does anyone has experience welding thermocopuples with P91 pipes?

by cesar
(peru)

Hello All
I need to weld wires of thermocouples with P91 pipes, but i am not sure if i should make a PHWT.
I will spot weld.
A person told me about a ASME CODE CASE 2578, but i dont get this documment and i dont know if it needs or not.. does anyone know some info?
Greetings

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