Gábor Vértesy

Nondestructive magnetic measurement methods can be successfully applied to determine the embrittlement of nuclear pressure vessel steel caused by neutron irradiation. It was found in previous works that reasonable correlation could be obtained between the nondestructively measured magnetic parameters and destructively determined ductile-to-brittle transition temperature. However, a large scatter of the measurement points was detected even in the cases of the non-irradiated reference samples. The reason for their scattering was attributed to the local inhomogeneity of material. This conclusion is verified in the present work by applying three different magnetic methods on two sets of Charpy samples made of two different reactor steel materials. It was found that by an optimal magnetic pre-selection of samples, a good, linear correlation can be found between magnetic parameters as well as the ductile-to-brittle transition temperature with low scattering of points. This result shows that neutron irradiation embrittlement depends very much on the local material properties.

Neutron irradiation-generated embrittlement of nuclear pressure vessel steel was inspected by a nondestructive magnetic method, called magnetic adaptive testing (MAT). This method is based on systematic measurement and evaluation of minor magnetic hysteresis loops. Result of MAT measurement was compared with the result of the traditional Charpy measurement. Good correlation was found between these parameters. One of the main findings of the present work is that the considerable part of scatter of points obtained by magnetic measurement can be attributed to local material inhomogeneity. Another important conclusion is that the embrittlement highly depends on the initial local material conditions, i.e., the initial microstructure, which are very different even within the same block of reactor steel material. By taking this into account, the magnetic descriptors obtain more precise determination of the local embrittlement than the traditionally used destructive mechanical parameters from Charpy data.

The influence of surface roughness on magnetic measurements of Reactor Pressure Vessel Steels was investigated by applying two types of magnetic, non-destructive measurement on nuclear reactor pressure vessel steel samples: magnetic adaptive testing (MAT) and magnetic Barkhausen noise measurement (MBN). The surface roughness was modified by primary and secondary machine cutting forces. Different settings of machine cutting produced different surface conditions. It was found that for both measurements a monotonic correlation was found to exist between magnetic parameters and surface roughness. Results of the MAT measurements found that the correlation depends on the speed (i.e., on the applied slew rate) of the magnetizing current. In a similar fashion, results from the MBN method show good agreement with MAT, where the response diminishes with an increase in surface roughness. The results show the importance of accounting for surface condition in the interpretation of results of non-destructive magnetic testing.