Kinetics Of Bainite Formation Research Articles

Modeling the effect of prior austenite grain size on bainite formation kinetics

While experiments show that refining the prior austenite grain size can either accelerate or decelerate bainite formation in steels, kinetic models based on the successive nucleation of bainitic ferrite subunits can only predict an acceleration. In this work we develop a physically-based model for bainite kinetics assuming a displacive growth mechanism which is able to reproduce both faster and slower bainite formation kinetics induced by austenite grain refinement. A theoretical analysis of the model and comparison against published experimental data show that slower kinetics for smaller grains is favored as the difference between the activation energy for grain boundary and autocatalytic nucleation of bainite increases, and as the austenite grain refinement results in finer bainite sub-units. We also theoretically analyze the density of initially present potential nucleation sites for bainite and show that the values of density used in other published bainite nucleation models are mostly underestimated. After using physically consistent values for the density of potential nucleation sites, we were able to calculate the apparent lengthening rate of bainite sheaves which were in line with experimentally measured lengthening rates.

Accelerating bainite transformation by concurrent pearlite formation in a medium Mn steel: Experiments and modelling

Bainite transformation has yet to be utilized and even thoroughly studied in medium Mn steels. Here, we investigate the isothermal bainite transformation in a 10Mn steel at 450 °C experimentally and theoretically, focusing on the effect of dislocations introduced by warm deformation. We show that the bainite transformation in the studied medium Mn steel exhibits extremely sluggish kinetics (on a time scale of days), concurrent with the pearlite formation. The introduced dislocations can significantly accelerate bainite transformation kinetics while also facilitating the pearlite reaction. This is likely the first report on the simultaneous occurrence of these two solid-state reactions in medium Mn steels. With respect to the roles of dislocations in the acceleration of bainite transformation observed in this work, we propose a new ‘carbon depletion mechanism’, in which dislocations-stimulated pearlite formation makes a twofold contribution: facilitating the formation of bainitic ferrite sub-units to further enhance the autocatalytic effect and preventing the carbon enrichment in the remaining austenite. On this basis, a physical model is developed to quantitatively understand the bainite transformation kinetics considering the effect of concurrent pearlite formation, revealing good agreements between model descriptions and experiment results. Our findings, herein, offer fundamental insights into the bainite transformation in medium Mn steels and uncover a previously unidentified role played by introduced dislocations in influencing the kinetics of bainite formation, which may guide its future application in manipulating microstructure for the development of advanced high-strength steels.

Isothermal decomposition of austenite in presence of martensite in advanced high strength steels: A review

The development of the quenching and partitioning (Q&P) process has prompted an interest in the process of isothermal transformation in presence of a pre-existing phase such as martensite. The presence of prior martensite is known to accelerate the overall kinetics of bainite formation, both in the 1-step and 2-step Q&P process. The underlying mechanisms behind this phenomenon are not fully understood. Also, the nature of the isothermal product (isothermal martensite and/or bainite) formed in presence of prior martensite seems to differ according to the thermodynamic and kinetic conditions. For certain thermodynamic conditions, depending on alloying, isothermal martensite may also form in the temperature range just above and below M s . In the event that both bainite and isothermal martensite formation are thermodynamically allowed, the competition in kinetics determines the observed transformation product. The effect of martensite on the subsequent isothermal transformation is reviewed with a focus on the nature of the transformation products and kinetics. In that context, the kinetics of the isothermal transformation in presence of other prior phases such as polygonal ferrite and bainite are also compared and discussed, together with the possible mechanisms behind the acceleration of the transformation kinetics. Guidelines for further investigation are also proposed. − Formation of both bainite and isothermal martensite is possible during isothermal heat treatment of austenite in presence of martensite. − Bainite formation kinetics is accelerated in presence of martensite, potentially due to an increase in the nucleation site density. − Opinions are differed whether bainite formed at or near the interface. Therefore the primary mechanism is not established yet. − Complexities in identifying the primary mechanism are discussed and possible research guidelines are proposed.

