Jim,
ASTM A335 Gr P5 is 5% chrome, 0.5 % moly steel. When welded to another A 335 Gr P5 it needs PWHT. Back in my days of erector engineer I took part in the erection of an oil refinery (B31.3) where there were dozens of tons of piping made of that kind of steel.
In ew's case it'll be welded to 316 stainless steel, for which PWHT is not applicable. I assume (just assume, not 100% sure) that it won't need PWHT due to the presence of 316 ss.
Giovanni S. Crisi
Sao Paulo - Brazil