Kinetics and microstructural evolutions during the tempering of martensitic and nano-bainitic low alloyed steel : in situ experimental study and modelling

Nano-bainitic steels represent a new class of alloys, whose microstructure consists of nanostructured bainitic ferrite formed at low temperature with a high amount of retained austeniteleading to a high ductility and high tensile strength of the steel. Formation of nanobainite has been studied thoroughly in literature as well as tempering of nano-bainitic steels. More recently it has been shown that adding carbide forming elements such as V and Mo increases the resistance to softening and hence the mechanical properties of nano-bainite at moderate temperature. Investigating the secondary carbide precipitation inside a nanobainitic microstructure is thus necessary to optimize the thermal treatments for this promising new class of steels. Three initial microstructures of the same steel composition are investigated: martensite, martensite + retained austenite and nano-bainite. Studying the more conventional case of martensite has served as a basis to better understand the microstructure evolutions inside the nano-bainitic steel. The microstructural evolutions during the tempering were followed by complementary experimental techniques including dilatometry, in situ high energy synchrotron X-ray diffraction (HEXRD), conventional and high resolution TEM. The sequence of carbides precipitation and dissolution (transition-iron-carbides, cementite, and alloyed carbides) both during heating and holding is shown similar for the three initial microstructures. The kinetics are similar as well as cementite chemical composition and size distributions of cementite and alloyed carbides. It has been shown too that the three microstructures present a high retained austenite stability. Moreover, the analyses of the lattice parameters evolutions all along the tempering treatment associated with carbon mass balances have allowed to better understand the carbon distributions between carbides and matrix phases (martensite, bainitic ferrite, retained austenite). The nucleation and growth model from a previous work was upgraded to take into account secondary precipitation and different new features (e.g. para-equilibrium interface condition for first stage of cementite growth, dislocation recovery kinetics based on HEXRD experiments, etc.). This model predicts the kinetics of precipitation, the particle densities and size distributions as well as matrix and carbides mean composition for different tempering conditions. Apart from the comparison with the experimental results that is discussed, it allowed to interpret the similar tempering behaviour for the three initial microstructures

Cinétiques et évolutions microstructurales pendant le revenu d’un acier faiblement allié martensitique et nano-bainitique : étude in situ expérimentale et modélisation

Nano-bainitic steels represent a new class of alloys, whose microstructure consists of nanostructured bainitic ferrite formed at low temperature with a high amount of retained austenite leading to a high ductility and high tensile strength of the steel. Formation of nano-bainite has been studied thoroughly in literature as well as tempering of nano-bainitic steels. More recently it has been shown that adding carbide forming elements such as V and Mo increases the resistance to softening and hence the mechanical properties of nano-bainite at moderate temperature. Investigating the secondary carbide precipitation inside a nano-bainitic microstructure is thus necessary to optimize the thermal treatments for this promising new class of steels. Three initial microstructures of the same steel composition are investigated: martensite, martensite + retained austenite and nano-bainite. Studying the more conventional case of martensite has served as a basis to better understand the microstructure evolutions inside the nano-bainitic steel. The microstructural evolutions during the tempering were followed by complementary experimental techniques including dilatometry, in situ high energy synchrotron X-ray diffraction (HEXRD), conventional and high resolution TEM. The sequence of carbides precipitation and dissolution (transition-iron-carbides, cementite, and alloyed carbides) both during heating and holding is shown similar for the three initial microstructures. The kinetics are similar as well as cementite chemical composition and size distributions of cementite and alloyed carbides. It has been shown too that the three microstructures present a high retained austenite stability. Moreover, the analyses of the lattice parameters evolutions all along the tempering treatment associated with carbon mass balances have allowed to better understand the carbon distributions between carbides and matrix phases. The nucleation and growth model from a previous work was upgraded to take into account secondary precipitation and different new features (e.g. para-equilibrium interface condition for first stage of cementite growth, etc.). This model predicts the kinetics of precipitation, the particle densities and size distributions as well as matrix and carbides mean composition for different tempering conditions. Apart from the comparison with the experimental results that is discussed, it allowed to interpret the similar tempering behaviour for the three initial microstructures.Les aciers nano-bainitiques représentent une nouvelle classe d’alliages, avec une microstructure bainitique nano-structurée obtenue à basse température avec une fraction importante d’austénite résiduelle conduisant à une limite d’élasticité et à une ductilité élevées. Récemment, il a été observé que l’addition d’éléments carburigènes comme le V et le Mo dans ces microstructures augmente la résistance à l’adoucissement ainsi que les propriétés mécaniques à moyennes températures. Ainsi l’étude de la précipitation des carbures secondaires dans les microstructures nano-bainitiques est nécessaire pour optimiser la microstructure et les traitements thermiques de revenu pour cette nouvelle classe d’aciers. Trois microstructures initiales ont été étudiées : martensite, martensite + austénite résiduelle et nano-bainite. L’étude de microstructures martensitiques plus conventionnelles a servi de base pour une meilleure compréhension des évolutions microstructurales dans la microstructure nano-bainitique. La précipitation pendant le revenu a été étudiée avec des techniques expérimentales : la dilatométrie, la diffraction des rayons X de haute énergie sur synchrotron et la microscopie électronique en transmission haute résolution. Il a été montré que les séquences de précipitation et de dissolution des carbures sont similaires pour les trois microstructures initiales. Les cinétiques sont similaires ainsi que la composition chimique de la cémentite et les distributions de taille de la cémentite et des carbures alliés. Nous avons aussi mis en évidence une importante stabilité de l’austénite résiduelle des trois microstructures. L’analyse des évolutions des paramètres de maille associé à des bilans de teneur en carbone ont permis de mieux comprendre sa distribution entre les carbures et les phases des matrices. Un modèle décrivant la germination et la croissance des précipités a été développé prenant en compte la précipitation des carbures secondaires, une cinétique de restauration basée sur les résultats expérimentaux et de nouvelles hypothèses comme le para-équilibre. Ce modèle prédit les cinétiques de précipitation mais aussi la densité de particules, les distributions de taille et la composition moyenne de la matrice et des carbures pour différentes conditions de revenu. Hormis la comparaison avec les résultats expérimentaux qui est discutée, le modèle a permis d’interpréter le comportement similaire des microstructures initiales au cours du revenu

