F255 uses F255 and the langley alloy GTAW wire with low sulfur is excellent.
GTAW works fine F255 to F255...Measure ferrite, do root and let cool, do not force cool.....
If you screw up, fail RT etc, then try a repair one time after that quicker to cut the weld out and start over.
Mechanical prep...not sure if you are doing pipe or plate?
F255 to carbon hmmm F255 is pretty tough...No real answer.