Originally Posted by
Andrew R Stewart
I'll quibble with the heat treated comment. To my knowledge all bike tubing (and all steel production) uses heating steps at different points along the process. The bike industry use of the term "heat treated" is an attempt to use engineering terms for marketing purposes. All "heat treated" tubing is is just the continuing use of thermal treatment to further affect the strength of the tube beyond the other offerings that company also makes. Andy (who does know what you meant, but feeling a bit randy today though)
So I think the way it works is this. The tube starts life as rolled and welded (but thick wall) or as a solid round for the higher-end tubes. It then gets smooshed over mandrels lots of times. In between it gets heated up and allowed to cool a few times. All this happens in a pretty big factory, which may even be the same place where they're making the actual steel, because the tubes are still very long at this point, so you need huge machines and big ovens.
Then it goes to the Reynolds factory (or equivalent) as seamless or DOM tube where they have a butting machine (about the size of a lathe), a hacksaw and an oven. They cut the tube and draw it a bit thinner (going from about 1mm to 0.8/0.5 or so) and butt it. This all happens cold. Then if it's a "heat-treated" tube (like 725 or 853, but Tange and others do this kind of thing as well) they cook it an oven in a special way that makes it stronger but less ductile. The hardness goes up from about 200 VPN to 400.
Tubes that are heat-treated like that are harder to dimple and bend and jack around with and this is a reason to prefer the non-heat-treated versions of the same tubes for the rear triangle.