Damage characterization in two reformer heater tubes after nearly 10 years of service at different operative and maintenance conditions

Reformer furnaces are used in petrochemical industry to produce hydrogen. Their most critical components are radiant tubes, where extreme temperature and pressure conditions required the deployment of high alloyed austenitic HP grade steels, owing to their superior strength to creep rupture and good corrosion resistance. Nevertheless, these high strength alloys undergo damage when process conditions allow coke deposition and maintenance procedures are not carried properly.Two radiant tubes, coming from two different plants and made of HP40Nb and HP40Nb microalloy steels respectively, are investigated here to highlight the endured damage after 85,000–96,000 h of service at maximum temperature of ∼950 °C. Light optical microscopy (LOM) and scanning electron microscopy (SEM) analyses both confirmed significant aging in both tubes, as revealed by a substantial microstructural evolution consisting of phases transformation and precipitation, as well as cavitation and incipient microcracking in the bulk. Mechanical testing demonstrated the considerable worsening of mechanical properties following such a microstructural deterioration.However, noticeable differences between the two tubes were discovered as far as type and distribution of damage, and extent of the aged zone. The small differences in alloys composition seem not sufficient alone to explain such dissimilarity in behaviour because, in addition to creep damage, one tube showed also clear evidence of carburization. Instead it is argued that differences in process conditions (i.e. temperature, pressure chemical composition of process fluids and steam–hydrocarbon ratio) and cleaning maintenance for decoking (i.e. type and frequency) may have synergistically triggered different damage mechanisms responsible for the different form of degradation on each reformer tube.