At Blackout Defense, we are thrilled to offer cryogenically hardened barrels as part of our custom rifle and upper builds. We have received a lot of questions about this process that we hope to answer here.
What is cryogenic hardening and why are we so excited about this option?
Cryogenic hardening, also known as deep cryogenic treatment, involves subjecting the metal to extremely low temperatures, typically below negative 300 degrees Fahrenheit (negative 185 degrees Celsius), for an extended period of time. This is done to enhance the metal’s mechanical properties, and in particular to increase its hardness and wear resistance. For stainless steel in particular, cryogenic hardened barrels exhibit improved properties over raw barrels, without the risks associated with black nitride.
Why choose cryogenic hardening over black nitride for stainless steel barrels?
Black nitride – also known as salt bath nitride, QPQ, Melonite, and Tennifer – is known to greatly increase metal hardness and wear resistance (and therefore increase a barrel’s expected life). It is an excellent finish that is very common across the industry. So why would anyone choose cryogenic hardening over this more common process?
The answer is that although black nitride offers many benefits, the process also involves risks that cryogenic hardening does not. In particular, it risks undoing the stress relief process that is performed during the manufacturing of the barrel blank. Black nitriding occurs at high temperatures – typically around 1000 degrees Fahrenheit – which is very close to the temperature at which stainless steel is stress relieved. If the stress relief temperature is exceeded, this can impart stress back into the barrel and seriously degrade the barrel’s accuracy.
For some shooters – particularly those who compete – the risk to accuracy that is inherent in the black nitride process is simply not worth it. The best alternative is usually to leave the barrel raw.
Why choose cryogenic hardening over unprocessed metal for stainless steel barrels?
For stainless steel in particular, cryogenically hardened barrels exhibit improved properties over raw barrels, and the cryogenic process itself does not come with any known downsides or risks. In particular, compared to their raw counterparts, cryogenically hardened stainless steel barrels exhibit:
- Increased hardness and wear resistance;
- More uniform and coherent barrel harmonic;
- Easier cleaning; and
- Increased muzzle velocity.
How does the cryogenic process actually enhance the mechanical properties of raw stainless steel?
In the same way that heat can affect a metal’s microstructure and properties, so can extreme cold. Here’s where things get a little sciencey.
416R stainless steel is a type of steel known as martensitic steel. The key characteristic of martensitic steel lies in its crystal structure, which gives it distinctively high hardness, strength, and wear resistance. In contrast, 300 series stainless steels such as 304 and 316 are a type of steel known as austenitic steel. The key characteristic of austenitic steel also lies in its crystal structure, which gives it a relatively high level of ductility, good formability, and excellent corrosion resistance.
Austenitic steels are interesting in that they can undergo “martensitic transformation” when subjected to rapid and extreme cooling (i.e., the cryogenic process). This transformation changes the crystal structure from austenitic to martensitic, resulting in an increase in hardness and a decrease in ductility.
Although 416R stainless is already a martensitic steel, cryogenic hardening still has an impact. In particular:
- Increased hardness and wear resistance: 416R stainless steel may still have some retained austenite. The cryogenic hardening process will transform any retained austenite into martensite, resulting in an increase in hardness and improvement in wear resistance.
- Reduced residual stress: The cryogenic hardening process not only does not risk imparting stress like the black nitride process, but it can also positively reduces residual stress. This, in turn, can improve the dimensional stability of the material and reduce the likelihood of distortion or warping during subsequent use.
- More uniform barrel harmonic, increased muzzle velocity, and easier cleaning: The cryogenic hardening process can also affect the distribution and morphology of carbides within the metal’s microstructure, resulting in a distribution that is more uniform in both axial and radial directions. This in turn may produce a more uniform and coherent barrel harmonic that generates a more sinusoidal action. It may also result in a smoother surface that increases muzzle velocity and is easier to clean.
Cryogenic hardening offers several benefits for 416R stainless steel barrels compared to raw and black nitrided barrels, including: increased muzzle velocity, easier cleaning, and less residual stress. At Blackout Defense, we perform this process in-house and we are happy to offer cryogenically hardened barrels as an option to our customers.