Cracking of a Steam Pipe Flange

The bottom flange of a vertical pipe coupled to an isolating valve in a steam supply line to a turbine failed. Steam pressure was 1,500 psi and the temperature 416 deg C (780 deg F). Multiple cracking occurred in the bore of the flange. A quarter-segment was cut out and examined. The cracks were located in the part of the flange that formed a continuation of the pipe bore. The majority of them originated at the end of the flange bore and extended axially along the pipe and radially across the flange face. Magnetic crack detection revealed a further number of cracks in the weld deposit. While the fracture in the weld metal was of the ductile type exhibiting a fine fibrous appearance, that in the flange material was of the cleavage type. Microscopic examination revealed that the cracks were blunt-ended fissures of the type characteristic of corrosion-fatigue. It was concluded that cracking was due to corrosion-fatigue, which arose from the combined effect of a fluctuating tensile stress in the presence of a mildly corrosive environment.