Welding P91 to Stainless Steel 2

[Nogard]

I am looking for guidance in understanding the potential drawbacks in performing a dissimilar weld between P91 and stainless steel pipe. This would be a socket weld joint.

A contractor is proposing to make these welds and then PWHT the P91 and stainless at the same time. I have read that the P91 side should be buttered then put through the PWHT cycle, then welded to the SS without PWHT. This doesn’t seem practical for a SW joint.
My concerns are:
1. Sensitization of the SS due to PWHT
2. Expansion issues. Would this type of transition (from P91 directly to SS) concern you?

Any advice? Thanks!

 

[stanweld]

Your best transition would be P91 socket fitting (preferabl;y buttered with Nickel based filler, PWHT'd and machined) to alloy 600 pipe to austenitic stainless steel socket coupling. Coefficient of expansion mismatch is the problem especially when thermal cycling will occur.

Whereas the pipe will no doubt operate above 1000F, sensitization of the SS is inevitable.

 

[Welder4956]

..or you could insert heavier wall P22 pipe in the F91 fitting, stress relieve the F91 to P22 joint, then weld P22 to stainless with a F22 coupling using Inconel electrode. Make sure you can size the P22 pipe and F22 fitting for the pressure/temperature involved.