An empirical data mining approach was employed to separate the effects of weld thermal cycles in the Heat Affected Zone (HAZ) and Post Weld Heat Treatment (PWHT) on Transition Temperature Shift (TTS) from other variables, using unirradiated and irradiated Charpy test data obtained from US RPV surveillance program results. HAZ data, which tend to have greater scatter due to microstructural variation in the HAZ, are separated into “high scatter” and “low scatter” categories to assess the effect of scatter on TTS trends. In general, the TTS behavior of the HAZ is comparable to the non-HAZ. However, the forging HAZ TTS diverges from the non-HAZ TTS as a function of copper content. This trend is also observed in the plate HAZ when looking at plate HAZ datasets with copper content and PWHT times less than forging copper content (≤0.16 wt. %) and PWHT time (≤20 h). Additionally, TTS of weld and HAZ materials increases with increased PWHT time. These trends are not well captured by modern embrittlement trend curves. For all but one heat evaluated herein, the irradiated transition temperatures of the HAZ are lower than those of the non-HAZ from the same heat in the same irradiated condition.