In this paper, a novel method for non-destructive evaluation of the type and severity of damage in HP40Nb steam reformer tubes is presented based on the eddy current method. Beyond simple defect detection, this method provides numerical representation of various types of microstructural damage. Field and laboratory inspections were performed on tubes from three reformer units of an Iranian petrochemical industrial unit with nominal operating lives of 5, 13, and 16 years. Eddy current inspection was performed by the point scanning method with an electromagnetic sensor with a T-shaped core at a frequency of 60 kHz using a handheld portable device. Microstructural studies of samples of these tubes were performed with optical and scanning electron microscopes equipped with EDS analysis. The results of the microstructural laboratory studies showed that the main damages in the damaged pipes include changes in the amount and type of carbides, reduction in the chromium content of the matrix, carburization, oxidation, and microcracking. Eddy current evaluations showed that the microstructural and the eddy current variations can be characterized by appropriate parameters. Intrinsic damage (DI) due to carburization and oxidation can be evaluated using the phase angle index and creep damage (DII), which is manifested by the formation of holes and cracks and can be evaluated using the impedance index. It has been observed that by increasing the amount of carburization and the conversion of M23C6 carbides to M7C3 and reducing the chromium content of the matrix, the phase angle index increases from 18.7 to a maximum of 63.2 degrees. Also, as the pipes' life increases and the creep cracks' nucleation and growth occur, the impedance index increases from a dimensionless value of 655 to 1064. Finally, by combining the two electromagnetic indices and presenting an empirical relationship, the total failure rate index (Dtotal) of each pipe is presented.