Metal Finishes and Treatments: What Do They Mean, and Why Are They Important?

“What lower will match the color of my upper?”

“Why should I buy an aftermarket trigger when I can do the ’15 Minute Trigger Job’ on my stock trigger or send it to someone for a cheap trigger job?”

“Why should I buy an upper that already has M4 feed ramps when I can do it myself with a Dremel?”

“Which bolt carrier group finish is best?”

If these exact questions aren’t being asked online or in gun stores on a daily basis, variations thereof are certainly bouncing through the minds of many AR-15 owners every day.

In order to understand the answers to the above questions, we must first understand what each finish or treatment is, and what it does. In future articles, I will discuss melonite, manganese phosphate, and electroless nickel (among others). For now, I want to cover heat treatment and anodizing. I would like to thank Tom Miller at Spike’s Tactical for much of the information (such as case hardening depth) provided below.

Case Hardening

Case hardening is a process used with steels that have a low carbon content – that is, below 0.25%. These are called “mild steels.” 8620 (carbon content of 0.18-0.23), which is used for the bolt carrier and the barrel extension, and Carpenter 158 (carbon content of 0.10), used for the bolt, are examples of this type of steel. As we read here, these steels cannot be “through hardened” because of the low carbon content – the surface layer must have its carbon content increased “by prolonged contact at a high temperature with a chemically reactive source of carbon.”

Then, the steel is heated to very high temperatures and rapidly cooled (“quenched”) at specific intervals. Because only the surface layer had its carbon content increased, only that layer reacted to the “quenching” process, and the hardening only occurred at that layer. The rest of the steel maintains its previous hardness.

Anodizing

Anodizing “is an electrochemical process that converts the metal surface into a decorative, durable, corrosion-resistant, anodic oxide finish.” It is essentially controlled oxidation, and comes in many types, classes, and colors. For the purposes of this article, discussion will mostly focus on Class II, Type III anodizing. Anodizing is used on aluminum components – namely, the upper and lower receivers, as well as the receiver extension tube. Because anodizing does not offer much in the way of lubricity, the interior of each anodized component should have a baked-on dry film lube from the factory. It does increase wear and abrasion resistance significantly – besides corrosion resistance, this is the primary reason for anodizing weapon components. Because exact shades are difficult to achieve, specifications for receivers will often call out a range of acceptable colors. Receivers from the same manufacturer and the same lot might have visibly different shades of anodizing.

Why Is Heat Treatment Important?

Although I’ll focus on the AR here, the same concepts also apply elsewhere.

So now that we have a very small understanding of each process, we can move on to their importance to the weapon system. As mentioned previously, the barrel extension and bolt carrier are case hardened – but so are parts such as the fire control group. This case hardening is generally around ten thousandths of an inch (0.010) deep – twelve thousandths for the barrel extension in the feed ramp area, and eight thousandths for the fire control group, specifically. In normal operation, with proper heat treatment, these components hardly ever wear out – generally, associated components wear out first, indirectly requiring the replacement of the unaffected components.

When heat treatment isn’t completed properly, however, bad things happen. See Failure Analysis of the M16 Rifle Bolt (V.Y. Yu*, J.G. Kohl, R.A. Crapanzano, M.W. Davies, A.G. Elam, M.K. Veach) – “The wear observed in the controlled experiment indicates the mechanism of why the corrosion pits formed near the locking lug fillet by exposing the Carpenter Steel 158 base metal to the environment. Vickers microhardness readings near the fillet region show that the bolt was not uniformly case hardened. Comparison of the microhardness readings near the fillet region and 10 mm from this region show a disparity of approximately 100 units. The softer, less carburized region near the fillet contributes to the formation of a wear area after firing just 1800 rounds.”

How can you reduce your chances of buying improperly heat treated parts? Well, for one thing, you can buy components from quality manufacturers that take the time to test and inspect – or require suppliers to certify that they inspect- each component individually.

