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International audience

Using in situ high energy X-ray diffraction study of austenite formation in hypoeutectoid steel with three different initial microstructures (ferrite-pearlite, tempered martensite and bainite), the lattice parameters of ferrite, cementite and austenite are examined on heating at 0.25, 10 and 100 °C/s. The lattice parameters of ferrite, cementite and austenite do not vary linearly with the temperature, especially, in the temperature range where the austenitization takes place. For the austenite, it is suggested that the deviation from the linearity is mainly associated to the carbon content variation. Using Dyson and Holmes equation, the carbon content in austenite is evaluated for any moment of the austenite formation for each initial microstructure and all heating rates. For the ferrite-pearlite microstructure heated at 0.25 °C/s, the carbon content in austenite after complete cementite dissolution corresponds to that of pearlite. Moreover, a rapid decrease in carbon content in the austenite is observed during the first stage of the austenitization (simultaneous dissolution of ferrite and cementite) followed by a slow further decrease during the transformation of the remaining ferrite. The obtained results are discussed using thermodynamic calculations.

International audience

This contribution first aims at providing a more detailed analysis of the cutting mechanisms of a normalized carbon steel from a microscopic and metallurgical point of view. SEM and EBSD are used to highlight, in the main intensive deformation zones of the chip, a drastic grain refinement induced by a dynamic recrystallization process. The second part of this study intends to emphasize the consequences of the latter on the workmaterial behaviour. A rheological study based on dynamic compression tests is briefly presented and enables to understand their occurrence as well as their influence on the flow stress of the material. A "metallurgy based" constitutive equation is identified to reach a better description of the flow stress. Finally, this one is implemented in a Finite Element code (Abaqus/Explicit) to assess the potential of this method. It is shown that microstructural evolutions could be relatively well predicted especially when considering the level of recrystallization.

The main objective of this paper is to clarify the deformation mechanisms of ferritic-pearlitic steels in metal cutting and correlate them to the associated thermo-mechanical loadings. Dry orthogonal cutting tests have been performed on a normalised AISI 1045 steel with coated carbide tools. Experimental evidences of a drastic grain refinement process in the main deformation zones are advanced on the basis of optical microscope, Field Emission Scanning Electron Microscope (FESEM) and Electron BackScattered Diffraction (EBSD). Microstructural evolutions leading to a grain size down to 200 nm and fragmented cementite are especially emphasized. A numerical approach is further employed to target and quantify the loadings applied to the machined material and extract further information on the Secondary Shear Zone (SSZ). Strains amplitude appears to be the driving parameter of these evolutions via a dynamic recrystallisation process promoted by an intense and localised heat generation. The present contribution highlights that in-depth and microscale investigations of chip formation including microstructural aspects are still required.

This paper questions the perfect thermal contact conditions usually assumed at the tool-chip interface in machining. Dry orthogonal cutting tests are first conducted on a AISI 1045 steel with TiN coated carbide tools. Tool-chip contact zones are analysed by SEM-EDS and sticking and sliding parts are dissociated. A formation mechanism of a Thermal Contact Resistance (TCR) is proposed from the real contact area extracted. A Finite Element (FE) model based on the Arbitrary-Lagrangian-Eulerian (ALE) approach is then employed to investigate the influence of such thermal contact conditions on the cutting process. Evolution of the main cutting outputs such as average cutting forces, average chip thickness, tool-chip contact length and thermal fields is assessed. It is demonstrated, on one side, that average cutting forces, chip thickness and tool-chip contact length are shown to be insensitive to a TCR. On the other side, heat flux transmitted to the tool, temperature distribution on the tool rake face as well as continuity of temperature across the tool-chip interface are clearly affected depending on its amplitude. This study emphasizes that the existence of a TCR at the tool-chip interface can completely modify local heat partition compared to a perfect thermal contact. The possible occurence of an imperfect contact in machining should be highly considered and modelled based on thermal exchange considerations. Local heat transfer models at the interface are still required to reach more reliable and physically based simulations.

