This paper presents results from a multi-frequency electromagnetic (EM) sensor used to characterise the microstructural changes in P9 and T22 power generation
steels after tempering and elevated temperature service exposure. The EM sensor can detect the microstructural changes in both steels due to the corresponding relative permeability values increasing from 33 to 67 and then 133 for P9 during the tempering and the long service exposure respectively while for T22 the permeability changes from 61 to 75 and then 86, as well as the relatively small per-centages of resistivity change. The changes in the rel-ative permeability values have been qualitatively corre-lated to the changes in microstructural features, which affect the domain wall movements during the multi- freq-uency EM sensor tests. The real inductance value at low frequencies Lo is particularly affected by differences in the relative permeability of the steels studied, which is found to increase exponentially with the Lo values in the range studied
