A welding flux for use in welding stainless steel parts to increase welding penetration, consisting essentially a base material obtained from manganese peroxide (MnO.sub.2), and an activator selected from a material group that includes zinc oxide (ZnO), silicon dioxide (SiO.sub.2), chromium oxide (CrO.sub.2), titanium dioxide (TiO.sub.2), molybdenum dioxide (MoO.sub.2), and iron oxide (Fe.sub.2O.sub.2).