Gradient study of a large weld joining two forged A 508 shells of the Midland reactor vessel

Results of the study the welds of the Forged A 508 shells are presented in this document. The low-carbon welds (WF67 and WF70) in the slab examined contained no abnormalities that would indicate fracture behavior different from that observed in bulk-material fracture tests. The A 508 material in the HAZ region, very close to the welds, contains small ({approximately}3 mm) regions adjacent to each layer of weld runs where grain coarsening and hardness elevation suggest reduction of cleavage initiation toughness. The degree of severity is largest where this local region coincides with a local elevation of carbide density in the A 508 material. The A 508 HAZ region adjacent to the topmost weld run may be the region most likely to assist cleavage-fracture initiation because of its location: close to a free surface, small cracks, and the HAZ region beneath the cladding. It was noted that the small cracks under the cladding have the appearance of prior austenite grain boundary separations that connect to austenite grain boundaries in the cladding. The extreme hardness of a narrow layer of cladding a the fusion boundary may be of interest in further studies of cladding toughness.