We report on apparatus for the magnetic material characterization of a wide range of steel grades of high and low magnetic permeability. The instrument has been used to detect thermal aging in Duplex steel by the measurement of the residual magnetization (Br) of test specimens. A method to estimate the value of Br by remote measurements up to 200 mm from a specimen is described. The results are compared with data from conventional hysteresis loop measurements.

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The magnetic properties of A533B low alloy steel irradiated up to 3 x 1023 n/m2 in a nuclear reactor were measured using a SQUID sensor. The SQUID sensor used in the test is the type with a first-order axial gradiometer. Test specimens were irradiated to 0.5 x 1023 and 3.0 x 1023 n/m2, and then magnetized in a 0.2 T field until saturation before measurement. The specimens were put on a scanning table in a hot laboratory and the SQUID output was recorded while each specimen was gradually moved in a horizontal direction under the SQUID sensor. A superconducting magnet was used to put the test specimen under a magnetic field that changed from +0.01 T to -0.01 T to observe the relation between magnetic field intensity and coercive force (B-H curve). The B-H curve shows a definite tendency towards a higher coercive force for test specimens with higher neutron fluence. The test result also shows a good relation between mechanical properties, such as yield strength, tensile strength, and the Charpy impact test result, with SQUID output. This proves that there is a very good chance of developing a non-destructive evaluation system for nuclear reactor structure. This kind of evaluation system would be a great help in making plant life management even more reliable and reasonable.

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