This is part 4 of David Pye's series on the heat treatment of small arms, guns and rifle barrels. Part 3 can be found here.

Even More on Howitzer Barrel Heat Treatment 

Surface Treatment

The current method of surface treatment in both the breech chamber and the rifled barrel is by hard-chrome electroplating. Electroplating is simply a deposition of hard chromium onto a clean, machined surface.

It should be noted that the hard-chrome plating procedure and its result will only be as good as the surface to which it deposited. In other words, absolute cleanliness of the internal surfaces is mandatory.

The hard-chrome plating will result in a surface hardness of approximately 700 HV (100-gram cross-transverse measurement). The deposition thickness should not exceed 0.2 mm.

It will also be a necessity to de-embrittle within a maximum time elapse of no greater than two hours after completion of the electroplating procedure, otherwise there is a risk of random crack initiation after the plating operation.

Components to Add to the Barrel 

To make the barrel into a useful and functional weapon, there are numerous components to be added. The components are itemized here, followed by sketches.  

The breechblock assembly and its mechanism seal the breech chamber once the projectile has been loaded and the breechblock closed. The breech-loading chamber can be expected to reach pressures up to 65,000 psi for a fraction of a second due to the explosion resulting from the projectile shell being struck and igniting.

This is dependent on the analysis of the propulsion charge within the shell casing. In addition to this, there will also be a rise in temperature on the breechblock in the chamber walls, which can vary (once again, dependent on the analysis of the propulsion charge) up to temperatures of 5000°F for a fraction of a second.

There is an extraordinary amount of both heat treatment/metallurgy and surface treatment conducted on the above artillery pieces. However, I am of the opinion that diffusion heat treatment/metallurgy is preferable to a surface-deposition treatment because no matter how good the surface-deposition treatment is a diffusion treatment is superior.