Ferrite Transformation Kinetics Research Articles

Influence of Solute Drag Effect and Interphase Precipitation of Nb on Ferrite Transformation.

The significant impact of Nb on ferrite transformation, both in terms of solute drag effect (SDE) and interphase precipitation, was investigated quantitatively. Ferrite transformation kinetics were characterized using thermal expansion experiments and theoretical calculations. The microstructures were characterized using high-temperature confocal laser scanning microscopy (CLSM), a field-emission scanning electron microscope (FESEM), and a transmission electron microscope (TEM). Under a higher driving force, interphase precipitations were observed in the sample with a higher Nb content. A three-dimensional (3D) reconstruction method was used to convert the two-dimensional (2D) image of interphase precipitation into a three-dimensional model for a more typical view. The SDE and interphase precipitation had opposite effects on the kinetics of ferrite transformation. A lower Nb content showed a strong contribution to the SDE, which delayed ferrite transformation. A higher concentration of Nb was expected to enhance the SDE, but the inhibition effect was eliminated by the interphase precipitation of NbC during interfacial migration. Both the experimental results and theoretical calculations confirmed this phenomenon.

Unraveling the effects of Nb interface segregation on ferrite transformation kinetics in low carbon steels

Addition of small amount of Nb can strongly retard the ferrite growth kinetics. The origin has been attributed to the solute drag effect (SDE) by Nb segregation at the migrating ferrite/austenite interface. However, due to limitation of characterization techniques, the relations between elemental segregation, SDE and interface velocity have not been quantitatively clarified yet. Meanwhile, the strong affinity between Nb and C atoms can induce carbide precipitation at the interface, namely interphase precipitation, which may influence the Nb segregation behavior and make the issue even more complicated. Therefore, in this study, the interface information including amount of Nb segregation, energy dissipation, NbC precipitates and interface velocity in Fe-0.08C-(0.035, 0.061)Nb (mass%) model alloys is quantitatively investigated. It reveals that the energy dissipation at the migrating ferrite/austenite interface decreases with longer holding time or higher transformation temperature. The Nb atoms prefer to segregate at the non K-S interface rather than the near K-S interface. Amount of Nb segregation at the non K-S interface increases with longer time, while raising bulk Nb content or lowering transformation temperature does not lead to a notable increment of segregation. The relations between Nb segregation, energy dissipation, and interface velocity can be well reproduced by the SDE model with optimized parameters (i.e., segregation energy, interface thickness and trans-interface diffusivity). Occurrence of NbC interphase precipitation affects the transformation kinetics indirectly by weakening the SDE via consumption of Nb solutes in ferrite. In contrast, their pinning effect plays a marginal role.

Gibbs energy minimisation model for the austenite-ferrite phase transformation in Fe-C-X-Y alloys

ABSTRACT A new model has been developed to predict austenite ferrite transformation kinetics in steels. For each alloying element, the concentration profile is computed solving a unique diffusion equation (including the 2 phases and the interface). The interface is described assuming linear variation of chemical potentials, saving thus computational time. Interface motion is driven by the minimisation of Gibbs energy. The model naturally reproduces the transition between thermodynamic equilibria (Para equilibrium, Local equilibrium with negligible partitioning, Local equilibrium) during heating. The validity of the model for reverse transformation has been validated on ternary and quaternary systems Fe-C-(Mn-Si-Mo) on decarburisation experiments.

Bayesian inference of ferrite transformation kinetics from dilatometric measurement

A Bayesian approach is presented for clarifying the best kinetic model explaining the transformation kinetics of a low-carbon steel under different continuous cooling conditions only from dilatometric curves. To estimate kinetic parameters as well as the model plausibility of candidate kinetic models, the exchange Markov chain Monte Carlo method was used. The effectiveness of the proposed method was demonstrated by metallographic investigations of the ferrite formation in a Fe-0.15C-1.5Mn alloy. It is shown that the method is successfully applied for clarifying ferrite transformation kinetics, such as transformation start temperatures, formation mechanisms, and fractions of microstructures. In comparison with a previous experimental study, it is also presented that the important parameter determining the ferrite nucleation rate can be estimated only from dilatometric curves without the help of intensive metallographic observations.

