Austenite-ferrite Transformation Research Articles

Phase‐Field Simulation of Carbon Diffusion during Banded Structure Formation in Low‐Carbon Steel

During the cooling process after rolling, low‐carbon steel frequently develops banded structures, primarily due to the segregation of alloying elements and the uneven diffusion of carbon under a slow cooling rate during phase transformation. The current work integrates experiment and simulation to discuss the influence of various cooling rates on ferrite and pearlite bands in Cr–Mn–Ti low‐carbon steel. Meanwhile, a multicomponent multiphase‐field model is established to quantitatively analyze carbon diffusion behavior during the evolution of austenite–ferrite. The results reveal that low cooling rates and the segregation of Mn, Si, and Cr elements can promote the formation of banded structure. Furthermore, in the solute‐poor regions, slow cooling rates allow sufficient time for carbon diffusion during austenite–ferrite transformation, which leads to the extension in both carbon diffusion distance and interface migration distance, thereby generating severe ferrite bands. Additionally, in the solute‐rich regions, as the cooling rate decreases, C concentrations at Ar3 temperature and Ar1 temperature increase, inhibiting the nucleation and growth of ferrite, thereby resulting in strong pearlite bands. This study further elucidates the formation mechanisms of ferrite and pearlite bands, enhancing the understanding of carbon diffusion behavior during banded structure formation.

Modeling solute drag during austenite–ferrite transformation with ab initio binding energies

The solute drag effect can have a considerable impact on the austenite to ferrite transformation. Most models describing the solute drag effect require knowledge of solute binding energies, which due to lack of accurate data typically are treated as fit parameters. In this study we use density functional theory (DFT) calculations to estimate the effective binding energies of the substitutional alloying elements in an ultra low carbon steel. For the first time these energies are used to calculate the solute drag effect considering site competition for the austenite to ferrite phase transformation. The depletion of elements in solid solution as a result from precipitation is computed and the solute drag contribution of each element is calculated. Comparison of the binding energies with the resulting solute drag effect to literature data shows reasonable agreement. The outlined approach points the way to future alloy development based on interface-controlled integrated computational materials engineering. Another field of application is given by the circular economy driven transition in the steel industry from the BF/BOF- to the EAF-based production route with increased utilization of scrap material introducing new tramp elements.

Solidification cracking nature and sequence of different stainless steels

For magnesium, nickel, and stainless steel, solidification cracking is a critical failure mode that has been extensively studied for decades. The nature of solidification cracking can be investigated through experiments or theoretical calculations. Different models have been proposed for these calculations, including the cracking susceptibility index (CSI), which was developed in the last decade. Building a CSI map for a group of metals facilitates referencing or comparison when the metal is applied in casting, welding, or developing new alloys. In this study, the CSI of various types of stainless steel was examined, and CSI maps were constructed. For each type of alloy, the calculations include more than ten experimentally determined compositions from commercial alloys. The CSI maps of stainless steel were constructed and compared with those of the Cr and Ni equivalents, as well as their respective phase diagrams. The CSI sequence matched the cracking results of previous studies on austenitic steels. The ferritic type had a lower CSI but a higher cracking rate, which could be attributed to the ductility gap during the austenite-ferrite transformation, as reported in previous studies.

In situ observation of solidification in peritectic steels and evaluation of the strains induced in the solidifying shell

In the continuous casting of peritectic steels, unevenness and cracks of the solidifying shell are typical casting defects. This study focuses on the ferrite–austenite transformation and the influence of the transformation on the solidifying shell deformation. In situ observations revealed that the austenite hardly nucleated and, consequently, the undercooled ferrite massively transformed to the austenite in the solid state. Simultaneously, the austenite rapidly solidified in the remaining liquid. The volume changes due to the ferrite–austenite transformation and the solidification shrinkage were measured using time-resolved tomography. On the basis of the observations and measurements, a model that includes the ferrite–austenite transformation and the austenite solidification was proposed to evaluate the volumetric strains in the solidifying shell. There was a sharp peak in the volumetric strains derived from the model at hypo-peritectic compositions. This work demonstrates the need to include the ferrite–austenite transformation in the solid state and the rapid austenite solidification for understanding the solidifying shell deformation in peritectic steels.

