Austenite Growth Kinetics Research Articles

Investigation of effects of aluminum additions on microstructural evolution and mechanical properties of ultra-high strength and vanadium bearing dual-phase steels

The effects of aluminum (Al) additions (0.04, 0.4, and 0.8 wt%) on the microstructural evolution, tensile properties, and sheared-edge ductility of dual phase (DP) steels were investigated. The DP steels were processed with distinct coiling temperatures after finish rolling, cold rolled, and subjected to two continuous galvanizing line (CGL) simulations: standard galvanizing (GI) and supercooling process (SC). Al expanded both the ɑ-ferrite and (ɑ+γ) phase fields at the expense of the γ-austenite region. Continuous cooling transformation (CCT) diagrams showed that Al increased ferrite start temperatures, broadened the ferrite transformation zone, and reduced austenite stability. Austenite growth kinetics during intercritical annealing, quantified using a modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, revealed Avrami exponents ranging from 0.72 to 0.81 and apparent activation energies increasing from 422.9 to 496.7 kJ mol−1 with Al additions. Microstructural characterization revealed larger ferrite grain sizes and higher ferrite volume fractions with higher Al levels. Ultimate tensile strength (UTS) was influenced by Al contents, coiling temperatures, and CGL simulation methods. Global ductility (total elongation, TE) was highest for the 0.8Al Steel with a high coiling temperature and GI annealing, while local ductility (hole expansion ratio, HER) was maximized for the 0.04 Al Steel with a low coiling temperature and SC annealing. The majority of the CGL simulated DP steels exhibited good strength-ductility combinations, with the 0.8Al Steels demonstrating superior properties compared to current automotive advanced high-strength steel (AHSS) grades.

Effect of Austenite Reversion and Its Stability on the Tensile and Impact Transition Behavior of High‐Strength Naval Steel

The present investigation correlates the austenite reversion and its stability during intercritical annealing (IA) with the tensile and Charpy impact properties of a low‐C naval grade steel. The as‐received steel is subjected to water quenching after austenitization at 950 °C, followed by IA at 620, 650, and 680 °C for 2 h. DICTRA simulation is used to predict the austenite reversion kinetics during the IA. Incorporation of cementite phase in the simulation correlates well with the experimental dilatometry results on the austenite growth kinetics. DICTRA simulation predicts a continuous rise in the reverted austenite fraction with increasing IA temperature. However, the percentage of reverted austenite, that gets retained after the subsequent quenching, decreases with increasing annealing temperature due to the reduction of austenite stability at elevated IA temperature. In other words, the amount of fresh untempered martensite increases rapidly with increasing IA temperature. Consequently, the sample annealed at an intermediate temperature of 650 °C (IA650) shows the best combination of strength, elongation, and low‐temperature impact properties as compared to the IA620 and IA680 steels due to an optimum balance of mechanically stable austenite and relatively lower untempered martensite fractions.

Numerical Investigations of Phase Transformations Controlled by Interface Thermodynamic Conditions during Intercritical Annealing of Steels

Austenite formation was numerically investigated using Thermo-Calc/DICTRA in a deformed ferrite/pearlite microstructure to produce dual-phase steels. This work aims to better understand how the interface conditions (local equilibrium with negligible partitioning—LENP—or local equilibrium with partitioning—LEP) control the austenite growth kinetics during the intercritical annealing. Inspired by our experimental observations, two nucleation sites were considered. The austenite formed from pearlite islands showed a regime transition from LENP to LEP when the holding stage started. For the growth of austenite from isolated carbides, three stages were identified during the heating stage: first, slow growth under LEP; then, fast growth under LENP; and finally, after dissolution of the carbide, slow growth again. LENP and LEP interface conditions may coexist thanks to these regime transitions. In the case of competition, LEP conditions hinder austenite growth while it is promoted by LENP interface conditions. Such differences in growth kinetics explain, in part, the morphogenesis of dual-phase microstructures.

Details of pearlite to austenite transformation in steel: Experiments and phase-field modeling

The austenitization of an initial pearlitic microstructure is simulated using the phase field model to achieve insight into White Etching Layer (WEL) formation in pearlitic railway steels. The simulations take into account the resolution of the cementite lamellae within a pearlite colony as well as the presence of pro-eutectoid ferrite. The austenite growth kinetics and morphology obtained via simulations are compared with dilatometry and microscopy observations. The influence of γ/θ and γ/α mobilities on the austenite growth morphology are studied. The simulations reproduce the microstructural features as well as the experimentally observed kinetics behavior of austenite formation, involving the correlation between mobilities and nucleation behavior.

