P9 to Stainless steel fillet weld

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Hi everyone,We are installing SS 310 skin thermocouple with heat shield in one of our fired heater. The tube material is A335 P9. Our integrity department insist we perform buttering and PWHT for the installation. My questions:1. Anybody have experience in such installation of skin TI where you have buttering and PWHT due to the dissimilar material weld? Does the TI's perform properly?2. Is it a practice to have buttering for fillet weld and pwht for the installation? Buttering is due to dissimilar metal welding and PWHT is the requirement due to P9 material.3. Our existing skin TI does not have buttering layer, and in our record, there is no PWHT done for the existing installation. Is it a practice? Any code or standards does it refers to? Thanks guys.