E309 or 309L would by my selection. The chemistry is as follows: Carbon - 0.23; Manganese - 0.5 - 1.5; Silicon - 0.4; Vanadium - 0.11; Columbium - 0.05; Sulfur - 0.035; Copper - 0.06; Nickel - 0.45; Chromium - 0.35; Molybdenum - 0.15. (all values are maximum, unless a range is shown).
The Cr-Moly may change things, but it seems to me 309 is the way to go.
I would be interested in knowing if there are any dissenters out there. My opinion is based on the assumption that this material is similar enough to carbon steel to be welded as we would weld (CS). Charles Hall
Mr. Hall is correct. The appropriate way to weld stainless to carbon is to butter each side with 309L and then weld them together with 309L. However, not too many people ever do this, they just weld them together. Particularly when they are thin like your material. Try not to get too much dilution with the base metal.
I agree with above postings. But please don't think this is a general rule !!! When you're welding carbon steel to stainless steel for use at elevated temperature (eg. pressure vessel, exhaust of a vehicle, furnace, ...) one must NOT use a 309 type ! An Inconel 600 filler is recommended. Nickel is a carbon barrier to prevent the embrittlement of the weld at higher temperature !!!
We ran into the same situation here at work. We had a lintel that was 304 stainless plate welded to the A992 beam . I was only guessing as what to weld this with.
Thanks for the entry, and the replies, this will help me as well.
We ran into the same situation here at work. We had a lintel that was 304 stainless plate welded to the A992 beam . I was only guessing as what to weld this with. 
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