Wear plates, if located on the outside of the vessel are not part of the vessel code boundary (pressure retaining).

So wear plates DO NOT need to be NACE MR0175 certified and Pads do not need to be normalized....correct?

Go back and carefully review the engineering specification for construction of the vessel. If the engineering specification for the construction of the vessel did not stipulate heat treatment condition for material outside of the pressure vessel code pressure boundary (like supports, saddle plates, etc) or dealt with support steel, you are correct.

David, I would be perhaps overly conservative, but I know that welding on the shell could generate structural material modifications, out of the normal control guidelines of NACE MR0175. Typically, no one would test the wear plate weld at OD for hardness, yet failures have occurred. Same thing with the welding of the reo pads at the OD. I have seen production separator failure on Duplex material at the reo pad OD, causing the shut down of a FPSO for investigation and repair. Many millions of dollars lost in production. Speaking for myself, I have always selected and recommended the wear plate to be the same material as the shell and have the same certification, including compliance to the NACE recommendations. The hydrogen can propagate through the shell thickness and find the weakness at the weld site. Oh yes, the code is not much help in this case.
Cheers,
gr2vessels

For gr2vessels, duplex is especially susceptible to hydrogen induced cracking when under high stress as the austenite acts as a H2 sink. This provides H2 for the ferrite, where the cracks start. Also, duplex will hold the hydrogen in it's structure for years at room temperature once removed from the source of H2. We tested some duplex material that cracked due to hydrogen after months of ambient temperature exposure in the shop, ended up with > 1000 ppm (ferrite needs much less than this to crack).

As for material at the OD of a vessel, I agree with gr2vessels if it is welded to the OD. Make sure the weld and HAZ complies with MR-0175 as this IS part of the pressure boundary (even at the OD).

Thanks OGMetEngr, valuable insight in the welding to the pressure envelope. As just a mechanical engineer I would like to emphasize the importance of the otherwise overlooked weld at the OD of reo pads and including the wear plates.
Cheers,
gr2vessels

The parts of the vessel that are not in contact with the process fluid are not under the scope of NACE MR0175 as well as the non pressure parts could be constructed by non normalized SA-516 Gr 70.
For NACE welding you could see an interesting post thread at this link:
http://www.weldinguide.com/gdanastasiadis/viewtopi...

Please follow NACE and customer's specifications, sometimes the customer will require the vessel non pressure parts be fabricated with the same materials
and it depends of the ambient where the vessel will be installed offshore/sour service may need consideration.