James Casper # 1 Are these bolts tourqed to max speck? Are they on the front end of an Automobile? If so welding is not adviseabled
what size of bolt (nut) are we talking about and what welding process? Unless you are concerned the head of the bolt may break off and go thru a machine what harm is a small hardened area.
If the bolt is large enough and weld process quick enough that may be the extent of harm. I doubt it greatly effect the strength but may may crack but then it would still serve as an Anti-rotation devise.