The microstructure evolution and influence factors of acicular ferrite in low alloy steels

The excellent combination of strength and toughness renders acicular ferrite to be a desirable microstructural contexture in C-Mn and high-strength low-alloy steels. The formation of acicular ferrite in steels is generally affected by factors such as composition, cooling rate, prior austenite grain size (PAGS), and impurity inclusions. Due to the complexity of multi-factor coupling, it is difficult to explore the influence of a single factor through actual experiments. In the current work, the influences of cooling rates, PAGS, and impurity inclusion density on the microstructure evolution of austenite-acicular ferrite transformation in low alloy steels are quantitatively evaluated by multi-phase-field simulation. The numerical results demonstrate that reducing PAGS during austenite-ferrite transformation, increasing cooling rate and increasing inclusion nucleation density can effectively refine acicular ferrite grain size. Moreover, the C element diffuses from acicular ferrite to austenite and accumulates at the phase interface during the phase transformation from austenite to ferrite. The findings in the current work contribute to better regulating and designing acicular ferrite in steels.

Modelling of the diffusional austenite-ferrite transformation

ABSTRACT The austenite-ferrite transformation is the key metallurgical tool to tailor properties of low alloyed steels and remains an active area of research. Models have yet to be developed with truly predictive capabilities for phase transformations in multi-component commercial steels. This review provides a critical analysis of the various austenite-to-ferrite diffusional transformation model approaches that have been significantly broadened over the past decade by modelling at different length scales, i.e. classical macro-scale models have been augmented with simulations at the meso-scale and atomistic scale. Both semi-empirical and fundamental models are reviewed with an emphasis on polygonal ferrite formation in low and medium carbon steels. Formation of ferrite with more complex morphologies (i.e. irregular/bainitic/Widmanstätten ferrite) is also discussed. In particular, approaches to describe the interaction of alloying elements with the austenite-ferrite interface are critically analysed. The paper concludes with an outlook on the proposed austenite-ferrite transformation modelling work for the next decade.

AUSTENITE – FERRITE TRANSFORMATION TEMPERATURES OF C MN AL HSLA STEEL

The article is aimed to investigate a shift of transformation temperatures of C-Mn-Al HSLA steel with different cooling rates. The transformation temperatures from austenite to ferrite have been determined by dilatometry using thermal-mechanical simulator Gleeble 1500D. To define the start and finishing temperatures of the austenite-ferrite transformation intersectional method was used. Effect of cooling rate on transformation temperature has been evaluated for 0.17, 1, 5, 10, 15, 20, 25°C.s-1. There was found out that rising the cooling rate results in moving transformation temperature range to lower temperatures. The transformation temperatures have been also compared with temperatures calculated using equations of several authors. Some of them have considered cooling rates only. Cooling rates have effect on final microstructure. The effect has been evaluated by measuring hardness (HV10) relating the cooling rates from 0.17 to 25°C.s-1. Increasing cooling rates resulted in increase of hardness. Moreover, Thermo-Calc software was used to determine the Ae3 and Ae1 equilibrium temperatures. Equilibrium transformation temperatures Ae3-Ae1 were higher than experimentally measured by dilatometric method using Gleeble 1500D.

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.

Development of ferrous-based weldable seismic damping alloy with prolonged plastic fatigue life

A new weldable Fe-Mn-Cr-Ni-Si seismic damping alloy with high fatigue resistance was developed by controlling phase transformation behaviour during both solidification and fatigue deformation. The solidification phase transformation was controlled by changing the Cr/Ni ratio to induce ferrite-austenite transformation and thus reduce the solidification cracking susceptibility. The Gibbs free energy difference between the austenite and ε-martensite was optimized to exhibit reversible transformation during fatigue deformation. The resultant alloy exhibited a prolonged fatigue life, and no solidification cracking was observed in a melt-run welding test. By using the newly developed alloy, a weld damper with excellent fatigue resistance can be fabricated.

