Trailing shield

Hot weld seams react with oxygen and nitrogen from the air. This causes them to oxidize and become brittle or lose their corrosion resistance. You push the trailing shield directly behind the torch over the fresh seam and supply shielding gas there. The gas displaces the air until the seam has cooled down. For reactive materials like titanium, this is mandatory; for stainless steel, it's necessary if the seam needs to stay clean. Make sure the shield is wide enough and let the gas run on a bit until the seam is no longer glowing.