Hi Stringer
If you are referring to ASTM A516, then the material is a standard pressure vessel grade C/Steel made for temperatures down to around -29°C. There are four grades within this spec. Gr 55; 60; 65; & 70, where the grade refers to the UTS in ksi. If this is what you are going to weld, then you do not need backing gas. ER70S-2; 4; 6 filler will do the trick.
If there is another 516 carbon, maybe someone else can tell us.
Regards
Niekie