Thermal aging of several duplex stainless steels was detected by an ultrahigh sensitive magnetic sensor using superconducting quantum interference device (SQU). It was found that the SQU output signal pattern in the presence of AC magnetic field applied to the specimen was sensitive to very small amout of material property changes due to thermal aging at the operating temperature of light water reactors (LWRs). The change in the SQU output strongly depended on aging temperatures and ferrite contents of unaged duplex stainless steel specimens.

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