See the latest paper published by the ASME section II committee on materials " Issues of concern to ASME BPV committee TG on creep-strength enhanced ferritic steels , and remedies under consideration" by J Henry, M Gold(chairman), and J Tanzosh.
Basically , the ASME code had assumed that the users, fabricators, and engineers would retain in-house metallurgists that would assume technical responsibility for understanding how to fabricate these alloys. The corpoate downsizing of the 80's and 90's led to a dumbing down of the technical expertise, such that correct fabrication of P91 /T23/P911/P92/P122 is the exception, and not the rule.
New rules being proposed are:
a) inital qualifying heats submitted for code acceptance must inlcude samples at both ends of the alloying range ( lean and rich) for testing
b)redefining the normalizing and tempering temperatures ,, and define what operations will requrie a N+T ( ie ,hot bending)
c)define the PWHT temp based on Ni + Mn content
d) define max coldwork permitted without heat treatment
e)define a broad harness range that, if it is nto met, would then require further testing to ensure correct crystal sructure.
f)worry about SCC, and keep part dry between welding and PWHT
g)define a tempering parameter, and a range of acceptable values to qualify a part for hi-temp service. Implicit is the need to monitor and archive ALL time vs temp histories of the P91 parts during fabrication. This montoring by itself willbe a big improvement in QC- we normally see these cucrves provided if teh part is forged overseas, but it does not seem to be done by domestic foundries.
Basically, they will finally read the Mannesman + Vallourec P91 book and follow those recommendations ( which were available circa 1990).