This is the first of several blogs that will describe some of the heat-treatment practices conducted on, handguns, rifle barrels, sniper barrels and artillery (155-mm howitzer). We do not offer anything “new” in the heat-treatment procedures, but we hope to increase understanding to the users of weapons. It is not a new technology but a matter of understanding thermal processing and surface treatments.

The writer has no doubt that the personnel working to process new weapon barrels will currently know these practices. However, not all of the general users recognize the need for heat treatment. The writer will commence with the subject apropos the particular weapon.

1. Reduction of general barrel wear and tear.

2. Reduction of distortion that can occur on the weapon.

3. Aesthetic appearance of the barrels.

 

When any barrel is used frequently, discharging many propulsion-charged projectiles, there is massive temperature generation due to the force that discharges the projectile. The temperature that can be generated for a fraction of a second will depend on the propulsion-charge analysis. However, the temperature can rise for a fraction of a second in the breech area of the barrel to 4500°F.

This temperature rise will occur in any barrel size ranging from a .22 barrel up to those used onboard military vessels – an aircraft or a howitzer-size barrel of 155 mm. However, the 155-mm barrels are not found in any commercial shop!

In the load area (the breech), the propulsion charge is subjected to the highest ignition temperature. The net result is that the breech area and the barrel length will begin to heat up. In addition to this, one needs to consider that the pressure generated within the breech area is massive ... but only for a fraction of a second.  

Because steel is a good conductor of heat, however, the barrel and firing mechanism will begin to heat up. This explosion in the breech area will cause the barrel to expand and contract, meaning that the breech-area expansion and contraction will generally exceed the capability of the chosen steel.

Relative to the barrel rifling, there will be a temperature rise on the surface of the rifling, but not as high a temperature as is generated in the breech area. Somehow the barrel and its breech must be allowed to cool down, but damage may well have commenced. 

Generally, the barrels are heat treated to reduce the risk of distortion as well as the wear and tear on the barrel. The heat-treatment procedure that is practiced is state-of-the-art and in the public domain in magazines and textbooks. The process chosen is generally nitriding or ferritic nitrocarburizing, which is a diffusion process and not a deposition process.

AISI 4140 is generally chosen as an appropriate steel for the manufacture of the barrel and breech. AISI 4140 is perhaps one steel that is used most frequently and is commonly known as ordnance steel or chrome-moly steel. This steel is also used frequently for functioning components in the handgun assembly (e.g., bolts, receivers).

An alternative steel (but more expensive) is AISI 4150, which is also known as ordnance steel. The carbon content is slightly higher than AISI 4140 with the carbon content at a nominal. This alloy contains molybdenum 0.50% plus a small amount of vanadium. Then there are the stainless steels that are often utilized. which can be 17-4 PH, 410 or 416.

 

Heat Treatment

There are three heat-treatment procedures that can be applied to the manufacture of the handgun barrel. The purpose of any of the briefly described heat-treatment procedures is to improve the surface-wear characteristics of the barrel and reduce the risk of premature corrosion.

The first heat-treatment procedure would be what is known as pre-hardening and temper, which is to create the primary mechanical properties necessary for the final heat-treatment procedure.  

The two options for the final heat treatment are nitriding and post-oxi-nitriding (sometimes known as Nitrox). These procedures can be performed by thermal gaseous procedures utilizing ammonia for the first stage of the process followed by a surface oxidation procedure.

Alternatively, the post-oxy-nitriding procedure can be conducted in molten salt baths followed by a thorough washing/cleaning to ensure that there is no surface contamination from the molten-salt residue. Another alternative procedure could be controlled nitriding using the pulsed-plasma thermal process (aka plasma-assisted nitriding).

Part 2 will discuss the heat treatment of rifle barrels and 155-mm howitzer barrels.