Mechanism Of Bainite Transformation Research Articles

Carbon solute drag effect on the growth of carbon supersaturated bainitic ferrite: Modeling and experimental validations

The carbon partitioning and lengthening rate of bainitic ferrite (αb) are excellent experimental parameters to estimate our level of understanding of the mechanism of bainitic transformation from a continuum perspective and our ability to capture it in analytical expressions. For Fe-C alloys and relatively simple steels the classical Zener-Hillert theory captures the bainitic transformation rather well but mispredicts the level of carbon in solution in the bainite and overestimates the lengthening rates for transformations at lower temperatures. To address this issue, this paper presents a new thermo-kinetic model based on the Zener-Hillert theory and the Gibbs energy balance concept to simulate the lengthening behavior of αb in the Fe-C and low alloyed steels. The model incorporates the effect of the temperature dependent carbon diffusion within the migrating interface via a temperature dependent ferrite/austenite interfacial energy and a temperature dependent diffusion coefficient but does not impose local equilibrium across the interface. The good agreement between the model predictions and nine sets of published experiments indicates that both the carbon supersaturation in αb and the slower lengthening rate are caused by carbon diffusion within the migrating interface. It is found that the degree of carbon supersaturation in αb increases significantly with decreasing temperature. Consequently, the enhanced carbon solute drag effect, resulting from carbon diffusion within the interface, strongly retards the lengthening rates of αb at lower temperatures. Transformation strain is shown to have a modest effect on the lengthening rates but to lower the degree of carbon supersaturation.

Effect of the cooling rate in the medium temperature zone on the phase transformation and microstructure of carbide-free bainitic steel

Controlling continuous cooling is considered an effective means of producing high-strength and high-toughness bainitic steel. A medium carbon bainite frog steel is designed. By combining a thermal dilatometer with in-situ high-temperature metallography, the influence of continuous cooling rate on the phase transformation of bainitic steel in the medium temperature range (500-365 °C) is studied. The microstructure and crystallographic orientation are characterized. The results indicate that with the increase of cooling rate in the temperature range for bainitic transformation, the initial temperature of bainitic transformation significantly decreases, and the transformation kinetics enhance, which causes the cooling rate for obtaining a maximum fraction of bainite significantly shifts to the left. As the cooling rate increases, the morphology of the bainite changes from a mixture of granular and coarse lath-like forms to fine lath-like forms. The volume fraction and average size of retained austenite as well as the size of bainite laths decrease with the increase of cooling rate. In addition, there are significant differences in the high-angle interface under different cooling rates, which is related to the variant selection mechanism of bainitic transformation at different cooling rates. The research results reveal the transformation process and microstructure evolution of bainite under different cooling rates in the medium temperature range, providing support for the microstructure control and performance optimization of bainite frog steel under continuous cooling control.

Bainite nucleation mechanism and mechanical properties in low carbon bainite deposited metals with different nickel additions

Exploring the transformation mechanism of bainite is necessary to achieve the matching of high-strength and accepted damage tolerance for low carbon bainite deposited metals. In this work, the effect of Ni content on the bainite transformation mechanism and mechanical properties has been investigated systematically with Ni contents change from 4 wt. % to 9 wt. %. Scanning electron microscope, transmission electron microscope, and electron backscatter diffraction were used for the characterization of the microstructure. Results indicated that the nucleation method of bainite ferrite (BF) laths changed from autocatalytic nucleation to classical nucleation when the Ni content exceeded 5.5%. The deposited metals with BF laths nucleated by autocatalysis had excellent combinations of strength and toughness while the toughness severely deteriorated as the classical nucleation replaced the autocatalytic nucleation. The presence of autocatalytic nucleation resulted in the refinement of microstructure, and the density of high-angle grain boundaries had been improved effectively. In particular, apparent variant selection tendency in microstructure occurred accompanied by the disappearance of autocatalytic nucleation. Moreover, the strong variant selection tendency resulted in numerous identical variants growing next to each other and coarsening the microstructure. In particular, the key factor for the generating of autocatalytic nucleation was that the lower interfacial energy for the new BF laths nucleated through autocatalytic nucleation, and resulted in a lower nucleation activation energy. Therefore, autocatalytic nucleation tends to occur in the deposited metals with low bainite transformation speed due to enough time and space for new BF laths nucleates at existing BF lath.

