STUDY ON FRACTURE BEHAVIOR AND ASSESSMENT FOR DISSIMILAR METAL WELD OF LOW ALLOY STEEL AND NI-BASE ALLOY WELD USING A BWR REACTOR PRESSURE VESSEL MATERIAL

Cases of stress corrosion cracking (SCC) in Ni-base alloy weld metals welded to low alloy steel (LAS) of reactor pressure vessel (RPV) components have led to discussions on the possibility of SCC propagating into the RPV. However, the fracture behavior and the assessment methodology have not been sufficiently studied for such dissimilar metal weld (DMW) components where embrittlement due to neutron irradiation damage is negligible and the operating temperature is above the upper shelf temperature range. In this study, the fracture mode of the DMW of LAS and Ni-base alloy weld was investigated by using a large, heavy forged steel part, called the bottom head ring, of a RPV, manufactured for a recent BWR. In addition to fracture toughness tests with compact tension specimens, fracture tests using plates cut from a DMW material with a semi-elliptical surface crack were conducted. The initial crack tip locations were designed and controlled to be in the heat affected zone (HAZ) in LAS, at varying distances from the weld fusion line. All the mechanical tests conducted showed highly ductile fracture behaviors and properties; the evaluated elastic-plastic fracture toughness was more than 400 kJ/m(2) in the fracture toughness tests. The fracture loads evaluated using the elastic-plastic fracture mechanics (EPFM) methodology showed fairly good agreement with the maximum loads in the fracture test. The results confirmed that the fracture assessment methodology based on EPFM is reasonably applicable to the DMW of RPV components where the operating temperature is above the upper shelf temperature range and the embrittlement is not a concerned mechanism of degradation.