This get tricky.
Shooting from the hip... weld the combination of B-2/C-2000 with ERNiCrMo-17 , you are matching the C-2000 material and overalloying the B-2. But it is not that simple, One really need to know the environment, trace chemical etc.
Several problems to think about:
1. Chromium Carbide formation in the C-2000 with the diluted carbon from the B-2 alloy.
2. Weld HAZ Stress Corrosion in the B-2 material
3. One also needs rapid heating to avoid some detrimental tranformation reactions of the B-2/C-2000 diluted material
http://www.haynesintl.com/weldinginformation.htmThe second question is, if we weld dissimilar material, do we have to come out with new WPS and PQR? Yes, and do some immersion and Stress Corrosion testing as well. Note that the SCC testing should be fairly quick. A failure by SCC can be catastrophic.