You use a safe-end and have to avoid sensitizing the 304H by ensuring the P91-P91 PWHT zone is far from the 304H-incolnel HAZ.
According to Mannesman-Vallourec :
" the best way to realize a transition from T/P91 and an austenitic steel is by making a transition joint ( safe end) at the shop. First nickel-base weld metal is deposited on one side of a t/P91 piece followed by an NT treatment similar to T/P91 base metal. This transitio piece can tehn be welded on site to T/P91 and to the austenitic steel . The T.P91 side is a regular similar weld with matching consumables and local PWHT ( without heating and sensitizing the 304H), whereas the austenic side is welded to the nickle base weld deposit by using a nickel base consumable without PWHT. "
