Characterization of precipitation-induced embrittlement of 15 NiCuMoNb 5 steel using micromagnetic techniques

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). Small-angle neutron scattering (SANS) measurements (performed by MPA Stuttgart) concluded that the service-induced hardening and decrease in toughness of WB 36 materials was caused by the precipitation of copper particles ranging from 1 to 3 nm in size. For the non-destructive characterization of the precipitation-induced property changes in WB 36, service exposure was simulated on a set of tensile test samples. The material's hardness was observed to rise by as much as 40 HV 10 as a result of the simulated service exposure. However, as conventional Vickers hardness measurements are not applicable repetitively and area-wide in this case, and as spot tests require information about critical test areas, early-detecting the hardness increase non-destructively is a most favourable solution of this problem. This way, electromagnetic surveillance of power plant components can inform the provider about the aging processes. Therefore, the suitability of micromagnetic NDE techniques for the characterization of the Vickers hardness was investigated. A measurement system was successfully calibrated for the prediction of HV 10 by Barkhausen noise and field upper harmonics analysis.