Full field modeling of recrystallization and grain growth thanks to a level set approach: towards modeling by industry

International audienceMetal forming modeling can be predictive only if the strain rate, strain and temperature dependency of the flow behaviour are correctly described. The mechanical properties and behaviour of metallic materials mainly depends on the content and structure of dislocation network, this points out the need to incorporate microstructure concepts into the numerical models. The goal is to correctly describe the main physical mechanisms occurring in metals during thermomechanical processes i.e. work-hardening, recovery, grain boundary migration, nucleation and grain growth related to dynamic, static or metadynamic recrystallization. Macroscopic and homogenized models are widely used in the industry, mainly due to their low computational cost. If this mean field framework is quite convenient, it can be synonymous, for a given material, with a large amount of experiments with advanced laboratory devices. Moreover, the homogenization of the microstructure does not permit to capture some very local phenomena

Modelling and simulation of hydrogen redistribution in a heterogeneous alloy during the cooling down to 200 degrees C

International audienceHydrogen embrittlement induced by internal hydrogen has been identified as a critical issue for heavy components manufactured from either plain or hollow ingots made of 16MnNIMo5 through 20MnNiMo5 alloys that have the particularity to be heterogeneous at various scales. Based on available data of solubilities, diffusivities and temperature dependent phase transformations for the matrix and segregated bands, a model of hydrogen distribution in such a heterogeneous microstructure has been developed. Additionally, penny-shaped thin cavities have been introduced to account for the presence of incoherent MnS inclusions leading to a potential decohesion at MnS/matrix interface. Pressure increase in such cavities has been evaluated using ideal gas behavior for hydrogen. A unidimensional Finite Difference numerical approach has been implemented in order to simulate hydrogen redistribution among matrix, segregation_band and cavity. This model has allowed to quantitatively evaluate the redistribution of hydrogen at the scale of the microstructural heterogeneities during the cooling down to 200 degrees C: first,, close to the component surface where important kinetic effects have been demonstrated due to high cooling rate and second, in the bulk, where hydrogen redistribution was close to the thermodynamic equilibrium during the whole cooling period. It has been shown that the final equilibrium pressure of hydrogen in the cavity at 200 degrees C is negligible even for a high initial hydrogen content. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved

Modelling of hydrogen induced pressurization of internal cavities

International audienceInternal cavities can constitute a crack initiation site especially if filled with hydrogen at high pressure. A new refined equation of state based on recent NIST database has been introduced in order to model the equilibrium pressure. It is based on a thermodynamic definition of fugacity and uses the NIST data relating hydrogen density and pressure to define a new fugacity pressure quadratic dependence. The resulting Equation Of State (EOS) is compared to the standard Abel Noble EOS and it is shown that for a given fugacity, imposed by a Sievert's law, the corresponding pressure is significantly higher. This new refined EOS was introduced into a previously developed numerical model of hydrogen diffusion and desorption and applied to evaluate the kinetics of pressure build-up within a cavity and its equilibrium pressure. It has been shown that the kinetics of pressure buildup at room temperature, which reaches values close to equilibrium in some hundreds of hours, is compatible with the industrial quality control procedures. The calculated pressures are in the range 4500-8650 bars depending on hydrogen solubility, which differs between the matrix and the segregation bands, and tend to equilibrium values obtained from mass balance approach. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved

Dynamic recrystallization in 304L steel: Full field and mean field simulation results compared to experimental data

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Coupled hydrogen and phosphorous induced initiation of internal cracks in a large 18MnNiMo5 component

International audienceThis study analyzed a crack in a scrapped 18MnNiMo5 80-tonne shell. The root cause of crack formation was the low fracture toughness caused by a combination of a high hydrogen content and phosphorous segregation, caused by an inappropriate heat treatment. Cracking initiated on MnS inclusion clusters enabled by hydrogen desorption at the MnS/matrix interfaces. Crack propagation was due to the internal hydrogen pressure causing the crack tip stress intensity factor to exceed the locally-reduced fracture toughness. This new hydrogen induced cracking (HIC) mechanism can be designated as Hydrogen Induced Localised Damage (HILD). Appropriate process controls can eliminate such defects

An extended mean field approach for modelling realistic grain size distribution evolutions during Discontinuous Dynamic RX and Post-Dynamic RX

