Welding SS 316L to SS 316L using 316L filler rod, this is a tight fit, basicaly a round plug into a tube welded from the outside using TIG with 2 passes. The tube is back filled with argon then welded. after welding and a hydrostatic test the inside shows some staining/rust. All tooling is clean and used only on stainless. the back side with the rust has no penetration as the weld is on the other side of the plug (thick) does not rquire full penetration. Any thoughts? I have a PDF of the layout but no luck putting in this post.