"AGE HARDENING" OF BRASS
Alex Findlay, Derek Peterson - 14 August 2020
We are frequently asked if brass cartridge cases harden with age. Anecdotally we hear reports of old case necks splitting due to age. To answer this question, we have:
- Re-tested older annealed cases from our extensive archive of sample cases.
- Asked Andrew Ouwejan from Metlab metallurgy consultants for an opinion, and
- Asked Derek Peterson, president of Peterson Cartridge Co. for industry specific expert information
The short answer is that correctly annealed and stored brass cartridge cases will not age harden over any practical time frame. Here is what we found:
AMP Annealing tests:
We have an archive of thousands of different sets of case samples. All our samples are stored in sealed zip lock bags. Our early customers supplied us with six sample pieces of their cases so that we could calibrate the correct annealing settings. As part of our testing procedure, each case was numbered 1 – 6. The case weights, neck wall thicknesses starting neck hardness etc, were all recorded. We then worked up to the correct annealed hardness and recorded all the data. Once the correct neck hardness was achieved, we published the program on our Settings page. Our target annealed neck hardness is 98 HV.
We have now gone back into the archives, and pulled out the correctly annealed cases from several batches of samples. The single most common cartridge we have tested is 308W, and the most common brand is Lapua. We re-tested the neck hardness of the correctly annealed case from three different sample packs of Lapua 308W. Three readings were taken on each sample. The averages are below.
The first pack was originally tested on 22 Sept. 2017, nearly four years ago. Lapua lot # was P00930802/0210316. The unannealed neck hardness as supplied by the customer was 145 HV. When we calibrated the correct Standard program for our data base, case #4 was correctly annealed to 97 HV using Standard program 71. Re-testing it now, that case neck is still at exactly the same hardness.
The third pack was originally tested on 9 Feb. 2018. The customer did not supply the lot #. The annealed neck hardness of Case #2 also remains unchanged at 98 HV.
Our conclusion is that well stored brass will not "age harden” at least for the four years that we have on our records. We cannot comment on very old brass, or brass stored in proximity to chemicals or fumes etc.
We then asked Metlab for feedback. This is Andrew’s response:
An interesting point to ponder.
Intuitively, my initial thoughts are that If the brass is correctly annealed, I’m surprised that it would age harden as the structure is completely ‘relaxed’. If, on the other hand, the brass has some residual stress in it then some age hardening could occur as nitrogen from the air acts to restrict atomic dislocations as it diffuses into the brass. Being in a zip-lock bag might act to limit the amount of air (nitrogen) present.
I wonder if you find any age hardening on properly annealed cases exposed to the air….?
Thanks for sharing the info. This stuff certainly gets the mind working.
In response to that, we then tested some virgin 223 Lapua cases which we had bought in 2015. The box was opened then, and it has been sitting closed but unsealed since then. We didn’t test for hardness back then, but Lapua is usually around 100 HV. On re-testing five years later, they average 101 HV, so effectively they haven’t age hardened at all.
We then forwarded all the above to Derek Peterson. In response, we believe that Derek has provided us with a definitive article on the subject. He starts with an excellent explanation of work hardening and annealing, and then explains why correctly annealer brass will not age harden. He also explains what factors can cause brass deterioration over time.
Brass is a unique metal in many ways. Strong enough to contain the explosion inside a rifle’s chamber, but soft enough to form to the chamber walls to prevent gas leak. When it comes to hardening and softening C260, or cartridge brass, there are only 3 factors that affect hardness. The factors are; #1 The amount of work done to the brass. #2 The amount of heat (temperature) applied to the brass. #3 The duration of time the brass remains at the elevated temperature.
With that said, the only way to make cartridge brass harder is to work it. Specifically, cold work. An example of cold work can be seen below in this graphic:
As mentioned above in the last paragraph, the amount of cold work / reduction is what makes brass harder. Specifically, what causes the brass to become harder after receiving cold work, has to do with the grain size in the brass. Cartridge brass receives its strength from the amount of grain boundaries contained in the particular sample you are testing. Basically, more grain boundaries equal stronger material, and/or higher tensile strength. Hardness and tensile strength are very closely tied, but not identical. Exactly how that is possible, I’m not sure of, but I have been told that on a few occasions by reputable industry professionals. However, with all of my testing, harder brass has equaled higher tensile strength.
