عنوان مقاله [English]
The eddy current nondestructive evaluation was investigated to estimate the service life of cast austenitic stainless steel tubes in the reformer unit of the direct reduction iron ore plan. This evaluation was performed at 60 kHz based on the impedance plane and as a point inspection method. Reference tubes with specified microstructure and service life were used for calibration. The optical and SEM microscopes were used for microstructural studies. The microscopic examination showed that the as-cast microstructure has austenitic matrix with eutectic cells containing chromium and niobium carbides. It was also observed that with increasing operating time and increasing the intensity of aging, the microstructural changes occur more severely. These changes include oxidation and carburization of surface, creating of destructive phases, and depletion of chromium in the austenite matrix, variation of eutectic cells morphology and formation of the creep microcracks. The results of eddy current evaluation showed that microstructural changes due to increasing the service life have a significant effect on the electromagnetic response of the tube. As the life of the tubes increases, the self-inductance resistance index increases sharply and the resistivity index decreases. The interaction of these two index is indicated as a point on the impedance plane of for each tube. Comparison of the position of these points in the impedance plane indicates the existence of linear changes between electromagnetic and aging behaviors. Therefore, the service life of the reformer tubes can be estimated from the electromagnetic responses in the eddy current method.