Failure analysis of furnace radiant tubes exposed to excessive temperature

Radiant tubes of an ethane pyrolysis furnace at a petrochemical plant failed after a fraction of service life. The tubes developed longitudinal cracks that in extreme cases penetrated the entire tube thickness. The tubes were manufactured from HP 45 heat-resistant steel casting. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the microstructure and chemical composition of the tube samples. Microhardness testing was used to evaluate the mechanical strength while structural analysis was conducted in an X-ray diffractometer. The experimental results showed that the mode of tube failure was a combination of high temperature carburization attack and creep damage leading to intergranular cracking. The cause of failure was overheating likely during decoking operations. Better control of furnace temperature was recommended in order to avoid overheating. However, if the furnace temperature exceeds 900 °C during operation, alloy replacement must be considered. In this case, steel casting material more resistant to carburization and creep damage such as the HP + W grade was suggested as an alternative.