Kinetic Model of Isothermal Bainitic Transformation of Low Carbon Steels under Ausforming Conditions

Carbide-free bainitic steels show attractive mechanical properties but are difficult to process because of the sluggish phase transformation kinetics. A macroscopic model based on the classical nucleation theory in conjunction with the modified Koistinen–Marburger relationship is proposed in this study to simulate the kinetics of incomplete bainitic and martensitic phase transformations with and without austenite deformation. A 0.26C-1Si-1.5Mn-1Cr-1Ni-0.003B-0.03Ti steel and a 0.18C-1Si-2.5Mn-0.2Cr-0.2Ni-0.02B-0.03Ti steel were investigated with different levels of ausforming. The concept of ausforming is expected to accelerate the onset of the bainitic transformation and to enhance the thermodynamic stability of austenite by increased dislocation density. The phase transformation kinetics of both steels is quantitatively analyzed in the study by dilatometry and X-ray diffraction so that the carbon concentration in the retained austenite and bainitic ferrite, as well as their volume fractions, is determined. A critical comparison of the numerical and experimental data demonstrates that the isothermal kinetics of bainite formation and the variation of driving energy can be satisfactorily described by the developed model. This model captures the incompleteness of the bainite phase transformation and the carbon enrichment in the austenite well. A fitting parameter can be used to elucidate the initial energy barrier caused by the ausforming. An increase in austenite stability can be described by the nucleation reaction and the thermodynamic energies associated with the change of dislocation density. The proposed model provides an in-depth understanding of the effect of ausforming on the transformation kinetics under different low-carbon steels and is a potential tool for the future design of heat treatment processes and alloys.

Dilatometric study of the effect of soluble boron on the continuous and isothermal austenite decomposition in 0.15C–1.6Mn steel

Abstract The effect of soluble boron on the phase transformations during either cooling and/or isothermal holding has been studied by means of dilatometry. Significant differences in the transformation behaviour were found for all austenite phase transformation reactions. In particular, the morphologies of ferrite and bainite were strongly affected by B alloying. The kinetics was studied in detail for the austenite decomposition reactions. Soluble B was found to be very effective in suppressing the carbide formation. It was also found to interact with the Mn partitioning to the austenite. As a result, the presence of Mn-rich regions in the final microstructures decreased strongly the Ac1 temperature during reheating. Isothermal transformations in the 450–660 °C temperature range showed that the incubation times for ferrite and pearlite formations were increased. The soluble B was found to affect strongly the nucleation rate. The growth kinetics was slower due to a solute drag effect caused by the partitioning of Mn. The kinetics of bainite formation was not affected by the presence of soluble B. Upper bainite was found to be acicular ferrite in the CMnB steel as a result of the heterogeneous nucleation of ferrite on TiN precipitates.

Effects of quenching temperature on bainite transformation, retained austenite and mechanical properties of hot-galvanized Q&P steel

Over the last decade, demand has increased for developing the hot-galvanized quenching and partitioning (Q&P) steel to overcome the disadvantages associated with vehicle safety, fuel consumption and corrosion resistant. The present work aims to elucidate the effects of quenching temperature on phase transformation kinetics, microstructure evolutions and mechanical properties of a hot-galvanized Q&P steel (0.225C-0.85Si-2.02Mn-0.91Al, in wt.%) by combining modeling and experimental research. Using dilatometry, SEM, EBSD, EPMA, TEM, PED, XRD and Image-Pro Plus (IPP) software, we quantitatively investigated the microstructure evolution at different quenching temperatures. Results indicated that a larger fraction of primary martensite at lower quenching temperature could strongly promote subsequent bainite transformation kinetics, which is attributed to more martensite-austenite interfaces and defect density. By fitting the dilatometer curves and establishing the equation of transformation rate vs. quenching temperature, a Kolmogorov-Johnson-Mehl-Avram (KJMA) equation was established to describe insufficient bainite formation kinetics during high-temperature short-time overaging. Furthermore, a modified CCE model taking into account intercritical ferrite and short-time bainite transformation was proposed and the predicted RA fractions are more consistent with the experimental values. As the quenching temperature decreases, small-sized blocky RA along martensite boundaries and filmy RA between martensite laths increase, while coarse lath/blocky RA inside bainite structures or at bainite boundaries decreases gradually. In addition, the YS decreases from 763 MPa to 431 MPa with the increase of quenching temperature, while the UTS varies in a narrow range between 967 MPa and 1036 MPa. A stable TEL of 22.6–25.7% can be obtained at a wide range quenching temperature (150–275 °C), which is attributed to the joint effects of TRIP effect and multiphase structure. This research would be of guiding significance for the industrial practice.

Bainite precipitation in a Cu–Al–Mn-Gd shape memory alloy

In this study, the aging effects and bainite formation kinetics in the Cu71.2Al18.6Mn10.2 and Cu69.7Al18.9Mn10.3Gd1.10 shape memory alloys are evaluated using different experimental techniques. The results show that the Cu71.2Al18.6Mn10.2 and Cu69.7Al18.9Mn10.3Gd1.10 alloys exhibit aging hardening during bainite precipitation, reaching hardness values at about 370 HV. It is verified that the ferromagnetic β3 phase is produced during the bainite precipitation on the aging. The kinetic process is evaluated, and for the Cu69.7Al18.9Mn10.3Gd1.10 alloy, the activation energy value is ~42 kJ.mol−1 while the alloy without Gd presents an activation energy value at about 62 kJ mol−1. Gadolinium modifies the reaction mechanism analyzed due to their interaction with β1 and α phases, and the phase decomposition with Gd. In the studied alloys the bainite formation shows to be strongly related to the diffusive processes.

