This is part 2 of David Pye's series on the heat treatment of small arms, guns and rifle barrels. Part 1 can be found here.
Howitzer Barrel Heat Treatment
The history of the manufacture of the howitzer gun barrel goes back to the early times of the Indian method of manufacturing large-bore cannons and has continued to be developed and improved upon. I believe it is important to understand the activity that takes place within the howitzer barrel when discharging a shell projectile. It is the breech section of the howitzer barrel where most activity takes place. The breech into which the projectile and discharging explosion occurs is where the most mechanical damage and metallurgical changes occur.
For a minute fraction of a second there is a massive explosion, and it is the analysis of the propulsion charge mixture that will determine how far the projectile will travel. For a fraction of a second in the breech area, there is a massive temperature created that can rise up to 5000°F.
The temperature rise will be entirely dependent on the analysis of the discharge material located within the shell casing, which moves the projectile through the barrel to continue its fight to its assigned target.
A pressure buildup (once again for a minute fraction of a second) also takes place, which means that the internal breech-block walls are subjected to a compressive stress and the external surfaces of the breech area are subjected to tensile stress. Once again, this occurrence will only occur for a split second.
However, the fact remains that there is a temperature rise and mechanical stress originating from inside the breech chamber. It is therefore the breech projectile loading chamber that has the potential to both suffer from internal compressive stresses and external tensile stresses. The design and the heat treatment of this particular area of the weapon is critical to its functionality; i.e., the distance that the shell projectile is ejected from the combustion shell to its impact point.
The life of the breech area with the mechanical stresses and breech metallurgy will be determined by the analysis/composition of the explosive charge material contained in the shell casing. As the shell is ejected from the casing, it enters the machine rifled barrel. The barrel is rifled as seen in figure 3.