Atom-probe tomographic and dilatometric studies of phase-transformations after inter-critical annealing of a low-carbon dual-phase steel

Phase-transformations and microstructural evolution during and after inter-critical annealing of a low-alloy, dual-phase steel are studied utilizing dilatometric measurements, and microstructural characterization methods, including scanning electron microscopy, synchrotron X-ray diffraction, and atom-probe tomography. Dilatometric measurements reveal that the sample contains ~72% austenite, after annealing for 5-min at 871 °C, which agrees with the water-quenched microstructure, and ~90% after further annealing to 1 h, which agrees with thermodynamic predictions. A continuous-cooling transformation (CCT) diagram is constructed, within the cooling-rate range between 1 °C/s and 70 °C/s. The incubation time for the ferrite-transformation is <1 s, after inter-critical annealing, and the secondary-phases are mainly pearlite for cooling-rates of <5 °C/s, and martensite for cooling-rates >10 °C/s, which is confirmed utilizing scanning electron-microscope observations. The microhardness increases as the cooling rate increases, and an abrupt slope-change is explained by the major secondary-phase changing from martensite to pearlite between 5 °C/s and 10 °C/s. Atom-probe tomography demonstrates the coexistence of martensite and pearlite as secondary-phases for 30 °C/s and a switch in austenite-ferrite transformation mechanisms from para-equilibrium (70 °C/s) to partitioning local-equilibrium (30 °C/s). A microstructural map is constructed to reveal the austenite decomposition products in the cooling-rate range between 1 °C/s and 70 °C/s. Based on composition measurements of the matrix, utilizing APT, the effects of cooling-rates on solid-solution-strengthening are estimated, and the results demonstrate that solid-solution strengthening, especially carbon-redistribution, accounts for the majority of the strength variations due to different cooling-rates.

Effect of rolling reduction below γ non-recrystallization temperature on pancaked γ, microstructure, texture and low-temperature toughness for hot rolled steel

To optimize rolling schedules for producing heavy plates with a limited slab-to-plate reduction ratio, we investigated the effect of rolling reduction below the non-recrystallization temperature (Tnr) of austenite (γ) on microstructure, texture, and low-temperature toughness for crack arrest heavy-gauge EH47 steel for shipbuilding. Four plates were hot rolled with 30%, 49%, 60% and 67% reduction below Tnr respectively, of which the total rolling reduction ratio was the same as 3.57. The microstructures and texture were characterized, and the low-temperature Charpy impact toughness was tested as well for each plate. Variations in toughness were significant among the steels, being highly correlated with the effective grain size (EGS), amount of high angle grain boundaries (HAGB), and texture. The steel with 60% reduction below Tnr showed the best toughness down to −80 °C with the impact absorbed energy of ~233 J for the minimum EGS of 3.4 ± 4.2 μm, the maximum HAGB proportion of 51.6%, {110} slip planes of 39.1%, and the strongest texture of {113}~{112}<110>, {112}<131>, {332}<113>. The theoretical calculation shows that the nucleation sites for austenite-ferrite transformation are mainly affected by the size of γ after rolling in the γ recrystallization region and the deformation substructure generated below Tnr. The changes of the pancaked γ can be quantified interpreted by the theory of effective interface area per unit volume. The relation of microstructures, texture, and low-temperature toughness to the rolling reduction below Tnr could be well interpreted by evaluating effective interfacial area per unit volume of austenite to ferrite during transformation.

Hardness testing and why it matters

In this work we concentrate on the in situ dynamics of interphase boundary motion during transformations. In-situ high temperature electron-back scatter diffraction (HT EBSD) was employed to study the ferrite-austenite-ferrite transformation in low carbon steel. A novel method was designed to derive the velocity of the interphase boundaries from the EBSD phase maps. It is concluded that the motion of the transformation front occurs in a jerky-type motion, i.e. not continuous in time on a microscale, as the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation would predict on the macro-scale. It is shown that HT EBSD is capable of providing relevant supplementary insight in the ferrite-austenite phase transformations with adequate spatiotemporal resolution, which would remain hidden in volume averaging experimental techniques such as X-Ray diffraction. In particular, ferrite-austenite phase boundary velocities between 1.4 ± 0.3 and 4.0 ± 0.2 nm/s were detected during isochronal heating with 0.5 °C/min. The mean interphase boundary velocity was ranging between 5.0 ± 0.2 and 6.4 ± 0.2 nm/s for austenite-ferrite transformation with a cooling rate of 1 °C/min. Ledge growth at isothermal conditions, resulted in velocity of 23 nm/s along phase boundary. A strong dependency of interphase boundary mobility and parent-daughter phase boundary misorientation angles was not found.

