Mod 9Cr-1Mo Steel (grade 91) is a material candidate for pressure vessels and some internal structures of Gas Cooled Reactors. Some of these structures will operate in the creep range and have to be designed relatively to creep-fatigue damage. In order to characterize the creep-fatigue behavior of this steel and to validate the creep fatigue design rules proposed in nuclear design codes (mainly RCC-MR and ASME), specific fatigue-relaxation (strain controlled during hold time) and creep-fatigue (stress controlled during hold time) tests have been carried out at CEA on this steel. All these tests are conducted at 550°C, but several loading conditions are considered and the influence of the position of hold time in the cycle (either in a tensile or a compressive state) is investigated. This paper presents the experimental program undertaken and describes the main results obtained. Then the creep-fatigue life evaluations are performed using creep-fatigue design rules and material data codified in RCC-MR and ASME and the predictions are compared to experimental results.

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