Rate Of Bainite Transformation Research Articles

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.

Effect of High‐Temperature Deformation on Bainite Transformation of a Low‐Carbon Bainite Steel

Unlike medium‐carbon and high‐carbon steels, the effect of the high‐temperature deformation process on the bainite transformation of low‐carbon steels is yet to be fully understood. Processes with varying high‐deformation temperatures and reductions are designed to investigate the bainitic transformation of low‐carbon steel. The results indicate that compared with the undeformed samples, for all deformation temperatures (850, 950, and 1050 °C) in the austenite temperature range, a small reduction of 5% accelerates the bainite transformation rate corresponding to the samples, and this can be attributed to the increase in the driving force. The promoting effect is increased with a decrease in deformation temperature. However, a relatively large reduction of the samples by 25% and 50% decreases the bainite transformation rate, and the inhibiting effect increases with decreasing deformation temperature. In addition, the amount of bainite increases at 1050 °C under all strains, and the amount is higher than the amount observed in undeformed samples. For the deformation temperature of 850 °C, the bainite amount increases under conditions of small reduction (5%), whereas the bainite amount decreases under large strain conditions (50% at 850 and 950 °C). The results provide a theoretical basis for optimizing the processing parameters associated with the production of low‐carbon bainite steels.

Effect of Austenite Grain Size on the Bainitic Transformation in a 690 MPa Grade High-Strength Multi-Functional Construction Steel

A high-strength low-carbon construction structural steel was investigated in the laboratory. The various austenite grain sizes were obtained by austenitizing the steel at different temperatures. The effect of austenite grain size on bainite transformation was studied by the dilatometer. The results show that the microstructure of high-strength low-carbon structural steels mainly includes granular bainite, lath-like bainite and martensite-austenite (M-A). The microstructure changes from granular bainite to lath-like bainite with the increase in austenitizing temperature or austenite grain size. When the samples were heated at the lower temperature of 860 °C, the bainite starting temperature was relatively high, which was mainly attributed to the promotion of the granular bainitic nucleation and the formation of the solute-depleted regions in the austenite. Compared to 860 and 1260 °C, the bainite transformation rate in the specimen austenitized at 1000 °C is the highest because of the small prior austenite grain size and larger transformation driving force.

Effect of martensite pre-quenching on bainite transformation kinetics, martensite/bainite duplex microstructures, mechanical properties and retained austenite stability of GCr15 bearing steel

In this work, effect of martensite pre-quenching on the subsequent bainite transformation kinetics of GCr15 bearing steel has been investigated by means of dilatometry. The results show that the pre-quenched martensite is significantly accelerate nucleation of subsequent bainite transformation in comparison with direct bainite austempering. Meanwhile, specimen of martensite pre-quenching at 200 °C has the shortest incubation period of bainite transformation at 240 °C. Moreover, the transformation rate of bainite transformation after martensite pre-quenching is faster than direct bainite austempering in the initial growth stage, and decrease with the decreasing of martensite pre-quenching temperature. Compared with the conventional quenched and tempered (QT) specimen, the tensile strength is slightly decrease from 2170 MPa to 2143 MPa, but the impact toughness is obviously increase from 43 J to 71 J in specimen heat-treated by martensite pre-quenching at 200 °C and subsequent bainite transformation at 240 °C. In addition, the mechanical stability of retained austenite in all martensite pre-quenching specimens are higher than that of QT specimen.

Effect of isothermal quenching on microstructure and hardness of GCr15 steel

Through hardness measurement and microstructure observation, the effect of isothermal quenching on the bainite transformation, the size and area fraction of carbides in GCr15 steel was studied. Results show that, at the solid solution temperature of 860 °C, as the isothermal quenching temperature increases from 200 °C to 230 °C, the bainite transformation rate in GCr15 steel increases. At the initial stage of isothermal quenching, there is relatively less bainite transformation in GCr15 steel, but the increasing rate is relatively larger. With the extension of the isothermal time, the bainite content increases, and the increasing rate gradually becomes slower. The above phenomena are related to the existence of the incubation period of bainite transformation, the driving force of bainite ferrite nucleation, and the stability of undercooled austenite. As isothermal time increases, the carbide content decreases, while the carbide size generally increases, which is related to the dissolution of some undissolved carbides due to the change of the surrounding carbon concentration and the Ostwald ripening mechanism. The hardness of GCr15 steel is obviously reduced at the stage of the first 1 h, while between 2 and 4 h the hardness decrease of GCr15 steel tends to be gentle, which is related to the increase of bainite content, the weakening of dispersion strengthening effect resulted from the increase of carbide size, and the self-inhibiting behavior of the bainite transformation.

