Maybe a little more metallurgy. Steel has several characteristics we need to keep in mind; compressive strength, tensile strength, shear strength, hardness, wear resistance, surface finish, machinabiity, ductility and brittleness. Classic guns have been made of mild steel. It is relatively low in all strengths, hardness, wear resistance, and brittleness. It is high in surface finish, machinability, and ductility. Medium and high carbon steels are much stronger, harder, wear resistant and brittle. They have lower machineability and ductility. All plain carbon steels have about the same modulus; the amount of force needed to deflect (bend) the piece. Springs could be made from mild steel but would have to be limited in deflection to avoid permanent bending; high carbon steel can stand much greater deflection without permanent bending if properly tempered after hardening
Mild steel is nice to work by means of relatively simple (hand) tools and has good surface finish. It is made strong enough for gun parts by designing the part large enough so as not to permanently deform under design loading (cycling and firing forces). Permanent deformation of any gun part is considered failure; breakage is not required. Springs can be an exception by way of small deformation without loss of utility or breakage. Wear resistance is dealt with via case hardening wherein a thin layer of medium/high carbon steel is formed by soaking in a carbon rich environment at elevated temperature.
With higher carbon steel, we now must balance hardness against brittleness. Brittleness is the characteristic of breaking/cracking prior to permanent deformation (ductility). Medium and high carbon steels are subject to heat treating. Hardening is accomplished by heating above a critical temperature and then freezing the steel’s structure by dunking into a quench medium. This produces a very hard, but excessively brittle piece. Some hardness can be traded for less brittleness by tempering, reheating the steel into the 300 – 700 degree F region, depending on the hardness/brittleness balance needed. This same process applies to the core and case of a case hardened piece. If the case is very thin, brittleness doesn’t seem to matter. However, when the case becomes “thick” relative to the thickness of the piece, then brittleness can become an issue. Recasing will thicken the case layer and may increase the percent of carbon (medium to high carbon).
So, annealing will make the case layer soft. Annealing, along with hardening, will have no effect on the mild steel core. Warpage can/does occur with any steel that is heated; it doesn’t take much to affect a precision fitted joint. Fortunately, those craftsmen good at case hardening have found ways to minimize warpage. If, however, a gun part is exposed to multiple case hardening cycles, temper will become an issue. Per CJO above, a tang (relatively thin) can become so carborized as to suffer a brittle fracture unless tempered. I’m not aware of anyone doing post quench tempering on case hardened gun parts; although I think it would be a good idea.
