Chromoly is the nickname for steel alloys containing, among other elements, chromium. Depending on the alloy and heat treatment, they can be far stronger than the hi-ten steel Brompton uses, and combined with tweeking the tubing geometry will result in a bicycle frame that's both stronger and lighter. Top quality steel bicycles moved from hi-ten to chromoly (or similar performing manganese-moly) in the 1930s~70s.
(Do I recall Mr. Ritchie once mentioning he chose hi-ten steel because his original frame manufacturing rig couldn't bend the stronger chromoly?)
The general manufacturing industry of all types of mechanical things generally moved from brazing to welding in ~1930s as well, but because bicycle frames are typically made of relatively thin wall tubing, the bicycle industry stuck with brazing, either by hand (as is still practiced @ Brompton) or in automated brazing stations. Gradually into the 1980s steel alloys and specialized techniques became available to allow reliable welding of thin wall bicycle tubing as well, creating stronger and lighter frames. This welding can be done by hand or, as is implemented in most modern bicycle factories, by robotic arms.
We had a thread were various posters offered some 20 things about the Brompton that could be improved, all while remaining true to the core and fundamentals of the bike - and, thoughtfully implemented, completely retrofittable. I only mentioned these here because Mr. Butler-Adams brought them up.
Last edited by tcs; 04-20-17 at 07:18 AM.