relative stress relief condition - steel shaft material

[Tmoose]

We make some steel shafts 9 to 12 inch diameter, up to 100 inches or so between bearings. A single row of Several 1 inch keyways are cut ~ inline along one side.
Historically a few shaft have reportedly "bent" in service. It is unclear if the shafts were straightened or needed straightening during manufacturing process.

Physical properties are satisfied with normalized or HR or annealed materials.

Which condition(s) is certain to resist distortion when asymetrically machined, as with a row of keyways?

thanks

Dan T