Bainite Transformation Kinetics Research Articles

Experimental and Modelling Research on the Effect of Prior Ferrite on Bainitic Transformation in Medium-Carbon Bainitic Steel

In this study, we investigate the impact of prior ferrite on the bainite transformation kinetics and microstructure of medium-carbon steel interrupted by an intercritical annealing (IAA) process. It was found that the incubation time and completion time decreased from 687 s and 6018 s to 20 s and 4680 s, with the volume fraction of ferrite increasing from 9.5% to 28.6%, while the maximum transformation rate increased from 00271 μm/s to 0.0436 μm/s. The ferrite/austenite interface is introduced, and the nucleation sites are increased to accelerate the subsequent bainite transformation due to the formation of prior ferrite. However, there is a competitive relationship between the number and activation energy of bainite nucleation. According to the experimental results and theoretical calculations, the activation energy of the bainite transformation in the medium-carbon bainite steel decreases gradually with an increase in the volume fraction of prior ferrite.

Ultra-Fine Bainite in Medium-Carbon High-Silicon Bainitic Steel.

The effects of austenitizing and austempering temperatures on the bainite transformation kinetics and the microstructural and mechanical properties of a medium-carbon high-silicon ultra-fine bainitic steel were investigated via dilatometric measurements, microstructural characterization and mechanical tests. It is demonstrated that the optimum austenitizing temperature exists for 0.3 wt.%C ultra-fine bainitic steel. Although the finer austenite grain at 950 °C provides more bainite nuclei site and form finer bainitic ferrite plates, the lower dislocation density in plates and the higher volume fraction of the retained austenite reduces the strength and impact toughness of ultra-fine steel. When the austenitizing temperature exceeds 1000 °C, the true thickness of bainitic ferrite plates and the volume fraction of blocky retained austenite in the bainite microstructure increase significantly with the increases in austenitizing temperature, which do harm to the plasticity and impact toughness. The effect of austempering temperature on the transformation behavior and microstructural morphology of ultra-fine bainite is greater than that of austenitizing temperature. The prior martensite, formed when the austempering temperature below Ms, can refine the bainitic ferrite plates and improve the strength and impact toughness. However, the presence of prior martensite divides the untransformed austenite and inhibits the growth of bainite sheaves, thus prolonging the finishing time of bainite transformation. In addition, prior martensite also strengthens the stability of untransformed austenite though carbon partition and enhances the volume fraction of blocky retained austenite, which reduces the plasticity of ultra-fine bainitic steel. According to the experimental results, the optimum austempering process for 0.3 wt. %C ultra-fine bainitic steel is through austenitization at 1000 °C and austempering at 340 °C.

Effect of prior martensite on bainitic transformation of high Si hypereutectoid steel

The paper discusses the effect of prior martensite on the bainitic transformation of a high Si hypereutectoid steel. The research uses a combination of experimental techniques, high-resolution dilatometry and X-ray diffraction analysis, and theoretical calculations to investigate the impact of martensite on the transformation kinetics and microstructure evolution. The study shows that the presence of martensite, regardless of the amount, accelerates the bainitic transformation, particularly in the early and fastest stages, and at higher isothermal transformation temperatures. At lower temperatures, the preconditioning of the austenite prior to the bainitic transformation is such that its mechanical stability increases to the point where the whole transformation is inhibited. It is also suggested that the formation of cementite due to the tempering of martensite at the isothermal stage controls the final fraction of bainitic ferrite. The findings provide valuable insights into the role of prior martensite in bainitic transformation kinetics and microstructure development.

Effect of applied stress on bainite transformation, microstructure, and properties of 15CrMo steel

The effect of external stress on the isothermal bainite transformation kinetics and microstructure of 15CrMo steel was studied using dilatometer, optical microscopes (OM), electron back-scatter diffraction (EBSD), scanning electron microscopes (SEM) and X-ray diffraction (XRD). The results show that the additional mechanical driving force provided by the external stress promotes the transformation of bainite, resulting an increase volume fraction of bainite-ferrite (BF) and causing the BF laths to become slender. The application of external stress can also induce selective orientation during the bainite transformation process, and the orientation relationship between the prior austenite and the BF under stress is closer to the Kurdjumov-Sachs (K-S) relationship. There is also an incomplete transformation of bainite transformation under stress. At the same time, the Vickers hardness of 15CrMo steel would increase after applying external stress. The mechanical driving forces calculations demonstrate that when tensile stress and compressive stress are of similar magnitudes, they provide comparable mechanical driving forces. As a result, they exert similar effects on the kinetics of bainite transformation, leading to no discernible differences in the resulting bainite microstructure.

