 to 24 BHN of many commercial cast bullets. 1440
x 18 BHN and a minimum pressure of 26,000 PSI is needed. This very
near max 357 Mag load (9" barrel, 190 gr. bullet @ 1550 fps) is about
9,000 PSI over the minimum 26,000 PSI for 18 BHN and is where it shoots it's
best with zero leading. This does not mean that you must be as much as
9,000 PSI over minimum BHN but this revolver with this bullet/load
An interesting side note of this testing was mixing different BHN bullets
within the same 5 shot group. Using a BHN range such as 17-18 or 15-16 didn't
effect groups but the worst groups fired throughout the tests were with a wider variation in
the BHN. When groups were fired with BHN ranges such as 15-20 BHN or 18 to 25 BHN
bullets wouldn't even stay on
the 150 meter target much less group. BHN variation opens up groups and
the more variation in BHN the larger the groups.
lead/antimony/arsenic alloys is a highly useful tool for bullet casters. A
BHN range can be selected for any given load/firearm combination we are
loading for and BHN variation will be kept to an absolute minimum, the trick is to not over do it. Wheel weight alloy with an
average composition of: 1/2% tin, 3-4% antimony,
1/4% arsenic and 951/4%
lead can be heat treated to well over 30 BHN but it's a
rare bullet that needs to be this hard unless your shooting very top end 454
loads at 65,000 PSI.
Conventional wisdom has it that
the industry has been reducing the antimony content of wheel weights and
my own experiments in heat treating seem to confirm this. The result
hasn't "yet" been softer heat treated bullets but rather bullets that took considerably longer to age harden after heat treating. The
article by "Key To Metals" confirms this by stating "The
alloy containing 2% Sb clearly does not respond
sufficiently" in referring to age strengthening/time curve after heat
treating and quenching. Recent batches of heat treated bullets took from 7
to 14 days to reach 18 BHN which is up from 2-3 days of previous batches.
According to the manufacturer Lawrence, magnum shot is supposed to contain
4% antimony and 11/4%
to the percentage of antimony in wheel weight alloy:
I emptied the Magma 40 pound pot and re-filled it with 35 pounds of ingots
from the same batch of wheel weight alloy, cast 500 bullets and re-did the
heat treating at 420o. The hardening/time curve returned to the
predictable 17 BHN in 48 hours. It seems that rather than a major change
in wheel weight alloy I deleted some of the antimony with improper fluxing
results were the same regardless of why the antimony percentage was low,
greatly increased time for hardening to occur. I couldn't have planned a
better test, the reduced antimony was an error on my part but helped to
prove the hardening/time curve of reduced antimony. Yes, that lumpy stuff
floating on top of the melt when you first heat up the pot is antimony, do
not remove it, flux it back in.
The composition of wheel weights is
nearly ideal for responding to heat treatment, (lead/antimony/arsenic).
The higher the tin content of the
alloy the less the alloy will respond to heat treating and the faster it
will age soften. (2-3% is fine
unless your goal is 30+ BHN which is very rarely needed)
The lower the antimony
content the slower age hardening from heat treating will occur.
Antimony is the key to heat treating
by providing the fine crystalline structure but antimony is extremely
brittle. The industry recommends 4% Sb for an optimum hardening/time
curve. Linotype at 12% Sb can be a poor choice for hunting bullets or
for use on steel targets. Monotype (19% Sb) and Foundry Type (23% Sb) bullets
are so brittle they can actually break in two when chambering the round.
Arsenic is a catalyst to a greatly
improved strength/time curve in heat treating & quenching
Magnum shot is a great source of both
arsenic and antimony for enriching your alloy. Linotype is a much better
source of antimony (12%) but not of arsenic.
Yes, it's easy to over do it and make
your alloy much harder than is required for your loads pressure/time curve and
velocity in your firearm.
The 150 meter revolver group testing
was instrumental in determining the effect of BHN on grouping. Detecting
differences with groups fired at 50 yards or less would have been
difficult or even impossible. The higher the velocity and the
longer the range the more obvious the differences become.
When I first started heat
treating wheel weight alloy I used a gas fired conventional cook oven and
the results were effective. The precise range of BHN wasn't all that easy
to achieve though and it wasn't until I switched to an electric convection
oven that I saw a remarkable difference (improvement). For the first
bullets heat treated in the convection oven I set the controls to the same
setting as the gas conventional oven (460o) and used the same
oven thermometers to help assure that I had the same temperature, but I
didn't get the 18 BHN bullets that I needed. In two days they were 25 BHN,
much too hard for my application. I repeated the test and the results were
the same, 25 BHN. Next the thermometers and more bullets went into the gas
conventional oven at 460o and the predictable 18 BHN was the
result. In the convection oven a setting of 440o achieved a BHN
of 20 and was repeatable. 420o in the convection oven produces
the 18 BHN needed for my near max long range revolver load. With a much better
temperature control and even heat throughout the oven for the entire time,
convection ovens it seems are much more efficient for heat treating. No wonder top
chefs insist on using them. Except where noted all of the heat treating results
listed in the chart were obtained in a convection oven.
