Originally Posted by
BikeWise1
The tubes used prior to the availability of air-hardening steels weakened significantly when exposed to the heat of welding. The only way to compensate was to make the tube thicker, therefore heavier. The new classes of steels actually get stronger with the heat, so the tubes can be quite thin. My welded Waterford R-33 has a downtube that is .7MM at the welds. Amazing. And the frame itself, made of S-3 steel weighs 2.9 lbs. Also, with the advent of laser tube mitering, tubes can be cut to fit precisely together, so that a very small weld bead is required.
Air hardening tubes are certainly a common thing in exotic bikes, but plain 4130 and most other common less expensive steel alloys are
not air hardening, and welding does, in fact, weaken the tubes on these. And TIG welding has still been the preferred method of joining these for 20+ years.
Here are some comments made online by Keith Bontrager:
http://search.bikelist.org/getmsg.as....9911.0067.eml
Basically, the pertinent info presented here is that welding does weaken tubes, but since the heat is applied and removed quickly, the HAZ is small and easier to deal with. Brazing requires either heating a larger area (generally with lugs) or heating for a longer time (as with fillet brazing).