Austenite Ferrite Phase Transformation Research Articles

Comparisons of {100} texture improvement and formability in hot-rolled non-oriented electrical steel by austenite–ferrite phase transformation and shear deformation

Comparisons of {100} texture improvement and formability in hot-rolled non-oriented electrical steel by austenite–ferrite phase transformation and shear deformation

Synchrotron EDXRD strain-temperature measurement during laser welding

Synchrotron EDXRD strain-temperature measurement during laser welding

Evaluation of Austenite-Ferrite Phase Transformation in Carbon Steel Using Bayesian Optimized Cellular Automaton Simulation.

Austenite-ferrite phase transformation is a crucial metallurgical tool to tailor the properties of steels required for particular applications. Extensive simulation and modeling studies have been conducted to evaluate the phase transformation behaviors; however, some fundamental physical parameters still need to be optimized for better understanding. In this study, the austenite-ferrite phase transformation was evaluated in carbon steels with three carbon concentrations during isothermal annealing at various temperatures using a developed cellular automaton simulation model combined with Bayesian optimization. The simulation results show that the incubation period for nucleation is an essential factor that needs to be considered during austenite-ferrite phase transformation simulation. The incubation period constant is mainly affected by carbon concentration and the optimized values have been obtained as 10-24, 10-19, and 10-21 corresponding to carbon concentrations of 0.2 wt%, 0.35 wt%, and 0.5 wt%, respectively. The average ferrite grain size after phase transformation completion could decrease with the decreasing initial austenite grain size. Some other parameters were also analyzed in detail. The developed cellular automaton simulation model combined with Bayesian optimization in this study could conduct an in-depth exploration of critical and optimal parameters and provide deeper insights into understanding the fundamental physical characteristics during austenite-ferrite phase transformation.

Thermomechanical and Microstructural Analysis of the Influence of B- and Ti-Content on the Hot Ductility Behavior of Microalloyed Steels

The effects of the combined addition of B and Ti, as well as the influence of different strain rates on the hot ductility behavior of low carbon, continuously cast, microalloyed steels were investigated in this work. Tensile tests, microstructure analyses, and thermokinetic simulations were performed with in situ melted samples. Furthermore, prior austenite grain evaluations were carried out for the two different microalloyed steels. Increasing the strain rate brought improvements to the ductility, which was more significant in the steel with the leanest composition. The steel containing more B and Ti presented a better hot ductility behavior under all conditions tested. The main causes for the improvements rely on the precipitation behavior and the austenite–ferrite phase transformation. The preferential formation of TiN instead of fine BN and AlN was seen to be beneficial to the ductility, as well as the absence of MnS. Grain boundary segregation of free B that did not form BN retarded the ferrite formation, avoiding the brittleness brought by the thin ferrite films at the austenite grain boundaries. Furthermore, it was revealed that for the steels in question, the prior austenite grains have less influence on the hot ductility behavior than the precipitates and ferrite formation.

Effects of Rare Earth on Austenite–Ferrite Phase Transformation in a Low-Carbon Fe–C Alloy

Effects of Rare Earth on Austenite–Ferrite Phase Transformation in a Low-Carbon Fe–C Alloy

Interaction of various factors in controlled cooling process and its influence on stress and strain of steel plate

ABSTRACT In this paper, the theoretical models used to describe the phase transformation effects are clarified through theoretical research, and the functions of ABAQUS software to analyze phase transformation effects are expanded by developing USDFLD, HETVAL, UEXPAN and TRIP subroutines. The measured value of the DIL805A dilatometer is in good agreement with the calculated result, which proves the feasibility of the model. Through the specific study of low carbon bainitic steel Q690D controlled cooling by austenite–ferrite phase transformation process of latent heat, transformation dilation and transformation induced plasticity of the interaction and the effect of the controlled cooling process on steel plate, so as to more accurately get the value of steel plate stress and strain under the effect of phase transformation, provide a theoretical basis for the production guidance.

