Damage investigation on welded tubes of a reforming furnace

In this work the creep damage of radiant tubes of a reforming furnace has been investigated. The considered furnace contains a battery of tubes constructed by butt welding three spun cast pieces, made of ASTM 608 HP-Nb alloy. They are designed to operate at temperatures of about 900°C, pressures of about 30 bars and times of the order of 100000 h. Tubes were inspected during the plant stops scheduled every two years, in order to identify and replace the damaged ones with the aim to ensure conditions of safe operation in the furnace. They were selected though a criterion based on measures of the internal diameter deformation performed in situ by Laser Optic Tube Inspection System (LOTIS). For a verification of this method, optical and scanning electron microscopy observation, Vickers microharndess and creep tests have been carried out on samples taken from tubes put out of service.