Continuous Cooling Transformation Behaviour and Bainite Transformation Kinetics of 23CrNi3Mo Carburised Steel

In this study, the phase transformation behaviour of the carburised layer and the matrix of 23CrNi3Mo steel was comparatively investigated by constructing continuous cooling transformation (CCT) diagram, determining the volume fraction of retained austenite (RA) and plotting dilatometric curves. The results indicated that Austenite formation start temperature (Ac1) and Austenite formation finish temperature (Ac3) of the carburised layer decreased compared to the matrix, and the critical cooling rate (0.05 °C/s) of martensite transformation is significantly lower than that (0.8 °C/s) of the matrix. The main products of phase transformation in both the carburised layer and the matrix were martensite and bainite microstructures. Moreover, an increase in carbon content resulted in the formation of lamellar martensite in the carburised layer, whereas the martensite in the matrix was still lath. Furthermore, the volume fraction of RA in the carburised layer was higher than that in the matrix. Moreover, the bainite transformation kinetics of the 23CrNi3Mo steel matrix during the continuous cooling process indicated that the mian mechanism of bainite transformation of the 23CrNi3Mo steel matrix is two-dimensional growth and one-dimensional growth.

Study on Bainite/Martensite Transformation in Reheated Weld Metals

A new combination of laser scanning confocal microscope (LSCM) and electron backscattering diffraction (EBSD) with a field-emission scanning electron microscope (SEM) is utilized to study the mechanism of bainite transformation in reheated low carbon bainitic weld metal. The LSCM observations show that laths grow on the surface at various rates, from 30 μm/s to 240 μm/s, which is greatly larger than those referred in literature for bainite. In order to confirm that the laths are bainite and not surface martensite, additional experiments were performed. The crystallographic characteristics of surface bainite were compared with those of bulk bainite obtained during isothermal treatments and those of bulk martensite obtained by water quenching. By means of a dedicated EBSD data-treatment software, orientation relationship, variant selection and packet groups were identified; it was shown that both the surface laths and bulk bainite share the same misorientation, habit plane, and have similar variant distribution. Experiments are running to compare these features with those of bulk martensite. If the distinction between martensite and bainite is successful, the very high growth rates of the surface laths could be used to discuss the displacive/diffusive nature of bainitic transformations.

The bainite transformation behavior, the influencing factors and control strategy in ferritic-pearlitic forged crankshafts with coarse grains

Microstructural characterization of the bainite in a ferritic–pearlitic forged crankshaft was carefully investigated. A Gleeble thermo-mechanical simulator as well as a high resolution dilatometer were also used to analyze the effect of cooling rate on the bainite formation and the bainite transformation mechanism in steels with different austenite grain sizes. Results show that the fine structure of the bainite mainly consists of bainitic ferrite and martensite. No segregations are found where bainite forms. Bainite tends to form in the slower cooled inner part of the crankshaft with an austenite grain size exceeding 100 μm. The formation of bainite is mainly affected by the austenite grain size as well as the cooling rate in the crankshaft studied. As the austenite grain size increases, ferrite start, pearlite finish and bainite finish temperatures are decreased. More bainite forms when bainite finish temperature decreases. The critical cooling rate of bainite transformation is increased from 0.34 °C⋅s-1 to 0.44 °C⋅s-1, if the maximum austenite grain size is refined from 216 μm to 100 μm. For ferritic–pearlitic crankshafts, or other bulky products, the elimination of bainite can be achieved through austenite grain refinement.

Diagnostics of the bainite transformation mechanism and the effect of normalizing and tempering on the hardness and microstructure of the new Cr-Mo-V-Ti steel for operation at elevated temperature

The Cr-Mo-V-Ti based low alloy steels are widely used in thermal power plants because of their ability to withstand elevated temperatures and high pressure under continuous service. In the present work conventional heat treatment like normalizing and tempering of the alloys has been performed. The material used in this study was the laboratory prepared experimental low alloy Cr-Mo-V-Ti steel. Samples were austenitized at 980oC for 0.5 hour air cooled and tempered at 500, 550, 600, 650, 700 and 750oC for 1 hour. Mechanism of bainite transformation has been studied in Fe-C-Cr-Mo-V-Ti steel using high speed dilatometry. These experimental data indicate that bainitic ferrite forms by a displacive transformation mechanism, but soon afterwards, excess of carbon is partitioned into the residual austenite. The changes observed in the microstructure of the steel tempered at the higher temperature, i.e. 750°C were more advanced than those observed at the temperature of 500°C. Performed microstructural investigations have shown that the degradation of the microstructure of the examined steel was mostly connected with the processes of recovery and polygonization of the matrix, disappearance of lath bainitic microstructure and the growth of the carbides. The magnitude of these changes depended on the temperature of tempering.

Cementite Precipitation of a H21 Tool Steel after Hot Compression and Double Temper

The cementite precipitation behavior in the martensite and banite of the H21 tool steel under high temperature axisymmetric compression test and double temper was investigated. The main purpose on this work is to develop a better understanding regarding the transformation mechanism of bainite and martensite in a H21 tool steel. The selected deformation temperatures were 1100 oC and 1000 oC and the double temper process was carried out at 650 oC for 1 hour respectively. The results showed that the cementite was sensitive to the stress. The applied stress has affected the Fe3C precipitation behaviour by decreasing the number of variants carbides in tempered martensite and decreasing the number of a single variant carbides in tempered lower bainite. The results were in agreement with a displacive mechanism of martensite and bainite transformation. It was also found that hot deformation temperatures selected in this work have the same contribution in decreasing number of variant carbides in tempered martensite and decreasing number of single variant carbides occurred in tempered lower bainite.