Influence of Q&P-Parameters and Al-Content on the Microstructural Evolution of Lean-Medium-Mn-Steels

Abstract This report investigates the impact of different heat treatment parameters and varying Al-contents on the microstructure of Quenching & Partitioning (Q&P) steels. Therefore, three lean-medium-Mn-steels with Al-contents between 0.3 and 0.9 wt-% underwent heat treatments according to Quenching & Partitioning regimes. For comparison, the steels were subjected to a transformation induced plasticity (TRIP) aided-bainitic-ferrite (TBF) process. In both cases, the samples were fully austenitized at 900 °C for 120 s, using dilatometry. For the Quenching & Partitioning process, the quenching temperature (TQ) ranged from 210 °C to 330 °C, while the transformation induced plasticity (TRIP) aided-bainitic-ferrite samples were cooled to 360 °C. Afterwards, the specimen were re-heated to the partitioning temperature (TP) of 400 °C and isothermally held for partitioning times (tp) of 40, 120 and 200 s. Subsequently, these steels were analyzed with regard to their phase fractions and hardness. The results indicated that in the Quenching & Partitioning process, the microstructure was primarily influenced by the partitioning temperature (TQ), while partitioning times (tp) played a minor role due to the time-independent martensitic transformation during quenching. In general, rising quenching temperature (TQ) led to an increase in retained austenite (RA) fraction. In the transformation induced plasticity (TRIP) aided-bainitic-ferrite (TBF) process, a substantial influence of partitioning times (tp) was found, which can be explained by the kinetics of the isothermal bainitic transformation. Regardless of the heat treatment concept, an increasing Al-content contributed to elevated retained austenite contents.

Effects of two-step austempering processes on the wear resistance and fatigue lifetime of high-carbon nanostructured bainitic steel

The previous research on two-step austempering processes have been mainly focused on the bainitic transformation kinetics, with few details the relationships between the parameters of two-step austempering process and the wear properties and fatigue lifetime. In this paper, the effects of two-step austempering process on wear resistance and fatigue life of high carbon nano-bainite steel were investigated. The results show that the first-step bainite with high volume fraction prolongs the total bainite transformation time. The two-step austempering samples contain finer bainitic ferrite plate and higher volume fraction of retained austenite, as well as resent excellent hardnesses, tensile properties, wear resistance and fatigue lifetime. In particular, the wear resistance of the 50%-290 °C sample is better than that of the 210 °C sample, this is caused by the transformation of the high fraction of retained austenite in the two-step austempering samples into hard martensite and the microstructure refinement zone that forms on the surface of the sample during the wear process. The 50%-290 °C sample have better fatigue properties at low total strain amplitudes (0.7% and 0.8%). When the total strain amplitude is 0.7%, the fatigue life of the 50%-290 °C sample is 57.5% higher than that of the 210 °C sample, which is because of the higher yield strength and finer microstructure of the 50%-290 °C sample. These results support the later study of controlling two-step austempering process for high-carbon nanostructured steels.

Laser Powder Bed Fusion Fabrication of a Novel Carbide-Free Bainitic Steel: The Possibilities and a Comparative Study with the Conventional Alloy

A newly developed medium-carbon carbide-free bainitic steel was fabricated for the first time utilizing the laser powder bed fusion (L-PBF) technique. Process parameters were optimized, and a high density of 99.8% was achieved. The impact of austempering heat treatment on the bainite morphology and transformation kinetics was investigated by high-resolution microstructural analysis (SEM, TEM, and EDS) and dilatometric analysis, and results were compared with conventionally produced counterparts. Faster kinetics and finer microstructures in the L-PBF specimens were found as a consequence of the as-built microstructure, characterized by fine grains and high dislocation density. However, a bimodal distribution of bainitic ferrite plate thickness (average value 60 nm and 200 nm, respectively) was found at prior melt pool boundaries resulting from carbon depletion at such sites.

