Stress distribution and cleavage analysis in a 16MnNiMo5 bainitic steel X-ray diffraction and multiscale polycrystalline modelling

Abstract

Many tensile tests have been realized on the 16MnNiMo5 bainitic pressure vessel steel at low temperatures [-196°C, -60°C]. The damaging processes (ductile/fragile) are observed with a scanning electron microscope (SEM), while X-ray diffraction (XRD) is used to determine the internal/residual stresses within the ferritic phase during loading (in-situ) and after unloading: stress states are lower in ferrite than in the bulk material due to cementite particles, the difference never exceeding 150 MPa. A polycrystalline modelling with a two-level homogenisation is also developed concurrently with the experimental characterization. It correctly reproduces the stress distribution in each phase, the intergranular strain heterogeneity as well as the macroscopic fracture stress and strain in relation to temperature, considering a constant number of grains (7%) reaching an experimentally identified crystallographic criterion of cleavage.