The only exception to "cold work being the only way to increase brass hardness”, is during the annealing process of the cartridge brass. When you anneal brass, the brass actually goes through 3 phases. Recovery, Recrystallization, and finally Grain Growth. You can see the 3 phases below in the illustration:
Depending who you ask about this topic, the names of the phases might change, but regardless of the names used, there are only 3 phases.
Note: this illustration shows slow furnace annealing. "Flash” annealing the neck and shoulder of a case is much faster and at a higher temperature.
What you can see in the above graphic, is the 3 phases the metal goes through during the annealing process. When I say that the brass can get slightly harder during the annealing process, I am specifically talking about the recovery stage. Right at the beginning of the recovery stage, the duplex grains in the brass actually strengthen. This strengthening is what causes the tensile strength, and therefore hardness, to actually go up. This recovery stage only lasts for a short time, and its effects are erased when the temperature continues to rise. As the temperature continues to rise, the brass enters Re-crystallization, followed by Grain Growth, and subsequently begins to decrease in hardness. The temperature, very roughly, at which cartridge brass enters recovery, is around 500 – 550 degrees Fahrenheit. Or so I have been told, and the graphs I have studied illustrated.
The time needed, at the increased temperature, for the brass to enter the recovery stage, and therefore harden slightly, normally + 10HV, depends on the thickness of the metal. This is what I meant above when I said annealing depends on the temperature you anneal the brass at, and the amount of time the brass is allowed to anneal at that increased temperature. Generally speaking, higher temperatures require less time to reach the same hardness as a lower temperature for a longer time. However, generally speaking, brass heated to below 500 degrees Fahrenheit will not soften or harden regardless of the amount of time the brass is left at that increased temperature.
The only proof that I actually have of the slight hardening happening, when the brass is in the recovery stage, is due to the hardness testing we perform during our manufacturing process. Specifically, when we anneal the un-tapered tubes before they enter our tapering machines. As you know, we use inductive annealing in our production process. During our annealing process, the inductive coils we use create an electromagnetic field around our parts, which we use to heat and anneal our rifle casings. The fields can be modified to create a heat gradient, which we use to then create the proper hardness gradient in our casings.
What I have noticed, only sometimes, is that towards the outskirts of the annealing zone generated by our inductive annealing coils, you can stumble upon a recovery zone which is actually harder than the brass was to begin with, compared to before we ran the part through the annealing process. The only explanation I have been able to come up with is that on the edge of the Re-crystallization zone / Grain Growth zone, there is a Recovery zone.
It should be noted that I have only found the above anomaly in the tapered parts before they reach the final mouth and shoulder anneal. It is my understanding that this small recovery zone tends to get blended during the final mouth and shoulder anneal, which brings its hardness back down, but I am not sure of that. I have drawn a very crude sketch, and included it below:
The only other way I have heard of people hardening brass, without physically working it, is by using ultrasonics. I am guessing that the theory is if you use ultrasonic waves to act on the brass, you can increase the hardness, but I have seen no proof of this when I studied it awhile back. In theory I would think someone could test this by buying an ultrasonic cleaner, filling it with water and casings, and then let it run for a week constantly, and see if the brass increases in hardness.
So, after all of that explanation, back to your main question. Does brass harden with age? It is a classic human trait to observe an effect, and then try to assign a cause to it without any scientific testing. But, in the end, isn’t that how almost all topics are treated in the reloading stratosphere?
What people are likely experiencing is called season cracking. One of the tests that is performed on brass casings is called the "advanced aging test”. The process of the test is to put a small amount of, Mercurous Nitrate, or ammonia spray, on the necks of the casings being tested. The Mercurous Nitrate, or ammonia, is allowed to sit on the casings for a predetermined amount of time. What these chemicals do is attack the brass. If brass has not been properly re-annealed after being tapered (in the case manufacturing process), the chemicals will cause a crack to form, almost always in the neck and/or shoulder of the casings. This is due to the thinness of the brass in those two areas.
