TIG welding stainless steel to carbon steel

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I am a public art sculptor. I am starting a large outdoor steel sculpture. (currently in the design and material sourcing phase) The sculpture will be 3 "box-constructed" triangular arcs that are arranged in a circle. The Arcs will be 14 ft tall and the circle they form at their base will be approximately 12ft in diameter. The arc-forms are formed by two separate plates rolled to the same arc, brought together at their top and spaced 18" at the base. both plates will be 1/4" thick. The interior rolled plate material will be 304 Stainless Steel and the exterior rolled plate will be (CORTEN style) weathering steel A588 steel. I will be TIG welding the two together at the top and edges. I know a structural engineer would never weld stainless steel to carbon steel, but this is not a bridge it is a sculpture. The pairing of stainless steel with rusting steel in the structure is very much a part of the aesthetic. I will be TIG welding all welds. My question is: what fill metal should I use to weld the two together?? I will attach an image of the sculpture sketch-up for context. Thank you for any input, expertise, or direction to find resources on filler and dissimilar steel welds!
Reply:I think 309L would be an appropriate choice for filler rod for this application.
Reply:Contact Dale @ www.dalerogersstudio.com/About.html.  309 seems logical.Last edited by ManoKai; 06-12-2014 at 05:11 PM."Discovery is to see what everybody else has seen, and to think what nobody else has thought" - Albert Szent-Gyorgyi
Reply:yup 309L, and that's all she wrote. 1st on WeldingWeb to have a scrolling sig! HTP Invertig 400HTP Invertig 221HTP ProPulse 300HTP ProPulse 200 x2HTP ProPulse 220MTSHTP Inverarc 200TLP HTP Microcut 875SC
Reply:Carbon to stainless welds are common in the superheater sections of boilers.  Normally welded with 309.
Reply:Also, be aware that the rusting of the Cor-ten will leave rust streaks on the stainless. Most of the rusting takes place in the first few weeks of exposure, but it is an ongoing process.JohnA few weldersA lot of hammersA whole lot of C-clamps
Reply:Some other considerations if you're still sourcing material.  A lower carbon grade and/or heat of your weathering steel will reduce the likelihood of weld/weld HAZ corrosion issues.  309L is certainly a strong filler metal candidate.  But 321 or 347 might buy you some extra insurance against carbide precipitation.  A lot depends on how you opt to weld the joints together and how much dilution you create between the carbon steel and the stainless steel in the weld joint.  Those stabilized grades of stainless steel filler metal will be more expensive; and this might also be an important consideration.Also, you might want to consider some kind of clear finish overtop the entire structure once you've finished fabricating it and allowed the weathering steel's patina to form.  Wikipedia has a nice brief commentary on the issues related to working with weathering steel.http://en.wikipedia.org/wiki/Weathering_steel"Using weathering steel in construction presents several challenges. Ensuring that weld-points weather at the same rate as the other materials may require special welding techniques or material. Weathering steel is not rustproof in itself. If water is allowed to accumulate in pockets, those areas will experience higher corrosion rates, so provision for drainage must be made. Weathering steel is sensitive to humid subtropical climates. In such environments, it is possible that the protective patina may not stabilize but instead continue to corrode. For example, the former Omni Coliseum, built in 1972 in Atlanta, never stopped rusting, and eventually large holes appeared in the structure. This was a major factor in the decision to demolish it just 25 years after construction. The same thing can happen in environments laden with sea salt. Hawaii's Aloha Stadium, built in 1975, is one example of this. Weathering steel's normal surface weathering can also lead to rust stains on nearby surfaces.The U.S. Steel Tower in Pittsburgh, Pennsylvania was constructed by U.S. Steel in part to showcase COR-TEN steel. The initial weathering of the material resulted in a discoloration of the surrounding city sidewalks, as well as other nearby buildings. A cleanup effort was orchestrated by the corporation once weathering was complete to clean the markings. A few of the nearby sidewalks were left uncleaned, and remain a rust color. This problem has been reduced in newer formulations of weathering steel. Staining can be prevented if the structure can be designed so that water does not drain from the steel onto concrete where stains would be visible."Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:I was curious so I did some digging.  A Google search of clearcoat for CorTen steel yielded some excellent links to discussions of architectural uses for weathering steel and dealing with cosmetic issues.  Finishing.com is a great website dealing with coatings, surface prep, and corrosion issues.  They have a good discussion with some good photos.http://www.finishing.com/169/45.shtmlBenson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:You guys are fantastic. What an empowering resource this is. I really appreciate the information! And especially thank you @A_DAB_will_do for the info and resource on CORTEN. You directed me to some very helpful information. Thank YOU!