Welding Carbon Steel To 316 Ss. It should be noted that dilution of these highly alloyed fillers by carbon steel may have a number of effects. For joining stainless to mild steel use a 309L filler.
Give the mild steel a good coating but you probably already know that. The same filler metal or AWS E309LER309L is commonly used to join duplex stainless steels to carbon and low alloy steels. Use a 75 argon25 CO2 gas.
It will tend to reduce the weld metal.
Look also at welding of stainless steel and other joining methods NIDI pub no 9002 there is a chap on dissimilar welding. If higher Cr content is desired then type 312 is used. Table 2 Stainless steels with high alloy content of 309L and 312 are suitable for welding stainless steel and carbon steel. Joining an austenitic stainless steel such as 304 to a carbon steel is very common and is relatively simple.