Degradation detection of materials used in nuclear power plant components applying probe-type eddy current sensor. Pt. 1. Detection of thermal embrittlement in duplex stainless steel

Material degradation caused by aging of nuclear power plant components includes fatigue, neutron irradiation embrittlement and thermal embrittlement of duplex stainless steel. Detecting of these degradation in an earlier stage for prevention of possible accidents may contribute to improve availability and reliability of nuclear power plants. Eddy current testing method has been widely used in nondestructive testing field because the apparatus and sensor are light in weight and compact in size. It also allows testing without contact and provides excellent recordability because eddy current is generated by electromagnetic induction. In this research, material degradation by thermal embrittlement of duplex stainless steel was measured with eddy current changing frequency in steps of 1 kHz, output signal from coil was processed by phase detection technique and displayed as a trajectory on complex plane. By comparing the trajectory of a non-degraded material with that of a degraded material, degradation was evaluated quantitatively. Drawing such trajectory enabled us to capture the eddy current behavior totally and thus to select a proper method of evaluation. (author)