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Experimental study and simulation of cyclic softening of tempered martensite ferritic steels

Abstract : The present work focuses on the high temperature mechanical behaviour of 9%Cr tempered martensite steels, considered as potential candidates for structural components in future Generation IV nuclear power plants. Already used for energy production in fossil power plants, they are sensitive to softening during high-temperature cycling and creep-fatigue. This phenomenon is coupled to a pronounced microstructural degradation: vanishing of subgrain boundaries, decrease in dislocation density, nucleation and/or growth of precipitates and new phases. This study aims at (i) linking the macroscopic cyclic softening of 9%Cr steels and their microstructural evolution during cycling and (ii) proposing a physically-based modelling of deformation mechanisms in order to predict the macroscopic mechanical behaviour of these steels during cycling. Mechanical study including uniaxial tensile tests and cycling at 550 °C was performed on a Grade 92 steel (9Cr-0,5Mo-1,8W-V-Nb). Examination of tensile specimens suggested that the physical mechanism responsible for softening is mainly the evolution of mean subgrain size, which increases by more than 15 % compared to the as-received state. The evolution of macroscopic stress during cycling shows that cyclic softening is due to the decrease in kinematic hardening. TEM observations highlighted that the mean subgrain size increases by 65 to 95 % while the dislocation density decreases by more than 50 % during cycling, compared to the as-received state. A self-consistent homogenization model based on polycrystalline elastoviscoplasticity, predicting the mechanical behaviour of the material and its microstructural evolution during deformation is proposed. This model takes the physical deformation mechanisms into account and only two adjustable parameters (activation energy and activation volume) linked to the viscoplastic deformation mechanisms are used. The value of other parameters were either experimentally measured or deduced from computations available in literature. The model correctly predicts the macroscopic softening behaviour and gives a good trend of the microstructural evolution during cycling. The parametrical study shows that the predictions of the model are rather stable with respect to the variation of the physically-based parameters. Finally, some hypotheses which permit us to improve the model are presented and torsion tests (with or without constant superimposed tensile stress) are modeled.
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  • HAL Id : pastel-00710628, version 1

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Pierre-François Giroux. Experimental study and simulation of cyclic softening of tempered martensite ferritic steels. Materials. École Nationale Supérieure des Mines de Paris, 2011. English. ⟨NNT : 2011ENMP0087⟩. ⟨pastel-00710628⟩

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