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

Abstract

• 'Fish mouth’ plastic deformation appearance, was observed in the failure area. • The outer diameter of the damaged tube increased up to 18 percent. • Microstructure of the rupture area was ferritic-bainitic devoid of carbides. • Increased scale thickness on inner surface of tube raises the risk of overheating. • The tube damage root cause: short-time overheating from burner or steam flow issues. 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.

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