Microstructural Characterizations and Kinetics Model of Isothermal Bainite Formation with the Presence of Prior Martensite in High‐Carbon Nanostructured Bainitic Steel

Quenching and subsequent isothermal bainite transformation (QBT) and direct isothermal transformation (DIT) procedures are performed to comparatively evaluate the impact of the introduced prior martensite on bainite transformation behaviors and microstructural characterizations in a high‐carbon nanostructured bainitic steel. A thermodynamic model under a QBT process based on displacive theory is also developed to expound the bainite formation kinetics. The results indicate that the modified kinetics model, by considering austenite/martensite interface nucleation, austenite grain boundary nucleation, and autocatalytic nucleation, evolved here can be used to accurately describe the bainite formation behavior in the presence of prior martensite. A certain fraction of prior austenite/martensite interface can serve as the preferential nucleation site for the bainite transformation and assist in an acceleration of transformation rate. The bainite nucleation event initially takes place at prior martensite plate boundary in the QBT process instead of austenite grain boundary in the DIT process and subsequently continues through autocatalytic nucleation at austenite/bainite interface generated at either austenite/martensite interface or austenite/austenite interface. The introduced prior martensite slightly refines bainite sheaves and lessens the blocky retained austenite size, presenting a feasible approach to shorten the required time for producing a nanosized bainitic microstructure.

Revealing the Dynamic Transformation of Austenite to Bainite during Uniaxial Warm Compression through In-Situ Synchrotron X-ray Diffraction

In this work, the microstructural evolution during the dynamic transformation of austenite to bainite was directly observed by in-situ high energy synchrotron X-ray diffraction measurements during warm uniaxial compression performed at the P07 beamline of PETRA III, DESY (Deutsches Elektronen-Synchrotron). Plastic deformation triggers the phase transformation, which is continuously stimulated by the introduction of dynamic dislocations into the austenite. This scenario accelerates the kinetics of bainite formation in comparison with conventional isothermal treatment. No mechanical stabilization of austenite was observed during dynamic transformation. Evidence of carbon partitioning between phases during plastic deformation was obtained. Further post-process investigations suggest that the bainitic microstructure developed during compression is oriented perpendicular to the loading direction. The findings open up new possibilities to design carbide-free bainitic microstructures directly via thermomechanical processing.

The role of grain-boundary cementite in bainite formation in high-carbon steels

Low temperature bainite formation in high-carbon steels leads to highly refined microstructures. However, the rate of bainite formation becomes impractically low as the bainite formation temperature decreases. Thus, it is important to create strategies to accelerate the low-temperature bainite formation kinetics in high-carbon steels. In this work, it is observed that decorating the parent austenite grain boundaries with grain-boundary cementite prior to bainite formation leads to an appreciable acceleration of subsequent bainite formation kinetics due to decrease in activation energy for bainite nucleation at cemenite/austenite interfaces.

Impact of austenite grain boundaries and ferrite nucleation on bainite formation in steels

The rate of bainite formation depends on several factors such as austenite grain size, decomposition of austenite into other phases and austenite composition. Although studies have been carried out to understand various factors affecting bainite formation, open lines of investigation still remain. In this work, the effect of ferrite formation prior to bainite formation as well as the effect of parent austenite grain boundary composition on the bainite formation kinetics is investigated. With this aim, bainite formation treatments directly after complete austenitization and in combination with an intermediate heat treatment step prior to bainite formation were applied to a low-carbon silicon-containing steel. The intermediate heat treatment step leads to ferrite formation at and/or elemental segregation to austenite grain boundaries which were characterized using scanning electron microscopy, electron backscatter diffraction analysis and atom probe tomography. The results indicate that the kinetics of bainite formation can be accelerated with the help of an intermediate heat treatment step prior to bainite formation. The acceleration of bainite formation is mainly due to increase in the density of bainite nucleation sites.

