Failure of tube used in high pressure steam generator due to incrustation deposits

In this work a failure analysis was conducted to investigate the premature failure of ASTM 106-Gr.B seamless tube used in steam generators. The failure occurred after 3 years of operation, due to a macroscopic crack with approximately 50 mm in the longitudinal direction. Incrustration deposits found in the region of failure were identified as sodium aluminum silicates (albite and nepheline). Micro-hardness profile showed a sharp increase of hardness near the crack, which was associated to microstructural changes observed by optical microscopy. Local over-heating of the tube has occurred due to the incrustation deposits. As a consequence, creep damage was produced and caused the crack of the tube. Temperatures in the austenitic field were achieved, since martensitic, bainitic and Widmansttäten ferrite structures were observed near the crack. In the crack tip, at the moment of leaking, the austenite underwent rapid cooling from the high temperature, producing hard microstructures in the crack neighborhood. Correct water treatment is recommended to avoid future failures in this type of tube.

► Steam generator tube failed by creep due to over-heating.
► Over-heating was caused by analcite incrustation in the internal wall.
► Water leaking provoked quenching and hardening of the crack tip.