Effect of weld microstructure on brittle fracture initiation in a thermally aged boiling water reactor pressure vessel head weld metal
9 September 2020
Revised: 20 October 2020
Accepted: 18 November 2020
Available online: 26 November 2020
Abstract: The effect of the weld microstructure and inclusions on the brittle fracture initiation is investigated in a thermally aged ferritic high-nickel weld of a reactor pressure vessel head from a decommissioned nuclear power plant. As-welded regions consist mainly of acicular ferrite and reheated regions mainly of polygonal ferrite. The fractographic examination of Charpy V-notch impact toughness specimens reveal large inclusions (0.5–2.5 μm) at the brittle fracture primary initiation sites. Higher impact energies were measured for specimens where brittle fracture initiates from a smaller inclusion or an inclusion further from the V-notch. The density, geometry and chemical composition of the primary initiation inclusions were investigated. The brittle fracture crack initiated as a microcrack either within the multiphase oxide inclusions or from the debonded interfaces between the uncracked inclusions and weld metal matrix. The primary fracture site can be determined in all specimens tested in the lower part of the transition curve, at and below 41 J reference impact toughness energy, but not above, due to the change of the fracture mechanism, and hence the changes in the fracture appearance.