Analysis of the Kinetics of Isothermal Bainitic Transformation in Alloy Steels

Analysis of the Kinetics of Isothermal Bainitic Transformation in Alloy Steels

Effects of above- or below-AC3 austenitization on bainite transformation behavior, microstructure and mechanical properties of carbide-free bainitic steel

The isothermal bainite transformation kinetics, microstructure, crystallography and mechanical properties of a carbide-free bainitic steel, subjected to pre-quenching treatment, followed by austenitization at various temperatures above or below AC3 and then by above-MS austempering, have been investigated via dilatometric measurements, microstructural characterization and mechanical tests. The results show that, as the austenitization temperature decreases from 980 °C to 860 °C (above AC3), bainite transformation first decelerates and then accelerates; this is primarily a result of the competition between the availability of parent austenite grain (PAG) boundaries as nucleation sites for bainite transformation and the shear resistance of bainite growth, both of which increase with decreasing the PAG size but play a reverse role in bainite transformation rate. Further decreasing the austenitization temperature down to 820 °C and 800 °C (below AC3) even more notably accelerates bainite transformation and shortens the transformation completion time, since the intercritical ferrite (or tempered martensite), which is formed in the intercritical region, favor the nucleation of initial bainitic laths. The resulting microstructure after austempering is composed of bainite, retained austenite and intercritical ferrite and/or fresh martensite, the feature of which depends on the austenitizing temperature below or above AC3. Below-AC3 austenitization largely refines the microstructure and decreases the amount of large, brittle fresh martensite/austenite blocks, mainly attributed to the synergetic role of the refinement of austenite grains and pre-existence of intercritical ferrite prior to austempering. In addition, the intercritical ferrite and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block. As compared with above-AC3 austenitization, below-AC3 austenitization shows large increases in elongation, especially for specimens subjected to shorter time austempering, despite some decreases in yield and tensile strengths.

Study on the Influence of Pre-Formed Phase on Accelerating Bainitic Transformation

Bainitic transformation goes through three stages: incubation period, nucleation, and growth. The long transformation time is not conducive to industrial production, which restricts its development. Therefore, research on accelerating bainitic transformation is based on the situation of energy saving and efficiency saving. The methods to accelerate bainitic transformation include the control of alloy elements, heat treatment control, etc. This paper focuses on the influence of the pre-formed phase on the kinetics of accelerated bainite transformation. Combined with existing research and experiments, it clarifies the influence of pre-formed ferrite, pre-formed martensite, and precipitated second-phase particles on the kinetics of bainitic transformation. Moreover, it clarifies the characteristics of bainitic transformation in terms of microstructure characterization.

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.

Bainite transformation under the effect of large stress in a carbide-free bainite steel

The effects of large stress on bainite transformation and microstructure were investigated by dilatometry, SEM and EBSD. The results show that the effect of large stress on bainite transformation includes two parts: mechanical driving force and prior deformation. A new finding is that bainite transformation is inhibited by large stress when the negative effect of prior deformation surpasses the positive effect of mechanical driving force. In addition, the bainite transformation kinetics model established by van Bohemen and Sietsma is modified to describe the bainite transformation under the effect of stress more accurately. Moreover, significant variant selection occurs under the effect of large stress, but the selected bainite variants in adjacent austenite twins are different. Furthermore, the hardness of the steel increases with the stress because of the increase in dislocation density and martensite amount.

