Boiler Tube Failure-Short Term Overheating

The simplest explanation for all "short-term" overheating failures is: when the tube metal temperature rises so that the hoop stress from the internal steam pressure equals the tensile strength at elevated temperature, rupture occurs.
Regardless of the location within the boiler that these failures occur, the appearance is similar.
Short-term overheating frequently exhibits a thin-lipped longitudinal rupture, accompanied by noticeable tube bulging, which creates the large fish-mouth appearance shown in the photomacrograph.

These failures display considerable ductility; the thinning at the failure lip may be more than 90% of the original wall at the instant of rupture. The microstructures throughout the failure will usually indicate, in the case of ferritic steel, the peak temperature at the time of failure. For ferritic steels there is a transformation from ferrite and iron carbide or pearlite, to ferrite and austenite. This temperature is referred to as the lower-critical transformation temperature.
While these microstructures and the estimated peak temperature at the time of failure cannot predict the sole cause of the failure, the metallurgical analysis can suggest the kind of boiler-operational problem that is likely to be the cause of the rupture.