Don't Buy a Nitride Bolt

Salt bath nitriding, often branded as Melonite or QPQ (a variant), has become a common surface treatment in the AR-15 world. Its benefits (corrosion resistance, wear resistance, a sleek black finish, cost-effectiveness) make it attractive to both manufacturers and consumers. Yet when applied to one of the most critical components in the rifle (the bolt) it introduces more problems than it solves. Beneath the appealing surface lies a metallurgical compromise that can undermine reliability, toughness, and fatigue resistance. This article explains why nitriding is a poor choice for finishing an AR-15 bolt.

What does the TDP say?

The Colt Technical Data Package (TDP) is our North Star when it comes to the AR platform. The TDP explicitly prohibits “carbo-nitriding” (i.e. ferritic nitrocarburization) of the bolt head.

Why would it specifically call this finish out?

Let’s get into it.

To understand why ferritic nitrocarburization (FNC) is not permitted for the bolt, we need to understand a little about two processes:

The production of the bolt
Ferritic nitrodcarburization

Everything about the incompatibility of FNC with the bolt comes down to processing temperatures.

To Manufacture a Bolt...

The bolt undergoes a lot of thermal processes as it moves from bar stock to a finished bolt. These thermal processes have very specific purposes.

The alloy will be forged into bar stock at a temperature up to 2100°F (1149°C) and then cooled slowly.
The steel is generally annealed around 1250-1280°F (677-693°C). The metal is cooled slowly. This annealing process softens the metal for easier and more precise machining.
The annealed steel will be machined into the desired shape around room temperature.
The part will be stress relieved at a temperature around 850°F (454°C) and then air cooled.
After machining, the part undergoes carburization at a temperature around 1550-1650°F (843-899°C). Carburization increases the carbon content at the surface of the part to increase the case hardness. The longer the metal is carburized, the deeper/thicker the surface hardness penetrates.
After carburization, the part undergoes heat treatment at a temperature around 1425-1500°F (774-816°C). The part is held at this temperature to allow the carbon to homogenize in the iron lattice and to diffuse toward the core of the part. Note that the hardening heat treatment step can be combined with carburization if the next step is performed immediately.
The heat treatment is immediately followed by oil quenching (oil temp around 150°F/66°C). The sudden drop in temperature traps carbon molecules between the iron molecules, thereby increasing the hardness throughout the part.
Per the TDP recommended procedure, the bolt will undergo an initial tempering at 350-375°F (177-191°C). This tempering process reduces internal stresses and reduces the brittleness of the metal, without significantly compromising the hardness.
After the initial heat treatment, the bolt will undergo cold treatment at -100°F (-73°C). This cold treatment further reduces internal stresses and improves the toughness of the bolt.
After cold treatment, the part will be retempered at a temperature of 350-375°F (177-191°C).

If we plot the sequence of this thermal cycling following the machining of the bolt, we get something that looks like this:

That all seems somewhat straightforward. In the end, we have a case-hardened bolt that has a core that is hard enough to withstand the punishment that the bolt experiences, but just ductile enough resist fracture from the high cycle fatigue stress.

Next, we will take a closer look at NFC.

What is Salt Bath Nitride (Ferritic Nitrocarburization)?

Salt bath nitride, technically salt bath ferritic nitrocarburization (FNC), is a chemical process that results in a chemical modification of the surface of ferrous parts, thereby increasing the case hardness of the metal.

The principle of FNC is to diffuse nitrogen and carbon into the case (the surface; typically 10-20 microns thick with a transition zone of 100-200 microns toward the core) of a ferrous part. This finish results in:

Increased surface hardness.
Reduced wear and friction.
Improved corrosion resistance.

These properties make nitride a great finish for many parts of the AR.

How Is Nitride Applied?

Salt bath FNC involves soaking a part in a molten salt bath to infuse nitrogen and carbon into the surface of the part. Salt bath FNC is typically performed at 560-580°C (1040-1075°F). This is where we run into a problem for the bolt.

Why is Nitride a Bad Finish For a Bolt?

If you recall, the last thing we did during the production of our raw bolt was to temper some of the brittleness out of the core, without significantly reducing hardness. We did so at a temperature of 350-375°F (177-191°C). Obviously, 1000+ degrees is more than 350-375 degrees. But why is this an issue?

We are about to get a little technical, but this is really important.

The hardening process had a very specific goal: achieve core hardness. When we heated the steel to 1425-1500°F (774-816°C), the lattice of iron molecules in the bolt relaxed (becoming austenitic), allowing the carbon molecules at the surface (from carburization) to migrate between the iron molecules toward the core. When we quickly quenched the bolt in oil, we froze those carbon molecules into a lattice of iron molecules. This process transforms austenite into martensite. Martensite is extremely hard, but brittle (we will deal with this next). The crystalline structure undergoes the following transformation:

The tempering process also had a very specific goal: reduce the brittleness (increase the toughness) of the martensitic steel. To achieve this, we carefully heat the steel to a temperature (well below the austenitic threshold) to alter the microcrystalline structure. The temperature used to temper the steel determines the crystalline changes that occur and has a great deal of impact on the final mechanical properties of the part.

