I have seen flame cut edges on A36 get hard, how hard is too hard?
If You have to machine it [as We did] it depends on the operation, and what the cutter material is. With carbide face milling taking a cut deep enough to get under the hardened material usually solves the problem.
To start a bandsaw cut You can grind a notch with an angle grinder to get through to the softer metal.
Drilling on a machine tool [large drill press or boaring mill] was seldom a problem, use low speed and lots of pressure.
The oxide on the surface is half the battle, especially with high speed steel cutters, always cut deep enough to get under that.