Investigation on the Formation of Cr-Rich Precipitates at the Interphase Boundary in Type 430 Stainless Steel Based on Austenite–Ferrite Transformation Kinetics

The Cr-rich precipitates at the interphase boundary in stainless steels not only lead to the sensitization, which further induces the intergranular corrosion and intergranular stress corrosion cracking, but also significantly deteriorate the ductility and toughness. In this work, the formation of Cr-rich precipitates at the interphase boundary in type 430 stainless steel was investigated from the perspective of austenite–ferrite transformation kinetics. Cyclic heat treatment was firstly conducted to reveal the kinetic mode of transformation behavior, i.e., local equilibrium or para equilibrium. Subsequently, interrupted quenching during continuous cooling was carried out, which illustrated clearly the relevance of the formation of interphase Cr-rich precipitates to the Cr enrichment adjacent to the interphase boundary as revealed by line scanning of energy dispersive spectroscopy (EDS). Finally, this enrichment of Cr was interpreted by DICTRA simulation, which is based on the determined kinetic mode for austenite–ferrite transformation. This work has, for the first time, established the correlation between the formation of interphase Cr-rich precipitates and the austenite–ferrite transformation kinetics.

Precise Flow Stress Analysis for the Occurrence of Dynamic Ferritic Transformation and Dynamic Recrystallization of Austenite in Low Carbon Steel

There have been previous attempts to observe the occurrence of dynamic ferritic transformation at temperatures even above Ae3 in a low-carbon steel, and not only in steels, but recently also in titanium alloys. In this study, a new approach is proposed that involves treating true stress-true strain curves in uniaxial compression tests at various temperatures, and different strain rates in 0.1C-6Ni steel, which is a model alloy used to decelerate the kinetics of ferrite transformation from austenite. The initial flow stress up to peak stress was used to analyze the change in dynamic softening phenomena, such as dynamic recovery, dynamic recrystallization, and dynamic transformation. It is worth mentioning that for predicting the occurrence of dynamic transformation, flow stress before reaching peak stress is much more sensitive to the change in the dynamic softening rate due to dynamic transformation, compared to peak stress. It was found that the occurrence of dynamic ferritic transformation could be successfully obtained even at temperatures above Ae3 once the deformation condition was satisfied. This deformation condition is a function of both the strain rate and the deformation temperature, which can be described as the Zener - Hollomon parameter. In addition, the driving force of dynamic ferritic transformation might be much less than that of the dynamic recrystallization of austenite at a given deformation condition. By applying this technique, it is possible to predict the occurrence of dynamic transformation more sensitively compared with the previous analysis method using peak stress during deformation. Key words: low-carbon steel, hot deformation, dynamic recrystallization, dynamic transformation, flow stress analysis

The effect of molybdenum on interphase precipitation and microstructures in microalloyed steels containing titanium and vanadium

Despite much research into steels strengthened through interphase precitation, there remains much that is not clear, such as the role of a range of elements, particularly Mo, in the interphase precipitation process. Four steels were manufactured with identical composition, but with variations in Ti, V, Mo and N content to investigate the effect of composition on interphase precipitation. Alloys were rapidly cooled from the single austenite phase field and isothermally transformed at 630 °C and 650 °C for 90min. The addition of Mo was found to significantly reduce the austenite to ferrite transformation kinetics, particularly for the V steel. Interphase precipitation was observed in all alloys at both transformation temperatures. For the Ti bearing steel, the two types of precipitate were observed throughout the sample, namely TiC (finer) and Ti2C (coarser), while for the V bearing steels, VC (finer) and V4C3 (coarser) were observed. Where Mo was present in the alloy, it was found dissolved in all carbide types. The (Ti,Mo)C and (V,Mo)C formed by classical planer interphase precipitation (PIP) while the (Ti,Mo)2C and (V,Mo)4C3, that had a much wider row spacing, formed through curved interphase precipitation (CIP). Each adopted one variant of the Baker-Nutting orientation relationship. The Ti-microalloyed steels exhibited the smallest precipitates of all the steels, which were approximately the same size irrespective of whether Mo was present in the alloy and irrespective of the transformation temperature. However, the addition of Mo to the V bearing steels resulted in a significant increase in precipitate volume fraction and a reduction in precipitate size. The mechanisms of interphase precipitation leading to the coincident production of two different precipitate types is considered and the role of Mo on the interphase precipitation process is discussed. The resultant effect on strength is considered.

