Premature Grade 91 failures — worldwide plant operational experiences

•Four different case studies of premature Grade 91 material failures•Creep life calculations are not necessarily conservative.•Using Weld Strength Reduction Factors is not a guarantee for low risk.

Four different case studies of premature failures of Grade 91 materials in Heat Recovery Steam Generators (HRSG) and power piping at combined cycle power plants throughout the world are described. The impact of plant design, the metallurgical aspects of the failures and results from finite element simulations are incorporated. The failure modes discussed are brittle, creep failure, creep and creep-fatigue (CF) failures. All of the investigated cases had life calculations showing a creep life greater than the intended service life of the components. However, since all cases suffered pre-mature failures, it can be concluded that sufficient design life using design temperature and hoop stress is not a guarantee for safe design. The failures can be accredited to improper material properties associated with deficient fabrication and Post Weld Heat Treatment, increased service loading due to geometrical effects not included in design calculations and increases in local temperatures due to gas turbine (GT) load changes. Recommendations to mitigate the risk of component failures are presented.