Carbon Enrichment of Austenite during Ferrite – bainite Transformation in Low-alloy-steel

The bainitic transformation kinetics and carbon enrichment of austenite during isothermal holding at 723–923 K were investigated for an Fe-0.1mass%C-0.5mass%Si-2.0mass%Mn alloy. The transformation progressed rapidly until approximately 50 s, after which transformation stasis was observed at 823 K. The carbon concentration of austenite increased as the transformation proceeded, and showed an almost constant value during stasis. It reached approximately 0.45–0.50% at 823 K, which corresponds to the carbon concentration at the T0’ composition with an additional strain energy of 100 J/mol associated with the transformation. After stasis, a slight increase in the ferrite or bainitic ferrite fraction was observed. The carbon concentration of austenite also increased and reached approximately 0.60%, clearly exceeding the carbon concentration at the T0 composition. These results imply that at the first stage, the bainite transformation occurs and shows the incomplete transformation, following which at the second stage, diffusional ferrite transformation proceeds. The additional strain energy associated with the transformation calculated from the carbon concentration at stasis due to the incomplete bainite transformation tends to decrease as the holding temperature increases. This indicates that strain relaxation due to the transformation occurred at higher holding temperatures.

Effect of cold deformation on the bainite transformation kinetics, microstructural evolution and mechanical properties of austempered GCr15 bearing steel

In this work, the effect of prior cold deformation on the bainite transformation kinetics of austempered GCr15 bearing steel was investigated by means of dilatometry. It is found that prior cold deformation retards bainite transformation by increasing the incubation and growth time and decreasing the transformation rate, indicating that the effect of carbon content in parent austenite on bainite transformation kinetics is larger than the refinement of prior austenite grain size. Meanwhile, it also found from microstructures analysis that the prior cold deformation is beneficial to refine prior austenite grain size, facilitate dissolution of spherical carbides and improve its aspect ratio. After the heat treatment of austenitizing at 850 °C for 30 min and following austempering at 240 °C for 30 min, a marginal improvement of tensile strength and significant improvement of impact toughness were achieved in specimen with 30% prior cold deformation, which is increased by 6.2% and 29.3%, as compared to that of the specimen without prior cold deformation. In addition, the effect of prior cold deformation on microstructural evolution is also discussed.

Roles of cooling rate of undercooled austenite on isothermal transformation kinetics, microstructure, and impact toughness of bainitic steel

At present, the application of extra thick super bainite steel has attracted considerable attention. In this paper, medium carbon bainite steel is taken as the research object, and the effects of the cooling rate of undercooled austenite on the transformation kinetics, microstructure and mechanical properties of bainite steel are studied systematically. The results show that the incubation period of bainite shortens and the transformation kinetics accelerates with the decrease of the cooling rate of undercooled austenite. The segregation of solute atoms at a slow cooling rate shortens the incubation period. When bainite nucleates and grows by an autocatalytic method, the size of the untransformed austenite is large and the strain compatibility is strong, leading to low activation energy of its autocatalytic nucleation and accelerating the kinetics of bainite transformation. The size of block structure and the length of bainite sheaves decrease with the increase of cooling rate. The impact toughness of the sample with a cooling rate of 30 °C/s is twice as high as that of the sample with a cooling rate of 0.3 °C/s. As the smallest unit to control the toughness of bainite steel, block structure affects the impact toughness. The impact toughness of bainite increases with the decrease of its block size. The content of film-like retained austenite increases with the increase of cooling rate, which is also one of the factors of excellent impact toughness of bainite at high cooling rate.

Interrupted quenching and bainitising below Ms temperature of EN X37CrMoV5-1 hot-work tool steel: Bainitic transformation kinetics, microstructure and mechanical properties

A series of dilatometric experiments were performed to investigate the influence of martensite content on the kinetics of bainitic transformation during interrupted quenching and bainitising (iQ&B) treatment in EN X37CrMoV5-1 hot-work tool steel. Three iQ&B treatments were designed and conducted based on experiments to evaluate the mechanical properties and microstructure. The proposed heat treatment leads to multiphase structure providing tensile strength up to 1966 MPa, uniform elongation up to 11.33% (3.31% for quenching and tempering) and impact toughness up to 10.3 J/cm2 (6.9 J/cm2 for quenching and tempering). Obtained microstructures were favourable due to nanobainitic regions and a high amount of retained austenite (above 30 vol%) – percolating and susceptible to the TRIP effect. Prior martensite was found not only to improve the strength of the material but also to accelerate bainitic transformation. When bainitising is conducted at temperatures not above 260 °C bainitic transformation is accelerated at the very beginning. The reason for this acceleration effect, called the “left arm uplift effect”, is seen in the occurrence of “fresh” martensite-austenite interfaces with a small level of carbon enrichment, which act as favourable nucleation sites for bainitic ferrite plates.

