Abstract: In response to the steam leakage issue observed in the reheater main stop valve cover bolts of a 660 MW ultra-supercritical unit after approximately four years of service, this paper presents a systematic material test analysis on two serviced 1Cr11Co3W3NiMoVNbNB (hereinafter referred to as Co?W?B) bolts. The results show that while the chemical compositions of the two bolts meet the requirements of DL/T 439-2018, their mechanical properties at both room and elevated temperatures, as well as their hardness values, are significantly below the specified limits. The stress-rupture life under 660 °C/245 MPa is less than 5 hours. The microstructure reveals coarsened M??C? carbides and Laves phases, indicating a softening-type failure dominated by precipitate coarsening. Combined with site installation records, the bolts were found to have been installed with deviations in torque and elongation measurement, leading to steam leakage after a period of operation. The unit continued to operate under leakage conditions for approximately three months, which may have exerted additional effects on the microstructural evolution of the bolts. Based on the failure facts, this paper further analyzes the applicable boundary of Co?W?B material under such service conditions, pointing out that its sensitivity to Laves phase precipitation is particularly pronounced during long-term service at 620 °C. The axial temperature gradient, together with abnormal service conditions, constitutes the key factors accelerating microstructural degradation. Accordingly, engineering countermeasures are proposed, including reducing the local temperature at the screwed-in end, establishing an in-service monitoring system based on hardness gradient, prudently defining the upper-temperature applicability limit of the material, and strictly controlling the installation torque. These recommendations provide a reference for material selection and life management of high-temperature bolts in similar units. Keywords:Ultra-supercritical unit; High-temperature bolts; Co?W?B; Microstructural degradation; Laves phase; Failure analysis