Brazing

it depends on the amount of heat, if you get it hot enough you heat treat it which makes it stronger, but if you don't get it that hot (or get it that hot in one point, but cooler around it as a torch does) you are weakening more then you are strengthening. Also bike tubing is quite thin, and easy to burn a hole through if your not careful, or don't know what your doing.

c'mon chris, you should know better than making broad brush statements like this. Heating can do two things dependent on what follows: if you cool the metal fast it goes harder but more brittle, cool it slowly - in warm air, say - and you can make it softer and less brittle. Fast heating or cooling make the metal structure less predictable if I remember correctly.