Ferritic Steel / CSEF - Estimation of Creep Strength of ASME Grade 91 Type Steel in Actual Service Conditions

The creep strength of base metal and welded joint of ASME Grade 91 type steel under actual service conditions was investigated using long-term used materials in this work. A series of creep tests were conducted on the materials used for hot reheater or main steam piping at ultra-super critical fossil power plants. The remaining creep life of each material under actual service conditions was calculated from a fitting curve using the Larson-Miller parameter. Then, the total creep life of each material was estimated as a summation of the service time at the plants and the remaining creep life. Such estimation was conducted for the base metal and welded joint used at about ten plants. This information was useful for examining the validity of the life evaluation formula under actual service conditions because it is extremely difficult to obtain creep rupture data under such conditions owing to test time durations. Therefore, the estimation results were compared with creep rupture life curves of the Grade 91 type steel proposed by the Assessment Committee on Creep Data of High Chromium Steels in 2015. Regarding the base metal, it was estimated that Grade 91 type pipe steel generally has a life shorter than that shown in the 99% confidence lower bound curves of the material in 2015. This finding indicated that the life evaluation formula of the Grade 91 type steel base metal proposed in 2015 should be reviewed. On the other hand, it was estimated that the welded joint of Grade 91 type steel generally has a life longer than that shown in the 99% confidence lower bound curves of the material in 2015, indicating that there is no need to review the life evaluation formula of the Grade 91 type steel welded joint proposed in 2015.