Hot rolled and cold rolled medium manganese steel: Mechanical properties and microstructure

The current work aims at obtaining optimum amount of austenite after processing of a medium manganese steel in order to invoke partial twinning induced plasticity (TWIP) mechanism during deformation and to maximise elongation. Steel chemistry was optimised using thermodynamic calculations and the same was processed in different ways which includes continuous cooling from single phase austenitic region, quenching + austenite reverted transformation (ART) annealing and austenite reverted transformation (ART) annealing. DICTRA and Thermo-Calc simulation was done to understand the austenite to ferrite transformation kinetics during continuous cooling and the mode of equilibrium that is operative. The study suggests that quenching + austenite reverted transformation (ART) annealing is the ideal approach for processing this material in order to get the desired microstructure and properties. Thermo-Calc and Koistinen and Marburger's (KM) equation was coupled to estimate the fraction of austenite to be retained after isothermal holding at different intercritical temperatures and the optimum temperature found was used for annealing this steel. Microstructural characterization along with tensile test was done to ascertain the effect of processing route, initial microstructure on the mechanical properties. It was found that the morphology of austenite was greatly affected by the initial microstructure of the steel. Mechanical properties obtained after cold rolling (CR) and austenite reverted transformation (ART) annealing was found to be superior to that of hot rolling (HR) and austenite reverted transformation (ART) annealing. The difference in mechanical properties was correlated with the austenite morphology and the final microstructure.

Austenite to polygonal-ferrite transformation and carbide precipitation in high strength low alloy steel

AbstractBy differential scanning calorimetry measurement and microstructural examination, the effects of cooling rates on the kinetics and microstructure of polygonal ferrite transformation have been investigated in a high strength low alloy steel. The peak temperature of ferrite transformation exhibits an evident dependency on cooling rate. With the cooling rate decreasing, the ferrite fraction corresponding to the maximum transformation rate is decreased, as well as the size of polygonal ferrite grains. More ferrite is nucleated at lower cooling rate. On the basis of the effects of austenite grain size and carbide precipitation on ferrite formation, it is suggested that carbide precipitation plays a more dominant role in the kinetics of ferrite transformation. During austenite–ferrite transformation, the ferrite nucleation still proceeds, and the nucleation would not be saturated by pre-existing nuclei.

Influence of Carbon Equivalent Content on Phase Transformation During Inter-critical Heating of Dual Phase Steels Using Discrete Micro-scale Cellular Automata Model

In this paper, a cellular automata method based model is proposed for simulating phase transformation kinetics of inter-critical heating of dual phase (DP) steel. This developed model deals with the kinetics of pearlite dissolution, ferrite transformation and austenite grain growth based on carbon diffusion process. Diffusion equation is discretized and solved by finite difference method whereas austenite grain growth is controlled by transition rules applied in cellular automata algorithm. The model is operated in the temperature range of 730–890 °C for four different specimens of DP steel. This model predicts appropriately the microstructure and volume fraction of formed austenite during inter-critical heating of DP steel. In addition, this study shows that the presence of carbon and alloying elements enhances carbon equivalent of DP steel, helps in austenite formation.

Effect of austenite grain size on kinetics of dynamic ferrite transformation in low carbon steel

The effect of austenite grain size on kinetics of dynamic ferrite transformation above Ae3 in a 6Ni–0.1C steel was studied. As the austenite grain size decreased, the onset of dynamic transformation was accelerated. The increase in the fraction of dynamically transformed ferrite was in good agreement with the change in flow stress, i.e. dynamic softening. The kinetics of dynamic transformation could be evaluated by an Avrami-type formula.

