Modelling of a Grade 91 power plant pressurised header weldment under ultra super-critical creep conditions

This paper is concerned with the creep-damage modelling of a Grade 91 pressurised header, which was observed to undergo in-service cracking in the weldments. A multi-axial creep damage model of Kachanov type, with a single state damage variable, has been implemented into finite element analysis to study the creep damage responses of weldments and the sub-zones i.e. the base metal (BM), weld metal (WM) and heat-affected zone (HAZ). Material properties for each weld constituent were obtained from the results of accelerated creep tests on materials extracted from the header. Predictions of crack initiation were made for sections of the stub to header welds. This analysis was also used to estimate creep failure life of the header weldment under ultra-super-critical conditions. Further, creep crack growth behaviour was predicted based on time-dependent critical damage growth. The predicted damage distributions and failure mode of the cross-weld creep test specimens were in good agreement with the reported experimental observations. The predicted damage distributions and cracking in the header correlate reasonably well with the reported industrial observations.

Automated summary

This paper focuses on the creep-damage modeling of a Grade 91 pressurized header to address in-service cracking in the weldments. A multi-axial creep damage model and accelerated creep tests were used to predict crack initiation and estimate creep failure life. The predicted damage distributions and cracking in the header were in good agreement with experimental observations.