Now, if this is more of a hobby for you, there’s nothing wrong with getting something replaced under warranty. However, if you depend on your rifle for more than just an afternoon of fun every month or so, it would behoove you to avoid cheap parts. Proper heat treating and inspection procedures are often first on the chopping block when some manufacturers look for ways to save money. An improperly heat treated bolt may fail within a few hundred rounds by breaking at the cam pin, or it may lose a lug or two a few thousand rounds earlier than it would have otherwise.

“What about the barrel extension?” you ask. Well, it’s hardened to 58-60 Rockwell (the “C” scale) to a depth of 12 thousandths where the feed ramps are located. The base metal, 8620, has a much softer base hardness. When people dremel away at the barrel extension, they are most certainly removing more than 12 thousandths of material.

“But hey,” you say, “that doesn’t really matter. It’s made of steel. It’s not going to wear out.” Well, if that’s the case, why bother heat treating the barrel extension at all? If it’s not important there, where is it important? Improperly heat treated barrel extensions can and do fail. See below:

“But the fire control group,” you say, “can’t it be improved in just 15 minutes?”

Not unless you really know what you’re doing (Some who think they do – don’t). There are countless reports of cheap trigger jobs failing – this is mainly because the layer of surface hardening was removed and the soft metal underneath exposed. These will fail – it may take 50 rounds, or 5000 – but they will fail sooner than properly heat treated (and unmolested) parts.

Again, to all those who say heat treatment isn’t important, or those who say that it might be kinda important, but still dremel and file on their rifles – I invite you to organize a group buy of non-heat treated components. You’ll save money, and you can file and dremel away to your heart’s content.

Anodized Receivers

The upper and lower receiver, among other parts, are anodized. Anodizing, like heat treatment, is quite thin – 2 thousandths of an inch total. As mentioned above, abrasion and wear resistance are the primary objectives, with corrosion resistance close behind. Even in extreme use, the Type III anodized receivers of the AR-15 will outlast many other components – the barrel and the bolt, for instance. However, when the anodizing is intentionally removed (and it only takes a moment with a Dremel to do so), the relatively soft aluminum below is exposed. As with the fire control group, this will cause premature failure – most likely after thousands of rounds have been fired, but this is far, far sooner than it would otherwise occur. An example is below:

While the factory intended the dremeled feed ramps to aid feeding, the manner in which the work was done resulted in the soft aluminum being pushed up over the barrel extension (which was, thankfully, not dremeled). Because receivers and barrel extensions are the same cost whether they have extended feed ramps or not, there’s no excuse for manufacturers to not properly match the two items without the use of hand or power tools.

Unless an issue such as overlap caused by a standard barrel extension protruding over the extended feed ramps machined into an upper is present, there is actually little pressing need to dremel in feed ramps – and this can be remedied in a much more simple manner by trading one of the components with someone who needs what you have and has what you need.

In extreme cases, the use of a dremel can cause a safety hazard (by weakening the locking lugs), as this sloppy excuse for a “feed ramp matching” job shows:

Yes, the manufacturer did try to cover up the hack job with a Sharpie or other such instrument. Here’s another angle, without the “makeup”.

Amazingly, the manufacturer saw no issue with the “work,” and only agreed to replace the affected uppers (yes, uppers) under public pressure from members of an internet forum.

What Does This Mean To Me?

Let’s face it – most ARs purchased on the civilian market will probably never see more than a case or two of ammunition in their owners’ lives. The majority of even the cheapest AR-15s will probably make it through this “service life” without experiencing the failure of a major component. Their owners will probably spend more time cleaning and looking at the rifles than they will shooting and carrying them. For these people, proper heat treatment and anodizing is not crucial. Even if a failure such as a broken bolt is experienced, having the weapon sidelined  for a week or two while a replacement is delivered is not a big deal.