A novel 3D mixed-mode multigrain model with efficient implementation of solute drag applied to austenite-ferrite phase transformations in Fe-C-Mn alloys

A computational 3D model that accounts for both nucleation and interface migration is a very useful tool to monitor and grasp the complexity of microstructure formation in low-alloyed steels. In the present study we have developed a 3D mixed-mode multigrain model for the austenite-ferrite and the austenite-ferrite-austenite formation capable of following diffusional phase transformations under arbitrary thermal routes. This new model incorporates the solute drag effect of a substitutional element (in this case Mn) and ensures an automatic change in transformation direction when changing from heating to cooling and vice-versa. An analytical solution for calculating the energy dissipation of solute drag together with multiple regression approximations for chemical potentials are proposed which significantly accelerate the computation. The modelling results are first benchmarked for an Fe-0.1C-0.5Mn (wt.%) alloy under different continuous cooling and isothermal holding conditions. The model revealed relatively large variations in transformation kinetics of individual grains as a result of interactions with neighboring grains. Then the model is applied to predict the transformation kinetics of a series of Fe-C-Mn alloys during cyclic partial phase transformations. The comparison with experimental dilatometer results nicely validates the predictions of this model regarding the change in overall transformation kinetics of the ferrite transformation as a function of the Mn content. New features of this model are its efficient algorithm to compute energy dissipation by solute drag, its capabilities of predicting the microstructural state for spatially resolved grains and the minimal fine tuning of modelling parameters. The code to implement this model is publicly available.

In Situ High-Temperature EBSD and 3D Phase Field Studies of the Austenite-Ferrite Transformation in a Medium Mn Steel.

In this research, in situ high-temperature electron backscattered diffraction (EBSD) mapping is applied to record and analyze the migration of the α/γ interfaces during cyclic austenite-ferrite phase transformations in a medium manganese steel. The experimental study is supplemented with related 3D phase field (PF) simulations to better understand the 2D EBSD observations in the context of the 3D transformation events taking place below the surface. The in situ EBSD observations and PF simulations show an overall transformation behavior qualitatively similar to that measured in dilatometry. The behavior and kinetics of individual austenite-ferrite interfaces during the transformation is found to have a wide scatter around the average interface behavior deduced on the basis of the dilatometric measurements. The trajectories of selected characteristic interfaces are analyzed in detail and yield insight into the effect of local conditions in the vicinity of interfaces on their motion, as well as the misguiding effects of 2D observations of processes taking place in 3D.

Modelling study on the three-dimensional neutron depolarisation response of the evolving ferrite particle size distribution during the austenite–ferrite phase transformation in steels

The magnetic configuration of a ferromagnetic system with mono-disperse and poly-disperse distribution of magnetic particles with inter-particle interactions has been computed. The analysis is general in nature and applies to all systems containing magnetically interacting particles in a non-magnetic matrix, but has been applied to steel microstructures, consisting of a paramagnetic austenite phase and a ferromagnetic ferrite phase, as formed during the austenite-to-ferrite phase transformation in low-alloyed steels. The characteristics of the computational microstructures are linked to the correlation function and determinant of depolarisation matrix, which can be experimentally obtained in three-dimensional neutron depolarisation (3DND). By tuning the parameters in the model used to generate the microstructure, we studied the effect of the (magnetic) particle size distribution on the 3DND parameters. It is found that the magnetic particle size derived from 3DND data matches the microstructural grain size over a wide range of volume fractions and grain size distributions. A relationship between the correlation function and the relative width of the particle size distribution was proposed to accurately account for the width of the size distribution. This evaluation shows that 3DND experiments can provide unique in situ information on the austenite-to-ferrite phase transformation in steels.

Effect of C and N and their absence on the kinetics of austenite-ferrite phase transformations in Fe-0.5Mn alloy

Investigating the partitioning effect of substitutional and interstitial elements on the migration of transformation interfaces during austenite-ferrite phase transformation in steels with the conventional experimental method is extremely challenging due to interaction between the solute atoms and the transformation interfaces. Additionally, the simultaneous nucleation of new phases during phase transformations limits the accuracy of extracted growth rates from experimental kinetics measurements of phase fractions. In a novel experimental approach, the cyclic partial phase transformation concept is used to avoid the effect of nucleation on total kinetics of phase transformations. In this study, a Fe-0.5Mn alloy in the presence and absence of interstitial C and N additions is subjected to different cyclic transformation routes to examine the possible interaction between solute atoms and migrating interfaces. The experimental results are in semi-quantitative agreement with modelling predictions made by the local equilibrium approach and provide indirect evidence of Mn partitioning at austenite/ferrite interface in absence of any interstitial elements. It is also confirmed that the presence of interstitial elements promotes Mn interaction with the interface, whereas N promotes more Mn partitioning at transformation interface compared to C.

