FE modelling of bainitic steels using crystal plasticity

International audienceModels classically used to describe the probability of brittle fracture in nuclear power plants are written on a macroscale. Physical phenomena are not naturally captured by this type of approach, so that the application of the models far from their identification domain (temperature history, loading path) may become questionable. To improve the quality of the prediction of resistance and life time, microstructural information, describing the heterogeneous character of the material and its deformation mechanisms has to be taken into consideration. The purpose of the paper is to propose a model able to describe local stress and strain fields in 16MND5 bainitic steel. These data will then be used as critical variables for multiscale failure models. The microstructure of 16MND5 steel is made of bainitic packets coming from former austenitic grains, which are not randomly oriented. Knowing the macroscopic stress is thus not sufficient to describe the stress-strain state in ferrite. An accurate model must take into account the actual microstructure, in order to provide realistic local stress and strain fields. After providing some observations and the analysis of the bainitic microstructure, the paper shows a quantitative model of the morphology and the crystallography, then a finite element analysis involving crystal plasticity

Effect of matrix microstructure and second-phase particles on the damage behaviour of hot stamped martensitic steels for automotive applications

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Hot Deformation and Recrystallization Mechanisms in a Coarse-Grained, Niobium Stabilized Austenitic Stainless Steel (316Nb)

International audienceThe hot deformation behavior and associated microstructural evolution of a coarse-grainedNb-bearing austenitic stainless steel (316Nb) has been investigated by the means of torsion testsat high temperature [1223 K to 1423 K (950 C to 1150 C)] followed by microstructuralcharacterization. The starting microstructure was varied by applying prior annealing. Except forstrains below 10 pct, the resistance to viscoplastic flow is not sensitive to the startingmicrostructure. On the other hand, prior annealing at higher temperatures increases theresistance to incipient viscoplastic flow and strongly impedes dynamic and post-dynamicrecrystallization by delaying the grain boundary bulging phenomenon. It also affects the staticrecrystallization behavior during further annealing. The influence of the amount of availableniobium atoms, in particular close to grain boundaries, is discussed

Austenite grain growth in a 2.25Cr-1Mo vanadium-free steel accounting for Zener pinning and solute drag : experimental study and modeling

International audienceAustenite grain size has been experimentally determined for various austenitization temperatures and times in a 2.25Cr-1Mo vanadium-free steel. Three grain growth regimes were highlighted: limited growth occurs at lower temperatures [1193 K (920 °C) and 1243 K (970 °C)]; parabolic growth prevails at higher temperatures [1343 K (1070 °C) and 1393 K (1120 °C)]. At the intermediate temperature of 1293 K (1020 °C), slowed down growth was observed. Classical grain growth equations were applied to the experimental results, accounting for Zener pinning and solute drag as possible causes for temperature-dependent limited growth. It was shown that Zener pinning due to AlN particles could not be responsible for limited growth, although it has some effect at lower temperatures. Instead, limited and slow growths are very likely to be the result of segregation of molybdenum atoms at austenite grain boundaries. The temperature-dependence of this phenomenon may be linked to the co-segregation of molybdenum and carbon atoms