Near‐Atomic Scale Characterization of Mn‐Gradient in Reverted Austenite in Hot‐Rolled Medium‐Mn Steels

Herein, the Fe–5Mn–2Al–0.1C (wt%) and micro‐alloyed Fe–5Mn–2Al–0.1C–0.1Nb–0.2V (wt%) medium Mn steels are processed by a novel flash‐austenite‐reverse‐transformation (ART) annealing. The core–shell Mn‐gradient in austenite is observed and compositionally characterized at near‐atomic scale by using atom probe tomography. Compared with the conventional ART annealing, flash‐ART process doubled the austenite fraction in both materials, based on electron backscattering diffraction measurement. The rapid austenite reversion in flash‐ART annealed MMnS is attributed to the negligible partitioning local equilibrium controlled austenite kinetics at the flash temperature. The formation of core–shell Mn‐gradient is due to the different austenite growth kinetics and interface mobilities. Microalloying elements are found to form nano‐carbides, which leads to decelerating austenite reversion because of depleted carbon content in the initial martensitic microstructure, and retarding austenite growth according to pinning effect.

Kinetics of austenite growth and bainite transformation during reheating and cooling treatments of high strength microalloyed steel produced by sub-rapid solidification

Kinetics of austenite growth and bainite transformation during reheating and cooling treatments of high strength microalloyed steel produced by sub-rapid solidification

Effect of Thermal Deformation Regimes on the Austenite Growth Kinetics and Recrystallization of Medium Carbon Low-Alloyed Steel for Large-Sized Steam Turbine Rotors

Effect of thermal deformation regimes on the austenite grain growth is studied in thearticle. The austenite grain growth curve based on the experimental data is constructed.Conclusion about temperature, deformation degree and rate effect is made.

Austenite growth and stability in medium Mn, medium Al Fe-C-Mn-Al steels

Fe-C-Mn-Al alloys are promising materials to respond to the new European regulations on car CO2 emissions. The austenite growth kinetics for a mid-Mn, mid-Al alloy have been investigated by DICTRA calculations. Our results establish that Al only affects the thermodynamic properties of the system, its diffusivity does not influence the kinetics. Calculations for several heating temperatures have shown that the austenite fraction is a direct function of the final martensite fraction, for austenite fraction above 40%.

Mechanism of Austenite Formation from Spheroidized Microstructure in an Intermediate Fe-0.1C-3.5Mn Steel

The austenitization from a spheroidized microstructure during intercritical annealing was studied in a Fe-0.1C-3.5Mn alloy. The austenite grains preferentially nucleate and grow from intergranular cementite. The nucleation at intragranular cementite is significantly retarded or even suppressed. The DICTRA software, assuming local equilibrium conditions, was used to simulate the austenite growth kinetics at various temperatures and for analyzing the austenite growth mechanism. The results indicate that both the mode and the kinetics of austenite growth strongly depend on cementite composition. With sufficiently high cementite Mn content, the austenite growth is essentially composed of two stages, involving the partitioning growth controlled by Mn diffusion inside ferrite, followed by a stage controlled by Mn diffusion within austenite for final equilibration. The partitioning growth results in a homogeneous distribution of carbon within austenite, which is supported by NanoSIMS carbon mapping.

Thermodynamics and kinetics of solution nitriding

In order to clarify the thermodynamics and kinetics of solution nitriding, the non-uniform nitrogen distribution formed by solution nitriding was investigated in austenitic (18Cr–10Ni) and ferritic (25Cr) stainless steels, and then it was simulated with DICTRA. The main issue for austenitic stainless steel is the surface reaction determined by the mass transfer from gas to solid, while that for ferritic stainless steel is the ferrite to austenite transformation and the austenite growth kinetics induced by nitrogen absorption. DICTRA simulations gave close agreement with the experimental results and demonstrated that solution nitriding is obviously controlled by mass transfer of nitrogen from gas to solid as well as diffusion in the solid, and austenite growth is controlled by nitrogen diffusion in austenite. In addition, the analysis suggests that nitrogen should have a stronger interaction with chromium than that treated in the thermodynamic database and austenite nucleation induced by nitrogen absorption takes place without any change in Cr composition.

Austenite Grain Refinement and Isothermal Growth Behavior in a Low Carbon Vanadium Microalloyed Steel

The austenite grain refinement through control of the grain growth during reheating process after thermo-mechanical controlled process (TMCP) in a vanadium microalloyed steel was achieved. The formation of ultra-fine grained austenite was attributed to the high density of austenite nucleation at the ferrite/martensite structure and to the inhibition of austenite growth by (Ti,V)C particles at the relatively low reheating temperature. Corresponding with the precipitation behavior of (Ti,V)C with temperature, the growth behavior of austenite in the vanadium microalloyed steel could be divided into two regions. At lower reheating temperature, austenite grains grew slowly, and ultra-fine grained austenite smaller than 5 μm was successfully obtained. By contrast, the austenite grains grew rapidly at high temperature due to the dissolution of (Ti,V)C particles. According to the measured and predicted results of austenite growth kinetics, two models were developed to describe the growth behavior of austenite grains in two different temperature regions, and the apparent activation energy Qapp for grain growth was estimated to be about 115 and 195 kJ/mol, respectively.

