A Proposal for Post-Assessment Test of Long-Term Creep Rupture Strength of Grade 91 Steel

Predictions as to 10⁵ hrs creep rupture strength of grade 91 steel have been made recently. The predictions should be verified by some means, since they are based on certain assumptions. A formula for predicting long-term creep rupture lives should correctly describe long-term data points used in its formulation. Otherwise the formula cannot properly predict further longer-term creep rupture lives. On the basis of this consideration, the predictions are examined with long-term creep rupture data of the steel. In the predictions three creep rupture databases were used: data of tube products of grade 91 steel reported in NIMS Creep Data Sheet (NIMS T91 database), data of T91 steel collected in Japan, and data of grade 91 steel collected by an ASME code committee. Short-term creep rupture data points were discarded by the following criteria for minimizing overestimation of the strength: selecting long-term data points with low activation energy (multi-region analysis), selecting data points crept at stresses lower than a half of proof stress (σ_0.2/2 criterion), and selecting data points longer than 1000 hrs (cut-off time of 1000 hrs). In the case of NIMS T91 database, a time-temperature parameter (TTP) analysis of a dataset selected by the multi-region analysis can properly describe the long-term data points. However, the TTP analyses of datasets selected by the σ_0.2/2 criterion and by the cut-off time of 1000 hrs from the same database overestimate the long-term data points. The different criteria for data selection have more substantial effects on predicted values of the strength of the steel than difference of the databases.