Assessment of creep damage and residual creep life of a cast HP 40 Nb Mod. Reformer tube was performed, wherein the experimental Larson–Miller diagram and area fraction of creep voids were adopted. The state of damage of the tube in service was metallographically analyzed by using light and electron microscopy. Samples from the serviced reformer furnace tube were cut and prepared for void examination and creep test at 940ºC-1000ºC under 20-30 MPa stress. Microstructural examination was carried out with an Scanning electron microscope with secondary and backscattered electron detectors. Inter-granular voids in the microstructure of the worked tube as a result of a creep phenomenon are ranked relating to the remaining life.