Quantitative study on area fraction of precipitated carbides of HP40Nb steel as a function of service period

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.