Above 500 deg C CaesarII will apply a reduction factor on the allowable sustained stresses when using a SIF-factor.
In the the table underneath for temperatures from 500 deg.C upto 710 deg.C the allowable sustained stresses have been visualized with and without a SIF-factor. At 710 deg C the reduction factor reduces the allowable stress with factor 0.67 when applying a SIF.

Where does this reduction factor come from? I’m using ASME B31.3 and cannot find anything what’s relating to this issue in the design code.

Allowable sustained stresses S in CAESAR II acc. ASME B31.3 for A-312 TP304.

Temperature Allowable sustained Reduction factor
stress S (S+SIF)/(S-SIF)
With SIF Without SIF
S+SIF S -SIF
[oC] [N/mm2] [N/mm2] [--]
500 99.78 99.78 1.00
520 96.21 97.80 0.98
540 89.66 94.30 0.95
560 79.28 86.34 0.92
580 66.58 75.18 0.89
600 54.23 63.59 0.85
620 44.01 53.67 0.82
640 35.54 45.14 0.79
660 28.53 37.80 0.75
680 22.81 31.59 0.72
700 18.35 26.63 0.69
710 16.43 24.42 0.67

Any help is welcome.

You're talking about the "weld joint efficiency factor (w)" right? This was introduced in the 2006 Edition and made optional in the 2008 Edition of B31.3.