Cette etude expose les premiers elements d'une demarche visant a determiner les performances d'usinage d'un acier C45 pour un outil de coupe donne a l'aide d'un modele numerique. Elle aborde les points fondamentaux de (i) la caracterisation mecanique du materiau usine; (ii) l'identification d'une loi de comportement et (iii) son implementation dans un modele numerique de coupe. L'evolution de la contrainte d'ecoulement du materiau, sur une large plage de deformations et temperatures, est investiguee par des essais de compression dynamique realises sur un simulateur Gleeble ® . L'identification d'une loi de type Johnson-Cook est ensuite realisee sur l'ensemble de ces donnees. Les resultats des simulations basees sur cette derniere sont compares a ceux obtenus par des lois issues de la litterature. Des essais de coupe orthogonale ont ete egalement realises et servent de reference pour la validation des simulations effectuees. Il apparait que, malgre les faibles vitesses de deformations employees lors de la caracterisation (

International audience; no abstract

International audience; We present a mathematical method for identifying, separating and quantifying the 3D significant distortions. Measurement of a gas quenched C-ring type sample is performed by a Coordinate Measuring Machine (CMM). Quenching simulation is done with the commercial software Forge 2008 TTT. After comparisons, we notice that the distortions and their tendencies are the same but not exactly the amplitudes. We only focus on distortions which have a physical origin, like the pincers opening, often observed in the literature. As a first explanation, we underline the role of phase transformations and volume dilatation of the steel during quenching.

Pearlitic steels are widely used in the industry for their good balance between strength, ductility and machinability. Sometimes, the classical lamellar structure can degenerate in a so-called globular structure which may significantly affect the behavior of the steel. To our knowledge, no size sensitive model is available in the literature to predict their mechanical behaviors and work-hardenings. Inspired by their analogy with ferrite–martensite dual-phase microstructures, we propose a new microstructure-based law for globular pearlite. It describes the evolution of the dislocation density into ferritic grains which is enhanced by the presence of hard particles. The model is as a consequence sensitive to ferritic grain but also to coarse carbide sizes after globularization. The model has been calibrated on a conventional steel grade and appears to be a practical tool for microstructure design. Keywords: Steel, Pearlite, Behavior, Carbide, Grain size

International audience

Improving the solidification of large industrial steel ingots remains a scientific challenge. During casting chemical heterogeneities (macrosegregation), sometimes in the scale of meters, can arise. Solid grain motion is one phenomena responsible for macrosegregation. Inoculation allows the number, size and morphology of these grains to be modified through additions to the liquid metal. In this work 42CrMo4 and 34Cr4 grade steel ingots of several sizes were modified with potential inoculants. Characterization techniques (2D and 3D) were developed to compare dendritic solidification structures before and after inoculation. The first series of small ingots (50 g) showed grain refinement for powder additions with a low lattice misfit between the inoculant and solidifying melt, and a coarsening effect when ferrocerium was added. The best inoculant candidates (TiN, CeO2, Si3N4 powders and cerium ferroalloy) were tested in medium sized ingots, after which the cerium addition was tested with a 6,2 t industrial ingot. Cerium is responsible for coarsening the grain size in all sizes of cast ingots investigated. This is likely due to a modification of the solid/liquid and liquid/mold interfacial energy along with the replacement of existing sites with cerium inclusions which are not active during nucleation.; L’amélioration de la solidification de lingots d’acier industriels de plusieurs tonnes demeure un défi scientifique. Lors de cette étape cruciale, des hétérogénéités chimiques – à l’échelle du mètre – peuvent se développer. Le mouvement des grains solides qui se forment et se déplacent dans le bain liquide est un des leviers d’action sur la macroségrégation. L’inoculation permet d’agir sur le nombre, la taille et la morphologie de ces grains via des ajouts dans le métal liquide. Dans ce travail, les nuances 42CrMo4 et 34Cr4 ont été inoculées pour plusieurs formats de lingots. Des techniques de caractérisation (2D et 3D) ont été développées pour la comparaison des structures de solidification dendritiques avant et après inoculation. Une première série de lingotins (50 g) a montré l’effet affinant de poudres à bonne cohérence cristallographique avec la ferrite et l’effet grossissant d’un ferroalliage de cérium. Après une étude à plus grande échelle (8 kg) pour les meilleurs candidats (poudres de TiN, CeO2, Si3N4 et ferroalliage de cérium), un ajout de cérium a été fait dans un lingot industriel de 6,2 t. Le cérium, responsable de la croissance des grains équiaxes pour l’ensemble des lingots caractérisés. Il agit probablement sur les tensions interfaciales solide/liquide et moule/liquide tout en remplaçant les sites de germination nativement présents dans le métal liquide par des inclusions au cérium qui ne servent pas de sites de germination.

