Ligament cracking in a 13CrMo44 superheater header

Superheater header, with end-caps removed for inspection


  • Superheater outlet headers, fabricated from 13CrMo44 (1Cr0.5Mo) steel, in two identical 60MW units
  • Problem:

    • Extensive ligament cracking after 4 years service (~400 starts). This lay predominantly in the circumferential direction in the bottom pair of each set of four ligaments. Crack depths were up to 80% of wall.


    • Fracture mechanics assessment
    Ligament cracking


    • A leak-before break analysis showed that, under the pressure stress alone, a through-wall crack spanning the bottom pair of ligaments would be stable - with a reserve factor greater than 2. Even a crack spanning all four ligaments would be stable with a 10% reserve.
    • The strong directionality of the cracking implied the presence of significant additional system loading.
    • A simple fatigue crack growth calculation indicated that a stress range of around 150MPa was required to explain the observed crack history.
    • Under loads compatible with the crack development, a crack spanning the bottom pair of ligaments would be unstable, failing by collapse.


    • The headers could not continue in service
    • Replacement should include a measure of redesign to reduce the system loads

    Plant Integrity