Metals Used in Firearms - IV

In our last three posts, we looked at usage of steel and stainless steel in firearms. In today's post, we will look at the usage of alloys of another metal: aluminum.

Aluminum is one of the most abundant elements on the earth and is widely found in many minerals. In fact, it is a very commonly used metal in today's world and we find it in soda cans, aluminum foil, vehicles, aircraft, windows, doors etc. However, throughout most of mankind's history, people did not know how to extract the aluminum metal out of ores. The first successes were discovered in the mid 1850s or so, but the yield was small and slow. In fact, before the mid 1880s, pure aluminum was more expensive than even gold! It is no coincidence that the top of the Washington Monument was topped off by an aluminum cap stone when it was first built in 1884. Napoleon III of France held banquets where honored guests were supplied with aluminum utensils, whereas less honored guests were given gold utensils! Think of that the next time you throw a soda can into the trash.

Once the process of extracting aluminum from ores via electrolysis was discovered in 1886 and factories using this method started to open shortly after, the price of aluminum began to drop. The invention of airplanes made the demand for the metal even more and after World War II, aluminum prices dropped even more. In modern times, there are many factories around the world producing aluminum.

The main advantage of aluminum is that it is pretty strong compared to its weight. It can be easily shaped and offers pretty good corrosion resistance. Aluminum is usually never used in its pure form, but usually in the form of an alloy, with other elements such as copper, zinc, magnesium, silicon etc. added. These other elements improve the mechanical properties of the aluminum alloy. When aluminum is exposed to air, it forms a thin layer of aluminum oxide, which prevents further oxidation of the inner layers and gives it corrosion resistance.

The two common aluminum alloys used in firearms are 6061 and 7075 aluminum. 6061 aluminum is about 95.8-98.5% aluminum and contains magnesium and silicon as its major alloying elements. It exhibits ease of machining and welding. It also exhibits good corrosion resistance and is used for cans, boats, scuba tanks etc. 7075 aluminum is an alloy that contains about 5.6-6.1% of zinc as its major alloying element. It is much stronger than 6061 aluminum, but is harder to machine and weld than 6061 aluminum. It is also more expensive.

Aluminum is used to construct the frames and receivers of some pistols and rifles, most notably the M16 family. It is also used for magazines, sight rings, scope bodies etc.

In Vietnam, the original M16 used aluminum receivers made of 6061 aluminum originally, but later switched to 7075 aluminum. The reason given was that when the receiver was forged from 6061 aluminum, the forging process made them prone to intergranular exfoliation in environments of high temperature and humidity, such as that found in the jungles of Vietnam, especially when combined with human sweat. Upon a suggestion by Eugene Stoner, the receivers were changed to use 7075 aluminum instead.

Of course, after the machining process, the aluminum has to be hardened to withstand stress forces. This is done by a process called anodizing. The parts to be anodized is connected to a positive electrode (the anode) and dropped into a tank containing an acid solution. Direct current is applied to the anode and cathode and a layer of aluminum oxide forms on the piece. The coating forms a thick layer pretty quickly, much quicker and thicker than if the aluminum was to be exposed to air directly. The layer is very hard, but it contains pores, which could let air or water go through into the inner layers of the piece. Therefore, a sealant is applied after anodizing to seal off the pores.

US military specifications for aluminum alloys used in firearms talk about 7075-T6. The T6 at the end specifies the treatments applied to the aluminum at the end (heat treated and artifically aged). US military spec MIL-A-8625 specifies how the anodization of aluminum should be done.

Metals Used in Firearms - I

Over the years, we've briefly discussed the properties of some of the metals used to construct a firearm (such as here and here). In today's post, we will revisit the topic of metals used in firearm construction in greater detail.

In today's post, we will skip over metals used from a bygone era (e.g.) brass, iron, bronze, gunmetal etc. and restrict ourselves to metals that are used in modern firearms. The main metals and alloys used are: steel, stainless steel and aluminum. Of course, there are different grades of these, such as AISI 4140, AISI 4150 etc. and we will study what all this means in today's post.

Briefly speaking, a metal used for gun barrels should be capable of handling large stresses, because it will experience large amounts of pressure (over 50,000 pounds per square inch or 340,000 kpa for metric speakers). It should be strong and elastic and ideally, it should be easy to machine and somewhat cheap. With that said, let's look at the first of the metal alloys: steel.