Improving the solidification of large industrial steel ingots remains a scientific challenge. During casting chemical heterogeneities (macrosegregation), sometimes in the scale of meters, can arise. Solid grain motion is one phenomena responsible for macrosegregation. Inoculation allows the number, size and morphology of these grains to be modified through additions to the liquid metal. In this work 42CrMo4 and 34Cr4 grade steel ingots of several sizes were modified with potential inoculants. Characterization techniques (2D and 3D) were developed to compare dendritic solidification structures before and after inoculation. The first series of small ingots (50 g) showed grain refinement for powder additions with a low lattice misfit between the inoculant and solidifying melt, and a coarsening effect when ferrocerium was added. The best inoculant candidates (TiN, CeO2, Si3N4 powders and cerium ferroalloy) were tested in medium sized ingots, after which the cerium addition was tested with a 6,2 t industrial ingot. Cerium is responsible for coarsening the grain size in all sizes of cast ingots investigated. This is likely due to a modification of the solid/liquid and liquid/mold interfacial energy along with the replacement of existing sites with cerium inclusions which are not active during nucleation., L’amélioration de la solidification de lingots d’acier industriels de plusieurs tonnes demeure un défi scientifique. Lors de cette étape cruciale, des hétérogénéités chimiques – à l’échelle du mètre – peuvent se développer. Le mouvement des grains solides qui se forment et se déplacent dans le bain liquide est un des leviers d’action sur la macroségrégation. L’inoculation permet d’agir sur le nombre, la taille et la morphologie de ces grains via des ajouts dans le métal liquide. Dans ce travail, les nuances 42CrMo4 et 34Cr4 ont été inoculées pour plusieurs formats de lingots. Des techniques de caractérisation (2D et 3D) ont été développées pour la comparaison des structures de solidification dendritiques avant et après inoculation. Une première série de lingotins (50 g) a montré l’effet affinant de poudres à bonne cohérence cristallographique avec la ferrite et l’effet grossissant d’un ferroalliage de cérium. Après une étude à plus grande échelle (8 kg) pour les meilleurs candidats (poudres de TiN, CeO2, Si3N4 et ferroalliage de cérium), un ajout de cérium a été fait dans un lingot industriel de 6,2 t. Le cérium, responsable de la croissance des grains équiaxes pour l’ensemble des lingots caractérisés. Il agit probablement sur les tensions interfaciales solide/liquide et moule/liquide tout en remplaçant les sites de germination nativement présents dans le métal liquide par des inclusions au cérium qui ne servent pas de sites de germination.

Correct prediction of composition heterogeneities and grain structure across a steel ingot is critical in optimizing the industrial processing parameters for enhanced performance. The columnar to equiaxed transtion (CET) is a microstructural transition which is strictly controlled as it affects the mechanical properties of the final product along with the macrosegregation patterns. Larger equiaxed regions are preferred for most industrial applications. CET is significantly affected by the number density of equiaxed grains and by the nucleation undercooling. 8 kg 42CrMo4 alloy steel ingots (240 mm x 60 mm x 60 mm) were cast. The cast structure was characterized by ASCOMETAL. The experiments were simulated with a process-scale model of solidification that incorporates a multiscale description of the microstructure formation. The goal of the present study is to show the capabilities of such a process-scale solidification model to explain the observed structure distributions (extent of the columnar and equiaxed zones, equiaxed-to-columnar transition).

International audience; Dans une pièce métallique, des pores résiduels non désirés sont toujours présents après les opérations de fonderie. Pour refermer ces pores de taille généralement négligeable par rapport à la taille de la pièce, les industriels utilisent des procédés à chaud tels que le forgeage libre ou le laminage. Dans la littérature, plusieurs modèles à champ moyen prédictifs en terme d'évolution du volume des pores traitent de cette problématique. Récemment un nouveau modèle à champ moyen, appelé Cicaporo4, a été élaboré et continue d'être amélioré afin de modéliser la refermeture des pores. Ce modèle se base sur une approche multi-échelles2 utilisant des simulations sur Volume Elémentaire Représentatif (VER). La plupart des modèles classiques étudient la refermeture en fonction de latriaxialité des contraintes et de la déformation plastique équivalente. Pour l'élaboration du modèle Cicaporo, les effets de la morphologie des pores et de leur orientation dans la matrice ont été investigués.