On that same front, the basic transportation needs (for life) of those same people would probably be adequately met by a cheap economy car, and they would be able to survive without ever eating a 16 ounce prime rib cooked medium rare. However, at one point or another, they will probably aspire to have a nicer car, and will not eat only rice and beans for years on end. Similarly, even if an immediate and pressing need for a highly dependable rifle is not apparent, they might wish to buy a high quality rifle – and there’s nothing wrong with that.

Those who do have an immediate and pressing need for a highly dependable rifle – law enforcement officers, for example – should also seek out quality. High quality rifles will have all applicable components heat treated and anodized, and no machining will be done after those processes are complete. Many other things go in to making a quality rifle, but proper heat treatment and “unbroken” anodizing are often overlooked by potential purchasers.

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11 Comments

Filed under AR-15

11 responses to “Metal Finishes and Treatments: What Do They Mean, and Why Are They Important?

  1. Bill

    Good write up. All that stuff is beyond me 🙂

  2. ZM

    Thank you. Good write up. This should be posted on every gun store glass case where AR parts are sold.

  3. tom

    Wonderful write up! Invaluable knowledge!!

  4. John Jackson

    Is there an independent resource for determining if a particular manufacturer of rifles, or parts, makes/uses properly hardened steel or aluminum? An M4 “The Chart” for metals? (I am aware, through this blog, of ST’s posted certifications)

    • Doc Bob

      With the mechanical properties of steel; toughness, hardness, ductility, strength, and brittleness being directly relational to the required performance of a steel (AR15 extension and bolt,) a hard surface (toughness,) coupled with a non-brittle core (strength and ductility,) creates a metal conducive to destructive high pressure stresses of a 223 combustion chamber. The samurai sword comes to mind with its hard edge and ductile center. The low carbon steel is a good choice for the extension and bolt. My only comment on the feed ramps is that a polish never hurt and the removal of burrs on ramps and their bolt lug cutouts always helps (IMHO) eliminate FTF problems. Running brass over even 40 RC harness shouldn’t wear anything out.
      But I do agree that what was once a finesse artisan trade of gunsmith / armorer, often eludes the uninitiated and produces a bad result with their heavy hand. I would also enjoy a more in depth answer to the steel used, harness method, and mechanical properties of the FCG.

      • Andrew (Vuurwapen Admin)

        Unfortunately, FCG composition and manufacturing processes are closely guarded by the company/companies that manufacture parts for the military.

        As for dremeling the barrel extension – From the research I conducted, the base hardness of 8620 is under 15 HRC.

      • Doc Bob

        While I concur that center core RC can perhaps drop to 16 RC;
        http://www.interlloy.com.au/data_sheets/case_hardening_steels/case_pdf/Interlloy_Grade_8620_Case_Hardening_Steel.pdf
        (this in regards to all said processes applied,) I still believe that a polish and light grind could not approach 15 RC by removing burrs and machine marks. But of course without controlled experiment with mean averages compiled in relation to depth of polish grind and resulting surface hardness, the result is relative.
        Yes, the Secret of FCG mechanical properties lies hidden in the Valley of Proprietary Profits, right next to the Hills of Garden Variety Data Sheets!
        IMO I think FCG all travel down a mesh conveyor into a furnace the same way with oil quench.

        PS: Greeting from Northern Appalachia, the birthplace of US steel and oil.

  5. sadmin

    fantastic writeup, thank you for the information and your time.

  6. Redchrome

    Good information! Thanks much for the writeup!

  7. Pingback: Metal Finishes and Treatments, Part 2: Melonite and Spray-On Finishes « Vuurwapen Blog

  8. Darren S

    Good info. However, the “15 Minute Trigger Job” that is commonly referred to is simply a modification of the hammer and trigger springs. If done improperly, it simply results in light strikes, not an unsafe condition and can easily be reversed by replacing the modified srpings with a couple of inexpensive stock springs. This is in contrast to mucking with your sear/disconnect engagement areas as referenced in your post.

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