Good day to all
We are going to weld on calorized (Aluminum diffusion) tubes. The base material is 21/4Cr –1/2Mo steel. I have the original welding procedure when the tubes was installed 20 years ago, however I would like to know what the latest in welding these materials is. Our biggest concern is to preserve the aluminum diffusion layer next to the root, during the welding process. In the past the root was welded with an electrode 4390A (23Cr-Nb), this is apparently a Japanese specification, can anybody help me with some information? It might be a super ferritic stainless steel? Using austenitic stainless steel for the root might not be possible due to the possibility of polyphonic cracking.

Thank you
Joe