Influence of martensite/austenite interfaces on bainite formation in low-alloy steels below Ms

Bainite formation in steels typically starts at austenite grain boundaries and continues through nucleation of bainite at newly formed bainitic ferrite/austenite interfaces. Recent experimental evidence has pointed out that austenite to bainite transformation can also proceed in the presence of martensite. Studies suggest that the presence of athermal martensite formed prior to bainite formation can accelerate the kinetics of bainite formation with the martensite/austenite interfaces acting as potential nucleation sites. In this work, a kinetic model based on the displacive mechanism of bainite formation is adapted to isolate the impact of martensite/austenite interfaces on the overall rate of bainite formation when bainite formation occurs in the presence of previously formed martensite. This adapted kinetic model is validated using dilatometer studies published in the literature on a silicon-containing low-carbon steel in which bainite formation experiments are performed both below and above the Ms temperature. The results suggest that the formalism of the existing kinetic theory can describe the effects of martensite/austenite interfaces on the bainite formation.

Unravelling the mechanical behaviour of advanced multiphase steels isothermally obtained below Ms

The initial formation of athermal martensite was proven to accelerate the subsequent bainite formation kinetics during isothermal holdings below the martensite-start temperature (Ms). The presence of prior athermal martensite (PAM) within the phase mixture is expected to modify the overall mechanical response of these newly-designed multiphase steels. Differences stem not only from the balance of product phases, but also from the effect of tempering of the PAM with variations in the applied holding time. This work investigates the effect of tempering time on the mechanical behaviour of the PAM and, as consequence, on the overall mechanical response of these microstructures. Results show that, for short holding times (several minutes), PAM yields similar to as-quenched martensite while, for longer holding times, its yielding behaviour becomes comparable to the one exhibited by typical tempered martensite. Furthermore, the use of Kocks-Mecking curves for the analysis of the mechanical performance confirms the bainitic character of the product phase isothermally formed below Ms. Tailoring the bainitic-martensitic microstructure with variations of the holding time below Ms enables to obtain advanced multiphase steels with comparable mechanical properties to those exhibited by conventional bainitic steels, but in shorter processing times due to the acceleration of bainite formation.

Effect of cooling rate and composition on microstructure and mechanical properties of ultrahigh-strength steels

The influence of cooling rate on the microstructure and mechanical properties of two new ultrahigh-strength steels (UHSSs) with different levels of C, Cr and Ni has been evaluated for the as-cooled and untempered condition. One UHSS had higher contents of C and Cr, while the other one had a higher Ni content. On the basis of dilatation curves, microstructures, macrohardness and microhardness, continuous cooling transformation diagrams were constructed as a guide to heat treatment possibilities. Cooling rates (CRs) of 60, 1 and 0.01 °C/s were selected for more detailed investigations. Microstructural characterization was made by laser scanning confocal microscopy, field emission scanning electron microscopy combined with electron backscatter diffraction, electron probe microanalysis and X-ray diffraction. Mechanical properties were characterized using macrohardness, tensile and Charpy V-notch impact tests. UHSS with the higher C and Cr contents showed lower transformation temperatures and slower bainite formation kinetics than that with the higher Ni content. Higher cooling rates led to lower volume fractions and carbon contents of retained austenite together with finer prior austenite grain size, as well as effective final grain size and lath size. These changes were accompanied by higher yield and tensile strengths. The best combinations of strength and toughness were obtained with martensitic microstructures and by avoiding the formation of granular bainite accompanied by proeutectoid carbides at low CR. For the cooling rates studied, UHSS with the higher C and Cr contents showed the higher hardness and strength but at the cost of toughness.

Kinetics and microstructural change of low-carbon bainite due to vanadium microalloying

Noticeable strength increases, up to 120 MPa, have been obtained in hot strip and linepipe bainitic steels by microalloying with vanadium. However, there is no consensus on the mechanisms proposed in the literature to explain this effect. As such, this study has investigated the root cause and various strengthening contributions of vanadium additions to bainitic linepipe steels. Two variants of a X100 linepipe steel with identical compositions but different vanadium contents, i.e. a reference steel with residual amount of vanadium and a microalloyed variant with 0.06 wt% vanadium, have been subjected to dilatometric study to determine the kinetics of bainite formation during continuous cooling and cooling-coiling scenarios. Further, a novel in-situ hot compression experiment was developed to determine the strength of fresh bainite (prior to coiling) and its softening during simulated coiling at 450 °C. Various microstructural features of the bainite, including the morphology, dislocation density and lath size, as well as the presence of microalloyed carbonitrides have been quantified by TEM characterization. It was found that the kinetics of the bainite transformation during cooling and the tempering of fresh bainite upon coiling simulations were substantially influenced by vanadium microalloying. Vanadium in solution shifted the bainite transformation to lower temperatures (by 30–40 °C) during cooling at 1–50 °C/s, refining the lath structure and increasing the dislocation density of bainitic ferrite. The V-added steel demonstrated a higher strength of fresh bainite (about 50 MPa) compared to the reference alloy. This superior strength has been explained explicitly by accounting for the contributions from the increased dislocation density and the refined lath structure of V-added variant. No evidence of fine VC or VN precipitation occurring during coiling at 450 °C was found. Further, the complex TiNbV(CN) particles that were observed were too coarse and too scarce to make any strengthening contribution. The fresh bainite of both X100 variants tends to soften rapidly upon coiling at 450 °C; about 110 MPa strength drop was noticed for the reference alloy. The implications of the findings for alloy design and processing of advanced linepipe steels are discussed.

