Please review ASME Section II, Part D, Mandatory Appendix 1. The information is there to determine allowable stress value for the time dependent regime below.

(b) At temperatures in the range where creep and
stress rupture strength govern the selection of stresses,
the maximum allowable stress value for all materials is
established by the Committee not to exceed the lowest
of the following:
(1) 100% of the average stress to produce a creep
rate of 0.01%/1,000 hr
(2) 100Favg% of the average stress to cause rupture
at the end of 100,000 hr
(3) 80% of the minimum stress to cause rupture at
the end of 100,000 hr

and there is also a weld creep strength reduction factor applied if there is a longitudinal seam weld or a weld that is not totally a true circumferential butt weld ( such as a saddle weld). A conservative practice may be to also consider the weld creep strength reduction factor on all welds that also restrain other continuous secondary loads, such as gravity loads.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

Only one point to add, weld strength reduction factor is not applicable for carbon steels as per PW-26 of Section I

Javed Nasir
Inspection Engineer
Technip-France Abu Dhabi