Detection of Copper Precipitates in 15 NiCuMoNb 5 (WB 36) Steel Using Micro-Magnetic NDE Techniques

摘要

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 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 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 favorable 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 [1, 2]. 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 outside-effects like plastic deformation and tensile loads. In all cases, high correlation (0.90 < r{sup}2 < 0.99) and low deviation (5 HV10 < error bandwidth < 10 HV10) between prediction and aim were achieved. Dynamic magnetostriction measurements (using electromagnetic acoustic transducers, EMAT) 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. Extensive investigations of the tensile load dependence of electromagnetic measurands were performed in order to determine and quantify precipitation-induced changes in the micro residual stress state of WB 36. An approach for the determination of micro residual stresses with an accuracy of ±1.5 MPa by evaluation of the tensile load dependence of Barkhausen noise confirmed an integral stress difference of 11 MPa between service-exposed (57000h/350°C) and recovery-annealed (+3h/550°C) state of the melt E59. For the melt E2, however, an integral stress difference of 4.5 MPa was found between service-simulated (up to 3000h/400°C) and "as-delivered" state.