Abstract
Grade 91 steel, while increasingly popular in high-temperature power plants for both retrofit and new construction applications, faces significant challenges with Type IV cracking at the outer parent side edge of the weld heat affected zone. This structural integrity issue has led to extensive weld inspection requirements and, in severe cases, the premature replacement of grade 91 retrofit headers before their intended design life. This paper presents a method for estimating Type IV cracking timelines in operating grade 91 components by analyzing crossweld Type IV data to determine when Type IV life deviates from parent life. By combining test results from various temperatures, the method generates a generalized prediction of Type IV life that can be extrapolated to any temperature of interest, providing a practical lower bound estimate for service life of the weakest grade 91 material. This approach, which can be applied to service operating conditions to establish realistic inspection timelines for plant components, has already successfully identified early-stage Type IV cracking in two retrofit headers and is being expanded to additional grade 91 components.
