Stability of retained austenite in high carbon steel under compressive stress: An investigation from macro to nano scale

Although high carbon martensitic steels are well known for their industrial utility in high abrasion and extreme operating environments, due to their hardness and strength, the compressive stability of their retained austenite, and the implications for the steels' performance and potential uses, is not well understood. This article describes the first investigation at both the macro and nano scale of the compressive stability of retained austenite in high carbon martensitic steel. Using a combination of standard compression testing, X-ray diffraction, optical microstructure, electron backscattering diffraction imaging, electron probe micro-analysis, nano-indentation and micro-indentation measurements, we determined the mechanical stability of retained austenite and martensite in high carbon steel under compressive stress and identified the phase transformation mechanism, from the macro to the nano level. We found at the early stage of plastic deformation hexagonal close-packed (HCP) martensite formation dominates, while higher compression loads trigger body-centred tetragonal (BCT) martensite formation. The combination of this phase transformation and strain hardening led to an increase in the hardness of high carbon steel of around 30%. This comprehensive characterisation of stress induced phase transformation could enable the precise control of the microstructures of high carbon martensitic steels, and hence their properties

Atom probe tomography of the austenite–ferrite interphase boundary composition in a model alloy Fe–C–Mn

International audienceA tomographic atom probe analysis has been developed to study the interfacial conditions during isothermal austenite transformation to ferrite at 700 °C in an Fe–C–Mn model alloy. The interfacial conditions lead to different alloying element profiles across the interface, and a comparison is made between this experimental result and the DICTRA software predictions under the various conditions

Kinetics of bainite transformation in heterogeneous microstructures

International audienceIn classical models for microstructural evolution, the natural dispersion existing in the samples is often neglected. In this paper, we propose a general procedure that takes into account the distribution of austenite grain size on kinetics of bainite transformation in steels. Through this example and by comparison with experimental data, it is shown that the extended approach provides improved predictions of the overall kinetics

Atom probe tomography of the austenite–ferrite interphase boundary composition in a model alloy Fe–C–Mn

International audienceA tomographic atom probe analysis has been developed to study the interfacial conditions during isothermal austenite transformation to ferrite at 700 °C in an Fe–C–Mn model alloy. The interfacial conditions lead to different alloying element profiles across the interface, and a comparison is made between this experimental result and the DICTRA software predictions under the various conditions

Effect of intercritical annealing time on microstructure and mechanical behavior of advanced medium Mn steels

in : Thermec 2011, ed. T. Chandra, M. Ionescu and D. MantovaniInternational audienceThe study about the influence of intercritical annealing time on a cold rolled 0.1%C - 4.6%Mn (wt.%) steel was performed. The tensile tests show an interesting balance between strength and ductility especially after 7 hours annealing at 670°C. A part of this good result can be explained by the presence of rather high fraction of metastable austenite at room temperature. On the other hand a very complex microstructure combining lath-like and polygonal features was observed making the interpretation complicated

Effect of interstitial carbon distribution and nickel substitution on the tetragonality of martensite: A first-principles study

By using first principles calculations, the effects of carbon distribution and of 25 at.% of nickel substitution on the tetragonality of ferrous martensite have been investigated. Different carbon concentrations have been considered (0≤x c ≤ 12.5 at.%). All calculations were based on the Projector Augmented Wave (PAW) method of the Density Functional Theory (DFT). The Special Quasirandom Structure (SQS) method has been used to model the disordered structures of the Fe 16 C 2X and (Fe,Ni) 16 C 2X systems. Our calculations demonstrate that the lattice parameters a and c vary linearly with carbon concentration, up to the high value of x c = 12.5 at.% (for X = 1). The nickel substitution enhances the expansion of parameter c of the Fe 16 C 2X system up to 37%. This trend is correlated with the shear moduli calculated for bcc-iron and the disordered Fe25 at.%Ni structure. The mixing energies of the Fe 16 C 2X and (Fe,Ni) 16 C 2X systems indicate that the unmixing of martensite by spinodal decomposition is favoured by the nickel substitution. It is shown that carbon-carbon interactions are mostly repulsive until the distance of two bcc-iron lattice parameters.Etude de la décomposition spinodale dans les aciers martensitiques Fe-X-

Phase transformation controlled architectures in steel alloys

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Effect of interstitial carbon distribution and nickel substitution on the tetragonality of martensite: A first-principles study

International audienceBy using first principles calculations, the effects of carbon distribution and of 25 at.% of nickel substitution on the tetragonality of ferrous martensite have been investigated. Different carbon concentrations have been considered (0≤x c ≤ 12.5 at.%). All calculations were based on the Projector Augmented Wave (PAW) method of the Density Functional Theory (DFT). The Special Quasirandom Structure (SQS) method has been used to model the disordered structures of the Fe 16 C 2X and (Fe,Ni) 16 C 2X systems. Our calculations demonstrate that the lattice parameters a and c vary linearly with carbon concentration, up to the high value of x c = 12.5 at.% (for X = 1). The nickel substitution enhances the expansion of parameter c of the Fe 16 C 2X system up to 37%. This trend is correlated with the shear moduli calculated for bcc-iron and the disordered Fe25 at.%Ni structure. The mixing energies of the Fe 16 C 2X and (Fe,Ni) 16 C 2X systems indicate that the unmixing of martensite by spinodal decomposition is favoured by the nickel substitution. It is shown that carbon-carbon interactions are mostly repulsive until the distance of two bcc-iron lattice parameters

Effect of interstitial carbon distribution and nickel substitution on the tetragonality of martensite: A first-principles study

By using first principles calculations, the effects of carbon distribution and of 25 at.% of nickel substitution on the tetragonality of ferrous martensite have been investigated. Different carbon concentrations have been considered (0≤x c ≤ 12.5 at.%). All calculations were based on the Projector Augmented Wave (PAW) method of the Density Functional Theory (DFT). The Special Quasirandom Structure (SQS) method has been used to model the disordered structures of the Fe 16 C 2X and (Fe,Ni) 16 C 2X systems. Our calculations demonstrate that the lattice parameters a and c vary linearly with carbon concentration, up to the high value of x c = 12.5 at.% (for X = 1). The nickel substitution enhances the expansion of parameter c of the Fe 16 C 2X system up to 37%. This trend is correlated with the shear moduli calculated for bcc-iron and the disordered Fe25 at.%Ni structure. The mixing energies of the Fe 16 C 2X and (Fe,Ni) 16 C 2X systems indicate that the unmixing of martensite by spinodal decomposition is favoured by the nickel substitution. It is shown that carbon-carbon interactions are mostly repulsive until the distance of two bcc-iron lattice parameters.Etude de la décomposition spinodale dans les aciers martensitiques Fe-X-