The ductility of the steel was determined during the blending of the constituent metals (and other stuff) at the mill.
The ductility of the tube was changed and refined at the rolling mill by external pressure during rolling.
Boring the tube does not change the ductility of the steel.
The ductility of the tube was proved in testing, pre-sale by the manufacturer.
Unless you remove material from the as delivered barrels you have the same formula and ductility forever.
Firing millions of shells would slowly change the ductility, but, everything else would go into the trash before it happened.
It's a non issue for any shotgun tube close to original wall thickness, using the loads it was designed for.
Now, if a dent or bulge enters the picture, because the coefficient of elasticity was exceeded in that area, metallurgical changes have occurred there.
In all my reading, I have never heard of heat being applied to a dent repair, so, I don't believe the matrix of the steel is returned to original in a dent repair. But I also believe that the loss of ductility around the repaired dent is small enough that the lifespan of the tube while shortened, is still decades in excess of every other part of a shotgun.
Again, where the dent occurs being more important than the change in ductility due to the gigantic number of cycles required to fracture unmolested steel tubes.
The proof is in the the thousands, perhaps millions, of dents having been removed in the traditional fashion, not including heat.
I threw in the dent segue, because dents (and bulges) do change the lattice of the steel around them. And no one should forget that.