Effect of cooling rate on precipitation behaviour and transformation characteristics of a novel FeCrVBC cast alloy

This work presents the investigation of the solidification and cooling behaviour of a newly developed Fe92.65Cr4.2V2.1B0.05C1 wt% (FeCrVBC) tool steel with special emphasis to phase formations and transformations. JMatPro simulations were compiled to predict the characteristic temperatures of phase crystallizations and transformations under equilibrium conditions. Differential thermal analysis (DTA) was used to cool down samples from 1500 °C to room temperature with various cooling rates (5, 10, 20, 40, and 50 K/min), whereby differences in the number and position of the resulting peaks occurred. To correlate the peaks with specific phase reactions, the microstructure of the variously cooled samples was analysed by means of optical (OM) and scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), wavelength-dispersive X-ray spectroscopy (WDS), electron backscatter diffraction (EBSD) and microhardness tests. The systematic analyses show that the microstructure evolution of the FeCrVBC alloy strongly depends on the cooling rate. For low cooling rates up to 10 K/min, the austenite-ferrite transformation and a marked formation of M7C3 and M23(C,B)6 secondary carbides were observed. At rates of 20 K/min and above, the austenite-martensite transformation takes place, accompanied with a strong increase of the hardness in dendritic areas. By cooling with rates of 40 K/min and higher, the precipitation of M7C3 or M23(C,B)6 secondary carbides cannot be observed anymore, whereas primary M3(C,B) besides MC carbides were detected. Furthermore it was determined, that the very small content of boron (0.05 wt%) strongly influences the (trans-)formation of special carbides, shown by the comparison to its base alloy without boron, the formerly investigated Fe92.7Cr4.2V2.1C1 wt% (FeCrVC). Consequently, strongly depending on the formed carbides the materials hardness and wear resistance can be significantly affected by boron addition. Concluding, the obtained knowledge about the thermal behaviour of the FeCrVBC alloy and the strong impact of a small addition of boron offers the possibility for tailored adjustment of phases and respective properties for a load-adapted tool design.

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.

Model of phase transformations in steels subject to heating-cooling thermal cycles in continuous annealing line

ABSTRACTThe need for fast mean field models describing phase transformations in varying temperatures was the motivation for the research. The aim was to propose an austenite-ferrite transformation model which can be successfully substituted for the JMAK model and eliminate its weak points. The model consists of a second order differential equation which describes the kinetics of both the austenite-ferrite and the reverse transformations. The main advantage of such an approach is the ability to perform calculations for varying temperature without using the additivity rule. The coefficients in the equations were determined by performing the inverse analysis for the dilatometric data. The inverse approach was transformed into an optimisation task, which was solved using the Particle Swarm Optimization (PSO) algorithm. The first part of the work is devoted to the description of the model and is completed with the calculation of the coefficients obtained for two DP steels. The second part consists of numerical tests and the validation of the model, which was performed by simulations of two industrial thermal cycles. The final part of the work is devoted to the optimisation of the thermal cycle for continuous annealing, in order to obtain the required phase composition by changing parameters of the thermal cycle. This part also contains the results of sensitivity analysis of final phase distribution.

Effects of Fe3P Addition on Sintering Behaviors and Magnetic Properties of Fe-P Alloys Sintered at Low Temperatures

In this paper, we demonstrate that trace amounts of P addition can activate the sintering of carbonyl powder and influence the magnetic properties of the sintered materials. Fe-x P (x = 0, 0.6, 0.8, 1.0, 1.2, 1.4, and 1.6 wt.%) samples were fabricated by doping carbonyl powder with different amounts of Fe3P. They were sintered at 1000 °C in argon for 2 h. The sintering of the Fe-0 wt.% P sample was interrupted by the ferrite-austenite transformation at 912 °C due to the low diffusion rate of the austenite. The addition of P can stabilize the ferrite, and suppress the ferrite-austenite transformation. Therefore, all the P-containing samples shrank continuously throughout the whole sintering process, which showed improved sintering densities compared to the P-free sample. However, the sintering density did not increase monotonously with increasing P content. The Fe-1.4 wt.% P and Fe-1.6wt.% P samples easily got oxidized during sintering, and the densification process was thus influenced by the P-containing oxide particles. As a result, the Fe-1.2 wt.% P sample exhibited the highest sintering density (7.664 g/cm3) and the best magnetic properties (coercive force 172 A/m).

