The influence of irradiation damage and recovery of properties by thermal annealing for low upper-shelf WF-70 weld metal has been evaluated. The test material was obtained from the beltline and nozzle course girth welds of the Midland Unit 1 reactor pressure vessel. The two welds, ostensibly made with the same heat of weld wire and the same lot of submerged-arc flux, were determined to be separable for evaluations by virtue of differing copper contents. Conventional transition temperature methods of drop-weight nil-ductility transition and Charpy V-notch (CVN) transition curves could not distinguish a difference in transition temperature. However, a recently developed master curve method of data evaluation revealed the difference by virtue of achieving better sensitivity using only fracture mechanics-based test data.
Irradiation embrittlement after a fluence of 1.0 × 1019 n/cm2 was evaluated by both the conventional American Society of Mechanical Engineers (ASME) Code method and by the more recent American Society for Testing and Materials (ASTM) master curve method. Again, the Code method rated the postirradiation transition temperatures of the two materials the same. The master curve method indicated that the postirradiated properties of the two materials differed significantly. The U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide 1.99, Rev. 2, reasonably predicted the experimentally obtained CVN transition temperature shift at the 41 J reference level for the beltline weld.
Postirradiation annealing at 454°C (850° F) for 168 h almost completely restored the as-received transition temperature properties. The recovery of the transition temperature compared to unirradiated values after 343°C (650°F) for 168-h recovery anneal was 52% by the 41 J CVN temperature, which conflicts with the theoretical recovery of 17% calculated by NRC Regulatory Guide 1.162. The ΔTo from the master curve method indicated 21% recovery of transition temperature.