The Babcock & Wilcox Company used Mn-Mo-Ni automatic submerged-arc welds with the Linde 80 flux type for the fabrication of 14 currently operating PWR reactor vessels. Due to the fact that the weld wire was copper-coated (high copper content) and that they have low upper-shelf toughness properties, the welds are limiting with respect to pressurized thermal shock analysis. The welds (reactor vessel, surveillance, and nozzle dropouts) were exposed to stress relief times (postweld heat treatment) ranging from 10–53 h between 593°C (1100°F) and 621°C (1150°F). In analyzing recently acquired master curve reference temperature data (ASTM E1921), it was discovered that there is a stress relief time effect on the transition temperature. The stress relief time effect can be seen in the drop weight TNDT data, but its effect is less clear in the 41 J (30ft-lb) Charpy impact data. In addition, the stress relief time has an effect on yield stress. The stress relief time may have an effect on the irradiation embrittlement of the welds, but the trend is not conclusive. Since there is an effect of stress relief time on the initial transition temperature, it is felt that it is important to test surveillance welds with similar stress relief times as the vessel welds that are being evaluated. Surveillance welds with longer stress relief times can be used to conservatively represent the vessel welds.