Thermal ratcheting of a P91 steel cylinder under an axial moving temperature distribution

Abstract The progressive inelastic deformation, commonly referred as ratchetting, is a major concern in the design of components and structures submitted to mechanical and thermal cyclic loads in the plastic range. In the RCC-MRx code, the design assessment against ratcheting is performed by applying a simplified method based on the elastic analysis of the structure. The design rule uses a diagram, called efficiency diagram, elaborated essentially on the basis of the results of tension/torsion experiments. In this work the application of the RCC-MRx efficiency diagram to P91 steel has been investigated. It has been considered the thermal ratcheting of a cylinder subjected to a moving axial temperature gradient with no primary stresses applied. The thermal loads have been produced via hot liquid thermal shocks, by dipping a cylindrical mock-up in a molten eutectic mixture of sodium and potassium nitrates. The experimental results show that the present efficiency diagram in RCC-MRx is not suited for P91 steel. Its use for the design would foresee cumulative strains lower than those observed experimentally which is clearly not conservative. This result confirms the results obtained in previous tension-torsion ratcheting tests and provides additional data for the development of a modified efficiency diagram suited for 9Cr steel.