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In the present work, several specimens were obtained from microalloyed HP40Nb reformer tubes with the different serviced period. The creep tests were conducted at a temperature of 950 °C and under stress levels in the range 25–55 MPa. Results indicated that the degradation of the creep life was due to the increase of the serviced period. The microstructural degradation and creep voids were observed at the grain boundary regions, attributed to the main reason leading to the premature creep failure. The creep exponent started decreasing after the servicing and approached a constant value after 7.2 years. The creep coefficient was obtained to be a function of time when the creep exponent was constant. Norton's creep power law equation was used to derive an equation describing the relationship between the diametrical expansion and serviced period of the reformer tubes. Larson-Miller curves were obtained from creep test data. Amount of carbides from the different serviced period was also calculated. [ABSTRACT FROM AUTHOR]