Super duplex is tricky to weld. You need an over alloyed consumable, which you should be able to get from any respectable consumable supplier, and possibly some guidance on technique.
You must keep it cool, no preheat and very low interpass temperature. Small-bore tubes require extreme care, weld opposite quadrants.
You should also consider additional PQR tests such as: -
G48 corrosion test, gives a very good qualitative indication of a successful welding technique. Recommended by BS4515-2, the only standard I have found that specifically covers duplex.
Ferrite counts can also be carried out, which are expensive if done properly using a microscope, also checks for the presence of deleterious phases such as Sigma and Chi.
Impact tests can also give a good indication of a successful welding technique.
I agree with John's comments.
One can also apply the requirements of the ASTM standard A923-01 titled "Standard Test Methods for Detecting Detrimental Intermetallic Phase in Wrought duplex Austenitic/Ferritic Stainless Steels".
If your weldment is not going to recieve a PWHT, an electrode is used that is similar to the base metal in composition, but slightly overmatches. If your super-duplex base metal contains copper, then you probably should use an electrode designed to weld that base material, that contains copper also.