When we heated the steel to 350-375°F (177-191°C), we made the entire part tougher (less brittle) without compromising the hardness that we achieved in the prior step.

Back to our FNC process.

By increasing the temperature of the bolt to 1040-1075°F (560-580°C), FNC results in over-tempering of the bolt. Remember, we only tempered to 350-375°F (177-191°C). The FNC processing temperature is way over this temperature and it pushes us into the “high-temperature tempering” range. While we aren’t reaching the austenitic temperatures (which would completely reverse the martensitic transformation), reheating to this temperature will have negative effects on the hardness and strength of the bolt.

The chart below adds our FNC thermal cycle to the thermal life of the bolt. This diagram puts the FNC processing temperature in perspective.

If we look at the effects of tempering temperature on hardness, strength, and ductility, we can see that FNC (580°C) may not have the most desirable effects on the AR bolt, whether C158 or 9310.

But my nitride bolt is fine...

A nitride finished bolt is not going to explode in your gun. Let’s get that out of the way.

The nitride bolt will have a lower yield strength (practical strength) and a lower ultimate strength (failure strength).

The lower yield strength coupled with increased ductility will result in a bolt that stretches and permanently deforms with less applied stress. The bolt stretches at the weakest point: the cam pin hole. As the material flanking the cam pin hole stretches, it becomes drawn out and weaker until it ultimately fractures (i.e. a 1-piece bolt becomes a 2-piece bolt).

We are not saying that you won’t get use out of a nitride bolt. Just understand that a nitride bolt will not last as long as one that is finished at lower temperatures.

If you prescribe to Chad Albrecht’s decades of experience on the matter (which we do), once a bolt’s cam pin hole begins to stretch to a detectable level, you are roughly a few hundred rounds from failure. Now, a nitride bolt will stretch even faster and, once the stretching becomes detectable, will fail faster.

If you’re running a nitride bolt, ride it out until it starts to fail. Let that be the last nitride bolt you buy.

Conclusion: No Nitride for Us

So why are nitrided bolts so common? Because they look better, cost less to finish, and carry strong marketing appeal. Consumers see corrosion resistance and high surface hardness and assume it’s superior. But when form replaces function, the buyer pays the price in reduced bolt life and rifle reliability. The deeper metallurgy tells a different story—one that the spec sheets won’t show.

Nitriding offers surface-level advantages but creates structural liabilities where it matters most. For AR-15 bolts—a part defined by mechanical stress and fatigue exposure—surface hardness is not enough. The correct finish must preserve core strength, maintain ductility, and enable long-term durability. Manganese phosphate may be old-school, but it’s still the one of the best finish for bolts where reliability matters. Don’t be fooled by blacker, harder, or slicker. When it comes to bolts, metallurgy wins.

Alternative Bolt Finishes

If you want to graduate from a phosphate bolt (not that there is anything wrong with a phosphate bolt…), there are other alternatives that won’t ruin the core hardness of your bolt. No other standard finish (including phosphate) requires you to over-temper your bolt.

Manganese Phosphate: Manganese phosphate is applied at 190-210°F (88-99°C) and is the Mil-Spec standard.

Hard Chrome: Chrome is applied at 120-140°F (49-60°C) and is a great bolt finish.

NP3: NP3 is applied at about 200°F (93°C) and then baked at 250-300°F (121-149°C) and is an excellent bolt finish.

DLC: DLC is applied at 150-200°F (66-93°C) and is probably the all-around best performing finish for a bolt.

To hammer home the difference between these finishes and FNC, here is the thermal lifecycle chart for each.

Item #1

Totten, G. E., Howes, M. A. H., & Inoue, T. (2002). Handbook of Residual Stress and Deformation of Steel. ASM International.
ASM International. (1991). Heat Treater’s Guide: Practices and Procedures for Irons and Steels (2nd ed.).
Davis, J. R. (Ed.). (2002). Surface Hardening of Steels: Understanding the Basics. ASM International.
Carpenter Technology Corporation. (2005). Carpenter 158® Steel Technical Datasheet.
Schaeffler, W. A. (1949). Constitution Diagram for Stainless Steels. Metal Progress, 56(11), 680–686.
Gschneidner, K. A., & Eyring, L. (Eds.). (1990). Physical Metallurgy of Steel. North-Holland.
NATO Army Armaments Group – Land Group (NAAG-LG). (2016). Small Arms Fatigue and Failure Reports, 2006–2015. NATO STANAG 4179 Subcommittee Archive.
US Army ARDEC. (2011). High Cycle Fatigue Testing of AR-15 and M4 Bolt Materials [Technical Report]. Picatinny Arsenal, NJ.
Shot Peening and Surface Treatment Journal. (2008). Effect of Surface Treatments on Fatigue Resistance in Hardened Steels, 17(2), 34–42.
Ricker, R. E. (2006). Corrosion Fatigue: Mechanisms and Mitigation Strategies. NACE Corrosion 2006 Conference.
MIL-DTL-16232G. (1999). Military Specification: Phosphate Coating, Heavy, Manganese or Zinc Base.
P. M. Unterweger et al. (2014). Fatigue Behavior of Nitrided Steel with Varying Case Depths. International Journal of Fatigue, 65, 129–136.