Originally Posted by
guy153
He could have got it very hot in spite of the only 15cm of paint damage. In any case he will have destroyed the heat treatment, which was the idea, so he could bend it. So you now basically have a partly 631 chainstay which is probably OK unless it was super thin to start with. In principle therefore it should be OK to cold-bend it a bit more but I would be careful. Also further out from the part he heated it will still be hardened.
Hmmpf
A moderate heat might improve ductility without necessarily destroying the heat treatment. The 831 is supposed to be air hardening, but may ultimately be similar to the 631 after welding/brazing.
Are the chainstays simple tapered, or butted?
If there is no butting, then the weak spot will be at the bottom bracket which already was welded/brazed.