GOKHAYAMAN, check out the link below, there is a chart for making dissimilar welds. All you need now is the proper WPS/PQR and a qualified welder and you should be good to go with the 309L.

Duplex stainless steels can be welded to other duplex stainless steels, to austenitic stainless steels, and to carbon and low alloy steels.
Duplex stainless steel filler metals with increased nickel content relative to the base metal are most frequently used to weld duplex stainless steels to other duplex grades.
When welding duplex stainless steels to austenitic grades, the austenitic filler metals with low carbon and a molybdenum content intermediate between the two steels are typically used. AWS E309LMo/ER309LMo is frequently used for these joints. The same filler metal or AWS E309L/ER309L is commonly used to join duplex stainless steels to carbon and low alloy steels. Because austenitic stainless steels have lower strength than duplex grades, welded joints made with austenitic filler metals may not be as strong as the duplex base metal.
When welding the highly alloyed austenitic stainless steels, nickel-base fillers are used. The nickel-base filler metals are not normally used for duplex stainless steels, but if they are, they should be free of niobium (columbium). Although not thoroughly documented, there have been suggestions that the ENiCrMo-3 filler (625) has been less than satisfactory, possibly because of interaction of the niobium from the filler with the nitrogen from the duplex base metal.

Table I summarises filler metals frequently used to weld duplex stainless steels to dissimilar metals. These examples show the AWS bare wire designation (ER), but depending on the process, joint geometry and other considerations, electrodes (AWS designation E), and flux-cored wire may be considered. Hope this helps, have a good day

http://www.stainless-steel-world.org/duplex/fabweld.asp

The filler metal to be used not only depends on your base metal types, but also on your application. For instance, if there is going to be any thermal cycling (such as power plant tubing), you may want to consider nickel based filler such as NiCrFe-2 instead of 309. The coefficient of thermal expansion for 309 is higher than either base metal, and could cause thermal fatigue. 309 will also form a thin layer of martensite between iteself and the carbon steel. This may or may not be detremental to you application.