Bainite isothermal transformation of Si–Mn–Mo alloy by phase structure factor calculation under different rare earth contents

The effect of rare earth (RE) on bainite transformation mechanism is not yet unified in the academia. Therefore, further studying the effect of RE on bainite transformation mechanism will have important guiding significance for the development of bainite steels. In this paper, using differential dilatometer (DIL805A/D), heat treatment was performed and C-curve was drawn. The effect of phase structure factor on bainite transformation under two different fractions of RE in Si–Mn–Mo bainite steels was investigated by the empirical electron theory (EET) of solids, molecules, dilatometry, and metallography. Experimental results show that RE makes Fe–C-RE segregation structural unit of Si–Mn–Mo bainite steels increase, i.e., the values of phase structure factor (nA, FCD) increase, and delay bainite transformation due to the drag effect resulting from the segregation of RE ferrite island interphase and the retardation of RE to carbon diffusion in Si–Mn–Mo bainite steels. Beside this, C-curve is shifted to right and down, and microstructures of bainite ferrite and substructures are finer.

In situ measured growth rates of bainite plates in an Fe-C-Mn-Si superbainitic steel

The growth rates of bainite plates in an Fe-C-Mn-Si superbainitic steel were investigated by in situ observation. The lengthening rates of ferrite bainite during both cooling and isothermal holding processes were observed and the growth rates of bainite plates nucleating at grain boundaries, within grains and on preformed bainite were measured. It is indicated that the lengthening rates of bainite plates during the cooling and isothermal processes were different, and that the growth rates of bainite plates nucleating at different types of sites also demonstrated diversity. The bainite plates initiating at grain boundaries during cooling grew the fastest, while the plates nucleating on preformed bainite did the slowest. However, the growth rate of the bainite plates nucleating at grain boundaries during isothermal transformation decreased the most, whereas the bainite plates initiating within grains grew the fastest. In addition, the growth rate of ferrite bainite in the study supported the diffusion transformation mechanism of bainite from the viewpoint of growth rate.

Crystallography of surface precipitates associated with shape change in a Ti–5.26 wt.% Cr alloy

The crystallographic features of surface α precipitates accompanied by surface tilt(s) in a Ti–5.26wt.% Cr alloy have been comprehensively studied by transmission electron microscopy of samples prepared using a focused ion beam. For comparison, the bulk precipitates formed far below the free surface in the same alloy have also been examined. It is found that both the surface and the bulk α precipitates exhibit a lath-shaped morphology and their habit plane always contains a single set of misfit dislocations with the Burgers vector [11¯1]β/2|[21¯1¯3]α/6. However, the surface precipitates differ from the bulk ones in terms of their orientation relationship with the matrix, the habit plane and the long axis direction. As a result, the interphase interface between the surface precipitates and matrix contains glissile dislocations and the interface of bulk precipitates is associated with sessile dislocations. Such a glissile interface is one of the major common features of displacive-diffusional and martensitic transformations and can be used to further understand the mechanism of bainitic transformation in steels and other alloy systems.

Quantum chemical study of organic inhibitors of corrosion and hydrogen absorption An investigation of the mechanism of bainite transformation in experimental 0.2c-1v-2mn steel An investigation of the mechanism of bainite transformation in experimental 0.2c-1v-2mn steel

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An investigation of the mechanism of bainite transformation in experimental 0.2c-1v-2mn steel

An investigation of the mechanism of bainite transformation in experimental 0.2c-1v-2mn steel

Advanced Bainitic and Martensitic Steels with Carbide-Free Microstructures Containing Retained Austenite

