That's an interesting point, something I've wondered for a long time. Say you're welding 6061 T6. Because of the heat of the welding process, the metal on either side of the bead will be annealed back to the T0 condition. So about how far from the weld is the metal's strength/hardness affected? Is that really an issue for everyday projects, or is it only critical where high strength/weight applications are vital such as in bicycle frames?