Originally Posted by
dblA
Interestingly the bolt appears to be utterly undamaged, at least to the naked eye. Having a bolt that much harder than the thread is of course a recipe for disaster. The hanger is part of the frame, not detachable. Otherwise the problem would be smaller I guess.
On any bike frame, the bolt is always going to be harder than the frame material, even for a steel frame and even more obviously for aluminum or carbon. There have been aftermarket aluminum rear derailleur mounting bolts available in the past but those were made to break away in the event of an impact so as not to damage the frame. They were not intended to protect the hanger threads.