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On the determination of the ductile to brittle transition temperature from small punch tests on Grade 91 ferritic-martensitic steel

The exposure of structural materials to the environment in nuclear power plants leads to aging through thermal and/or irradiation effects. One of the consequences of this aging is the shift of the ductile to brittle transition temperature (DBTT) towards higher temperatures. Characterising the aging of structural materials is therefore a safety relevant topic especially in the context of long term operation of nuclear plants. For testing in service materials and to reduce the health risks and costs associated with handling of irradiated materials miniature testing techniques are being developed. The small punch (SP) technique is one of these methods. In a tensile/fracture SP test, a punch with a hemispherical tip is pushed with a constant displacement rate through the center of a small cylindrical disk specimen. A series of tensile SP tests on the ferritic/martensitic steel Gr. 91 at two different displacement rates show no impact of the displacement rate on the DBTT. A new method using fracture energies normalized to the maximum force for deriving the DBTT from force-deflection data is presented. Post-test analysis by means of X-ray computed tomography is used for deriving the DBTT from fracture strains. Both methods give consistent values for the transition temperature. However, the DBTT determined in this study is higher than what can be found in the literature. Supplementary tests indicate that the punch diameter has an effect on the SP defined DBTT.