Numerical and experimental study of creep of Grade 91 steel at high temperature

Abstract : Grade 91 steel is a suitable candidate for structural components of the secondary and the vapour of generation IV nuclear reactors. Their in-service lifetime will last for 60 years. It is necessary to consider the mechanisms involved during long-term creep to propose reliable predictions of creep lifetimes. Necking is the main failure mode for creep lifetimes up to 160 kh at 500°C and 94 kh at 600°C. Necking modelling including the material creep softening leads to two bound equations including experimental lifetimes of a large number of tempered martensitic steels loaded up to 200 kh at temperature 500-700°C. The observed creep intergranular cavities are shown to affect very weekly creep strain rate. The prediction of the cavity evolution will allow estimating creep lifetimes out of experimental data domain. Their nucleation and growth, supposed to be associated to vacancy diffusion, are modelled using two classical models. The first one considers instantaneous nucleation (Raj and Ashby) and the second one continuous nucleation obeying the Dyson law (Riedel). The second one leads to two bound equations, more stable with respect to the parameter values. It allows predicting final size of cavities in reasonable agreement with measured ones. Yet, the nucleation rate should still be estimated from measured cavity densities. Nucleation of cavities by diffusion is simulated using the Raj model. This model does not allow predicted final cavity densities in agreement with the measured ones, even by considering cavity nucleation at precipitates/ Laves interfaces experimentally observed and the maximum local stress concentration of a factor 2 computed using finite element calculation in a 2D plane strain hypothesis based on either simulated or real microstructures containing triple points or precipitates/Laves phases. The use of the Dyson model allows us to propose predictions of long-term creep lifetimes. Lifetime predicted using the diffusion-induced growth model of one creep test under low stress still in progress is approximately in agreement with the experimental lifetime estimated based on the fraction of tertiary stage
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Rattanak Lim. Numerical and experimental study of creep of Grade 91 steel at high temperature. Materials. École Nationale Supérieure des Mines de Paris, 2011. English. ⟨NNT : 2011ENMP0083⟩. ⟨pastel-00687235⟩

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