Fast algorithms for phase transformations in dual phase steels on a hot strip mill run-out table (ROT)

The hot rolling of dual phase steel, alloyed with 0.064% C, 0.05% Al, 0.48% Cr, 0.93% Mn, and 0.41% Si, was simulated by a deformation dilatometer. To model the ferrite transformation behaviour on the run-out table, the rate law approach proposed by Leblond and Devaux was employed. It models the ferrite transformation kinetics as a function of holding temperature and austenite conditioning, which is achieved by varying the austenite grain size and retained strain. Its transformation kinetics during cooling to the coiling temperature was investigated by varying the carbon content of the remaining austenite. The martensite transformation kinetics was also modelled by Leblond and Devaux's approach with a parameter coupled to the Koistinen–Marburger model. The modelling shows that this alloying concept allows a large process window for both temperature and austenite grain size. Finally, a selected set of processing parameters was chosen and transferred to a pilot rolling mill. The mechanical properties of the rolled sheets are satisfactory.

The Effect of High Temperature Grain Refinement on the Isothermal Ferrite Grain Growth Kinetics in Steel S460

Plate steel S460 is intercritically rolled during the final stages of industrial processing. A series of experiments to represent the preliminary stages of an intercritical simulation were completed and the isothermal austenite to ferrite transformation kinetics investigated. The growth of the ferrite grains was interpreted using the classic JMAK model and the effect of processing history also examined. A double austenite deformation at 1323 K, to a true strain of 0.2, led to the most acceptable starting microstructure for the extended simulation. The role of niobium in this preliminary simulation is also considered.

Ferrite Transformation Kinetics of Severely Hot-Deformed Austenite

The ferrite transformation kinetics of severely hot-deformed austenite has been studiedby considering ferrite nucleation from dislocation cell blocks inside austenite grains. The size ofdislocation cell blocks and ferrite grain size just after phase transformation are acknowledged to beinversely proportional to the square root of dislocation density. It is found that the ferrite nucleationrate in this area can reach the saturated state at a high temperature just under Ae3, and the ferritetransformation finishes within a very short time. The kinetics of ferrite volume fraction and theferrite grain growth after phase transformation for plain carbon (0.1%C, 0.2%Si, 1.0%Mn) steelhave been studied using a THERMECMASTER hot-compression testing machine. These modelscan be applied to the hot and warm forming processes of plain carbon steel to predict the ferritetransformation from severely deformed austenite.

A JMAK-like approach for isochronal austenite–ferrite transformation kinetics in Fe–0·055 wt-%N alloy

The kinetics of the isochronal austenite γ to ferrite α transformation of Fe–0·055 wt-%N alloy was investigated for cooling rates in the range of 5–15 K min− by high resolution dilatometry. In accordance with thermodynamic characteristics of γ→α transformation investigated in this study and previous kinetic theory, a Johnson–Mehl–Avrami–Kologoromov (JMAK)-like approach for the kinetics of isochronal phase transformations was developed that incorporates three overlapping processes: site saturation nucleation, alternate growth modes (transition from interface- to diffusion-controlled growth) as well as impingement for random distribution nuclei. The JMAK-like approach has been employed to fit the experimental results, and the fitting results show that the γ→α transformation of Fe–0·055 wt-%N alloy does have two stages: a first, short interface-controlled growth stage and a second, long diffusion-controlled growth stage. In addition, soft impingement effect has been recognised to become serious in the later part of the second stage.