RÉSUMÉ. Certains traitements thermiques génèrent sur les pièces mécaniques des déformations, locales ou globales, plus ou moins importantes selon le traitement et la nuance d'acier utilisés. Des opérations de finition, souvent coûteuses, sont alors nécessaires pour respecter les tolérances fonctionnelles spécifiées. Afin d'optimiser la nuance et le traitement, et d'ajuster les modèles de simulation, les variations géométriques de pièces " test " traitées thermiquement sont qualifiées et quantifiées par une analyse dimensionnelle. Les travaux présentés se centrent sur les stratégies de mesure et de dépouillement. Nous évoquons aussi l'identification de déformations issues de trempe d'essais sur deux nuances d'aciers étudiées. ABSTRACT. Some heat treatment generates on workpieces, local or global distortions which are more or less significant according to the treatment and the steel grade. Finishing operations are then necessary to respect the required functional tolerances. This may increase the cost of the product. In the objective to optimize both the steel grade and the heat treatment and to adjust the numerical simulation models, the geometrical variations of heat treated "test" parts are qualified and quantified by a dimensional analysis. In this article, we focus on part measurement and data processing strategies. We also evoke the identification of the distortions resulting from testing quenching on two studied steel grades.

Nowadays, heat treatment distortion is still difficult to control because of the lack in the corresponding mechanisms comprehension. Indeed, many technological and physical parameters are involved in the distortion formation and evolution, their coupled effects influencing the final results in a complex manner. In this context, the developed method defines a background suitable to study the influences of all parameters on a part distortion. This method is threefold. One point refers to the distortion significant components identification related to specific 3D part geometry: "distortion phenomena". They are quantified and their accuracy is evaluated using reliability indicators. The second point concerns the comparison of the experimental results with the numerical simulation ones. The numerical simulation prediction of the distortion is investigated by comparing the magnitudes of the distortion significant components. The third point is focused on the distortion mechanisms comprehension. The thermal, metallurgical and mechanical effects on each "distortion phenomenon" magnitude are examined in order to dissociate their influences on the final distortion results. Applied to the C-ring specimen, the developed method allows us to analyze the influence of two parameters on distortion: steel grade and quenching media. Two categories of "distortion phenomena" were detached: homogeneous dimensional variation due to the thermomechanical effects and heterogeneous dimensional variation due to the metallurgical and mechanical effects. Furthermore, the method was adapted to the analysis of numerical simulation results. Thus, distortion numerical simulation results were compared to the experimental ones. A good prediction is obtained for the distortion significant components, even for the local ones. Investigating the numerical simulation results allowed us to understand the distortion mechanisms and to highlight the strong sensibility of some "distortion phenomena" to the experimental parameters variation, used as initial data for simulation.; La déformation en traitement thermique est associée à des mécanismes complexes du fait de l'interaction d'un grand nombre de paramètres technologiques et physiques. Dans ce contexte, la méthode développée dans ces travaux de thèse définit un cadre générique pour étudier l'influence de ces paramètres sur la déformation d'un produit. Cette méthode comporte trois facettes d'intérêt. La première est la qualification de la déformation au moyen de signatures géométriques 3D significatives, choisies pour leur réalité physique : les phénomènes de déformations. Ceux-ci sont ensuite quantifiés et leur pertinence est évaluée par un indicateur de niveau de confiance. La deuxième facette de la méthode est la possibilité de confronter directement les résultats expérimentaux et ceux issus de la simulation numérique. La prédiction de la déformation est ainsi vérifiée en comparant les amplitudes des signatures géométriques. La troisième et dernière facette est la compréhension des mécanismes de la déformation. Elle s'effectue par le suivi des amplitudes des phénomènes de déformations en fonction des effets thermiques, métallurgiques et mécaniques survenant lors du traitement thermique. Appliquée à l'éprouvette croissant, la méthode a permis d'illustrer l'influence de deux paramètres sur la déformation : la nuance d'acier et le milieu de trempe. Pour cela, les phénomènes de déformations ont été séparés en deux catégories : la première correspond à la variation homogène des dimensions dues aux effets thermomécaniques, la seconde, à la partie hétérogène due aux effets métallurgiques et mécaniques. L'application de la méthode à un cas issu de la simulation numérique 3D a révélé la prédiction satisfaisante des signatures géométriques, y compris celles locales. Les investigations numériques ont permis d'appréhender les mécanismes de la déformation du croissant et de mettre en évidence la forte sensibilité de certains phénomènes de déformations face à la variation des paramètres expérimentaux, utilisés comme données initiales dans les simulations.

