Unexpected low temperature crack propagation in nuclear post-shutdown water chemistry of Alloy 52 with potential effects of hydrogen

Constant-displacement bolt-loaded compact tension specimens of Nickel-based Alloy 52 were exposed to boiling water reactor environment for 12 years, followed by an additional 3 years in post-shutdown cold water conditions in a Swedish nuclear power plant test loop, under a stress intensity factor of 20 MPa√m. After outer surface decontamination and specimen opening, unexpected crack extensions of 3–4.5 mm were observed. The fracture surface and the cross-sectional deformation microstructure were examined by electron microscopies techniques down to the nanoscale. The oxide layer in the region exhibiting unexpected crack growth was notably thin, suggesting that it formed after exposure to elevated operating temperatures. The dominant fracture mode is transgranular, propagating along close-packed {111} planes. The grains contained heterogeneous microstructures with regions enriched in nanometer-sized Ti(N,C) and the zigzag crack paths did not traverse these regions strengthened areas. Extensive shear bands were present near the crack tips, indicating pronounced localized plasticity. Hydrogen reduces stacking fault energy, results in localized plasticity and enhances shear bands formation. Low temperature crack propagation with evident effects of hydrogen was considered as the potential cause of crack propagation in Alloy 52 in the absence of external dynamic loading under post-shutdown cold water chemistry.