a b s t r a c tThe 304L stainless steel is a major component of residual heat removal circuits of pressurized water reactors (PWRs). The main purpose of this study is to understand the risk of thermal fatigue damage resulting from the machining of the 304L steel pipes inner surface (pre-hardening gradient, residual stresses and scratches), at the scale of the microstructure. This work is based on previous results obtained for pipe specimens thanks to a macroscopic elasto-visco-plastic model. Applied to the pipe specimens, this modelling showed that a thermal loading with temperature gradient, induced a cyclic non-linear biaxial loading at the inner surface of the pipe. In this paper, a polycrystal plasticity model, implemented in a Finite Element (FE) code, is adapted to cyclic loading. An elementary volume (3D aggregate), representing the inner surface and sub-surface of the 304L steel tube, is built from successive polishings and orientation mappings thanks to an Electron Back Scattering Diffraction method. At the grain scale, the polycrystal model is used as a ''numerical microscope'' to compute the local mechanical fields. Different fatigue criteria are tested to determine their sensitivity to surface properties (roughness, residual stress and pre-hardening) and to the microstructure of the material (crystallographic orientation and grain size). Pre-hardening leads to a lower and more homogeneous distribution of local strain amplitudes in the aggregate, but slightly higher stresses when compared to initial material without hardening. By contrast, surface roughness leads to large localized strain and stress fields in grains located at the bottom of scratches. To determine the surface micro-structural ''hot spots'' features and to test the sensitivity of different surface conditions, three different fatigue criteria (Manson-Coffin, Fatemi-Socie and Dissipated Energy criteria) have been computed. We point out that the pre-hardening may have a complex effect on fatigue resistance, since it reduces local plastic strain amplitudes, but increases local stresses. Moreover, the pre-hardening has a positive effect on fatigue since it delays damage initiation. By contrast, the surface roughness leads to a negative effect. However, we have shown that the three different fatigue criteria do not deliver similar quantitative predictions. Relevant criteria for high cycle fatigue, such as stress based criteria, are not considered in this paper, since the thermal loading used for computation is large enough to reduce cyclic plastic strain straining within all grains of 304L pipe inner surface for midlife of experiments.

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The main purpose of this study is to determine, via a three dimensions Finite Element analysis (FE), the stress and strain fields at the inner surface of a tubular specimen submitted to thermo-mechanical fatigue. To investigate the surface finish effect on fatigue behaviour at this inner surface, mechanical tests were carried out on real size tubular specimens under various thermal loadings. X ray measurements, Transmission Electron Microscopy observations and micro-hardness tests performed at and under the inner surface of the specimen before testing, revealed residual internal stresses and a large dislocation microstructure gradient in correlation with hardening gradients due to machining. A memory effect, bound to the pre-hardening gradient, was introduced into an elasto-visco-plastic model in order to determine the stress and strain fields at the inner surface. The temperature evolution on the inner surface of the tubular specimen was first computed via a thermo-elastic model and then used for our thermo-mechanical simulations. Identification of the thermo-mechanical model parameters was based on the experimental stabilized cyclic tension-compression tests performed at 20°C and 300°C. A good agreement was obtained between numerical stabilized tractioncompression cycle curves (with and without pre-straining) and experimental ones. This 3 dimensional simulation gave access to the evolution of the axial and tangential internal stresses and local strains during the tests. Numerical results showed: a decreasing of the tangential stress and stabilization after 40 cycles, whereas the axial stress showed weaker decreasing with the number of cycles. The results also pointed out a ratcheting and a slightly non proportional loading at the inner surface. The computed mean stress and strain values of the stabilized cycle being far from the initial ones, they could be used to get the safety margins of standard design related to fatigue, as well as to get accurate loading conditions needed for the use of more advanced fatigue analysis and criteria.