The Integration of Process and Product Metallurgy in Niobium Bearing Steels

A review of the technological integration of both the process and physical metallurgical advancements of value-added niobium (Nb) microalloyed thermo-mechanical controlled process (TMCP) steels have evolved into the development of higher quality steels for more demanding end user requirements. The connection of process and physical metallurgy is evolving through the integration of research that is aimed at improving product quality. However, often the connection of the process metallurgical parameters is not reported, especially with industrial data. The importance of this innovative metallurgical connection is validated by the market demand for reduced fuel consumption, improved quality, and CO2 emissions in both the automotive and construction sectors. This situation has further increased the demand for new higher quality Nb-bearing steel grades. This integrative process/physical metallurgical (IP/PM) approach applies to both low and high strength steel grades in numerous applications. Often, the transition from laboratory melted and TMCP to the production scale is challenging. The methodology, process control, and key production steps that are required during the melting, ladle metallurgy, continuous casting, thermal, and hot rolling production conditions often vary significantly from the laboratory conditions. Understanding the reasons and corrective action for these variations is a critical product development success factor. These process metallurgy parameters for the industrial melting, casting, reheating, and hot rolling of Nb grades are connected and correlated to the resultant microstructures, physical metallurgy, and mechanical properties. These advanced high strength steels are microalloyed with Nb, V, Ti and/or other elements, which affect the austenite-ferrite transformation. Niobium enables the achievement of substantial grain refinement when the plate or sheet is rolled with the proper reheat, hot reduction, and thermal schedule. A recently developed key metallurgical transition is in progress applying this integrative approach with the use of MicroNiobium. A reduction of Mn and C levels with the complementary refinement of the microstructural grain size through MicroNiobium additions improves the robustness of the steel to better accommodate some process metallurgy variations. Applications are evolving in lower strength steels with Nb to achieve complementary grain refinement.

Austenite – ferrite transformation temperature regression equations for low carbon steels with cooling rate account

The austenite-ferrite transformation temperatures evaluated by dilatometry using thermo-mechanical simulator Gleeble 1500D are investigated in this paper. The effect of cooling rates 1, 5, 10 and 15°C/s on the upper and lower critical transformation temperatures was evaluated for 30 specimens of six material groups. Considering the cooling rate from dilatometry tests and chemical composition (C ≤ 0.2%, Mn ≤ 2%, Si ≤ 0.26%) of particular specimens, the regression equations for both transformation temperatures were derived. These relations have to be satisfied to avoid the crack formation during continuous casting, as well as to provide the hot rolling control. The proposed regression equations are compared with 32 similar ones adopted from 1961 to 2017 and exhibit a good conformity and accuracy.

A molecular dynamics study of the nucleus interface structure and orientation relationships during the austenite-to-ferrite transformation in pure Fe

ABSTRACTMolecular dynamics simulations using an embedded-atom method potential for pure iron (Fe) were performed to investigate the solid-state transformation of a body-centred-cubic phase from a polycrystalline face-centred-cubic matrix. A Kurdjumov–Sachs orientation relationship accompanied with a step-ledge disconnection structure was detected in high mobility phase boundaries of a full 3D classical and barrier-free nucleation specimen. The results imply that interface coherency is an important factor for both nucleation and growth processes, which may significantly affect the transition rate of the austenite–ferrite transformation.

Interphase boundary motion elucidated through in-situ high temperature electron back-scatter diffraction

In this work we concentrate on the in situ dynamics of interphase boundary motion during transformations. In-situ high temperature electron-back scatter diffraction (HT EBSD) was employed to study the ferrite-austenite-ferrite transformation in low carbon steel. A novel method was designed to derive the velocity of the interphase boundaries from the EBSD phase maps. It is concluded that the motion of the transformation front occurs in a jerky-type motion, i.e. not continuous in time on a microscale, as the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation would predict on the macro-scale. It is shown that HT EBSD is capable of providing relevant supplementary insight in the ferrite-austenite phase transformations with adequate spatiotemporal resolution, which would remain hidden in volume averaging experimental techniques such as X-Ray diffraction. In particular, ferrite-austenite phase boundary velocities between 1.4±0.3 and 4.0±0.2nm/s were detected during isochronal heating with 0.5°C/min. The mean interphase boundary velocity was ranging between 5.0±0.2 and 6.4±0.2nm/s for austenite-ferrite transformation with a cooling rate of 1°C/min. Ledge growth at isothermal conditions, resulted in velocity of 23nm/s along phase boundary. A strong dependency of interphase boundary mobility and parent-daughter phase boundary misorientation angles was not found.