In this study failure investigation was carried out on two types (type ”A” and type “B”) of secondary superheater tubes of a boiler. The steam flows inside the tubes and there is hot flue-gas stream outside of them. The rupture occured usually at/around bend section of the tubes. Visual iection, chemical analysis, X-ray diffractometery, optical and scanning electron microscopy, and hardness measurement have been done on both virgin tubes and failed tubes. In order to investigate the possible role of mechanical stress in failure of the type ”A” tube which was ruptured in bend part, distribution of mechanical stress on the tube was simulated via finite element method. Results showed that the steel of both tubes is high temperature structural steel, but the type “B” tube steel is not suitable for superheater tube. In addition, molybdenum deficiency made it susceptible to creep failure. Based on thickness of the steam side oxide scale, average tube temperature was calculated 550°C for type “A” tube and 650°C for type “B” tube; while maximum allowable temperature for both steels is 530°. Therefore, type “A” tube has experienced slight overheating; 21 year service time of this tube confirms this point. In case of the type “B” tube, the temperature of 650°C is obviously high and caused its overheating. Short service life of 3 years for the mentioned tube along with heavy oxidation and severe material degradation support occurrence of overheating. Calculation of hoop stress considering metal overheating and wall-thinning (as a result of oxidation and erosion) showed that hoop stress exceeded creep rupture strength of the alloy; thus creep rupture is expected. Simulation outputs of mechanical stress distribution is in accordance with experimental findings, showing higher potential for damage on fireside surface of the failure region. It can be concluded that thermal stress, overheating and erosion by steam are main roots for ruptur of type “A” tube; where as severe overheating and oxidation together with thermal stress caused failure of type “B” tube. Mechanical stress acted as a secondary mechanism in damage of type “A” tube.
