Creep failures of overheated boiler, superheater and reformer tubes

Internally pressurised tubes are critical components in heat-exchanger applications, such as boiler water tubes, steam superheater elements and chemical plant reformer tubes. Tubes in such applications are vulnerable to temperature excursions: as a consequence the material may enter the creep regime, and creep deformation (bulging) and even fracture (longitudinal rupture) may subsequently occur, with serious consequences. It is estimated that 10% of all power-plant breakdowns are caused by creep fractures of boiler tubes. In general, 30% of all tube failures in boilers and reformers are caused by creep. This paper gives details of four case studies in which internally pressurised tubes failed by creep bulging and rupture (two boilers, one superheater and one reformer). The conditions of temperature and time under which the failures occurred are deduced from the morphology of fracture and the changes in microstructure, and are correlated with the deformation-mechanism and fracture-mechanism maps for the tube materials. (C) 2004 Elsevier Ltd. All rights reserved.