In the past, the US and British military have had a heck of a time with ammunition cracking when it was either stored for a long period of time, or stored improperly near either of those two chemicals. A lot of people don’t know that leaving the anneal line on the casing, besides actually physically testing each casing with a hardness tester, is the only practical method to determine if a casing has been re-annealed after the tapering process. The presence of an anneal line doesn’t mean that the casing was annealed to the exact proper hardness, but it does show that it was annealed after taper and likely won’t crack if it comes in contact with a harsh chemical or sits on a shelf for a long time. This is why we do it, and also why the US military requires its ammunition to have the anneal line left on it during the production process.
As mentioned earlier in this article, brass is hardened when it is worked. During the tapering operation in manufacture, we work the brass a tremendous amount. Especially in the neck and shoulder area. Those two areas see the most amount of reduction in the tapering operations. Since they see the most amount of work, their grains will consequently be smaller. For example, when we anneal a tube before it is put into the tapering process, we reduce the hardness in the top section of the tube, which will end up being the neck of the casing, to just below 100HV. After that same tube has been tapered, the hardness will normally have increased to between 150 - 180 HV, roughly. All of that work we put into the mouth and shoulder of the casing, also imparts a lot of stored stress in the metal. That is to say, the metal has been constricted to a much smaller diameter, and it isn’t happy about it.
As the casings go through re-anneal just before they exit our machinery, the metal in the newly formed neck and shoulder is softened, which causes it to relax. Basically, final neck and shoulder anneal causes the metal in the mouth and shoulder to accept its new shape, and provide proper tension so the mouth of the casing will properly and evenly grip the bullet when loaded, and evenly release the bullet and expand to the chamber walls to seal against gas leak when the cartridge is fired.
In my opinion, the highest likelihood scenario for someone to put away functionally intact casings one year, and come back to cracked necks and shoulders a few years later, would be if they shot the brass a bunch of times without running it through a re-annealer, like an AMP, and then put the casings on a shelf in a hard condition.
Again, the AMP annealer is far superior to any other one on the market for a bunch of reasons. Likely the biggest reason being that the AMP annealer hits the proper hardness for the neck and shoulder every time. Every time a casing is fired and reloaded, the neck and shoulder are worked. This working hardens the brass. If this wasn’t the case, there would be no need to re-anneal brass ever. Resizing brass, and firing it to some smaller extent, is a form of cold work as even during the firing process, the brass doesn’t reach the 600 degrees Fahrenheit required to begin the annealing re-crystallization process of the grains in the brass.
Lastly, the other very likely scenario for "age hardening” has to do with a little-known chemical called Windex. Or any other cleaning chemical with ammonia in it. I have actually seen this here at our shop. From time to time, I will be working on a problem in the machinery, and go into a conference room to draw something on a white board to help solve the problem. I normally take a casing into the conference room with me which shows the problem I am trying to fix. I tend to set those casings down on the table, where they are put into a small plastic ammo box, like we ship our finished product in for reloader sales. Each of our conference rooms have one of these boxes on the table for meetings when we need to show someone a casing. Inevitably, I will forget to take the casing out of the room with me, and I will leave it in the box. Almost none of the casings I take with me into the conference room have been through final neck and shoulder anneal, as we never have any problem with the machines which perform that function.
The casing will stay in the conference room for months in that plastic ammo box. Our cleaning person will then come in and spray the table down with Windex or some other ammonia laden glass cleaner. Some of the glass cleaner will then get on the casings from an overshot while they are cleaning the table top. I have then come in the conference room a few months later, and the casing has a big crack in the neck. But the casings in the same box that had been through final neck and shoulder anneal have never been cracked. This doesn’t happen every time, or even most times, but it has happened. That said, a properly annealed casing can handle almost any chemical you throw at it for a limited amount of time. In fact, I have seen casings fall into our soap containers from time to time, which is sulfuric acid based, and stay there for months without deteriorating. The surface of the casing will be pretty etched, but it will not have cracked.
Peterson Cartridge Co.
17 Leonberg Drive, Suite 100,
Cranberry Twp., Pa. 16066