Effect of prior austenite grain size on the formation of carbide-free bainite in low-alloy steel

ABSTRACT Inconsistencies exist in literature regarding the effect of prior austenite grain size (PAGS) on the extent and kinetics of bainite transformation. Attempts have been made in the present work to address these issues in a low-alloy carbide-free bainitic steel using dilatometry, over a range of PAGS. The bainite transformation kinetics in the above-mentioned conditions have been analysed quantitatively using established kinetic model to extract information related to the transformation mechanisms in such conditions. Greater obstruction from grain boundaries in fine-grained austenite restricts sheaves of bainite to develop completely and thereby reduces the volume fraction of bainite in comparison with coarse-grained austenite. Initial nucleation rate of bainite transformation increases with decreasing PAGS due to an increase in the nucleation site density. However, the maximum nucleation rate decreases consistently with decreasing PAGS due to gradual reduction in the autocatalytic factor.

The Influence of Vanadium Additions on Isothermally Formed Bainite Microstructures in Medium Carbon Steels Containing Retained Austenite

The influence of V additions on isothermally formed bainite in medium carbon steels containing retained austenite has been investigated using in-situ high energy X-ray diffraction (HEXRD) and ex-situ electron energy loss spectroscopy (EELS) and energy dispersive X-ray analysis (EDX) techniques in the transmission electron microscope (TEM). No significant impact of V in solid solution on the bainite transformation rate, final phase fractions or on the width of bainite laths was seen for transformations in the range 375–430 °C. No strong influence on the dislocation density could be detected, although quantitative analysis was impeded by ferrite tetragonality. A reduction in the carbon content of retained austenite Cγ that is not believed to be due to competition with VC or cementite precipitation was observed. No influence of V on the carbon supersaturation in bainitic ferrite Cb could be directly measured, although carbon mass balance calculations suggest Cb slightly increases. A beneficial refinement of blocky MA and a corresponding size effect induced enhancement in austenite stability were found at the lowest transformation temperature. Overall, V additions result in a slight increase in strength levels.

Effect of prior austenite grain size on isothermal bainite transformation in 65Cr steel

Isothermal bainite transformation was conducted on both ultrafine-grained (UFG) and normal-grained (NG) 65Cr steels. Both bainite morphology and transformation rate are greatly influenced by prior austenite grain size. Bainite transformation rate is greatly retarded in the UFG steel with average grain size of 4.6 μm. Lower bainite was obtained for both UFG and NG steels when austempered at 250 °C and 300 °C; however, the bainite morphology changed from lower to upper banite for UFG steel when austempered at 400 °C, and lower bainite was still obtained in NG steel. This phenomenon is caused by austenite yield strength increase due to grain refinement.

In Situ Observation of Bainite Transformation and Simultaneous Carbon Enrichment in Austenite in Low-Alloyed TRIP Steel Using Time-of-Flight Neutron Diffraction Techniques

An in situ neutron diffraction experiment during austempering of low-alloyed transformation-induced plasticity steel, Fe-1.48Si-1.52Mn-0.15C, in wt pct was conducted. In this study, time-of-flight neutron diffractometer with a large detector coverage, iMATERIA at J-PARC MLF, was employed. The phase fraction and carbon concentration in austenite could be quantitatively determined with a time resolution 1 minute although considerable textures existed for both ferrite and austenite. The carbon concentration in austenite during austempering was found to be inhomogeneous, resulting in a bimodal concentration distribution. The low-carbon region was consumed by bainite transformation whereas the high-carbon austenite slightly increased and even survived the final cooling to room temperature, forming a retained austenite. The rate of bainite transformation was affected by the state prior to the start of austempering. Consequently, different morphological features of the retained austenite were formed. More block-shaped austenite was observed in the case of slower bainite transformation, and it was determined that film-shaped austenite could also exist. The average carbon concentration was similar to that of high-carbon austenite during austempering. Hence, the film and block shapes of the retained austenite do not necessarily reflect the difference in carbon concentration.

Effect of cerium addition on the bainite transformation behavior of medium carbon microalloyed steel

The bainite transformation kinetics and microstructure evolution of medium carbon microalloyed steels containing 0, 0.0367%, 0.0559%, and 0.0792% of the rare earth element cerium (Ce) were investigated. Optical microscopy and scanning electron microscopy characterizations showed that increasing Ce concentration refines the transformed microstructures, increases the volume fractions of bainite, and reduces the average size of the martensite islands by increasing the carbon content of the bainite. Dilatometry data indicated that Ce promotes the bainite transformation of medium carbon microalloyed steel, increases the bainite transformation rate, and prolongs the total time that the reaction takes to reach completion. Based on the displacive mechanism of bainite formation, the effect of Ce on the transformation kinetics of bainite results from Ce refining the austenite grain and increasing the carbon content in bainite. A Ce content of 0.0792 wt% is also not excessive for the bainite transformation of medium carbon microalloyed steel.

