Seems that there are two possibilities:
1. The frame is actually case hardened steel. If this is true, then in my opinion weiser's suggestion will not work, because the carbon content of the surface layer of case hardened steel could be as high as 1 - 1,2 %, and even if the steel has softened down, weld will be impossible.
2. Dave is right and the frame is just medium carbon steel. In this case, I agree with his suggestion.
Giovanni S. Crisi