International audienceMean-field models for Discontinuous DRX (DDRX) have demonstrated for years their ability to reproduce stress and grain size evolutions during hot deformation. However most of them fail to predict grain size distributions, which is necessary to couple DDRX models with Post-Dynamic RX models. Such failure is a consequence of mean field assumptions that prescribe grains to follow the same path in the diameter dislocation-density space. In this paper, an alternative model based on a stochastic approach which permits to deal with the main advantages of mean field models without this limitation is proposed. Results of this new model are successfully compared with experimental data of DDRX and PDRX obtained on a home-made AISI304 deformed up to large strains by hot torsion tests within the range 950 °C – 1150 °C. A comparison with a level-set based full field method of PDRX run on a simplified case is also discussed

Une approche semi–topologique pour la modélisation en champs moyens des recristallisations dynamique et métadynamique

International audienceLa présente communication présente une partie de la thèse de Smagghe (2017). Elle se concentre sur le développement d'une approche originale semi-topologique pour la modélisation en champs moyens des recristallisations dynamique et métadynamique. Cette démarche générique, qui permet d'utiliser des modèles en champs moyens, chaînés entre eux dans les cas multipasses, pour une prévision précise des distributions de la taille des grains, est illustrée dans le cas particulier du forgeage libre de l'acier inoxydable austénitique AISI 304L pour de grosses pièces de chaudronnerie nucléaire

Simulation of the Austenite (γ) – Ferrite (α) transformation in steels within a finite element – level set numerical framework

International audienceThe microstructure characteristics have significant impacts on the mechanical properties of the metallic materials. During the formingprocesses, phase transformations (PT) could occur to respond to the external thermomechanical loads (i.e. heat, mechanical works …)and minimize the free energy of the system.Among these phenomena, Austenite-Ferrite transformation plays an important role in metal forming of steels, as it’s closely relatedwith the other phase transitions occurring at lower temperatures (e.g. bainitic, martensitic transformations). The final state of themicrostructure (including phase fraction, morphology and grain size distribution of each phase) is hence indirectly influenced by theaustenite-ferrite transformation occurred in the upper range of temperature.The kinetic of the austenite-ferrite transformation involves some mechanisms: thermodynamic fluctuations (responsible of the phasenucleation) and energy minimizing (i.e. interface migration, solute diffusion).The complexity to model at the polycrystal scale the austenite-ferrite transformation generally comes from the difficulties to representnumerically the nucleation, to simulate the interfaces migration and the discontinuity of the solute diffusion through the interfaces.This discontinuity at the interfaces is classically treated in the well-known Phase Field (PF) approach by introducing an artificialdiffusive interface, implicitly described through the spatial distribution of a non-conserved order parameter evolving in time in orderto minimize a free energy describing the thermodynamics of the considered configuration.In this work, we introduce, in a finite element context, a new level set numerical framework. The diffusive interface in the phase fieldmodels is replaced by a sharp interface helping to remove purely numerical parameters inherent to classical phase field formulations.The need to build free energy is no longer necessary as the kinetics of the interface migration is simply described through a linearrelationship between the migration velocity and the thermodynamics driving forces. Thanks to the simplicity of the kinetics relations, we also aim to suggest a level set model corresponding to more complexconfigurations, where the interface mean curvature, as well as the jump of stored energy accross phase interfaces contribute to thephase interfaces migration in parallel with the phase transformation chemical potential driving force

A semitopological mean-field model of discontinuous dynamic recrystallization: Toward a correct and rapid prediction of grain-size distribution

International audienceSome mean-field models are currently available for discontinuous dynamic recrystallization (DDRX). They offer an affordable way to predict the variation of averages altered by DDRX as flow stress, grain size and recrystallized fraction. They predict also a grain-size distribution, but it appears unrealistic, at least if a deterministic equation governs the migration of grain boundaries. The present paper exposes an extension to mean-field models which introduces some topological and stochastic features in the migration equation. This extension aims to bridge the gap between mean- and full-field models. It allows predicting realistic distributions of grain size by keeping the simplicity of mean-field approaches. The results obtained are consistent with experimental data on an 18% Cr–11% Ni stainless steel

Full field modeling of dynamic recrystallization in a global level set framework, application to 304L stainless steel

A new full field numerical approach for the simulation of dynamic and post-dynamic recrystallization will be detailed. A level Set framework is employed to link a crystal plasticity finite element method with the modeling of recrystallization. Plasticity is calculated through the activation of slip systems and provides predictions for both SSDs and GNDs densities. These predictions control the activation and kinetics of recrystallization. All the developments are applied on 304L stainless steel