This is part 5 of David Pye's series on the heat treatment of small arms, guns and rifle barrels. Part 4 can be found here.

The deposition treatment is only as good as the preparation and surface cleanliness is concerned. This necessitates intensive pre-preparation/cleaning prior to the deposition treatment being conducted.

Once the deposition surface treatment has any crack propagation and if the deposition begins to fracture, exfoliation can occur. If this is the condition of the deposition material (such as hard chromium), then the weapon will begin to lose, not only its range that the projectile will travel, bus accuracy will now be flawed.

A diffusion treatment control involves:

  • Process temperature can be accurately measured and maintained for temperature uniformity.
  • The depth of diffusion material can be controlled and monitored to be reproduced on every item of the construction of the artillery piece.
  • The analysis of the surface diffusion material (such as the process gases) can also be accurately controlled and repeated consistently.

It is recognized by the writer that the hard-chromium deposition treatment has been used for many years, both nationally and internationally. One of the principle reasons is that there will be no distortion occurrence on the 155-mm howitzer barrel. However, the barrel could be nitrided (which is a diffusion procedure) using either the gaseous nitriding procedure or the pulsed-plasma ion-nitriding procedure.


Gaseous Nitriding

This can be accomplished by the utilization of a long horizontal furnace (which will obviously be gas-tight) and with a gas delivery tube in the bore of the barrel connected to a manifold to deliver the process gas uniformly along the length of the bore of the barrel. This is not new technology; it has been available since it was first patented in the early 1900s. To insert a long tube with holes drilled through the wall of the tube and along its length is quite a simple operation and will ensure uniformity of the diffusion of nitrogen into the rifled portion as well as the breech area of the tube. The gaseous dissociation can be monitored either by the water dissociation method or the electronic analysis and control system using PC/PLC for control and monitoring the gas dissociation. In order to reduce the risk of distortion, the barrel would need to be stress relieved after each major machining operation and for the final nitriding heat-treatment procedure to be well supported to reduce the risk of distortion.