it's simply because just one pass is not enough, if the weld bead is wide and penetration is deep.
multipass, multiwire, that's common in submerged arc welding process.
of course, to finish a SAW process, you need welding fluxes and wires.
http://www.ylflux.com/

mild steel, low alloy steel, stainless steel welding need different types of wires and fluxes. welding fluxes for stainless steel contains alloying elements Cr, which gives hardness and resistance to rust. Check American Welding Society guidance, you will get instruction. http://www.aws.org/ http://...