This is part 3 of David Pye's series on the heat treatment of small arms, guns and rifle barrels. Part 2 can be found here.
More on Howitzer Barrel Heat Treatment
The next area of concern is the howitzer barrel. The rifled internal wall of the barrel can wear rapidly if it is not surface treated. As with any metallurgical surface treatment, if is not pre-heat treated to provide a foundation to support the diffused or deposited layer, the treated surface is likely to collapse no matter how good the surface treatment.
The typical material selection for the long 155-mm-bore barrels is that of a medium-carbon steel. A typical analysis of material would be: carbon-0.35%, chromium-1.4%, molybdenum-1.2%, vanadium-0.3%, nickel-0.8%.
The barrel is manufactured by a forging operation. Because of the forging cross-sectional area, I believe it is very important to normalize the forged barrel after forging. The reason is that the forging temperature of the barrel typically results in a very coarse grain structure.
So, normalizing now becomes a necessity. Once the preliminary heat treatment has been conducted, the barrel is then machined both externally and internally to accommodate the projectile. I would recommend, because of the extensive machining operations, to periodically stress relieve to reduce the residual machining stresses set up by the manufacturing operations of the barrel.
In order for the barrel to withstand the enormous pressures, temperatures and wear generated inside its bore, heat treat (austenitize, quench, temper) is needed to develop the necessary metallurgical and mechanical conditions. Once the barrel is in the desired metallurgical and mechanical condition, final machining can take place.