This is the sixth and final part of David Pye's series on the heat treatment of small arms, guns and rifle barrels. Part 5 can be found here.
Pulsed-Plasma Ion Nitriding
The pulsed-plasma nitriding method of heat treatment is based on similar principles as the electroplating method. That is to say, there is a cathode and an anode and the process gas as the carrier of the nitrogen for diffusion. The barrel would act as the cathode with a long anode rod, which would be inserted into the bore of the barrel complete with a process-gas inlet manifold.
Either of these two suggested processes will also improve the corrosion resistance of the outside diameter of the barrel as well as the internal bore of the barrel and breech area.
The comment may well be made by other experts in the field of heat treatment of the howitzer barrel and the breech component parts that there is a serious risk of distortion because of the process temperature necessary, which is generally up to 1000°F (540°C).
Once again, it must be stated that the plasma nitriding method is not a new untried method of nitriding. It was first developed by Drs. Wienheldt and Berghaus in Germany in the early 1920s. The procedure has now developed an accepted maturity level on an international basis.
However, the final temperature selection will be dependent upon the pre-heat-treatment process temperature for core properties of the barrel and its resulting hardness condition. But one is not locked into these temperatures. The process temperature can be lowered significantly to a temperature of 824°F (440°C). Obviously, the process will take a longer time to acquire the desired case depth.
Another advantage of the gas nitriding procedure or the plasma-ion nitriding procedure is that the complete barrel will acquire an improved resistance to corrosion. In addition, when using the plasma-ion nitriding system, the resulting surface metallurgy will show a much denser metallurgy of the formed case, which adds significantly to the resistance to crack initiation and exfoliation.
The plasma nitriding system will also improve the life of the barrel, thus making it more economical. I am not familiar with any work during the research of this presentation of the nitriding process being used for the barrel surface-hardness treatment.
The breech-block assembly and mechanism is the item that seals the breech explosion chamber once the projectile has been loaded. The breech loading chamber can be expected to reach 65,000 psi (as previously been stated). Breech operating mechanism is the sequence that actuates the breech block. The pressure development in the chamber will be dependent on the propulsion charge analysis inside of the shell casing.
It is the writer's opinion that the most important area of surface heat treatment will be the breech chamber. It is the breech chamber that sees high stress levels for the following reasons.
- Extreme chamber wall temperatures due to the analysis of the projectile propulsion material in the shell casing.
- The internal shell walls being immensely stressed by the pressure release from the shell containing the propulsion material for the projectile, which comes under a compressive stress.
- The external surfaces of the breech chamber area of the weapon also becomes stressed under tensile conditions resulting from the analysis of the shell, containing the propulsion material for the projectile.
The internal barrel rifling does not suffer the same operating conditions as the breech area of the weapon. It is likely to suffer wear conditions, however, which is why it needs to be surface treated.
It can be seen that the howitzer barrel assembly is the most important area to ensure long life. This means that the barrel needs high-precision surface treatments in order to ensure long life and projectile discharge accuracy to its intended target. Any physical change that occurs within the barrel will reduce accuracy and range.
The writer is of the belief that the success of the howitzer system lies not so much on deposition but on diffusion. The process of carburizing as a diffusion treatment is too risky from the point of view of potential distortion, which would most probably occur as a result of quenching. The nitriding procedure (whether it be gaseous nitriding or plasma- assisted nitriding) has the usage potential of accomplishing a superior condition of performance and life.