Steel is an alloy of iron mixed with other elements. The most important of these "other elements" is carbon. Pure iron is actually a soft metal and the addition of carbon allows the steel to be hardened much more than iron. However, an excess of carbon in the steel makes the steel brittle, so the quantity of it has to be carefully controlled. As we have studied previously, steel consists of crystals and it can exist with different crystalline structures. The shapes of these crystalline structures control the physical properties of the steel (such as hardness, elasticity, melting point etc.), The different crystal structures are sometimes called "phases" and there are several of these, such as ferritic phase, austenitic phase, martensitic phase, ledeburite phase, pearlite phase, cementite phase etc.

Steel Phase Diagram

Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported License by Christopher Dang Ngoc Chan.

Steel can be switched from one phase to another, by heating, adding or removing other elements and controlling the cooling rate. The diagram above shows different steel phases and how it changes from one phase to another one, based on the temperature and carbon content. However, carbon isn't the only element added to iron to make steel, there are also other elements added, which also help to change the properties of steel. For instance, adding nickel and manganese makes steel more elastic, vanadium adds hardness, chromium adds hardness, increases melting temperature and adds corrosion resistance. Adding tungsten keeps the steel from forming cementite and forming martensite instead.  Sulfur, nitrogen and phosphorus make the steel more brittle, so these are removed during steel manufacture etc.

In America, the Society of Automotive Engineers (SAE) is responsible for maintaining standards for different grades of steel. Some of these specifications were originally developed by the American Iron and Steel Institute (AISI), but since SAE and AISI were often developing standards for the same materials, they decided to combine their efforts and AISI has turned over maintenance of standards to SAE since 1995. In other countries, there are similar organizations, such as the British Standards Institution (BSI), European Committee for Standardization (EN), Japanese Industrial Standards (JIS), German standards (DIN) etc.

Per the SAE standards, the steel grades are labelled with a four-digit number (such as 1060, 4140, 4150 etc.). The first digit indicates the main alloying element of the steel. For instance 1xxx is carbon steel, 2xxx is Nickel steel, 4xxx is molybdenum steel, 7xxx is tungsten steel etc. The second digit indicates the secondary alloying element(s) in the steel and the last two digits indicate the amount of carbon in hundredths of a percent by weight. For instance, 1060 steel is a steel alloy that only contains carbon and has 0.60% by weight of carbon it it. Similarly, 4140 steel has molybdenum and chromium in it, with about 0.40% by weight of carbon and 4150 steel has molybdenum, chromium and about 0.50% by weight of carbon in it. In reality, there is a little leeway allowed. For instance, according to SAE, the allowed percentages by weight for 4140 steel are: Chromium: 0.8 - 1.1 %, Manganese: 0.75 - 1.0 %, Carbon: 0.380 - 0.430 %, Silicon: 0.15 - 0.30 %, Molybdenum: 0.15 - 0.25 %, Sulfur: up to 0.040 % allowed, Phosphorus: up to 0.035% allowed, Iron: 96.785 - 97.77 %. The standards for 4150 steel are similar to 4140, except that the carbon content allowed is 0.48 - 0.53 % by weight and the iron content is correspondingly reduced to 96.745 - 97.67 %, with all the other elements in the same proportions as 4140 steel.

Other countries have similar standards for steel grades. For instance, in Europe, the EN standard 42CrMo4 steel is about the same specifications as SAE 4140 steel, as are the British standard EN 19, Japanese standard SCM 440, German standard 42CrMo4 etc.

Now why did we mention 4140 and 4150 steels? That's because these are steel grades that are heavily used in the firearms industries to make barrels (they are also used to make gears, axles, connecting rods etc. by the automotive industry). These steels belong to the "chrome-molybdenum" or "chrome-moly" family. While these alloys do contain chromium, it is not as much as the chromium content found in stainless steel and therefore, they have less corrosion resistance compared to stainless steel. However, chrome-moly steels can be surface hardened, where the interior of the piece retains its properties, but the surface is hardened against wear and tear.

The standards for 4140 and 4150 grade steels has been around since about 1920 or so. As we noted a few paragraphs above, the difference between 4140 steel and 4150 steel is the carbon content (about 0.40% for 4140 steel and 0.50% for 4150 steel). So the difference between these two steel alloys is only that 4150 steel has approximately 0.1% more carbon. However, this extra 0.1% makes a big difference in the hardness, heat resistance and resistance to wear of 4150 steel, compared to 4140 steel. It also makes 4150 steel so much harder to machine and therefore increases the cost of manufacturing barrels.

The US military wants their barrels to last longer and work well under automatic fire, therefore they are willing to pay the extra costs associated with 4150 grade steel barrels. When you see barrels labelled "mil-spec", these are likely made of 4150 grade steel. That does not mean 4140 grade steel is bad -- in fact, it works well for civilian applications and does last for a long time as well, which is why you find so many manufacturers making barrels out of 4140 steel.

In the next few posts, we will study other metal alloys used in firearms.