Improvement of tensile properties by controlling the microstructure and crystallographic data in commercial pearlitic carbon-silicon steel via quenching and partitioning (Q&P) process

In the current research, a complex microstructure and crystallographic data were developed through quenching and partitioning (Q&P) process to improve tensile properties of commercial pearlitic carbon-silicon steel. Two-stage Q&P process, including full austenitization, quenching at 220 °C, followed by two different partitioning temperatures, was applied to the as-received specimen to generate a complex microstructure composed of tempered martensite, bainite, ultrafine carbides/martensite-austenite/retained austenite particles. Microstructure and crystallographic data were investigated by scanning electron microscopy, electron backscattered diffraction (EBSD), and X-ray diffraction techniques. Then, hardness and tensile properties were evaluated to confirm the improvement of mechanical properties. Dilatation-temperature curves exhibited the kinetics of martensitic and bainitic transformation during quenching and isothermal partitioning stages. The presence of nano-carbide particles inside athermal martensite was confirmed by electron microscopy due to the pre-formed martensite carbon depletion during the partitioning stage coupled with bainitic transformation. The formation of preferential atomic-compact <111> direction in BCC (martensite/bainite) plates characterized by EBSD, could enhance ductility by providing adequate slip systems. Point-to-point misorientation analyses demonstrated a slight dominance of low angle boundaries proportion in bainitic dominance structure in Q&P-220-375 specimen, which could be used in phase characterization. Results revealed that the development of nanoscale carbide dispersed in refined bainite/martensite matrix boosted the yield and ultimate tensile strength by over 100% and 110% compared to the initial pearlitic microstructure. However, ductility reduced to half value in Q&P-220-325 and Q&P-220-375 specimens.

Nanostructured bainite transformation characteristics in medium-carbon steel subjected to ausforming and isothermal holding below martensite start temperature

The influence of martensite on the transformation rate of bainite as well as enhancement of various properties is well established, yet the combined influence of both the primary martensite and low temperature ausforming below the martensite start temperature (MS) on the characteristics of microstructural evolution comprising ultrafine/nanostructured bainite, martensite and retained austenite (RA) in medium C steels has not been unambiguously clarified. A series of physical simulation experiments involving isothermal holding at various temperatures below and above the MS was conducted with or without prior ausforming (∼0.5 strain, 0.5 s−1 strain rate) in order to assess the influence of ausforming on subsequent transformation behaviour. The results suggest that when ausforming was applied above MS, it not only accelerated the bainite transformation kinetics, but also initiated strain-induced bainite/martensite formation. At temperatures below MS, prior ausforming did hinder the onset of bainite transformation, but the transformation rates were faster suggesting the strain-induced acceleration in kinetics. Due to a remarkable refinement of the bainitic structure achieved by prior ausforming in samples held below MS, the Vickers hardness improved appreciably unlike in the case of samples held above MS. Ausforming promoted significant refinement and uniformity of the bainitic plates, and also the orientations of the plates became more randomized. The dislocation density of RA decreased with the reduced content of secondary (fresh) martensite that formed during final cooling to room temperature.

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 prior martensite formation on the bainite transformation kinetics in high-strength 3% Mn multiphase steel

The work presents results on the effect of prior martensite formation on bainite transformation kinetics in a 3% medium-Mn multiphase steel. The material was subjected to two isothermal holding temperatures: 400 °C (without martensite) and 350 °C (with prior martensite). According to obtained dilatometric results, the formation of prior martensite leads to the acceleration of bainite transformation kinetics. The bainite formation starts and finishes much faster, when the prior martensite was present before the isothermal holding. The microstructural investigation of the steel after heat treatment was carried out using light and scanning electron microscopy. The microstructures were composed of fine bainitic laths with retained austenite and small amount of martensitic-austenitic islands at 400 °C. At 350 °C the presence of large tempered martensite laths was detected. The bainite is composed of a mixture of fine and coarse laths. The increase of the bainitic lath thickness is attributed to the coalescence process occurring at the lower holding temperature. The differences in the steel hardness after the two heat treatments were relatively small (~ 13 HV10).