I bent my stock rear axle flange couple of years ago. It bends right where it connects to the shaft, so effectively the flange is still flat, the axle is still straight, but the flange is on the piss. I've done exactly the same thing in other cars too. I've always just replaced them with upgrades axles.
I think you would have your work cut out for you straightening it with a hammer - you would be more likely to bend the flange disc out of flat. Doubt you could get it true enough anyway even if you could avoid bending the flange. The only possibly feasible way I would attempt is to make a jig to hold the axle vertical in a press and offset it to the press head so you could work the outer of the flange. You would need a good hydraulic press, 50 or 60 tonne.
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