Fatigue testing of metallurgically-bonded EBR-II superheater tubes

Fatigue crack growth tests were performed on 2¼Cr–1Mo steel specimens machined from ex-service experimental breeder reactor-II (EBR-II) superheater duplex tubes. The tubes had been metallurgically-bonded with a 100 μm thick Ni layer; the specimens incorporated this bond layer. Fatigue crack growth tests were performed at room temperature in air and at 400 °C in air and humid Ar; cracks were grown at varied levels of constant ΔK. In all conditions the presence of the Ni bond layer was found to result in a net retardation of growth as the crack passed through the layer. The mechanism of retardation was identified as a disruption of crack planarity and uniformity after passing through the porous bond layer. Full crack arrest was only observed in a single test performed at near-threshold ΔK level (12 MPa√m) at 400 °C. In this case the crack tip was blunted by oxidation of the base steel at the steel–nickel interface.