Metals Used in Firearms - III

In our last couple of posts, we looked at certain types of steel alloys which are used in firearm construction. In today's post, we will look at another type of steel alloy that was invented in 1912 and used in some firearms: stainless steel.

Stainless steel is a steel alloy that contains a high percentage of chromium (greater than 10.5% by weight). Unlike ordinary carbon steels, it has good resistance against corrosion and rusting. This is because of the high chromium content. What happens is that the chromium at the surface of the object reacts with the oxygen in the air, to form a thin layer of chromium oxide. This chromium oxide layer prevents oxygen from reaching the inner steel and therefore blocks rusting and corrosion. It must be remembered that while stainless steel is rust-resistant, it is not rust-proof.

Fittingly, the invention of stainless steel was actually related to firearms. Harry Brearley, an English chemist was working in Sheffield, England for Brown Firth research labs in 1912, trying to find a new steel that could resist erosion caused by high temperatures of gun barrels. It was already known at that time that adding a little chromium to steel increases the melting point of steel. He was trying to establish precisely, the relationship between melting points and chromium content of various steel samples. As part of this study, he was required to study the microstructure of the various steel alloy samples and to do this, he had to polish and etch the samples first. The standard way to do this was to use a weak solution of nitric acid and alcohol to do the etching, but as Mr. Brearley found, some of the samples were exceptionally resistant to these chemicals. After a bit of investigation, he determined that the high chromium content of these samples was responsible for the exceptional resistance to acid. From this research, a whole new industry of manufacturing stainless steels sprung up around the Sheffield area.

Like chrome-moly steels, there are also different grades of stainless steels and only some grades are used in the manufacture of firearms. For instance, SAE grades 410 and 416 are used for firearms barrels. They are both steel alloys with high chromium content (11.5 - 13.5% for 410 stainless steel and 12-14% for 416 stainless steel). The main difference is that 416 stainless steel contains a bit more sulfur in it, which makes it easier to machine than 410 stainless steel, which makes the barrels cheaper to produce. However, 410 stainless steel retains its toughness better and performs better in freezing conditions. Some companies make custom alloys, such as Crucible Specialty Metals' 416R, which is specially designed for precision steel barrels. Another stainless steel alloy used by some makers is 17-4 PH (PH standing for Precipitation Hardening).

Some of the other parts of the guns are also made of 400 or 300 series of stainless steels. The 300 series is more resistant to corrosion than the 400 series of steels, but cannot be hardened as easily, so it is used for parts that aren't exposed to huge forces.

The advantage of stainless steel alloys over chrome-moly steel alloys is that they are easier to machine and resist heat erosion better. However, they are a bit more expensive and cannot be blued using conventional methods. The US military prefers chrome-moly barrels, but most competitive target shooters prefer stainless steel barrels, because they can be machined more precisely and keep their accuracy longer. This is why the majority of match-grade barrels are made of stainless steel.

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.

Metal Treatments: Stainless steel

In most of the previous methods we've studied, the method of reducing corrosion has been to infuse a thin layer of corrosion resistant material on the outer surface of the iron or steel parts of a firearm. In this post, we will study another method, where the material used to make the firearm parts is itself corrosion-resistant. We're talking about stainless steel.

Stainless steel is basically a steel alloy with at least 10.5 % chromium content by mass, or more. It does not stain as easily as ordinary steel, which is why it got its name. It also does not rust or corrode as easily as ordinary steel. Despite what the name suggests though, it is not completely stain-proof (or rust-proof, for that matter).

Some of the early research into corrosion resistant steels happened in Germany in the 1890s and 1900s and Krupp engineers even patented an autenitic stainless steel in 1912. It is fitting though, that Harry Brearley of Brown-Firth research lab of Sheffield, invented a martensitic stainless steel alloy in 1912, while they were seeking a better alloy for the manufacture of firearms! The lab was given a project to try and study how to reduce the erosion on the inside of barrels that is caused by high temperatures. Harry Brearley was in charge of researching this problem. By a stroke of luck, he noticed that one of his samples had shown no signs of rust, even after being immersed in water and exposed to air for a long time. He analyzed this piece of steel and determined that it was the high chromium content that was causing it to not rust and thus "rustless steel" was born in 1912. The name was later changed to "stainless steel" due to a suggestion from a local Sheffield firm that specialized in making cutlery items.

There are many different grades of stainless steel with differing mechanical and chemical properties. For firearms, the grade usually chosen is SAE 416 grade. This is a martensitic steel with sulfur content, that has good machining properties and can be hardened later with heat treatment. Due to its good machining properties, barrels made with stainless steel exhibit better accuracy than made with chrome-molybdenum steel. This is why target shooters prefer stainless steel barrels for their weapons. Stainless steel is more expensive though and also has slightly less life than chrome-molybdenum steel barrels due to less hardness, which is why military weapons use chrome-molybdenum steel.

The finish looks very much like a nickel plated steel weapon. Like nickel-plate, the finish can be bright, brushed or matte. However, since nickel plated steel only has a thin outer finish, it is subject to flaking and peeling off of the plated layer. This is not the case with a stainless steel weapon.

While there were revolvers with some stainless steel parts before, the Smith & Wesson Model 60 revolver pictured above has the distinction of being the first regular production model revolver to be completely made of stainless steel. It was been in continuous production since 1965. When it was first introduced, the model was so popular that most gun shops had a waiting list of 6 months to purchase one.

The above Smith and Wesson M1911 model is also made of stainless steel. Feel free to compare and contrast this with the 1911s that have been nickel plated and chrome plated. Smith & Wesson did not produce the first all stainless steel 1911 model though. That honor goes to the Arcadia Machine and Tool company which produced their all stainless-steel model of the M1911 in 1977.

Stainless steel does look better on some weapons and it is easier to machine, so that makes it easier for manufacturers. However, it also has some disadvantages. While it is rust resistant, it is not completely rust-proof and so, care of the weapon is recommended. Also, it is not quite as hard as a carbon-steel weapon, so stainless steel with bend and scratch easier than an equivalent blued carbon-steel weapon. Light scratches are easier to buff out on stainless steel though and will not show as easily as a blued weapon. Stainless steel is only hardened to about 25-32 HRC on the Rockwell Hardness scale, so it is not so hard as some of the other metal treatments. Galling of the moving parts was an issue with early stainless steel weapons, but the problem has largely been solved now with better lubricants.