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Life assessment predictions of reformer tubes of HP40Nb grade steel used in the petrochemical industry were investigated by quantitative metallographic evaluations of the area fractions of precipitated carbides. Correlations were established between the field-service periods and area fractions of the carbides via microstructural analysis. The microstructures of the virgin and other tubes were examined using optical microscopy (OM) for service periods of 1.8, 6.0, 7.2, 8.5, 9.7, and 16.2 years at 950 °C. The area fractions of the precipitates were measured by image analysis using ImageJ software. The results showed that increase in the area fractions of the precipitates was almost proportional to increase their service times. The relationships between area fractions of the precipitates and service times were also discussed for a 6-year cracked tube and a tube that had been in service for 11.5 years at another furnace. The obtained relationship can thus be used via the surface replication method for on-site residual life assessments. [ABSTRACT FROM AUTHOR]

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]