International audience; In continuous casting (CC), thermomechanical simulation is essential to analyze important issues: gap formation; stress and deformation of the solidified shell; bulging of product between supporting rolls in the case of steel CC; size of final product, butt-curl defect in direct chill casting of aluminium. The numerical simulation package THERCAST has been developed with the objective of supplying an accurate analysis of those phenomena, permitting to define relevant process actuators. In this paper, some characteristic features especially developed for steel continuous casting are presented and illustrated by examples of industrial application.

Crack growth under mode II cyclic loading was investigated in maraging steel, ferritic–pearlitic steel and TA6V. When ΔKII exceeds a threshold value, cracks do not bifurcate but grow in mode II over a distance which increases with ΔKII. Shear mode crack growth was much more extensive in maraging steel than in TA6V and ferritic–pearlitic steel. This result is discussed in relation with the cyclic behaviour of the materials and the importance of friction along the crack faces. The maximum growth rate criterion is shown to be suitable for the prediction of crack paths when shear mode crack growth is likely to occur.

International audience; A “local approach” of bifurcation based on the application at the crack tip of two critical-plane fatigue criteria predicting either tensile-stress-dominated or shear-dominated fracture and on the choice of the fastest damage mode is proposed. In pure mode II, this approach is shown to successfully predict a transition from bifurcation, as predicted by usual criteria, to coplanar crack growth when DKII increases, which usual criteria fail to predict. It is also shown that the deceleration and final bifurcation of cracks propagating in shear-mode is not due to a stress/strain redistribution as the crack propagates through its plastic zone but can really be attributed to a friction-induced decrease in effective DKII. The proposed approach may also be used for unconstrained plasticity or non-proportional loading and it is sensitive to the loading path. An application to sequential mode I and II is presented. The synergy of mode I and mode II in that case is captured.

International audience; The crystallographic structure of vanadium carbide precipitates in iron is investigated using High Resolution Transmission Electron Microscopy (HRTEM) and conventional Selected Area Diffraction (SAD) analysis. After a two steps precipitation treatment (10 hours at 700°C and 10 days at 800°C) and different annealing treatments (from 870°C to 920°C) performed on an ultra-pure Fe-V-C model alloy, carbides exhibit unambiguously the ordered monoclinic form V6C5. The often reported V4C3 structure, that refers to the pioneer work by Baker and Nutting [R. G. Baker and J. Nutting, Precipitation processes in steels (1959)] is not encountered in the present investigation. Reasons for this contradiction will be discussed, and the conclusion is drawn that no literature data is available to unambiguously support the existence of precipitates with the V4C3 structure.

National audience; Deux méthodes de caractérisation du comportement anisotrope de tôles sont comparées. Les coefficients de Lankford de tumé : ôles en alliage d'aluminium-magnésium sont tout d'abord déterminés via des essais de traction à température ambiante. Un système de reconnaissance automatique de motifs aléatoires est utilisé pour analyser ces essais en termes de comportement mécanique et pour identifier les coefficients d'anisotropie. En parallèle, des essais d'emboutissage de type Nakazima ont été réalisés, puis analysés à l'aide d'un module d'analyse inverse. Ce modèle s'appuie sur une méthode d'optimisation automatique couplée à un logiciel éléments finis tridimensionnel. Le comportement anisotrope des tôles d'aluminium est modélisé en utilisant un critère de Hill. Les mesures de champs de déformation obtenues avec le système de corrélation d'images sont utilisées pour identifier les paramètres du critère de Hill. Les coefficients de Lankford ainsi déterminés sont comparés à ceux issus des essais de traction.