Analysis for Kinetics of Austenite Growth due to Isothermal Carburization of Ferrite

A diffusion model was used to analyze mathematically the kinetics for isothermal carburization of pure iron at annealing temperatures between 101 1 K and 1185 K. Under such carburization conditions, the austenitic γ phase is produced as a layer on the surface of the ferritic α phase. In the analysis, the diffusion coefficient of carbon in each phase is considered independent of the composition. According to the model, the square of the thickness of the γ phase is proportional to the annealing time. Such a relationship is called the parabolic relationship. The parabolic coefficient is a monotonically increasing function of the annealing temperature and the concentration of carbon on the surface of the γ phase. However, the dependence of the parabolic coefficient on the carbon concentration is less remarkable at higher annealing temperatures than at lower annealing temperatures. The parabolic coefficient varies depending on the annealing temperature in a complicated manner, even if the activity of carbon in the carburization atmosphere remains constant. Thus, the temperature dependence of the parabolic coefficient cannot be described by an Arrhenius equation in the whole annealing temperature range.

Growth of Individual Austenite Grains Measured with 3DXRD Microscopy

Studying austenitisation in steel, so far, was either limited to observations at the surface of the material or to the determination of the average grain growth behavior in the bulk. The development of the three-dimensional X-ray diffraction (3DXRD) microscope at beam line ID11 of the European Synchrotron Radiation Facility in Grenoble, France, made it possible to study the transformation kinetics in-situ and at the level of individual grains in the bulk of the material. Unique in-situ observations of austenite growth kinetics during continuous heating experiments were made for two commercial low-alloy steels (C22 and C35). The observed growth behavior of individual austenite grains gives a valuable contribution to understanding the phase transformations on heating, i.e. austenite formation from ferrite and pearlite.

Crystallography and structural evolution during reverse transformation in an Fe–17Cr–0.5C tempered martensite

The mechanism and the crystallography of austenite and δ-ferrite formation from tempered martensite at temperatures of 900–1200°C have been studied by means of transmission electron microscopy in an Fe–17Cr–0.55C alloy. It was found that austenite nucleates within ferrite at low angle, high angle and twin-related lath boundaries as well as at high angle equiaxed grain boundaries in contact with M 23C 6 grain/lath boundary carbides. The austenite grains are in a cube–cube relationship with the M 23C 6 carbide particles and bear the Kurdjumov–Sachs orientation relationship with at least one of the adjacent ferrite grains. They are often in the Kurdjumov–Sachs relationship with both ferrite laths separated by a high angle boundary as far as the laths had formed from the same austenite. The {111} A close packed plane of γ precipitate is parallel to the {110} F plane most parallel to the grain boundary. The close packed planes of some austenite grains nucleating at the high angle lath boundaries are parallel to the close packed planes of both ferrite laths. These crystallographic features often result in a single variant of austenite orientation at a grain boundary. After nucleation, the austenite grains grow by the migration of both semicoherent and incoherent interfaces. These results demonstrate that a specific orientation relationship is preferred for the austenite nucleation, but is not necessary for the subsequent growth. The kinetics of austenite growth are controlled by chromium diffusion. The δ-ferrite particles precipitate at high temperatures as a non-equilibrium phase. No rational orientation relationship between δ-ferrite and retained austenite was found. The experimental results are discussed qualitatively in terms of the thermodynamic predictions using the software ThermoCalc, assuming local equilibrium at the moving interfaces.

Pearlite to austenite transformation in an Fe–2.6Cr–1C alloy

The mechanism of austenite formation, the kinetics of cementite lamellae dissolution and the crystallography of the austenitization from pearlite have been studied in an Fe–2.6 wt% Cr–0.96 wt% C alloy. Austenite grains nucleate after rather long incubation time both at pearlite colony boundaries and at the ferrite/cementite interfaces within a pearlite. Characteristic morphologies of transformation products were observed at various stages of transformation. Particular attention was paid to the structural evolution close to the α/ γ interfaces. No indications of the diffusion ahead of the α/ γ interface were found. The kinetics of austenite growth is controlled initially by carbon diffusion but in later stages by chromium diffusion. The results are discussed by assuming local equilibrium at the moving interfaces with the software and database ThermoCalc. The method involves the driving force determination for the diffusion of carbon and substitutional element during austenitization. The orientation relationships between ferrite, martensite and cementite were also determined. The possible austenite orientations were evaluated assuming the α/ θ, α/ γ and γ/ θ orientation relationships so far obtained.