Brophy's "Plans and Specifications" book on p.21, on a drawing titled Straddle Mill Lug. gives the barrel material as approximately .40 carbon, and on page 204 in a table of materials for the various parts, lists the barrel as being made of SAE 1020 steel. In either case it is a low carbon steel and shouldn't be subject to hardening from heating and cooling. This is also verified by the fact that the lug and the rib extension are brazed to the barrels with no ill results. I believe that a skilled welder could make this repair by TIG welding and that it would be as strong as the original barrel.