Because the intent here is to
strengthen (harden) your bullets, when you process (smelt) your wheel
weights into ingots it is important to separate the clip-on weights from
the stick-on weights. Stick-on weights are nearly pure lead and by
including them in your alloy you are softening the entire batch by
diluting the antimony content. Additionally the quantity and size of
stick-on weights in each bucket will vary making it impossible to repeat
your alloy from lot to lot. Also, it's a bit of a waste of a very good
source of quite soft lead (about 6 BHN).
When most steel alloy's are
or "heat treated" they
become both harder and more brittle, when lead is "worked" it becomes
softer. Lead does not respond like steel, lead can be heat treated and
made harder without adding any brittleness. Bullets destined for heat treating should be
sized without lube and gas checked before they are heat
treated, not after. Sizing hardened bullets is not only tough on your
lubrisizer, tough on the bullets and tough on you, it also will work
soften the driving bands of your bullets, the very part you wanted to
strengthen. After a day or two for age hardening and to completely dry
out they can be run through the lubri-sizer using a die .0005" to .001"
larger than the die they were sized with.
Stand your sized and gas checked
bullets up on the base in a suitable pan. It's fine if they are touching
each other, in fact I group them together to aid in keeping them
upright. I use flat bottom spaghetti pans with the holes enlarged for
better water flow. Pre-heat your oven to the pre-determined
chart) for the
BHN you wish to achieve and then place the pan in the center of the oven
for one hour. At the end of the hour as quickly as possible remove them
and submerse in cool water (they are extremely soft at this point, try not
to bang them around). I don't know of any testing done to determine
an optimum time to be left in the water but I like to give them 10 minutes
or so just to assure they are completely cooled throughout. If nothing
else, it's a good excuse to drink a cup of coffee.
stacked high enough that the tops of the bullets don't touch
the bottom of the pan above.
oven, a thermometer and the spaghetti pans. Any bullets
too long for bottom or middle pan are done in top pan.
Once out of the water I lay
them out on an old terry cloth towel to dry out (completely, even under
the gas check) and just let
them sit there for a day or so. Now they can be run through the lubri-sizer, pan lubed, tumble lubed etc. to apply your favorite lube. If
additional time is needed to reach their final BHN it's fine to go ahead
and load them at this point and let them finish age hardening while loaded in the case.
We have all heard that heat
treated bullet alloy will age soften over time but how much and how fast
does this occur? While cleaning out the cabinet under my loading bench I
came across a couple of box's of 35 caliber, RCBS 200 gr. heat treated bullet's properly labeled
with the alloy (clip-on weights + 2% tin), the date and a BHN of 30.
They were over 10 years old so I figured they would be putty by now but
they tested at 26 BHN. 10 years, how is this possible? Taking from the
"Key To Metals" article the antimony content of at least 4% and a low
tin content controls the age strengthening and
age softening of the
alloy. It seems that if the percentage of tin had been higher or the percentage of
antimony lower (or both) age softening would have been faster. With the
box of heat treated bullets was a box of 7mm bullets of the same alloy
and with the same date but not heat treated, the label said 11 BHN. In
10 years they also age softened and now test at 10 BHN.
There you have it. It's that
simple to achieve the BHN that is proper for your load. Sufficient
antimony (4%) and only enough tin to aid in mould fill-out, the proper
convection oven temp and... exercise constraint. Do you really need
or want a 30 BHN bullet? With a little time spent oven & range testing
to see at what BHN your cartridge/firearm and load/pressure works it's
best, a day at the range with cast bullets can be pure joy, not an
evening of scrubbing lead from the bore. The proper BHN alloy for your
load/firearm will enable you to get the most velocity and best groups
possible with your cast bullets. Even terminal ballistics with hunting
bullets can be controlled for proper expansion or no expansion with heat
treated alloy's and hollow point designs can be controlled for the
amount of expansion desired.
Water quenching (dropping
bullets straight from the mould into a bucket of water) will harden your
lead, antimony, arsenic alloy but as the lead pot and mould temperature
varies so does the final hardness of the bullets. Additionally, you get
what you get (about 15-19 BHN with wheel weights). As the revolver tests
proved, varying the BHN effects both groups and the velocity extreme
spread. With convection oven heat treating a specific BHN range can be
attained and variation is held to a minimum.