Failure analysis of steam superheater boiler tube made of ASTM T22 steel

This study investigates the failure of T22 grade steel boiler tube, as per ASTM A 213/A A213M standards, which experienced significant plastic deformation with a characteristic 'fish mouth' appearance. Analysis revealed an increase in outer diameter up to 18 percent, coupled with a reduction in wall thickness from 8.8 mm to around 3 mm at the rupture site. Microstructural examination of samples taken away from the rupture site revealed a structure composed of ferrite and partially decomposed bainite, attributed to prolonged annealing, along with fine spherical carbide precipitates. Samples from the rupture area displayed ferritic-bainitic structure without secondary carbide precipitates, indicating exceeded transformation temperatures. Additionally, increased hardness in the rupture area suggested transitions of overcooled austenite into ferrite and bainite. Thickened scale, particularly on the inner surface, heightened the risk of overheating. Based on these findings, it was concluded that the tube failure resulted from short-time overheating.