Really though it's probably mild steel which is not heat-treatable. Mild steel fairly weak (for steel) but it doesn't get much stronger or weaker from heat exposure and isn't brittle.
High carbon, stainless, and fancy stuff is where you can run into problems. In general you won't see this on car or bike frames, too expensive in bulk.
If it's a newer bike it's probably aluminum which you can usually still weld with impunity. If it was high grade alloy the heat could matter, but as far as I know nobody cares that much about bike frames. Heat treatment, or "seasoning" in the case of some aluminum is expensive.
Source: used to cut up old bikes and weld the frames into crazy chariot contraptions we would tow around