This paper addresses a necking model used for predicting creep lifetimes of Grade 91. Creep results from more than 15 tests at 500-600°C on Grade 91 are used. One of them fractured after 160×10~3h at 500°C. Hart's necking model using the Norton power-law rule correctly predicts lifetimes up to 60×10~3h at 500°C. However, it overestimates lifetimes in all other loading conditions. The necking model including material creep softening, appearing during the tertiary stage, predicts lifetimes differing from the experimental results by less than 20% for lifetimes up to 160×10~3h at 500°C and 50% up to 94×10~3h at 600°C. These predictions are reasonable with respect to experimental scatter. The model predicts the time evolution of the necking section in agreement with an interrupted creep test at least up to 75% of the experimental lifetime. Two lifetime predictions are deduced from this necking model. For a large number of tempered martensitic steels, these two criteria bound the experimental lifetimes up to 200×10~3h at 500-700°C.
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