I have not yet seen any original documents that precisely describe the process(es) of heat-treating the bolt body of any Lee Enfield.
There would have been several such processes, as all four bearing surfaces of the lugs should have been sufficiently hard to minimise wear, but not to be brittle.
The main "body" of the lugs should have been "tough" to support this hardened skin and , particularly, to have some elasticity to cope with the forces transmitted during the shock of firing.
If this process were not done correctly, especially in the days before pyrometers etc, and given that, to the end, SMLES were made from various grades of CARBON steel, (NO "fancy alloys" allowed except under very special circumstances), the potential for "over-cooking" the metal was very much present.
The bolt body started out as a straight bar of steel and was FORGED into an "L" shape before being machined to final form. The forming of the long "tail" on earlier LE bolts must have been REAL fun!
The 1903 Spec (S.A. 242 (1903)) for the Mk1 SMLE states that the bolt body was made from "56A42 Crucible steel, oil-hardened, tempered and browned".56A42 is actually a low-carbon (0.67 to 0.77") steel and the stuff used in the bolts was to have vanishingly small traces of Manganese, Phosphorus, Sulphur and Silicon.
The 1938 Spec (S.A. 462P) specifies "D.D.8" steel, oil-hardened, tempered, polished and browned (blued" or oil-blacked.Both recipes seem to rely on selected Swedish ore for the base material. One source of problems is in the coal used to produce the coke in the furnace. Some coal contains significant traces of Phosphorus and Sulphur, neither of which are desirable.
Interestingly, at some, as yet unknown date, Lithgow started using what I suspect was "electric" heating for the heat-treating of the locking shoulders in the body. If you are lucky enough to have a "MINT" Lithgow SMLE (usually WW2 or later), or one of the WW2 XP carbines, take a close look at the exterior of the left and right sides, just behind the locking shoulders.
There you should find a small circle of discolouration and surface distortion, a bit like tiny "spot-welding" scars. I understand that these marks were left by the application of Carbon electrodes that delivered the current to "spot" heat the metal before quenching.
Thus, the remainder of the body would be in a "toughened" (strong, but NOT brittle) condition, and thus able to withstand the "shock load" of firing.
Later "FTR" activity seems to erase these little heat-treatment marks.
