It is a violation of DOT regulations to weld on heat treated frames e.g. semi's...

Most consumer vehicle frames are of mild steel construction..although there are places where HSLA steel is used..

The dead give away on the frame material is whether it is welded construction or riveted construction (on the heat treated frames)...

This document published by the Autrailian DOT is the most comprehensive and informative that I've found... It's for heat treated frames, but very good advice for other frames.

http://www.dotars.gov.au/transport/safety/road/bulletin/pdf/vsb_06_h.pdf