Effects of hot-deformation on grain boundary precipitation and segregation in Ti-Mo microalloyed steels

Hot-deformation can refine grain size and change the dynamics of austenite-ferrite phase transformation during thermo-mechanical processing of microalloyed steels. Here, atom probe tomography has been used to characterize nanoscale precipitates and segregation in Ti-Mo microalloyed steels processed with and without hot-deformation at 890°C. It provides a comprehensive understanding of solute redistribution in both the grain interior and grain boundary regions. The results show that coarse (Ti, Mo)C precipitates are formed at grain boundaries, whereas fine precipitates are densely distributed in grain interiors, regardless of deformation conditions. Precipitate-free zones are developed near grain boundaries in the undeformed Ti-Mo steel, but absent in the hot-deformed steel. The elimination of precipitate-free zone in the hot-deformed steel is attributed to the high dislocation density and accelerated γ→α transformation caused by hot-deformation. The majority of Ti, Mo, and C atoms partition into (Ti, Mo)C precipitates, but Mn, Si, and Al atoms are mainly in solute state in ferrite matrix and segregation at grain boundaries. The hot-deformation significantly changes the C segregation at grain boundaries, but has little effect on other solute elements.

Isothermal Austenite–Ferrite Phase Transformations and Microstructural Evolution during Annealing in Super Duplex Stainless Steels

Super Duplex Stainless Steels (SDSSs) are composed of α-ferrite and γ-austenite grains, the simultaneous presence of which forms an optimal microstructure to achieve the best combination of mechanical and corrosion resistance properties. Moreover, international quality standards are strict about the phase fraction ratio. The purpose of this work is the achievement of a better description of the phase ratio evolution taking place during annealing at 1080 °C in the super duplex stainless steels F53–S32750 and F55–S32760. The experimental results show a damped sinusoidal trend in the α/γ phase ratio evolution with the increase of the soaking time of thermal treatment. This can be described by coupling both the competitive coarsening growth regime and the concept of the local equilibrium phase transformations, pointing out a good correspondence with the experimental data. Further, recrystallization phenomena also play a major role. Finally, the additivity character of the observed processes has been proven.

Simulation Study of Heterogeneous Nucleation at Grain Boundaries During the Austenite-Ferrite Phase Transformation: Comparing the Classical Model with the Multi-Phase Field Nudged Elastic Band Method

In this work, molecular dynamics (MD) simulations have been used to study the heterogeneous nucleation occurring at grain boundaries (GBs) during the austenite (FCC) phase to ferrite (BCC) phase transformation in a pure Fe polycrystalline system. The critical nucleus properties (including size, shape, and activation energy) determined by classical nucleation theory are compared with those obtained by using a combination of the multi-phase field method (MPFM) and the nudged elastic band (NEB) method. For nucleation events that exhibit low-energy facets completely embedded within the parent FCC phase, there is a good agreement between the MD and the MPFM result with respect to the critical nucleus size, shape, and nucleation energy barrier. For systems where the emerging nucleus contains facets that cross the GB plane, the MPFM-NEB, when compared to MD, yields a better prediction than the classical approach for the nucleus morphology. New observations from the MPFM-NEB method indicate that the critical nucleus shape may change with volume and therefore depends on the nucleation driving force (undercooling).

Evolution and Interaction of the Microstructure and Texture at the Different Processing Steps for Ferritic Nonoriented Electrical Steels

The fabrication process of nonoriented electrical steels comprises casting, hot rolling, cold rolling, and final annealing. The development of new technologies for the fabrication of hot band offers new possibilities. In this paper, we will shortly describe the similarities and differences with respect to the evolution of microstructure (grain structure) and texture along the conventional processing route and thin strip casting. We will point out the most relevant features at the different processing steps, which are important for the optimum texture and microstructure of the finally processed material. Thereby, we will regard ferritic FeSi steels, where no homogenization of the microstructure appears due to the austenite–ferrite phase transformation.

Sensitivity analysis and identification of the phase transformation model based on the control theory

The aim of this work was to improve the previously developed model of austenite-ferrite phase transformation by its identification for selected steels and by performing sensitivity analysis. Created model allows prediction of phase transformation kinetics for non-isothermal conditions. Model is characterized by very short computing time and relatively good predictive capabilities. There are five input coefficients in the model, which should be identified for each steel on the basis of dilatometric tests. In the previous works model was used to predict phase transformation kinetics in various DP steels for different thermal cycles. In the first part of this work sensitivity analysis of the model was performed using three methods: quality method, factorial design method and Morris analysis method. Obtained sensitivity coefficients described how changes of the model input parameters influence the response of the model and which of these parameters are the most significant. The second part of the work was devoted to model identification for the selected steels. Identification problem was turned into optimization task which was solved using Hooke-Jeeves method. Obtained model’s parameters allowed describing austenite-ferrite phase transformation in the conditions of varying temperatures. Validation of the model was performed by comparison with the results obtained from the advances numerical model based on the solution of the diffusion equation in the austenite. Results obtain from both models for typical thermal cycled used to obtain multiphase microstructure were compared.