Recent decades have witnessed some remarkable advances in engineering steels driven by the need to respond to challenges posed, for example, by recovery and transmission of oil and gas, or enhanced vehicle safety and fuel economy. Foremost amongst these must surely be the extended application of carbon steels, achieved principally through ferrite grain refinement by the practice of microalloying coupled with controlled thermomechanical processing. Limitations to strengthening ferrite/pearlite structures further by grain refinement or precipitation, however, has focused attention back to acicular forms of microstructure. One of the most interesting advances in this area has been the development of bainitic steels, which have been almost dormant since the mid-20th century. This resurgence may partly be attributed to a better appreciation of the bainite transformation mechanism, and the experimental work for this which unexpectedly spawned some interesting bainitic microstructures which have seen further development and application. These are the so-called ‘carbide-free’ bainites, which employ alloying to replace carbides, principally cementite, with carbon-stabilized retained austenite. Particularly noteworthy has been the emergence of the transformation induced plasticity (TRIP) sheet steels with enhanced properties principally targeted for automotive use. It is worth mentioning also that a parallel development has produced similar microstructure in austempered ductile irons (ADI), another important ferrous alloy which has seen recent expanding interest in its application. Even more recently, as we proceed into the 21st century, the concept of employing steel microstructures containing carbon-enriched retained austenite, has been developed further by combining both alloying and novel heat treatment procedures to exchange ‘bainitic’ ferrite with ‘martensitic’ ferrite. Interestingly, this non-equilibrium ‘quenching and partitioning’ process route also offers the possibility to increase the retained austenite carbon concentration to very high levels, potentially revealing new and previously unobtainable properties.

On Bainite Transformation Kinetics and Mechanism

The platelike bainitic ferrite growth rates were calculated by two modified diffusional models. Good agreements between experimental and theoretical results are found in Fe-0.59C wt.%, Fe-0.81C wt.% and Fe-0.478C-4.87Ni wt.% alloys. A slowing down effect due to the alloying element Mo is emphasized in Fe-0.69C-1.8Ni-0.8Mo wt.% alloy. However, the experimental data are lower than theoretical ones about two orders in Fe-C-8.7Ni wt.% alloys. According to the discussion of the results, it is suggested that the bainite transformation mechanism may relates to steel composition and transformation temperature.

Structure and properties of high-speed steels after austempering

Some special features of the kinetics and mechanism of the bainitic transformation in high-speed steel HS18-0-1 have been studied. Austempering has been performed at temperatures 300°C and 270°C for 10–360 min, after heating up to 1100°C, 1200°C, and 1280°C. Optical metallographic analysis, electron-microscope analysis, X-ray phase analysis and testing of Rockwell hardness have been carried out. The bainitic transformation at temperatures 270°C and 300°C takes place according to the scheme: α + A(c). The temperature of heating (1100°C, 1200°C, 1280°C) does not influence the mechanism of bainitic transformation. It changes the kinetic process parameters only.

The mechanism of bainite transformation

Ultra-fine structures of bainite and its surface relief, and carbide nucleation inside austenite at the interface of ferrite/austenite are studied in this paper. The sympathetic nucleation and ledgewise growth theory is applied to explain the experimental results.

Experimental investigation of the influence of the austenite grain size on the mechanism and kinetics of the bainite transformation in steels

This study describes selected results of an extensive characterisation of the influence of the reduction of the austenite grain size on the mechanism of bainite transformation in several high carbon low-alloy steels. For quite large grain sizes, it is generally admitted that the transformation starts with the nucleation and growth of a first platelet at the austenite grain boundary. It further proceeds by the autocatalytic nucleation and growth of new platelets at the tip of the previous ones, bringing about the typical sheaf morphology. This process then stops when the sheaf reaches the grain boundary. Previous studies have shown that the increase of the grain boundary area resulting from smaller grains brings about an increase of the nucleation rate and thus faster kinetics for the bainite transformation. However, no systematic work was carried out for very small grain sizes, particularly when the austenite grain becomes of the same order as the length of the individual platelets of bainitic ferrite. This study shows how the conditions of nucleation and growth of the bainitic ferrite platelets as well as the carbon redistribution are influenced when the austenite grain size is ranging from I to 50 mum.

An analysis of retained austenite in austempered ductile iron

Data from the literature have been analyzed to understand aspects of the retained austenite in austempered ductile irons, especially its relationship with the transformation mechanism of bainite. The final and initial carbon concentrations in austenite, % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabm4qayaara% WaaSbaaSqaaiabeo7aNbqabaaaaa!38A0! $$\bar C_\gamma $$ and C γ 0 , respectively, are important in determining the maximum extent of reaction, and hence, the amount of austenite and and bainitic ferrite % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabm4qayaara% WaaSbaaSqaaiabeo7aNbqabaaaaa!38A0! $$\bar C_\gamma $$ and C γ 0 data have been expressed in terms of chemical compositions and reaction temperature, with reasonable agreement between experimental and predicted results. It is demonstrated that, in connection with the lever rule, the calculated % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabm4qayaara% WaaSbaaSqaaiabeo7aNbqabaaaaa!38A0! $$\bar C_\gamma $$ and C γ 0 values can be employed to predict the microstructural constituents of austempered ductile irons.

The mechanism of bainite transformation in steels

Sympathetic nucleation and ledgewise growth are competitive with each other throughout the bainite formation in steels. Many evidences are provided to support this mechanism as follows: ultra-fine structures of bainite and its surface relief, and carbide nucleation inside austenite at the interface of ferrite/austenite, etc.