The Effects of Hot Deformation of Austenite on the Bainite Transformation in a Fe-C-Mn-Si-Cr Steel

The effects of hot deformation conditions in the stable austenite state on the transformation kinetics and morphology of bainite were examined using dilatometry, electron back scatter diffraction (EBSD) and X-ray diffraction (XRD) measurements in a carbide-free bainitic steel with a composition of Fe-0.34C-2Mn-1.5Si-1Cr (in wt.%). Both uniaxial tensile and compression tests were applied to study the effect of the deformation mode. The temperature, strain and strain rate of deformation were varied in the ranges of 820-1000 °C, 0.1-0.6 and 0.001-0.1/s respectively. It has been revealed that hot tensile deformation retards the austenite transformation to lower bainite. The overall transformation kinetics slows down and the final attained amount of bainite decreases after completion of the isothermal transformation at 350 °C. However, hot compression deformation accelerates the bainite transformation, increasing both the bainite transformation rate and the final amount of bainite formed. The total amount of bainite increases with decreasing the strain rate irrespective of the mode of deformation. The effect of the deformation temperature and strain on the bainite transformation is in a complicated manner depending on the deformation mode.

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.

Low-temperature transformation to bainite in a medium-carbon steel

Abstract The consequences of reducing the carbon concentration from 0.8 to 0.6 wt.% in a bulk nanostructured bainitic-steel have been investigated with the aim of enabling processing. We observed that the average thickness of the product phase plates increased from below 20–40 nm to about 100 nm after isothermal transformation at the same temperature. The increase in plate size led to decrease in hardness as expected. However the hardness reduction was not as large as might be expected, a transition to lower bainite at temperatures below 250°C may have provided additional strengthening. Transformation just below the martensite start temperature resulted in a mixture of martensite and bainite, with no observed increase in the rate of bainite transformation.

Effect of Chromium and Vanadium Contents on Continuous Cooling Transformation Behavior of Multi-Component White Cast Iron

In this study, the multi-component white cast irons with 5wt% of Mo, W, 2 wt% of C, Co each and varying Cr content from 1 to 9 wt% and V content from 3 to 9 wt% independently were employed and effect of Cr and V contents on the behavior of continuous cooling transformation (CCT) was investigated. When the Cr content increases to 5wt%, the critical cooling rate of pearlite transformation (VC-P) does not change much, but the VC-P decreases gradually as the Cr content is increased over 5 wt%. The critical cooling rate of bainite transformation (VC-B) increases with an increase in the Cr content. In the case of V effect, on the other hand, the VC-P decreases as the V content increases. However, the VC-B decreases with an increase in the V content up to 4mass%, but over 4wt%V, it increases, and at 9wt%V, the precipitation of ferrite phase occurred. Ms temperature decreases continuously as the Cr content increases to 5wt%, and then increases. As the V content increases, Ms temperature rises gradually. In the case that Mf point appears, the Mf temperature goes up with an increase in the Cr and V contents.

A New Physical Model for the Kinetics of the Bainite Transformation

A model describing the bainite reaction is presented that takes into account the effect of very small austenite grain size. This model considers the displacive approach and uses thermodynamic criteria for the description of the nucleation and growth of bainite sub-units forming either at grain boundaries or autocatalytically on previous sub-units. The evolution of the austenite composition with the partitioning and redistribution of carbon is estimated as the transformation proceeds. The size of the sub-units is calculated for each composition and temperature. The transformation kinetics, as well as the incomplete reaction phenomenon, are correctly predicted. Furthermore, the influence of the austenite grain size on the bainite transformation rate is addressed in the case of austenite grains smaller than the length of an unconstrained bainite sub-unit. In this case, the observed enhanced nucleation rate is semi-empirically related to the austenite yield strength. This semi-empirical relationship is corroborated with kinetics data from different alloys.

Effects of austenitizing temperature on the kinetics of bainite reaction at constant austenite grain size in FeC and FeNiC alloys

The influence of austenitizing temperature on a subsequent diffusional transformation has been investigated. The overall bainite transformation rate in a slightly hypo-eutectoid Fe-7.6Ni-0.48C alloy is increased by a double austenitizing treatment. Optical microscope observations have revealed that this acceleration of bainite transformation is caused mostly by the acceleration of nucleation rate at austenite grain boundaries. Intergranular fracture surface observations have shown that a second austenitizing treatment enhances cementite precipitation on grain boundaries. It is considered that the enrichment of sulfur at grain boundaries by a second austenitizing treatment enhances the precipitation of cementite during the subsequent cooling to the transformation temperature. When the second austenitizing temperature is 750°C or below, disc shaped Fe-rich FeS precipitates are formed together with the cementites. The initial stage of bainite transformation has shown that the cementites and Fe-rich FeS precipitates act as preferential nucleation sites for bainite. The precipitation of cementite and Fe-rich FeS by a double austenitizing treatment is also observed in a Ni free Fe-0.61C alloy.