Originally Posted by guy153

You don't need to preheat cromoly when it's less than about 3mm thick, which is basically everything on a bike. I don't think anyone preheats when welding bike frames.

The cromoly itself is sometimes heat treated sometimes not but either way you just go ahead and weld it.

You can only weld it with TIG (and a fair bit of practice) because you need a high level of control because the metal is so thin.

Some low end bikes made of thicker carbon steel are actually MIG welded and this may be the lumpy welds the OP was talking about. If you have a cheap frame like that it's a good starting point for a tall bike or a cargo bike project etc and will easier to weld to.