Martensite and nanobainite transformations in a low alloyed steel studied by in situ high energy synchrotron diffraction

Martensitic and nanobainite transformations are studied in situ in a low alloyed, high-Si steel by using in situHEXRD, combined with dilatometry and SEM observations, and by considering the same steel composition andaustenitization conditions. The martensitic microstructure presents a mixed lath-plate morphology with largescatter of sizes whereas the bainite microstructure shows finer laths with more uniform sizes. Recently introducedmethods are used to track in situ by HEXRD, in one single experiment, the phase fractions, the distributionof the carbon and the evolution of the dislocation densities. The study of nanobainite revealed that about twothirds of the carbon partitions from the ferrite to precipitate into transition iron carbides or to enrich theaustenite. Both processes occur very fast after the formation of each nanobainite lath, but the ferrite remainslargely supersaturated in carbon. The dislocation density increases inside each new forming bainitic ferrite lath.It then decreases when recovery becomes preponderant, as described with a recovery model from the literature.After the martensitic transformation, the retained austenite ends up with high hydrostatic compressive stresses.Dislocation densities are higher than in nanobainite and probably more heterogeneous, because recovery is lesssignificant. No carbides were detected, contrary to the nanobainite. The carbon mass balance is analyzed in thelight of these new results and previous investigations on similar systems

Carbon heterogeneities in austenite during Quenching & Partitioning (Q&P) process revealed by in situ High Energy X-Ray Diffraction (HEXRD) experiments

International audienceBased on the evolution of the positions and intensities of the diffraction peaks, high energy X-ray diffraction (HEXRD) is recognized as the ultimate method to follow quantitatively in situ phase transformations in steels. However, the possible asymmetricity of diffraction peaks is seldom considered, and is known to bear information. A procedure for quantifying their skewness is proposed. In the case of a third generation high strength steel obtained by quench and partitioning

Carbon heterogeneities in austenite during Quenching & Partitioning (Q&P) process revealed by in situ High Energy X-Ray Diffraction (HEXRD) experiments

Based on the evolution of the positions and intensities of the diffraction peaks, high energy X-ray diffraction (HEXRD) is recognized as the ultimate method to follow quantitatively in situ phase transformations in steels. However, the possible asymmetricity of diffraction peaks is seldom considered, and is known to bear information. A procedure for quantifying their skewness is proposed. In the case of a third generation high strength steel obtained by quench and partitioning 1Partition du carbone dans les phases ferritiques nanostructurées: cinétiques et microstructuresDesign des Alliages Métalliques pour Allègement des Structure

Numerical investigations of the effects of substitutional elements on the interface conditions during partitioning in quenching and partitioning steels

International audienceIn quenched and partitioned steels, carbon partitioning is considered to be driven by a constraint para-equilibrium at the martensite/austenite interface. Using Thermo-Calc calculations, we investigated the effect of non-partitioned elements on the resulting interface condition. Among all tested elements, only aluminum and chromium have significant effects. From this numerical study, a practical composition- and temperature-dependent relationship describing interface tie lines was derived and calibrated for Fe-C-2.5Mn-1.5Si-X wt pct alloys (X = Cr or Al)

Thermodynamic effetcs of substitutional elements on the CPE interface conditions in Q&P steels

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Thermodynamic effetcs of substitutional elements on the CPE interface conditions in Q&P steels

International audienc

Thermodynamic effetcs of substitutional elements on the CPE interface conditions in Q&P steels

International audienc

High Resolution Reciprocal Space Mapping Reveals Dislocation Structure Evolution During 3d Printing

Dislocation structures are ubiquitous in any 3D printed alloy and they play a primary role in determining the mechanical response of an alloy. While it is understood that these structures form due to rapid solidification during 3D printing, there is no consensus on whether they evolve due to the subsequent solid-state thermal cycling that occurs with further addition of layers. In order to design alloy microstructures with desired mechanical responses, it is crucial to first answer this outstanding question. To that end, a novel experiment has been conducted by employing high resolution reciprocal space mapping, a synchrotron-based X-ray diffraction technique, in situ during 3D printing of a single-phase material. It reveals that dislocation structures formed during rapid solidification undergo significant evolution during subsequent solid-state thermal cycling, in particular during addition of the first few (up to 5) layers above the layer of interest