Continuous cooling transformation behavior and the kinetics of bainite formation in a bainitic–martensitic steel

Abstract The continuous cooling transformation diagram of a low carbon bainitic–martensitic steel was constructed using dilatometry and metallographic methods. It was found that as cooling rate increases, the structure changes from granular bainite to lath martensite. Three regions of different kinetic behavior were discerned for the bainitic–martensitic steel. One of the regions conformed to martensite formation and the other two comprised transformation to bainite. A non-isothermal type of Johnson–Mehl–Avrami–Kolmogorov kinetic equation of reaction rate was used to analyze the transformation behavior during continuous cooling of bainite formation. The Avrami exponent and activation energy values for different regions at cooling rates of 0.1 and 0.4 K s−1 varied from 1.5 to 4.7 and 71 to 84 kJ mol–1 respectively. Models obtained from such kinetic coefficients closely corresponded to experimental results.

Bainite formation kinetics in steels and the dynamic nature of the autocatalytic nucleation process

Over the years, a quantitative theory to explain bainite formation kinetics has been proposed based on the nucleation kinetics of bainitic sub-units. Although the theory shows acceptable correlation with experimental results, it is observed that the kinetic models show a certain degree of discrepancy with actual kinetics. It is identified that these mainly arise due to the inadequate estimation of autocatalytic nucleation, especially as a function of progress of bainite formation. With the help of this observation, the kinetic model is modified and a better insight into the process of autocatalytic nucleation, essential in bainite formation, is obtained.

Analysis of the kinetics of bainite formation below the MS temperature

The kinetics of the isothermal product formed below the MS temperature in 0.32C-1.78Mn-0.64Si-1.75Al-1.20Co (wt%) steel has been analyzed. It has been shown that the diffusion controlled growth rate of pearlite using Zener-Hillert model is too slow to explain the observed kinetics at such a low temperature. The isothermal product cannot be identified as the eutectoid phases. However, an existing model of carbide precipitation from supersaturated ferrite correctly predicts the precipitation observed in bainitic ferrite plate. The results indicate that bainitic ferrite is supersaturated with carbon when it first forms and therefore indicated a displacive mechanism of transformation.

Relative Influences of Aluminium and Silicon on the Kinetics of Bainite Formation from Intercritical Austenite

Bainite formation from intercritical austenite is of great practical importance for the production of TRIP-assisted steels. Silicon and aluminium play important roles during this transformation by delaying carbide precipitation, thus favouring the carbon enrichment of untransformed austenite, which makes its stabilisation down to room temperature possible. Previous studies have shown a strong dependence of bainite formation kinetics on both chemical composition and transformation temperature. In the present work, the effect of silicon and aluminium contents on bainite formation kinetics is investigated experimentally using dilatometry combined with microscopical observations. The experimental results are analysed by comparison with thermodynamic parameters, such as the activation energy G* for nucleation of bainite and the carbon content C-To corresponding to the T-o-curve. It is shown that the faster transformation kinetics induced by the substitution of silicon by aluminium can be ascribed (i) to a higher driving force for nucleation, (ii) to a higher carbon content C-To at the To-curve and (iii) to the precipitation of carbide in austenite in steels with a low At content.

Continuous cooling transformation behaviour and bainite formation kinetics of new bainitic steel

In order to avoid the appearance of soft particles composed of ferrite or pearlite in the actual production of new bainitic steel, the phase transformation behaviour and bainite formation kinetics were investigated by DIL805A dilatometer, optical microscopy, scanning electron microscopy and Vickers-durometer. The results show that the soft particles cannot appear when the cooling rate exceeds 0.025 K s−1, and this condition can be ensured by direct spray cooling in production. The local activation energy decreases with increasing transformed bainite volume fraction (fb), and the average energy is about 136.7 kJ mol−1. The local Avrami exponent mainly lies between 0.5 and 3 in a wide fb range, indicating that the dominating mechanism of bainite formation is two-dimensional and one-dimensional growth.