Thermal aging has been considered a secondary aging and degradation mechanism for the low-alloy steels used in nuclear power plants. Given the relative interest in lifetime extensions, this has to be revised considering the extent of time at temperature during a 60-year operation time or even beyond. Traditionally, the effect has been coupled to the nonhardening effects caused by phosphorus and sulphur diffusion to available phase or grain boundaries, or both, resulting in a marked embrittlement of these low-alloy steels. This phosphorus segregation is proposed to be the main embrittlement mechanism for the weld metals investigated in this study. In the high-manganese/nickel/silicon-bearing weld metal used in both pressurizers (PRZs) and reactor pressure vessels (RPVs), some of the material extractions from the Ringhals power plant show a larger than expected effect from thermal aging, possibly by the formation of agglomerates in the matrix due to higher nickel content. Further, some of the tested materials showed a hardening effect after in-service exposure at temperature. The results presented here show that even though there are elements to be included in the fitness-for-service analysis of the power plants, with traditional prerequisites for the RPV it will be limited regarding the operating window. An empirically based embrittlement trend curve, related to thermal aging, for the PRZ will be developed after further testing of these materials.