Experimental Simulation of Thermally Induced Stresses during Cooling of Continuously Cast Steel Slabs

Continuous casting is an established production route for steel slabs. The solidifying slab suffers a combination of internal stresses caused by thermal gradients, phase transformation, and external bending. The complex superimposed stress condition may produce local concentrations of tensile stresses that initiate cracks during cooling and deformation. Samples prepared from low-carbon steel slabs with and without Ni-content are compared and transformation stress phenomena are analyzed. In situ neutron diffraction experiments are carried out to reveal the stress condition within sections of cast steel slabs during cooling while passing the austenite–ferrite transformation temperature. The austenite–ferrite phase transformation is accompanied by an increase of volume of ≈1% between austenite-f.c.c. and ferrite-b.c.c., which causes internal stresses within the thermal stress gradients by cooling from the surface. The superposition of these thermal and transformation stresses is determined during cooling by an additional in situ diffraction experiment at constant macroscopic strain. The crack sensitivity of the Ni-containing slab is attributed to higher internal stresses within the austenite caused by the lower transition temperature than for the Ni-free steel.

Study of the interaction of solutes with Σ5 (013) tilt grain boundaries in iron using density-functional theory

Substitutional alloying elements significantly affect the recrystallization and austenite-ferrite phase transformation rates in steels. The atomistic mechanisms of their interaction with the interfaces are still largely unexplored. Using density functional theory, we determine the segregation energies between commonly used alloying elements and the Σ5 (013) tilt grain boundary in bcc iron. We find a strong solute-grain boundary interaction for Nb, Mo, and Ti that is consistent with experimental observations of the effects of these alloying elements on delaying recrystallization and the austenite-to-ferrite transformation in low-carbon steels. In addition, we compute the solute-solute interactions as a function of solute pair distance in the grain boundary, which suggest co-segregation for these large solutes at intermediate distances in striking contrast to the bulk.

Mobility of the austenite–ferrite interface under various states of loading

The massive austenite–ferrite phase transformation was simulated on an atomic scale for various states of loading: uniaxial, planar and hydrostatic. The simulated system involved the lateral growth of a two-dimensional ferrite seed at the ferrite–austenite phase boundary for a variable number of vacancies at the interface. The atomistic simulation was based on the application of a recently developed multi-lattice kinetic Monte Carlo method. The effects of the different states of loading and of the vacancy concentration at the interface were discussed in terms of their impact on the necessary local rearrangement of austenite atoms to unblock unoccupied ferrite–lattice sites. It followed that the net outcome of the effects of additional free volume and of vacancy pinning strongly depends on the state of loading.

Application of the cyclic phase transformation concept for determining the effective austenite/ferrite interface mobility

A series of cyclic partial austenite–ferrite phase transformation computer simulation experiments have been performed to elucidate the rate controlling dissipative processes during austenite-to-ferrite and the ferrite-to-austenite transformation in lean C–Mn steels. The transformation kinetics is analyzed by comparing the results of two complementary sharp interface models – one is based on the assumption of local equilibrium at the migrating interface − in the other model diffusion in the interface and the interfacial reaction is implemented by an effective interface mobility but substitutional diffusion in the bulk phases is neglected.Values for effective interface mobilities have been obtained for both the austenite-to-ferrite transformation and vice versa. By means of effective mobilities which depend only on initial composition and temperature, the transformation kinetics has been studied for other heat treatments than used to determine the effective interfacial mobility values. Although substitutional diffusion in the bulk is not taken into account, for the low Mn alloys it is possible to obtain similar trends by the effective mobility model as provided by the local equilibrium model. At modest to high interface velocities long range diffusion of the substitutional alloying elements can be ignored but then the effects of local diffusion processes near the interface need to be taken into account via an effective interface mobility. The effective mobility for the austenite-to-ferrite transformation differs from the effective mobility during the ferrite-to-austenite transformation in a rather essential manner.

Simulation of the massive austenite–ferrite transformation under uniaxial loading

The massive austenite–ferrite phase transformation has been simulated on an atomic scale by means of a multi-lattice kinetic Monte Carlo method. The simulated system involved a ferrite–austenite bicrystal under various uniaxial loads and for a variable number of vacancies at the interface. The results show that the massive transformation from austenite to ferrite is controlled by the local rearrangement of austenite atoms initially blocking unoccupied ferrite lattices sites. The growth mode is strongly dependent on the orientation of the interface. The effects of loading and vacancy concentration at the interface are discussed in terms of their impact on the necessary local rearrangement of austenite atoms. It is shown that local, relaxed clusters of atoms surrounding a vacancy play an important role for the kinetics of the transformation.