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Micromagnetic NDE techniques for the characterization of precipitation-induced embrittlement of 15 NiCuMoNb 5 (WB 36) steel

: Altpeter, I.; Dobmann, G.; Parlog, M.; Szielasko, K.

Green, R.E.; Djordjevic, B.B.; Hentschel, M.P ; Bundesanstalt für Materialforschung und -prüfung -BAM-, Berlin; Deutsche Gesellschaft für Zerstörungsfreie Prüfung e.V. -DGZfP-, Berlin; Center for Nondestructive Evaluation -CNDE-:
Nondestructive Characterization of Materials XI : Proceedings of the 11th International Symposium on Nondestructive Characterization of Materials
Berlin: Springer, 2003
ISBN: 3-540-40154-7
International Symposium on Nondestructive Characterization of Materials <11, 2002, Berlin>
Conference Paper
Fraunhofer IZFP ()
micromagnetic technique; heat resistant steel; nondestructive evaluation; embrittlement

The low-alloy, heat-resistant steel 15 NiCuMoNb 5 (WB 36, material number 1.6368) is used as piping and vessel material in boiling water reactor (BWR) and pressurized water reactor (PWR) nuclear power plants in Germany. After long-term service exposure at temperatures above 320 °C, damage was observed during operation (and in one case during in-service hydro-testing). Preliminary investigations concluded that the service-induced hardening and decrease in toughness in WB 36 materials was caused by the precipitation of copper.
For the non-destructive characterization of the precipitation-induced embrittlement of WB 36, service exposure was simulated on a set of tensile test samples. The material was observed to exhibit a peak-hardness of about 240 HV10 after about 1000 hours of service-exposure at 400°C. This is an increase of 40 HV10 with reference to the initial hardness of 200 HV10. As conventional hardness measurements are not applicable in this case, early-detecting the hardness increase non-destructively could enable the provider to stop any further service exposure of the material in time.
Therefore, the suitability of micromagnetic NDE techniques for the non-destructive characterization of the Vickers hardness was investigated. A measurement system was successfully calibrated for the prediction of HV10 by Barkhausen noise and field upper harmonics analysis. The practical applicability of this approach was shown by proving its independence on side-effects like plastic deformation and superimposed tensile loads. In all cases, high correlation (0.90 < r² < 0.99) and low deviation (10 HV10 < error bandwidth < 5 HV10) between prediction and aim were achieved.
Dynamic magnetostriction measurements and eddy current impedance analyses were found to offer additional possibilities for the non-destructive detection of the hardness changes, as both of them reflect changes in the material's conductivity and permeability.