As other folks have said, any steel works, but I like mild steel since it doesn't get hard in case I need to drill a hole later. With San Mai my usual process is to weld a 1" wide strip to billet as the edges are welded up. Then the weld is stress relieved during normal forge welding, normalization, and HT. While I've used stick in the past, now I use mig for welding the edges and tang. The tang gives a good place to hold billet with tongs during forge welding without affecting the San Mia billet itself.