Originally Posted by
WizardOfBoz
I would suspect chromemoly which would have superior strength/weight ration compared to HiTen. You can weld chromemoly, if you are willing to heat treat the whole frame afterwards. Otherwise, the steel in the heat-affected zone ends up being either too hard and brittle and/or too soft and weak. Neither good for bicycles. Which is why they use brazing, which is lower temperature and doesn't cause changes in metal temper.
Bromptons are made from chromoly steel. They are brazed because that's just how Brompton has chosen to join them. Chromoly, 4130, 531, 631, 853 and all of the other supersteel alloys used in bicycle frame building can be TIG welded with no heat treating afterward. There are a few BMX companies that heat treat their production frames after welding, but none of the custom framebuilders I know heat treat their steel frames. It's just not necessary to produce a frame whose strength far exceeds the loads placed on it.