Shooting from the hip. Some of my concerns are:
- With the tensile strength of the 347 weld material especially at elevated temperature. (the tensile properties of the 347 drops off faster than the 2CrMo at elevated temperatures)
- What are the trace materials that will be in the piping?
- Is there a potential for SCC of the weld, any Halides?

The next issue is Quality Control - How was the wrong filler material issued and used on the project? Beofre I would accept it you would have to provide a complete PQR for this combination.

Dissimilar metal weld failure caused by carbon migration along the weld fusion zone in the 2.25 Cr base metal. This will have a significantly shortened creep life and should be corrected otherwise, you will have an axial-oriented tube rupture.

Not to mention the thermal expansion stress caused by the difference in thermal expansion coefficients for 347 vs. 2CrMo.  Most likely will fail by fatigue first with 2CrMo material on both sides of the weld.