Influence of austenite deformation on ferrite growth in a Fe–C–Mn alloy

The kinetics of ferrite transformation in a Fe–0.2C–2Mn (mass%) alloy after deformation in undercooled austenite region were studied with regard to alloying element-partitioned and partitionless growth. According to the local equilibrium theory of diffusion-controlled growth, partitioned growth controlled by slow Mn diffusion occurs at a low undercooling, whereas partitionless growth at a larger undercooling is the much faster process, which is controlled by carbon diffusion. The alloy with or without deformation was isothermally reacted at temperatures that encompass the two growth modes. The ferrite growth rate at each temperature increases by austenite deformation. The acceleration at higher temperatures is more significant, probably due to the larger contribution of stored deformation energy in the total driving force for ferrite transformation. The transition temperature from partitionless to partitioned growth of ferrite as well as Ae 3 temperature increases after austenite deformation, resulting in acceleration of ferrite growth rates between the previous and new transition temperatures by 1–2 orders of magnitude.

Investigation of the Effect of Deformation on γ‐α Phase Transformation Kinetics in Hot‐Rolled Dual Phase Steel by Phase Field Approach

Dual phase steels, consisting of hard martensite particles in a ductile ferritic matrix, offer high strength and deformability at the same time. Additionally, they are cost effective by a dilute alloying concept. In industrial production, two manufacturing concepts have been implemented: intercritical annealing of cold rolled sheet, or hot rolling. The current work has investigated the effect of deformation on the γ-α phase transformation kinetics in the dual phase steel production using the hot rolling scheme. The pancaked austenite grains containing denser nucleation sites have a strong influence on the ferrite transformation kinetics. In addition, the multiplication of dislocations which results in the increase in elastic strain energy and dislocation core energy contributes to some acceleration in ferrite growth kinetics. A modelling approach for the γ-α phase transformation kinetics in dual phase steels has been developed employing the phase field theory. The nucleation behaviour, i.e. the number and size of nuclei developed after an elapsed time as well as their nucleation sites which were evaluated from microstructure analysis, and the increase in the driving force of grain growth were integrated into this model.

Influence of magnetic field on ferrite transformation in a Fe–C–Mn alloy

The kinetics of ferrite transformation in a Fe–0.10mass%C–2.94mass%Mn alloy in a strong magnetic field of 8 T were studied with regard to alloying element-partitioned and partitionless growth. According to the theory of diffusion-controlled growth, the slow Mn diffusion dictates partitioned growth that occurs at a low undercooling, whereas partitionless growth at a larger undercooling is rate-controlled by fast carbon diffusion. The alloy was austenitized and isothermally reacted at temperatures that encompass the two growth modes. The nucleation and growth rates of ferrite increased at all temperatures in the magnetic field, whereas the amount of increase was somewhat greater at lower temperatures. In the region of slow growth, besides its sluggish diffusion Mn possibly destabilizes the ferrite phase due to the influence on the magnetic moment and the Curie temperature of bcc Fe solid solution, and partially offsets the accelerating effect of transformation. The temperature of transition from the slow to the fast growth is predicted to increase, due to the shift in the ferrite/austenite phase boundaries in the presence of magnetic field.

Influence of V Precipitates on Acicular Ferrite Transformation Part 2: Transformation Kinetics

A combination of thermodynamic models as well as some physical metallurgical principles has been used to analyse the influence that V(C, N) precipitates have on the transformation kinetics of acicular ferrite by isothermal decomposition of austenite. Those precipitates were found absolutely necessary for the nucleation of acicular ferrite as reported in Part 1 of this work, now in Part 2, through the proper design of a heat treatment route we have studied the effect that those precipitates have on the kinetics of acicular ferrite transformation.

A calculation of partitional local equilibrium/negligible partitional local equilibrium boundaries in Fe–C–X systems

Local equilibrium theory has long been employed to explain the thermodynamics and kinetics of proeutectoid ferrite transformation in Fe–C–X systems. The growth kinetics of proeutectoid ferrite is decided by the diffusion behavior of the alloy element X during transformation. One of the key issues in application of this theory is the theoretical calculation and experiment determination of the partitional local equilibrium/negligible partitional local equilibrium boundary (PLE/NPLE transition temperature). In this study, we developed a simple analytic method to calculate the PLE/NPLE boundary. With this method, the PLE/NPLE boundaries in Fe–C–Mn and Fe–C–Ni systems are successfully computed and the results agree well with the formerly reported experimental results.