Atuuel,
EPRI is the leader in this testing and creep knowledge for heaters or boilers using creep materials such as P91, P92.  You can go to their web site and look at information they have. Since I don't know what material you are speaking about it is difficult to discuss.

Hi,

Jim pointed out EPRI as a great source of information and if my memory serves me it was also an EPRI technical report giving information about DMW's. What is your filler metal? If 3XX is used then carbon migration from ferritc side to SS weld metal due to big difference of Cr content is of more concerns than creep. A solution/practice is to use Ni-based filler metals. Both have thermal stress issue between austenitc weld metal and ferrtic base metal. To address this a special joint design may be required. I've seen some European manufacturer using short spool peices of DWM's (they called them black-and-white joints) with both internal and external surface machined to minimize stress risers.

Back to creep, boiler DMWs normally designed for joining Stainless tubes with ferritic tubes at high P & T so you will have high chance seeing Gr.91-3XX with Ni-based filler metal. Creep for 3XX and Ni-based filler metal is not a concern and Gr.91 is chosen for it good creep strength.

Anyway creep may not be the only concern when looking into the longterm service life of a boiler DWM~